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Physics and Astronomy â€ș Atomic and Molecular Physics, and Optics

Topological Materials and Phenomena

Works Count: 43420

Description

This cluster of papers explores the properties and applications of topological insulators and superconductors, including phenomena such as quantum spin Hall effect, Majorana fermions, Dirac and Weyl semimetals, photonic topological insulators, and quantum anomalous Hall effect. It also delves into the potential for topological quantum computation.

Keywords

Topological Insulators; Superconductors; Quantum Spin Hall Effect; Majorana Fermions; Dirac Semimetals; Weyl Semimetals; Photonic Topological Insulators; Quantum Anomalous Hall Effect; Chiral Anomaly; Topological Quantum Computation

Topological insulators and superconductors: tenfold way and dimensional hierarchy

2010-06-17
Shinsei Ryu , Andreas P. Schnyder , Akira Furusaki +1 more
It has recently been shown that in every spatial dimension there exist precisely five distinct classes of topological insulators or superconductors. Within a given class, the different topological sectors can 
 It has recently been shown that in every spatial dimension there exist precisely five distinct classes of topological insulators or superconductors. Within a given class, the different topological sectors can be distinguished, depending on the case, by a Z or a Z_2 topological invariant. This is an exhaustive classification. Here we construct representatives of topological insulators and superconductors for all five classes and in arbitrary spatial dimension d, in terms of Dirac Hamiltonians. Using these representatives we demonstrate how topological insulators (superconductors) in different dimensions and different classes can be related via dimensional reduction by compactifying one or more spatial dimensions (in Kaluza-Klein-like fashion). For Z-topological insulators (superconductors) this proceeds by descending by one dimension at a time into a different class. The Z_2-topological insulators (superconductors), on the other hand, are shown to be lower-dimensional descendants of parent Z-topological insulators in the same class, from which they inherit their topological properties. The 8-fold periodicity in dimension d that exists for topological insulators (superconductors) with Hamiltonians satisfying at least one reality condition (arising from time-reversal or charge-conjugation/particle-hole symmetries) is a reflection of the 8-fold periodicity of the spinor representations of the orthogonal groups SO(N) (a form of Bott periodicity). We derive a relation between the topological invariant that characterizes topological insulators/superconductors with chiral symmetry and the Chern-Simons invariant: it relates the invariant to the electric polarization (d=1), or to the magnetoelectric polarizability (d=3). Finally, we discuss topological field theories describing the space time theory of linear responses, and study how the presence of inversion symmetry modifies the classification.
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Quantum Spin Hall Effect in Silicene and Two-Dimensional Germanium

2011-08-09
Cheng‐Cheng Liu , Wanxiang Feng , Yugui Yao
We investigate the spin-orbit opened energy gap and the band topology in recently synthesized silicene as well as two-dimensional low-buckled honeycomb structures of germanium using first-principles calculations. We demonstrate that 
 We investigate the spin-orbit opened energy gap and the band topology in recently synthesized silicene as well as two-dimensional low-buckled honeycomb structures of germanium using first-principles calculations. We demonstrate that silicene with topologically nontrivial electronic structures can realize the quantum spin Hall effect (QSHE) by exploiting adiabatic continuity and the direct calculation of the Z(2) topological invariant. We predict that the QSHE can be observed in an experimentally accessible low temperature regime in silicene with the spin-orbit band gap of 1.55 meV, much higher than that of graphene. Furthermore, we find that the gap will increase to 2.9 meV under certain pressure strain. Finally, we also study germanium with a similar low-buckled stable structure, and predict that spin-orbit coupling opens a band gap of 23.9 meV, much higher than the liquid nitrogen temperature.
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Topological field theory of time-reversal invariant insulators

2008-11-24
Xiao‐Liang Qi , Taylor L. Hughes , Shou-Cheng Zhang
We show that the fundamental time-reversal invariant (TRI) insulator exists in $4+1$ dimensions, where the effective-field theory is described by the $(4+1)$-dimensional Chern-Simons theory and the topological properties of the 
 We show that the fundamental time-reversal invariant (TRI) insulator exists in $4+1$ dimensions, where the effective-field theory is described by the $(4+1)$-dimensional Chern-Simons theory and the topological properties of the electronic structure are classified by the second Chern number. These topological properties are the natural generalizations of the time reversal-breaking quantum Hall insulator in $2+1$ dimensions. The TRI quantum spin Hall insulator in $2+1$ dimensions and the topological insulator in $3+1$ dimensions can be obtained as descendants from the fundamental TRI insulator in $4+1$ dimensions through a dimensional reduction procedure. The effective topological field theory and the ${Z}_{2}$ topological classification for the TRI insulators in $2+1$ and $3+1$ dimensions are naturally obtained from this procedure. All physically measurable topological response functions of the TRI insulators are completely described by the effective topological field theory. Our effective topological field theory predicts a number of measurable phenomena, the most striking of which is the topological magnetoelectric effect, where an electric field generates a topological contribution to the magnetization in the same direction, with a universal constant of proportionality quantized in odd multiples of the fine-structure constant $\ensuremath{\alpha}={e}^{2}∕\ensuremath{\hbar}c$. Finally, we present a general classification of all topological insulators in various dimensions and describe them in terms of a unified topological Chern-Simons field theory in phase space.
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Observation of a large-gap topological-insulator class with a single Dirac cone on the surface

2009-05-10
Y. Xia , Dong Qian , David Hsieh +7 more
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A tunable topological insulator in the spin helical Dirac transport regime

2009-07-20
David Hsieh , Yipu Xia , Dong Qian +7 more
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Classification of topological quantum matter with symmetries

2016-08-31
Ching‐Kai Chiu , Jeffrey C. Y. Teo , Andreas P. Schnyder +1 more
Topological materials have become the focus of intense research in recent years, since they exhibit fundamentally new physical phenomena with potential applications for novel devices and quantum information technology. One 
 Topological materials have become the focus of intense research in recent years, since they exhibit fundamentally new physical phenomena with potential applications for novel devices and quantum information technology. One of the hallmarks of topological materials is the existence of protected gapless surface states, which arise due to a nontrivial topology of the bulk wave functions. This review provides a pedagogical introduction into the field of topological quantum matter with an emphasis on classification schemes. We consider both fully gapped and gapless topological materials and their classification in terms of nonspatial symmetries, such as time-reversal, as well as spatial symmetries, such as reflection. Furthermore, we survey the classification of gapless modes localized on topological defects. The classification of these systems is discussed by use of homotopy groups, Clifford algebras, K-theory, and non-linear sigma models describing the Anderson (de-)localization at the surface or inside a defect of the material. Theoretical model systems and their topological invariants are reviewed together with recent experimental results in order to provide a unified and comprehensive perspective of the field. While the bulk of this article is concerned with the topological properties of noninteracting or mean-field Hamiltonians, we also provide a brief overview of recent results and open questions concerning the topological classifications of interacting systems.
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A topological Dirac insulator in a quantum spin Hall phase

2008-04-01
David Hsieh , Dong Qian , L. Andrew Wray +4 more
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Generic New Platform for Topological Quantum Computation Using Semiconductor Heterostructures

2010-01-27
Jay D. Sau , Roman M. Lutchyn , Sumanta Tewari +1 more
We show that a film of a semiconductor in which $s$-wave superconductivity and Zeeman splitting are induced by the proximity effect, supports zero-energy Majorana fermion modes in the ordinary vortex 
 We show that a film of a semiconductor in which $s$-wave superconductivity and Zeeman splitting are induced by the proximity effect, supports zero-energy Majorana fermion modes in the ordinary vortex excitations. Since time-reversal symmetry is explicitly broken, the edge of the film constitutes a chiral Majorana wire. The heterostructure we propose---a semiconducting thin film sandwiched between an $s$-wave superconductor and a magnetic insulator---is a generic system which can be used as the platform for topological quantum computation by virtue of the existence of non-Abelian Majorana fermions.
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Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator

2013-03-15
Cui‐Zu Chang , Jinsong Zhang , Xiao Feng +7 more
The quantized version of the anomalous Hall effect has been predicted to occur in magnetic topological insulators, but the experimental realization has been challenging. Here, we report the observation of 
 The quantized version of the anomalous Hall effect has been predicted to occur in magnetic topological insulators, but the experimental realization has been challenging. Here, we report the observation of the quantum anomalous Hall (QAH) effect in thin films of Cr-doped (Bi,Sb)2Te3, a magnetic topological insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance reaches the predicted quantized value of h/e^2,accompanied by a considerable drop of the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes whereas the Hall resistance remains at the quantized value. The realization of the QAH effect may lead to the development of low-power-consumption electronics.
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Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices

2012-04-13
Vincent Mourik , Kun Zuo , Sergey Frolov +3 more
Majorana fermions are particles identical to their own antiparticles. They have been theoretically predicted to exist in topological superconductors. We report electrical measurements on InSb nanowires contacted with one normal 
 Majorana fermions are particles identical to their own antiparticles. They have been theoretically predicted to exist in topological superconductors. We report electrical measurements on InSb nanowires contacted with one normal (Au) and one superconducting electrode (NbTiN). Gate voltages vary electron density and define a tunnel barrier between normal and superconducting contacts. In the presence of magnetic fields of order 100 mT we observe bound, mid-gap states at zero bias voltage. These bound states remain fixed to zero bias even when magnetic fields and gate voltages are changed over considerable ranges. Our observations support the hypothesis of Majorana fermions in nanowires coupled to superconductors.
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Photonic Floquet topological insulators

2013-04-01
Mikael C. Rechtsman , Julia M. Zeuner , Yonatan Plotnik +6 more
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Topological Insulators in Three Dimensions

2007-03-07
Liang Fu , C. L. Kane , E. J. Melé
We study three-dimensional generalizations of the quantum spin Hall (QSH) effect. Unlike two dimensions, where a single Z2 topological invariant governs the effect, in three dimensions there are 4 invariants 
 We study three-dimensional generalizations of the quantum spin Hall (QSH) effect. Unlike two dimensions, where a single Z2 topological invariant governs the effect, in three dimensions there are 4 invariants distinguishing 16 phases with two general classes: weak (WTI) and strong (STI) topological insulators. The WTI are like layered 2D QSH states, but are destroyed by disorder. The STI are robust and lead to novel "topological metal" surface states. We introduce a tight binding model which realizes the WTI and STI phases, and we discuss its relevance to real materials, including bismuth.
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Observation of unidirectional backscattering-immune topological electromagnetic states

2009-10-01
Zheng Wang , Y. D. Chong , John D. Joannopoulos +1 more
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Anomalous Hall effect

2010-05-13
Naoto Nagaosa , Jairo Sinova , Shigeki Onoda +2 more
We present a review of experimental and theoretical studies of the anomalous Hall effect (AHE), focusing on recent developments that have provided a more complete framework for understanding this subtle 
 We present a review of experimental and theoretical studies of the anomalous Hall effect (AHE), focusing on recent developments that have provided a more complete framework for understanding this subtle phenomenon and have, in many instances, replaced controversy by clarity. Synergy between experimental and theoretical work, both playing a crucial role, has been at the heart of these advances. On the theoretical front, the adoption of Berry-phase concepts has established a link between the AHE and the topological nature of the Hall currents which originate from spin-orbit coupling. On the experimental front, new experimental studies of the AHE in transition metals, transition-metal oxides, spinels, pyrochlores, and metallic dilute magnetic semiconductors, have more clearly established systematic trends. These two developments in concert with first-principles electronic structure calculations, strongly favor the dominance of an intrinsic Berry-phase-related AHE mechanism in metallic ferromagnets with moderate conductivity. The intrinsic AHE can be expressed in terms of Berry-phase curvatures and it is therefore an intrinsic quantum mechanical property of a perfect cyrstal. An extrinsic mechanism, skew scattering from disorder, tends to dominate the AHE in highly conductive ferromagnets. We review the full modern semiclassical treatment of the AHE together with the more rigorous quantum-mechanical treatments based on the Kubo and Keldysh formalisms, taking into account multiband effects, and demonstrate the equivalence of all three linear response theories in the metallic regime. Finally we discuss outstanding issues and avenues for future investigation.
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Dirac semimetal and topological phase transitions in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>A</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>Bi (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>A</mml:mi><mml:mo>=</mml:mo><mml:mtext>Na</mml:mtext></mml:mrow></mml:math>, K, Rb)

2012-05-22
Zhijun Wang , Yan Sun , Xing‐Qiu Chen +5 more
Three-dimensional (3D) Dirac point, where two Weyl points overlap in momentum space, is usually unstable and hard to realize. Here we show, based on the first-principles calculations and effective model 
 Three-dimensional (3D) Dirac point, where two Weyl points overlap in momentum space, is usually unstable and hard to realize. Here we show, based on the first-principles calculations and effective model analysis, that crystalline ${A}_{3}$Bi ($A=\text{Na}$, K, Rb) are Dirac semimetals with bulk 3D Dirac points protected by crystal symmetry. They possess nontrivial Fermi arcs on the surfaces and can be driven into various topologically distinct phases by explicit breaking of symmetries. Giant diamagnetism, linear quantum magnetoresistance, and quantum spin Hall effect will be expected for such compounds.
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Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface

2009-05-10
Haijun Zhang , Chao‐Xing Liu , Xiao‐Liang Qi +3 more

Classification of topological insulators and superconductors in three spatial dimensions

2008-11-26
Andreas P. Schnyder , Shinsei Ryu , Akira Furusaki +1 more
We systematically study topological phases of insulators and superconductors (or superfluids) in three spatial dimensions. We find that there exist three-dimensional (3D) topologically nontrivial insulators or superconductors in five out 
 We systematically study topological phases of insulators and superconductors (or superfluids) in three spatial dimensions. We find that there exist three-dimensional (3D) topologically nontrivial insulators or superconductors in five out of ten symmetry classes introduced in seminal work by Altland and Zirnbauer within the context of random matrix theory, more than a decade ago. One of these is the recently introduced ${\mathbb{Z}}_{2}$ topological insulator in the symplectic (or spin-orbit) symmetry class. We show that there exist precisely four more topological insulators. For these systems, all of which are time-reversal invariant in three dimensions, the space of insulating ground states satisfying certain discrete symmetry properties is partitioned into topological sectors that are separated by quantum phase transitions. Three of the above five topologically nontrivial phases can be realized as time-reversal invariant superconductors. In these the different topological sectors are characterized by an integer winding number defined in momentum space. When such 3D topological insulators are terminated by a two-dimensional surface, they support a number (which may be an arbitrary nonvanishing even number for singlet pairing) of Dirac fermion (Majorana fermion when spin-rotation symmetry is completely broken) surface modes which remain gapless under arbitrary perturbations of the Hamiltonian that preserve the characteristic discrete symmetries, including disorder. In particular, these surface modes completely evade Anderson localization from random impurities. These topological phases can be thought of as three-dimensional analogs of well-known paired topological phases in two spatial dimensions such as the spinless chiral $({p}_{x}\ifmmode\pm\else\textpm\fi{}i{p}_{y})$-wave superconductor (or Moore-Read Pfaffian state). In the corresponding topologically nontrivial (analogous to ``weak pairing'') and topologically trivial (analogous to ``strong pairing'') 3D phases, the wave functions exhibit markedly distinct behavior. When an electromagnetic U(1) gauge field and fluctuations of the gap functions are included in the dynamics, the superconducting phases with nonvanishing winding number possess nontrivial topological ground-state degeneracies.
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Topological invariants of time-reversal-invariant band structures

2007-03-26
Joel E. Moore , Leon Balents
The topological invariants of a time-reversal-invariant band structure in two dimensions are multiple copies of the ${\mathbb{Z}}_{2}$ invariant found by Kane and Mele. Such invariants protect the ``topological insulator'' phase 
 The topological invariants of a time-reversal-invariant band structure in two dimensions are multiple copies of the ${\mathbb{Z}}_{2}$ invariant found by Kane and Mele. Such invariants protect the ``topological insulator'' phase and give rise to a spin Hall effect carried by edge states. Each pair of bands related by time reversal is described by one ${\mathbb{Z}}_{2}$ invariant, up to one less than half the dimension of the Bloch Hamiltonians. In three dimensions, there are four such invariants per band pair. The ${\mathbb{Z}}_{2}$ invariants of a crystal determine the transitions between ordinary and topological insulators as its bands are occupied by electrons. We derive these invariants using maps from the Brillouin zone to the space of Bloch Hamiltonians and clarify the connections between ${\mathbb{Z}}_{2}$ invariants, the integer invariants that underlie the integer quantum Hall effect, and previous invariants of $\mathcal{T}$-invariant Fermi systems.
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Topological insulators and superconductors

2011-10-14
Xiao-Liang Qi , Shou-Cheng Zhang
Topological insulators are new states of quantum matter which can not be adiabatically connected to conventional insulators and semiconductors. They are characterized by a full insulating gap in the bulk 
 Topological insulators are new states of quantum matter which can not be adiabatically connected to conventional insulators and semiconductors. They are characterized by a full insulating gap in the bulk and gapless edge or surface states which are protected by time-reversal symmetry. These topological materials have been theoretically predicted and experimentally observed in a variety of systems, including HgTe quantum wells, BiSb alloys, and Bi$_2$Te$_3$ and Bi$_2$Se$_3$ crystals. We review theoretical models, materials properties and experimental results on two-dimensional and three-dimensional topological insulators, and discuss both the topological band theory and the topological field theory. Topological superconductors have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions. We review the theory of topological superconductors in close analogy to the theory of topological insulators.
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Experimental realization of the topological Haldane model with ultracold fermions

2014-11-11
Gregor Jotzu , Michael Messer , RĂ©mi Desbuquois +4 more
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Topological insulators with inversion symmetry

2007-07-02
Liang Fu , C. L. Kane
Topological insulators are materials with a bulk excitation gap generated by the spin-orbit interaction that are different from conventional insulators. This distinction is characterized by ${Z}_{2}$ topological invariants, which characterize 
 Topological insulators are materials with a bulk excitation gap generated by the spin-orbit interaction that are different from conventional insulators. This distinction is characterized by ${Z}_{2}$ topological invariants, which characterize the ground state. In two dimensions, there is a single ${Z}_{2}$ invariant that distinguishes the ordinary insulator from the quantum spin-Hall phase. In three dimensions, there are four ${Z}_{2}$ invariants that distinguish the ordinary insulator from ``weak'' and ``strong'' topological insulators. These phases are characterized by the presence of gapless surface (or edge) states. In the two-dimensional quantum spin-Hall phase and the three-dimensional strong topological insulator, these states are robust and are insensitive to weak disorder and interactions. In this paper, we show that the presence of inversion symmetry greatly simplifies the problem of evaluating the ${Z}_{2}$ invariants. We show that the invariants can be determined from the knowledge of the parity of the occupied Bloch wave functions at the time-reversal invariant points in the Brillouin zone. Using this approach, we predict a number of specific materials that are strong topological insulators, including the semiconducting alloy ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ as well as $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ and HgTe under uniaxial strain. This paper also includes an expanded discussion of our formulation of the topological insulators in both two and three dimensions, as well as implications for experiments.
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Weyl Semimetal in a Topological Insulator Multilayer

2011-09-16
A. A. Burkov , Leon Balents
We propose a simple realization of the three-dimensional (3D) Weyl semimetal phase, utilizing a multilayer structure, composed of identical thin films of a magnetically-doped 3D topological insulator (TI), separated by 
 We propose a simple realization of the three-dimensional (3D) Weyl semimetal phase, utilizing a multilayer structure, composed of identical thin films of a magnetically-doped 3D topological insulator (TI), separated by ordinary-insulator spacer layers. We show that the phase diagram of this system contains a Weyl semimetal phase of the simplest possible kind, with only two Dirac nodes of opposite chirality, separated in momentum space, in its bandstructure. This particular type of Weyl semimetal has a finite anomalous Hall conductivity, chiral edge states, and occurs as an intermediate phase between an ordinary insulator and a 3D quantum anomalous Hall insulator with a quantized Hall conductivity, equal to $e^2/h$ per TI layer. We find that the Weyl semimetal has a nonzero DC conductivity at zero temperature and is thus an unusual metallic phase, characterized by a finite anomalous Hall conductivity and topologically-protected edge states.
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Photonic topological insulators

2012-12-14
Alexander B. Khanikaev , S. Hossein Mousavi , Wang-Kong Tse +3 more
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Majorana Fermions and a Topological Phase Transition in Semiconductor-Superconductor Heterostructures

2010-08-13
Roman M. Lutchyn , Jay D. Sau , S. Das Sarma
We propose and analyze theoretically an experimental setup for detecting the elusive Majorana particle in semiconductor-superconductor heterostructures. The experimental system consists of one-dimensional semiconductor wire with strong spin-orbit Rashba interaction 
 We propose and analyze theoretically an experimental setup for detecting the elusive Majorana particle in semiconductor-superconductor heterostructures. The experimental system consists of one-dimensional semiconductor wire with strong spin-orbit Rashba interaction embedded into a superconducting quantum interference device. We show that the energy spectra of the Andreev bound states at the junction are qualitatively different in topologically trivial (i.e., not containing any Majorana) and nontrivial phases having an even and odd number of crossings at zero energy, respectively. The measurement of the supercurrent through the junction allows one to discern topologically distinct phases and observe a topological phase transition by simply changing the in-plane magnetic field or the gate voltage. The observation of this phase transition will be a direct demonstration of the existence of Majorana particles.
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Helical Liquids and Majorana Bound States in Quantum Wires

2010-10-20
Yuval Oreg , Gil Refael , Felix von Oppen
We show that the combination of spin-orbit coupling with a Zeeman field or strong interactions may lead to the formation of a helical electron liquid in single-channel quantum wires, with 
 We show that the combination of spin-orbit coupling with a Zeeman field or strong interactions may lead to the formation of a helical electron liquid in single-channel quantum wires, with spin and velocity perfectly correlated. We argue that zero-energy Majorana bound states are formed in various situations when such wires are situated in proximity to a conventional $s$-wave superconductor. This occurs when the external magnetic field, the superconducting gap, or, most simply, the chemical potential vary along the wire. These Majorana states do not require the presence of a vortex in the system. Experimental consequences of the helical liquid and the Majorana states are also discussed.
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The birth of topological insulators

2010-03-01
Joel E. Moore

Topological Crystalline Insulators

2011-03-08
Liang Fu
The recent discovery of topological insulators has revived interest in the band topology of insulators. In this Letter, we extend the topological classification of band structures to include certain crystal 
 The recent discovery of topological insulators has revived interest in the band topology of insulators. In this Letter, we extend the topological classification of band structures to include certain crystal point group symmetry. We find a class of three-dimensional "topological crystalline insulators" which have metallic surface states with quadratic band degeneracy on high symmetry crystal surfaces. These topological crystalline insulators are the counterpart of topological insulators in materials without spin-orbit coupling. Their band structures are characterized by new topological invariants. We hope this work will enlarge the family of topological phases in band insulators and stimulate the search for them in real materials.
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Quantized Anomalous Hall Effect in Magnetic Topological Insulators

2010-06-04
Rui Yu , Wei Zhang , Haijun Zhang +3 more
Quantum Anomalous Hall Effect In addition to the Hall effect, which appears as a voltage change in conductors in response to an external magnetic field, ferromagnets exhibit the anomalous Hall 
 Quantum Anomalous Hall Effect In addition to the Hall effect, which appears as a voltage change in conductors in response to an external magnetic field, ferromagnets exhibit the anomalous Hall effect, which is often proportional to their magnetization and independent of the presence of the magnetic field. This effect, first observed more than a century ago, has not been realized in its quantized form. Yu et al. (p. 61 , published online 3 June) propose a realization of a quantum anomalous Hall system by magnetically doping thin films of three-dimensional topological insulators and calculate the effects of various dopants and film thicknesses. The resulting insulators are predicted to have long-range ferromagnetic order, potentially joining dilute magnetic semiconductors as candidates for spintronic applications.
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<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>Z</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>Topological Order and the Quantum Spin Hall Effect

2005-09-28
C. L. Kane , E. J. Melé
The quantum spin Hall (QSH) phase is a time reversal invariant electronic state with a bulk electronic band gap that supports the transport of charge and spin in gapless edge 
 The quantum spin Hall (QSH) phase is a time reversal invariant electronic state with a bulk electronic band gap that supports the transport of charge and spin in gapless edge states. We show that this phase is associated with a novel ${Z}_{2}$ topological invariant, which distinguishes it from an ordinary insulator. The ${Z}_{2}$ classification, which is defined for time reversal invariant Hamiltonians, is analogous to the Chern number classification of the quantum Hall effect. We establish the ${Z}_{2}$ order of the QSH phase in the two band model of graphene and propose a generalization of the formalism applicable to multiband and interacting systems.
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Quantum spin Hall effect in two-dimensional transition metal dichalcogenides

2014-11-21
Xiaofeng Qian , Junwei Liu , Liang Fu +1 more
We report a new class of large-gap quantum spin Hall insulators in two-dimensional transition metal dichalcogenides, namely, MX$_2$ with M=(Mo, W) and X=(S, Se, and Te), whose topological electronic properties 
 We report a new class of large-gap quantum spin Hall insulators in two-dimensional transition metal dichalcogenides, namely, MX$_2$ with M=(Mo, W) and X=(S, Se, and Te), whose topological electronic properties are highly tunable by external electric field. We propose a novel topological field effect transistor made of these atomic layer materials and their van der Waals heterostructures. Our device exhibits parametrically enhanced charge-spin conductance through topologically protected transport channels, and can be rapidly switched off by electric field through topological phase transition instead of carrier depletion. Our work provides a practical material platform and device architecture for topological quantum electronics.
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Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor

2014-10-03
Stevan Nadj-Perge , Ilya Drozdov , Jian Li +6 more
Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a 
 Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.
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Floquet topological insulator in semiconductor quantum wells

2011-03-13
Netanel H. Lindner , Gil Refael , Victor Galitski
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<i>Colloquium</i>: Topological insulators

2010-11-08
M. Zahid Hasan , C. L. Kane
Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator, but have protected conducting states on their edge or surface. The 2D topological insulator is 
 Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator, but have protected conducting states on their edge or surface. The 2D topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. A 3D topological insulator supports novel spin polarized 2D Dirac fermions on its surface. In this Colloquium article we will review the theoretical foundation for these electronic states and describe recent experiments in which their signatures have been observed. We will describe transport experiments on HgCdTe quantum wells that demonstrate the existence of the edge states predicted for the quantum spin Hall insulator. We will then discuss experiments on Bi_{1-x}Sb_x, Bi_2 Se_3, Bi_2 Te_3 and Sb_2 Te_3 that establish these materials as 3D topological insulators and directly probe the topology of their surface states. We will then describe exotic states that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana fermions, and may provide a new venue for realizing proposals for topological quantum computation. We will close by discussing prospects for observing these exotic states, a well as other potential device applications of topological insulators.
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Zero-bias peaks and splitting in an Al–InAs nanowire topological superconductor as a signature of Majorana fermions

2012-11-11
Anindya Das , Yuval Ronen , Yonatan Most +3 more
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Quantum Spin Hall Effect and Topological Phase Transition in HgTe Quantum Wells

2006-12-14
B. Andrei Bernevig , Taylor L. Hughes , Shou-Cheng Zhang
We show that the quantum spin Hall (QSH) effect, a state of matter with topological properties distinct from those of conventional insulators, can be realized in mercury telluride–cadmium telluride semiconductor 
 We show that the quantum spin Hall (QSH) effect, a state of matter with topological properties distinct from those of conventional insulators, can be realized in mercury telluride–cadmium telluride semiconductor quantum wells. When the thickness of the quantum well is varied, the electronic state changes from a normal to an “inverted” type at a critical thickness d c . We show that this transition is a topological quantum phase transition between a conventional insulating phase and a phase exhibiting the QSH effect with a single pair of helical edge states. We also discuss methods for experimental detection of the QSH effect.
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Experimental Realization of a Three-Dimensional Topological Insulator, Bi <sub>2</sub> Te <sub>3</sub>

2009-06-12
Y. L. Chen , James G. Analytis , Jiun‐Haw Chu +7 more
Three-dimensional topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. By investigating the surface state of 
 Three-dimensional topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. By investigating the surface state of Bi2Te3 with angle-resolved photoemission spectroscopy, we demonstrate that the surface state consists of a single nondegenerate Dirac cone. Furthermore, with appropriate hole doping, the Fermi level can be tuned to intersect only the surface states, indicating a full energy gap for the bulk states. Our results establish that Bi2Te3 is a simple model system for the three-dimensional topological insulator with a single Dirac cone on the surface. The large bulk gap of Bi2Te3 also points to promising potential for high-temperature spintronics applications.
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Berry phase effects on electronic properties

2010-07-06
Di Xiao , Ming-Che Chang , Qian Niu
Ever since its discovery, the Berry phase has permeated through all branches of physics. Over the last three decades, it was gradually realized that the Berry phase of the electronic 
 Ever since its discovery, the Berry phase has permeated through all branches of physics. Over the last three decades, it was gradually realized that the Berry phase of the electronic wave function can have a profound effect on material properties and is responsible for a spectrum of phenomena, such as ferroelectricity, orbital magnetism, various (quantum/anomalous/spin) Hall effects, and quantum charge pumping. This progress is summarized in a pedagogical manner in this review. We start with a brief summary of necessary background, followed by a detailed discussion of the Berry phase effect in a variety of solid state applications. A common thread of the review is the semiclassical formulation of electron dynamics, which is a versatile tool in the study of electron dynamics in the presence of electromagnetic fields and more general perturbations. Finally, we demonstrate a re-quantization method that converts a semiclassical theory to an effective quantum theory. It is clear that the Berry phase should be added as a basic ingredient to our understanding of basic material properties.
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Superconducting Proximity Effect and Majorana Fermions at the Surface of a Topological Insulator

2008-03-06
Liang Fu , C. L. Kane
We study the proximity effect between an $s$-wave superconductor and the surface states of a strong topological insulator. The resulting two-dimensional state resembles a spinless ${p}_{x}+i{p}_{y}$ superconductor, but does not 
 We study the proximity effect between an $s$-wave superconductor and the surface states of a strong topological insulator. The resulting two-dimensional state resembles a spinless ${p}_{x}+i{p}_{y}$ superconductor, but does not break time reversal symmetry. This state supports Majorana bound states at vortices. We show that linear junctions between superconductors mediated by the topological insulator form a nonchiral one-dimensional wire for Majorana fermions, and that circuits formed from these junctions provide a method for creating, manipulating, and fusing Majorana bound states.
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Type-II Weyl semimetals

2015-11-01
Alexey A. Soluyanov , Dominik Gresch , Zhijun Wang +4 more
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Discovery of a Three-Dimensional Topological Dirac Semimetal, Na <sub>3</sub> Bi

2014-01-17
Z. K. Liu , Bo Zhou , Yi Zhang +7 more
Three-dimensional (3D) topological Dirac semimetals (TDSs) represent a novel state of quantum matter that can be viewed as '3D graphene'. In contrast to two-dimensional (2D) Dirac fermions in graphene or 
 Three-dimensional (3D) topological Dirac semimetals (TDSs) represent a novel state of quantum matter that can be viewed as '3D graphene'. In contrast to two-dimensional (2D) Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. The TDS is also an important boundary state mediating numerous novel quantum states, such as topological insulators, Weyl semi-metals, Axion insulators and topological superconductors. By investigating the electronic structure of Na3Bi with angle resolved photoemission spectroscopy, we discovered 3D Dirac fermions with linear dispersions along all momentum directions for the first time. Furthermore, we demonstrated that the 3D Dirac fermions in Na3Bi were protected by the bulk crystal symmetry. Our results establish that Na3Bi is the first model system of 3D TDSs, which can also serve as an ideal platform for the systematic study of quantum phase transitions between rich novel topological quantum states.
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WannierTools: An open-source software package for novel topological materials

2017-10-18
Quansheng Wu , ShengNan Zhang , Haifeng Song +2 more
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Weyl and Dirac semimetals in three-dimensional solids

2018-01-22
N. P. Armitage , E. J. Melé , Ashvin Vishwanath
Weyl and Dirac semimetals are three dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three dimensional analogs of graphene, they have generated 
 Weyl and Dirac semimetals are three dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and -- despite their gaplessness -- to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. Here we review the theoretical foundations of these phases, their proposed realizations in solid state systems, recent experiments on candidate materials, as well as their relation to other states of matter.
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Topological photonics

2014-10-24
Ling LĂŒ , John D. Joannopoulos , Marin Soljačić
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Paired states of fermions in two dimensions with breaking of parity and time-reversal symmetries and the fractional quantum Hall effect

2000-04-15
N. Read , Dmitry Green
We analyze pairing of fermions in two dimensions for fully gapped cases with broken parity (P) and time reversal (T), especially cases in which the gap function is an orbital 
 We analyze pairing of fermions in two dimensions for fully gapped cases with broken parity (P) and time reversal (T), especially cases in which the gap function is an orbital angular momentum (l) eigenstate, in particular $l=\ensuremath{-}1$ (p wave, spinless, or spin triplet) and $l=\ensuremath{-}2$ (d wave, spin singlet). For $l\ensuremath{\ne}0,$ these fall into two phases, weak and strong pairing, which may be distinguished topologically. In the cases with conserved spin, we derive explicitly the Hall conductivity for spin as the corresponding topological invariant. For the spinless p-wave case, the weak-pairing phase has a pair wave function that is asympototically the same as that in the Moore-Read (Pfaffian) quantum Hall state, and we argue that its other properties (edge states, quasihole, and toroidal ground states) are also the same, indicating that nonabelian statistics is a generic property of such a paired phase. The strong-pairing phase is an abelian state, and the transition between the two phases involves a bulk Majorana fermion, the mass of which changes sign at the transition. For the d-wave case, we argue that the Haldane-Rezayi state is not the generic behavior of a phase but describes the asymptotics at the critical point between weak and strong pairing, and has gapless fermion excitations in the bulk. In this case the weak-pairing phase is an abelian phase, which has been considered previously. In the p-wave case with an unbroken $U(1)$ symmetry, which can be applied to the double layer quantum Hall problem, the weak-pairing phase has the properties of the 331 state, and with nonzero tunneling there is a transition to the Moore-Read phase. The effects of disorder on noninteracting quasiparticles are considered. The gapped phases survive, but there is an intermediate thermally conducting phase in the spinless p-wave case, in which the quasiparticles are extended.
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Anderson transitions

2008-10-17
Ferdinand Evers , A. D. Mirlin
The physics of Anderson transitions between localized and metallic phases in disordered systems is reviewed. The term ``Anderson transition'' is understood in a broad sense, including both metal-insulator transitions and 
 The physics of Anderson transitions between localized and metallic phases in disordered systems is reviewed. The term ``Anderson transition'' is understood in a broad sense, including both metal-insulator transitions and quantum-Hall-type transitions between phases with localized states. The emphasis is put on recent developments, which include multifractality of critical wave functions, criticality in the power-law random banded matrix model, symmetry classification of disordered electronic systems, mechanisms of criticality in quasi-one-dimensional and two-dimensional systems and survey of corresponding critical theories, network models, and random Dirac Hamiltonians. Analytical approaches are complemented by advanced numerical simulations.
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Experimental Discovery of Weyl Semimetal TaAs

2015-07-31
Bing Lv , Hongming Weng , Binbin Fu +7 more
Weyl fermions possess exotic properties and can act like magnetic monopoles. Researchers show that TaAs is a Weyl semimetal, demonstrating for the first time that Weyl semimetals can be identified 
 Weyl fermions possess exotic properties and can act like magnetic monopoles. Researchers show that TaAs is a Weyl semimetal, demonstrating for the first time that Weyl semimetals can be identified experimentally.
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Weyl Semimetal Phase in Noncentrosymmetric Transition-Metal Monophosphides

2015-03-17
Hongming Weng , Chen Fang , Zhong Fang +2 more
Based on first-principle calculations, we show that a family of nonmagnetic materials including TaAs, TaP, NbAs, and NbP are Weyl semimetals (WSM) without inversion centers. We find twelve pairs of 
 Based on first-principle calculations, we show that a family of nonmagnetic materials including TaAs, TaP, NbAs, and NbP are Weyl semimetals (WSM) without inversion centers. We find twelve pairs of Weyl points in the whole Brillouin zone (BZ) for each of them. In the absence of spin-orbit coupling (SOC), band inversions in mirror-invariant planes lead to gapless nodal rings in the energy-momentum dispersion. The strong SOC in these materials then opens full gaps in the mirror planes, generating nonzero mirror Chern numbers and Weyl points off the mirror planes. The resulting surface-state Fermi arc structures on both (001) and (100) surfaces are also obtained, and they show interesting shapes, pointing to fascinating playgrounds for future experimental studies.Received 12 January 2015DOI:https://doi.org/10.1103/PhysRevX.5.011029This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical Society
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Discovery of a Weyl fermion semimetal and topological Fermi arcs

2015-07-17
Su‐Yang Xu , Ilya Belopolski , Nasser Alidoust +7 more
Weyl physics emerges in the laboratory Weyl fermions—massless particles with half-integer spin—were once mistakenly thought to describe neutrinos. Although not yet observed among elementary particles, Weyl fermions may exist as 
 Weyl physics emerges in the laboratory Weyl fermions—massless particles with half-integer spin—were once mistakenly thought to describe neutrinos. Although not yet observed among elementary particles, Weyl fermions may exist as collective excitations in so-called Weyl semimetals. These materials have an unusual band structure in which the linearly dispersing valence and conduction bands meet at discrete “Weyl points.” Xu et al. used photoemission spectroscopy to identify TaAs as a Weyl semimetal capable of hosting Weyl fermions. In a complementary study, Lu et al. detected the characteristic Weyl points in a photonic crystal. The observation of Weyl physics may enable the discovery of exotic fundamental phenomena. Science , this issue p. 613 and 622
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Unpaired Majorana fermions in quantum wires

2001-10-01
Alexei Kitaev
Certain one-dimensional Fermi systems have an energy gap in the bulk spectrum while boundary states are described by one Majorana operator per boundary point. A finite system of length L 
 Certain one-dimensional Fermi systems have an energy gap in the bulk spectrum while boundary states are described by one Majorana operator per boundary point. A finite system of length L possesses two ground states with an energy difference proportional to exp(-L/l0) and different fermionic parities. Such systems can be used as qubits since they are intrinsically immune to decoherence. The property of a system to have boundary Majorana fermions is expressed as a condition on the bulk electron spectrum. The condition is satisfied in the presence of an arbitrary small energy gap induced by proximity of a three-dimensional p-wave superconductor, provided that the normal spectrum has an odd number of Fermi points in each half of the Brillouin zone (each spin component counts separately).
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Topological Insulators and Topological Superconductors

2013-12-31
B. Andrei Bernevig
This graduate-level textbook is the first pedagogical synthesis of the field of topological insulators and superconductors, one of the most exciting areas of research in condensed matter physics. Presenting the 
 This graduate-level textbook is the first pedagogical synthesis of the field of topological insulators and superconductors, one of the most exciting areas of research in condensed matter physics. Presenting the latest developments, while providing all the calculations necessary for a self-contained and complete description of the discipline, it is ideal for graduate students and researchers preparing to work in this area, and it will be an essential reference both within and outside the classroom. The book begins with simple concepts such as Berry phases, Dirac fermions, Hall conductance and its link to topology, and the Hofstadter problem of lattice electrons in a magnetic field. It moves on to explain topological phases of matter such as Chern insulators, two- and three-dimensional topological insulators, and Majorana p-wave wires. Additionally, the book covers zero modes on vortices in topological superconductors, time-reversal topological superconductors, and topological responses/field theory and topological indices. The book also analyzes recent topics in condensed matter theory and concludes by surveying active subfields of research such as insulators with point-group symmetries and the stability of topological semimetals. Problems at the end of each chapter offer opportunities to test knowledge and engage with frontier research issues. Topological Insulators and Topological Superconductors will provide graduate students and researchers with the physical understanding and mathematical tools needed to embark on research in this rapidly evolving field.

Weyl Semimetals: From Principles, Materials to Applications

2025-06-25
M. Zhong , Nam T. Vu , Wei Zhai +7 more | Advanced Materials
Abstract Weyl semimetals have attracted significant interest in condensed matter physics and materials science, due to their unique electronic and topological properties. These characteristics not only deepen the understanding of 
 Abstract Weyl semimetals have attracted significant interest in condensed matter physics and materials science, due to their unique electronic and topological properties. These characteristics not only deepen the understanding of fundamental quantum phenomena, but also make Weyl semimetals promising candidates for advanced applications in electronics, photonics, and spintronics. This review provides a systematic overview of the field, covering theoretical foundations, material synthesis, engineering strategies, and emerging device applications. This study first outlines the key theoretical principles and distinctive properties of Weyl semimetals, followed by an examination of recent advancements that enhance their functional versatility. Finally, this study discusses the critical challenges hindering their practical implementation and explore future development directions, along with the potential for expanding and enhancing their existing range of applications. By integrating discussions of both opportunities and obstacles, this review offers a balanced perspective on current progress and future directions in Weyl semimetal research.

Impact of tiny Fermi pockets with extremely high mobility on the Hall anomaly in the kagome metal ${\rm CsV_3Sb_5 }$

2025-06-25
NULL AUTHOR_ID | Physical Review Letters

Energy spectrum and dynamical properties of Dirac fermions in 3D SnTe(001) surface state under combined exchange and strain effects

2025-06-24
Diana Dahliah , Mohammad K. Elsaid , Khaled Abdulhaq | Journal of King Saud University - Science

Multi-Channel Conduction in Electron Transport Dynamics of Three Dimensional Topological Insulators

2025-06-24
Naresh Shyaga , Rajib Sarkar , Laxmipriya Nanda +6 more | Journal of Physics Condensed Matter
Abstract Three-dimensional topological insulators (3D TIs) have been the subject of extensive research, primarily emphasizing their surface and bulk properties. Surface state transport is highly sought after due to its 
 Abstract Three-dimensional topological insulators (3D TIs) have been the subject of extensive research, primarily emphasizing their surface and bulk properties. Surface state transport is highly sought after due to its spin-momentum locking; however, in real systems, mixing between surface and bulk states often occurs because of defect states present in the bulk band gap. In this study, we provide a detailed electronic characterization of the 3D topological insulator BiSbTeSe2 (BSTS), which exhibits pronounced surface transport effects. We observe a connection between the nonlinear Hall effect at first order and quantum coherent transport, as evidenced by weak anti-localization. Our results show that NLH is a more sensitive technique to multi-channel transport in 3D TIs, thereby clearly distinguishing the temperature regime over which bulk and surface states dominate the transport dynamics. Additionally, we investigate the influence of magnetic impurities within the crystal structure on this phenomenon. Our study systematically elucidates the significance of multichannel transport, which is essential to understand the transport properties in the presence of an external magnetic field.

Controllable creation of topological boundary states in topological insulator based Josephson corner junctions

2025-06-24
NULL AUTHOR_ID | Physical review. B./Physical review. B

Topological Surface State Dominated Nonlinear Transverse Response and Microwave Rectification at Room Temperature

2025-06-24
Q. SHEN , Jiaxin Chen , Bin Rong +7 more | Advanced Functional Materials
Abstract The nonlinear Hall effect (NLHE) has emerged as a powerful tool for probing symmetry and topological properties in quantum materials, offering exciting potential for applications in microwave rectification and 
 Abstract The nonlinear Hall effect (NLHE) has emerged as a powerful tool for probing symmetry and topological properties in quantum materials, offering exciting potential for applications in microwave rectification and energy harvesting. NLHE arises from mechanisms such as the Berry curvature dipole (BCD), skew scattering, and side jump. Unlike BCD, which is confined to materials with low symmetry, skew scattering can exist in systems with 3‐fold rotational symmetry, providing a broader platform for exploring nonlinear phenomena in topological materials. Here, a significant room‐temperature nonlinear transverse response in the 3D topological insulator Bi 2 Te 3 is reported. By preparing high‐quality Bi 2 Te 3 epitaxial films with thicknesses ranging from 7 to 50 nm, it is observed that the nonlinear susceptibility increases with thickness up to 25 nm and then saturates. This behavior is closely linked to the thickness‐dependent electrical conductance, carrier mobility, and crystallinity, underscoring the dominance of a robust topological surface state (TSS) in thicker Bi 2 Te 3 films. Additionally, microwave rectification in Bi 2 Te 3 is demonstrated at frequencies up to 16.6 GHz, showcasing its potential for high‐frequency applications. This work reveals that both linear and nonlinear transport phenomena in Bi 2 Te 3 are governed by the TSS, emphasizing its crucial role in enabling robust room‐temperature nonlinear effects.

Adiabatic preparation of a number-conserving atomic Majorana phase

2025-06-24
NULL AUTHOR_ID | Physical review. B./Physical review. B

Probing the dichotomy between Yu-Shiba-Rusinov and Majorana bound states via conductance, quantum noise, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>Δ</mml:mi><mml:mi>T</mml:mi></mml:msub></mml:math> noise

2025-06-23
NULL AUTHOR_ID | Physical review. B./Physical review. B

Sensitive strain effect on magnetic phase transitions in Sb-doped MnBi4Te7 magnetic topological insulator

2025-06-23
Huihui Zhang , Yu Zhang , Xuqi Li +2 more | Applied Physics Letters
Recent experiments have successfully fabricated ferromagnetic topological insulator MnBi4Te7 by Sb doping. However, distinct conclusions regarding magnetic phase transitions at different doping concentrations have emerged, alongside controversial interpretations of ferromagnetic 
 Recent experiments have successfully fabricated ferromagnetic topological insulator MnBi4Te7 by Sb doping. However, distinct conclusions regarding magnetic phase transitions at different doping concentrations have emerged, alongside controversial interpretations of ferromagnetic origin. Based on first-principles calculations, we demonstrate that the evolution of magnetism in Sb-doped MnBi4Te7 is not only attributed to Mn–Bi/Sb antisite defects, as observed in some experiments, but is also significantly influenced by strain effects. Furthermore, our results reveal that magnetic phase transition is accompanied by a change from n-type to p-type charge carriers at Sb doping concentrations exceeding 30%, in excellent agreement with experimental observation. Moreover, Sb substitution leads to an obvious difference in the effective mass of charge carriers, which qualitatively coincides with experimentally measured carrier mobility. Additionally, Sb doping results in more delocalized charge carriers due to the relatively weaker Sb–Te bonding. Based on these findings, we further discuss how the enhanced carrier delocalization influences interlayer magnetic coupling, leading to a nonmonotonic evolution of long-range magnetism from antiferromagnetic to ferromagnetic and back to antiferromagnetic phases with increased Sb concentration.

Stability of Majorana modes in Coulomb-disordered topological insulator nanowires

2025-06-23
NULL AUTHOR_ID | Physical review. B./Physical review. B

Off-resonant light-induced topological phase transition and anomalous thermoelectric transport in semi-Dirac materials

2025-06-23
NULL AUTHOR_ID | Physical review. B./Physical review. B

Intrinsic spin transport in a topological insulator thin film

2025-06-23
Sharadh Jois , Gregory M. Stephen , Nicholas A. Blumenschein +3 more | Applied Physics Letters
Topological insulators (TIs) are intriguing materials for advanced computing applications based on spintronics because they can host robust spin effects. For instance, TIs have intrinsically large spin generation enabled by 
 Topological insulators (TIs) are intriguing materials for advanced computing applications based on spintronics because they can host robust spin effects. For instance, TIs have intrinsically large spin generation enabled by their large spin–orbit coupling. Furthermore, topological surface states (TSS) with spin-momentum locking and Dirac dispersion lead to long spin diffusion. Future spintronic device technology will require scalable film growth of high-quality material. We grow epitaxial films of (Bi1−xSbx)2Te3−ySey (BSTS, x = 0.58, y = 1) and confirm the gapless band structure with optimal doping using angle-resolved photoemission spectroscopy. The temperature dependence of the longitudinal resistivity shows that bulk transport is suppressed as the temperature is decreased, and at low temperature, surface transport dominates. We evaluate the spin transport properties in BSTS without using ferromagnetic tunnel contacts via a non-local resistance experiment as a function of temperature and applied charge current. As expected, these experiments reveal the necessity of decreasing the bulk conduction to best enhance the spin transport. In the TSS, we find a charge-to-spin conversion efficiency (spin Hall angle, ΞSH∌1) and spin diffusion over several micrometers. Further development of high-quality TIs will make them viable candidates for efficient and lossless spintronics.

Surface Local Impurity Scattering as a Probe for Topological Kondo Insulators

2025-06-23
C.‐C. Joseph Wang , Jean‐Pierre Julien , Alexander V. Balatsky +1 more | Advanced Physics Research
Abstract Shortly after the discovery of topological band insulators, topological Kondo insulators (TKIs) is also theoretically predicted. The latter has ignited revival interest in the properties of Kondo insulators. Currently, 
 Abstract Shortly after the discovery of topological band insulators, topological Kondo insulators (TKIs) is also theoretically predicted. The latter has ignited revival interest in the properties of Kondo insulators. Currently, the feasibility of topological nature in SmB 6 is intensively analyzed by several complementary probes. Here by starting with a minimal‐orbital Anderson lattice model, the local electronic structure is explored in a Kondo insulator. It is showed that the two strong topological regimes sandwiching the weak topological regime give rise to a single Dirac cone, which is located near the center or corner of the surface Brillouin zone. It is further found that, when a single impurity is placed on the surface, low‐energy resonance states are induced in the weak scattering limit for the strong TKI regimes and the resonance level moves monotonically across the hybridization gap with the strength of impurity scattering potential; while low energy states can only be induced in the unitary scattering limit for the weak TKI regime, where the resonance level moves universally toward the center of the hybridization gap. These impurity‐induced low‐energy quasiparticles will lead to characteristic signatures in scanning tunneling microscopy/spectroscopy, which has recently found success in probing into exotic properties in heavy fermion systems.
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Missing odd-order Shapiro steps do not uniquely indicate fractional Josephson effect

2025-06-23
Po Zhang , S. R. Mudi , Mihir Pendharkar +7 more | SciPost Physics
Topological superconductivity is expected to spur Majorana zero modes-exotic states that are also considered a quantum technology asset. Fractional Josephson effect is their manifestation in electronic transport measurements, often under 
 Topological superconductivity is expected to spur Majorana zero modes-exotic states that are also considered a quantum technology asset. Fractional Josephson effect is their manifestation in electronic transport measurements, often under microwave irradiation. A fraction of induced resonances, known as Shapiro steps, should vanish, in a pattern that signifies the presence of Majorana modes. Here we report patterns of Shapiro steps expected in topological Josephson junctions, such as the missing first Shapiro step, or several missing odd-order steps. But our junctions, which are InAs quantum wells with Al contacts, are studied near zero magnetic field, meaning that they are not in the topological regime. We also observe other patterns such as missing even steps and several missing steps in a row, not relevant to topological superconductivity. Potentially responsible for our observations is rounding of not fully developed steps superimposed on non-monotonic resistance versus voltage curves, but several origins may be at play. Our results demonstrate that any single pattern, even striking, cannot uniquely identify topological superconductivity, and a multifactor approach is necessary to unambiguously establish this important phenomenon.
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Controllable and continuous quantum phase transitions in intrinsic magnetic topological insulator

2025-06-23
NULL AUTHOR_ID | Physical review. B./Physical review. B

Topological Landau Theory

2025-06-23
Canon Sun , Joseph Maciejko | Physical Review Letters

Solid-state analogue of gravitational redshift: Transport signatures of massless Dirac fermions in tilted Dirac cone heterostructures

2025-06-23
NULL AUTHOR_ID | Physical Review Research

Topological Berry Antenna on a Silicon Chip for Terahertz Wireless Communication

2025-06-22
Sonu Kumar , Ridong Jia , Yi Ji Tan +6 more | Advanced Photonics Research
Nonzero Berry curvature is central to the existence of topological edge states in electronic and photonic valley‐Hall systems. While manipulating the Berry curvature in condensed matter systems is challenging, valley‐Hall 
 Nonzero Berry curvature is central to the existence of topological edge states in electronic and photonic valley‐Hall systems. While manipulating the Berry curvature in condensed matter systems is challenging, valley‐Hall topological photonics offer unprecedented control, where the broken spatial inversion symmetry alters the Berry curvature. Herein, an all‐silicon Berry antenna is presented, using a continuously varying geometry corresponding to a gradual change in Berry curvature. The on‐chip topological edge mode with a tunable field extent is achieved to enhance effective antenna aperture, creating a high‐gain on‐chip photonic antenna with perfectly planar wavefronts. Experimentally, a maximum gain of 17 dBi that supports 20 Gbps chip‐to‐chip wireless communication is demonstrated, with active optical tunability of the antenna gain with modulation depths of 8 dBi. This Berry antenna paves the way for the development of complementary metal‐oxide‐semiconductor (CMOS) compatible topological Berry devices, with potential applications in integrated micro‐/nano‐photonics, next‐generation wireless communications (6G to Xth generation), and terahertz detection and ranging.

Investigation of Structural and Vibrational Properties of Topological Nodal‐Line Semimetal PtSn<sub>4</sub> under Pressure

2025-06-21
S. N. , Sathiskumar Mariappan , Thiagarajan Maran +4 more | physica status solidi (RRL) - Rapid Research Letters
This work investigates the influence of external pressure on the structural and vibrational properties of the topological nodal line semimetal (TNLS), PtSn 4 , respectively, through synchrotron X‐ray diffraction and 
 This work investigates the influence of external pressure on the structural and vibrational properties of the topological nodal line semimetal (TNLS), PtSn 4 , respectively, through synchrotron X‐ray diffraction and Raman spectroscopy. Systematic synchrotron high‐pressure (HP) diffraction data reveal that the ambient pressure orthorhombic crystal structure (space group Ccca) of PtSn 4 is stable at least until ≈24 GPa. However, the pressure dependence of the unit‐cell parameters is found to fall in different regimes: a low‐pressure regime with the bulk modulus ( B 0 ) of ≈83 up to ≈7.8 GPa and a pressure regime above with a significant increase in bulk modulus, B 0 of ≈235 GPa. This HP‐induced isostructural transition ≈7.8 GPa originates from the anisotropic compression of the in‐plane and out‐of‐plane axes of the orthorhombic lattice. HP Raman spectroscopy also shows the isostructural transition ≈7.8 GPa, coinciding with a distinct change in the pressure dependence of the Raman modes. Implications of the observed pressure‐induced isostructural transition on the TNLS nature of the PtSn 4 are going to be exciting to investigate through further experimental and theoretical tools.

Disorder Induced Dynamical Interband Response in Dirac Nodal Line Semimetals

2025-06-21
Vivek Pandey , Pankaj Bhalla | physica status solidi (RRL) - Rapid Research Letters
To obtain the total response of the system, the effect of disorder cannot be neglected, as it introduces a new contribution (i.e., extrinsic) in the total response of the system. 
 To obtain the total response of the system, the effect of disorder cannot be neglected, as it introduces a new contribution (i.e., extrinsic) in the total response of the system. In the study of dynamical (AC) effects, the interband response exhibits an exotic resonance peak due to interband transitions. Herein, the dynamical interband response of the Dirac nodal line semimetal is investigated by using the quantum kinetic approach. The scattering‐driven effect is analyzed under the first‐order Born approximation (i.e., in the weak disorder limit) and reveals a resonance peak at . In contrast, the field‐driven intrinsic response peak depends on both the mass () and chemical potential (). The results indicate that the total interband response of the 3D nodal line semimetals is mainly dominated by the disorder‐induced contributions.

Fluctuations and correlations of local topological order parameters in disordered two-dimensional topological insulators

2025-06-20
NULL AUTHOR_ID | Physical Review Letters

Probing Anisotropic Quasiparticle Dynamics and Topological Phase Transitions in Quasi‐1D Topological Insulator ZrTe<sub>5</sub>

2025-06-20
Yueying Hou , Linze Li , Sutao Sun +7 more | Advanced Science
Abstract The transition metal pentatelluride ZrTe 5 exhibits rich lattice‐sensitive topological electronic states, and demonstrates great potential in photoelectric and thermoelectric devices. However, a comprehensive investigation of electron‐phonon coupling and 
 Abstract The transition metal pentatelluride ZrTe 5 exhibits rich lattice‐sensitive topological electronic states, and demonstrates great potential in photoelectric and thermoelectric devices. However, a comprehensive investigation of electron‐phonon coupling and phonon scattering process remains limited, despite their importance for transport properties and device optimization. Here, the hot carrier dynamics and a 1.15 THz A g mode coherent phonon in ZrTe 5 are investigated by femtosecond transient spectroscopy across 10–300 K. Notably, polarization‐dependent measurements explicitly decouple a strong anisotropic transient response, which is attributed to the effects of excited‐state electron relaxation and reflectivity modulation by displacive excited coherent phonons. The temperature dependence of electron relaxation time in ZrTe 5 shows an inflection point, first offering the ultrafast dynamical signature of a temperature‐driven Lifshitz transition. At low temperatures, a long‐lived electron relaxation component emerges in the transient response, providing possible evidence of topological surface states in ZrTe 5 . In addition, the temperature‐dependent coherent phonon is also analyzed, revealing that its scattering is dominated by three‐phonon interactions and exhibits a relatively long lifetime compared to other modes. This work deepens the understanding of ultrafast processes in ZrTe 5 , resolves longstanding questions, paves the way for studying electronic phase transitions, and advances ZrTe 5 's application in optoelectronic and quantum devices.

Majorana edge states in s-wave kagome superconductors with Rashba interaction

2025-06-20
M. A. Mojarro , Sergio E. Ulloa | 2D Materials
Abstract We study kagome lattices with on-site and extended spin-singlet s-wave superconducting pairing and show that the inclusion of Rashba spin-orbit (RSO) interaction allows time-reversal-invariant topological superconducting states which support 
 Abstract We study kagome lattices with on-site and extended spin-singlet s-wave superconducting pairing and show that the inclusion of Rashba spin-orbit (RSO) interaction allows time-reversal-invariant topological superconducting states which support helical Majorana pairs at the edge. We calculate the Z2 topological invariant as a function of the pairing parameters for different chemical potentials. The rich phase diagrams reveal topological, nodal, and trivial superconducting states depending on the system parameters. We also consider a 2X2 time-reversal symmetry-breaking chiral flux phase, which has been demonstrated to be energetically favorable in the AV3Sb5 family of superconductors. Incorporating such symmetry-breaking order in our model leads to chiral Majorana edge states defined by a Chern number. We show how the RSO interaction allows for topological phases with even and odd Chern numbers for different system parameters. This work demonstrates how a simple s-wave kagome superconductor with RSO interaction can support helical and chiral Majorana edge states, and motivates the search for Majorana fermions in kagome superconductors.

Constructing valley Hall photonic topological insulator based on characteristic mode analysis

2025-06-20
Weihan Sun , K. Xu , Weiwen Li | Physica Scripta
Abstract The photonic valley-Hall topological insulator can support valley-polarized edge states at non-trivial domain walls. Similar to dielectric photonic crystal topological insulator, the designer surface plasmon (DSP) periodic structure can 
 Abstract The photonic valley-Hall topological insulator can support valley-polarized edge states at non-trivial domain walls. Similar to dielectric photonic crystal topological insulator, the designer surface plasmon (DSP) periodic structure can also realize the valley-Hall topological phase transitions However, the construction and choice for the DSP unit cells of photonic topological insulators lack guidance with intuitive physical significance. The DSPs can be achieved by texturing metal layer into periodic structure, and the periodic cell can be analyzed by using the theory of characteristic modes. We believe that the unit cell structures in some mode bands can realize the photonic topological phase transitions (PTPT) by selecting special degeneracy characteristic modes. In this paper, the hexagonal conductor element is constructed by co-locating two equilateral triangle layers to form a DSP periodic unit structure. The cell structure dimensions of topological phase transitions are determined by characteristic mode analysis (CMA). Energy band analysis shows that the proposed DSP crystal can achieve the valley-Hall PTPT. The corresponding domain wall is constructed, and the robust valley-polarized chiral transmission characteristics are experimentally verified. The photonic PTPT waveguide based on periodic metal structure is easily compatible to traditional microwave circuits and to be manufactured, which has a wider application.

Detection of Majorana zero modes bound to Josephson vortices in planar superconductor–topological insulator–superconductor junctions

2025-06-20
Katharina Laubscher , Jay D. Sau | Physical review. B./Physical review. B

Optimizing one dimensional superconducting diodes: interplay of Rashba spin-orbit coupling and magnetic fields

2025-06-20
Sayak Bhowmik , Dibyendu Samanta , Ashis Nandy +2 more | Communications Physics
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Planar Hall effect in ultrathin topological insulator films

2025-06-20
NULL AUTHOR_ID | Physical review. B./Physical review. B

Light-induced dissipationless states in magnetic topological insulators with hexagonal warping

2025-06-20
NULL AUTHOR_ID | Physical Review Applied

Magnetic field induced quantum metric dipole in Dirac semimetal Cd <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi/><mml:mn>3</mml:mn></mml:msub></mml:math> As <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi/><mml:mn>2</mml:mn></mml:msub></mml:math>

2025-06-20
NULL AUTHOR_ID | Physical Review Letters

Growth of bulk crystals of magnetic topological insulators under peritectic conditions

2025-06-19
Anton I. Sergeev , А. ĐĄ. Đ€Ń€ĐŸĐ»ĐŸĐČ , Nadezhda V. Vladimirova +7 more | Materials Chemistry and Physics
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On-chip amorphous terahertz topological photonic interconnects

2025-06-18
Rimi Banerjee , Abhishek Kumar , Thomas Caiwei Tan +6 more | Science Advances
Valley Hall photonic crystals (VPCs) offer the potential for creating topological waveguides capable of guiding light through sharp bends on a chip, enabling seamless integration with functional components in compact 
 Valley Hall photonic crystals (VPCs) offer the potential for creating topological waveguides capable of guiding light through sharp bends on a chip, enabling seamless integration with functional components in compact spaces, making them a promising technology for terahertz topological photonic integrated circuits. However, a key limitation for terahertz-scale integrated VPC-based devices has been the absence of arbitrary bend interconnects, as traditional VPC-designs restricted to principal lattice axes (i.e., only 0°, 60°, or 120°) due to crystalline symmetry. Here, we present an on-chip, all-silicon implementation of deformed VPCs that enable robust transmission along arbitrary shapes and bends. Although the lattice is amorphous and lacks long-range periodicity, the topological protection is sustained by short-range order. Furthermore, we show an amorphous lattice functioning as a frequency-dependent router, splitting input signals into two perpendicular output ports. We also demonstrate on-chip terahertz communication, achieving data rates of up to 72 Gbps. Our findings show that amorphous topological photonic crystals enhance interconnect adaptability while preserving performance.

Asymptotic Analysis for Bloch Electrons with Weyl Nodes

2025-06-18
Jianfeng Lu , Changhe Yang , Zhennan Zhou | Multiscale Modeling and Simulation
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Crossover from weak antilocalization to weak localization behavior in Bi2Te3/MnTe bilayer films

2025-06-18
Xudong Shi , Jian Gao , Tingting Li +4 more | Rare Metals

Mode-Shell correspondence, a unifying phase space theory in topological physics - Part II: Higher-dimensional spectral invariants

2025-06-18
Lucien Jezequel , Pierre Delplace | SciPost Physics
The mode-shell correspondence relates the number \mathcal{I}_M <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mstyle mathvariant="script"><mml:mi>ℐ</mml:mi></mml:mstyle><mml:mi>M</mml:mi></mml:msub></mml:math> of gapless modes in phase space to a topological \mathcal{I}_S <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mstyle mathvariant="script"><mml:mi>ℐ</mml:mi></mml:mstyle><mml:mi>S</mml:mi></mml:msub></mml:math> defined on a closed surface 
 The mode-shell correspondence relates the number \mathcal{I}_M <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mstyle mathvariant="script"><mml:mi>ℐ</mml:mi></mml:mstyle><mml:mi>M</mml:mi></mml:msub></mml:math> of gapless modes in phase space to a topological \mathcal{I}_S <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mstyle mathvariant="script"><mml:mi>ℐ</mml:mi></mml:mstyle><mml:mi>S</mml:mi></mml:msub></mml:math> defined on a closed surface - the shell - surrounding those modes, namely \mathcal{I}_M=\mathcal{I}_S <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mstyle mathvariant="script"><mml:mi>ℐ</mml:mi></mml:mstyle><mml:mi>M</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:msub><mml:mstyle mathvariant="script"><mml:mi>ℐ</mml:mi></mml:mstyle><mml:mi>S</mml:mi></mml:msub></mml:mrow></mml:math> . In part I, we introduced the mode-shell correspondence for zero-modes of chiral symmetric Hamiltonians (class AIII). In this part II, we broaden the correspondence to arbitrary dimension and to both symmetry classes A and AIII. This allows us to include, in particular, 1D <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>1</mml:mn><mml:mi>D</mml:mi></mml:mrow></mml:math> -unidirectional edge modes of Chern insulators, 2D <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>2</mml:mn><mml:mi>D</mml:mi></mml:mrow></mml:math> massless Dirac and 3D <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>3</mml:mn><mml:mi>D</mml:mi></mml:mrow></mml:math> -Weyl cones, within the same formalism. We provide an expression for \mathcal{I}_M <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mstyle mathvariant="script"><mml:mi>ℐ</mml:mi></mml:mstyle><mml:mi>M</mml:mi></mml:msub></mml:math> that only depends on the dimension of the dispersion relation of the gapless mode, and does not require a translation invariance. Then, we show that the topology of the shell (a circle, a sphere, a torus), that must account for the spreading of the gapless mode in phase space, yields specific expressions of the shell index. Semi-classical expressions of those shell indices are also derived and reduce to either Chern or winding numbers depending on the parity of the mode’s dimension. In that way, the mode-shell correspondence provides a unified and systematic topological description of both bulk and boundary gapless modes in any dimension, and in particular includes the bulk-boundary correspondence. We illustrate the generality of the theory by analyzing several models of semimetals and insulators, both on lattices and in the continuum, and also discuss weak and higher-order topological phases within this framework. Although this paper is a continuation of Part I, the content remains sufficiently independent to be mostly read separately.

Large magnetoresistance in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>ZrT</mml:mi><mml:msub><mml:mi mathvariant="normal">e</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math> crystals realized by growth control

2025-06-18
Yong Zhang , Yang‐Yang Lv , Yanyan Zhang +5 more | Physical Review Materials

Tunable fractional Chern insulators

2025-06-18
Yihang Zeng | Nature Materials

Unconventional Topological Weyl Dipole Phonon

2025-06-17
Jianhua Wang , Yang Wang , Feng Zhou +5 more | Advanced Science
Abstract A pair of Weyl points (WPs) with opposite topological charges can exhibit an additional higher‐order Z 2 topological charge, giving rise to the formation of a Z 2 Weyl 
 Abstract A pair of Weyl points (WPs) with opposite topological charges can exhibit an additional higher‐order Z 2 topological charge, giving rise to the formation of a Z 2 Weyl dipole (WD). Owing to the nontrivial topological charge, Z 2 WDs should also appear in pairs, and the WPs within each Z 2 WD can not be annihilated when meeting together. As a novel topological state, the topological Weyl dipole (TWD) phase has garnered significant attention, yet its realization in crystalline materials remains a challenge. Here, through first‐principles calculations and theoretical analysis, the existence of the nontrivial unconventional WD phase is demonstrated in the phonon spectra of the P 6 3 type Y(OH) 3 . Particularly, the nontrivial unconventional WD in this system is protected by a quantized quadrupole moment, and it is distinguished from conventional WD, as it comprises an unconventional charge‐3 WP with charge of –3 and three conventional charge‐1 WPs with charge of +1. Consequently, the nontrivial unconventional WD phase in Y(OH) 3 features unique 2D sextuple‐helicoid Fermi‐arc states on the top and bottom surfaces, protected by the topological charges, as well as 1D hinge states that connect the two nontrivial unconventional WDs along the side hinges, guaranteed by the quantized quadrupole moment.

Electron-Correlation-Induced Charge Density Waves and Magnetism-Related Energy Gap in Kagome FeGe Unraveled by STM/STS

2025-06-17
Xiaoqiu Yuan , Wanru Ma , Chengfeng Yu +7 more | Nano Letters
As the first magnetic Kagome material exhibiting charge density waves (CDWs), FeGe has garnered widespread research interest. In this work, we utilized low-temperature, high-magnetic-field scanning tunneling microscopy/spectroscopy to investigate both 
 As the first magnetic Kagome material exhibiting charge density waves (CDWs), FeGe has garnered widespread research interest. In this work, we utilized low-temperature, high-magnetic-field scanning tunneling microscopy/spectroscopy to investigate both the CDWs and magnetism in FeGe. We observed coexisting short-range 2 × 2 and √3 × √3 CDW patterns, which are spatially exclusive with Ge1 site defects in the Kagome layer, suggesting that CDW formation is related to Ge1-dimerization involving electron correlations. Using a spin-polarized tip, we identified the A-type antiferromagnetic (AFM) structure, which undergoes a gradual spin-flop transition with increasing magnetic field. The gap opened at the Fermi energy evolves with the magnetic structure transition but remains insensitive to the presence of CDWs. These results underscore the role of electron correlations in the formation of the CDWs in FeGe and identify the magnetism origin of the low-energy gap, revealing a compatibility between AFM order and CDWs in FeGe.

Imaging valley-vortex edge modes in a phononic crystal at ultrahigh frequencies

2025-06-17
Paul H. Otsuka , Motonobu Tomoda , Daiki Hatanaka +3 more | Journal of Applied Physics
We perform optical measurements and numerical simulations of guided phonon propagation in novel topological phononic crystal structures at ultrahigh frequencies. The structures support valley-polarized states that exhibit an energy vortex 
 We perform optical measurements and numerical simulations of guided phonon propagation in novel topological phononic crystal structures at ultrahigh frequencies. The structures support valley-polarized states that exhibit an energy vortex nature and propagate with high efficiency at domain boundaries because backscattering is suppressed due to conservation of time reversal symmetry. We extract frequency- and time-resolved spatial mode patterns and k-space images, together with dispersion relations. We investigate the conditions required for robust propagation along interfaces and thereby observe very high efficiency waveguiding.

Thermionics in Topological Materials

2025-06-16
Sunchao Huang , Zihao Zhang , Youfeng Yang +6 more | Advanced Materials
Abstract Thermionic emission is fundamental to many technologies and devices, including thermionic energy converters, X‐ray tubes, scanning electron microscopes, and transmission electron microscopes. The discovery of topological materials, particularly graphene, 
 Abstract Thermionic emission is fundamental to many technologies and devices, including thermionic energy converters, X‐ray tubes, scanning electron microscopes, and transmission electron microscopes. The discovery of topological materials, particularly graphene, has significantly advanced thermionics research. Thermionic emission in these materials deviates from the Richardson‐Dushman equation due to their linear energy dispersion. Various models are developed to accurately describe thermionic emission. Graphene, with its dangling bond‐free surface, can be stacked either vertically or laterally with materials to form heterostructures. The Schottky barrier height at the interface of heterostructures can be tuned from a few millielectronvolts to several electronvolts by selecting appropriate materials or adjusting the Fermi level of graphene. This low and tunable barrier height gives rise to a great potential in developing thermionic energy converters and photodetectors. While free‐standing single‐layer graphene exhibits high electron mobility, its thermionic emission capability is constrained by the low density of states. This constraint can be alleviated by using 3D Dirac materials, which also possess linear energy dispersion. Thermionic emission in 3D Dirac materials is further enhanced by the emergence of nodal‐ring semimetals and Weyl semimetals that exhibit linear‐like energy dispersion. This review highlights recent progress in thermionic emission and devices in graphene structures and other topological materials.

The in-plane anisotropy of topological semimetal Nb3SiTe6 investigated by angle-resolved polarized Raman spectroscopy

2025-06-16
Qinghang Liu , Xiaolan Zhang , Peng Zhu +5 more | Applied Physics Letters
The in-plane anisotropy of the layered topological semimetal Nb3SiTe6, owing to its unique lattice structure and nontrivial electronic states, may play an important role in low-power, multifunctional optoelectronic devices by 
 The in-plane anisotropy of the layered topological semimetal Nb3SiTe6, owing to its unique lattice structure and nontrivial electronic states, may play an important role in low-power, multifunctional optoelectronic devices by regulating anisotropy, and provide an ideal research platform for exploring quantum phenomena such as chiral anomaly and quantum Hall effect. In the above-mentioned research related to material anisotropy, it is necessary to determine the crystallographic orientation of thin-layer samples. In this paper, we systematically studied the optical in-plane anisotropy of the layered topological semimetal Nb3SiTe6 via angle-resolved polarized Raman spectroscopy. The Raman intensities of the 13 Raman peaks show clear polarization dependence. Based on the lattice symmetry of multilayer Nb3SiTe6, we accurately distinguished the two Raman modes of Nb3SiTe6 and corresponding Raman peaks. In addition, we proposed a method for quickly, accurately, and nondestructively determining the crystallographic orientation of multilayer Nb3SiTe6 by polarized Raman spectroscopy. This work provides a crucial foundation for exploring potential applications of the anisotropy of Nb3SiTe6 in thermoelectric and optoelectronic fields.

Observation of the Magnetic Field Induced Fermi Surface Expansion in Aperiodic Quantum Oscillations

2025-06-16
Anamika Kumari , Harsha Silotia , Sunit Das +4 more | Advanced Functional Materials
Abstract Magnetic field‐induced quantum oscillations in resistivity have been extensively used to explore Fermi surfaces in quantum materials. This is enabled by the robustness of the Fermi surface to a 
 Abstract Magnetic field‐induced quantum oscillations in resistivity have been extensively used to explore Fermi surfaces in quantum materials. This is enabled by the robustness of the Fermi surface to a magnetic field and the validity of the semiclassical Onsager's quantization relation for Landau levels (LLs). Challenging this conventional understanding, evidence of magnetic field‐induced expansion of the Fermi surface is presented. This is captured by our observations of magnetic field induced aperiodic quantum oscillations in resistivity at the conducting interfaces of LaVO 3 ‐KTaO 3 (LVO‐KTO) and EuO‐KTaO 3 (EO‐KTO). It is showed that these aperiodic oscillations occur in systems with a small Fermi surface, where the magnetic susceptibility‐induced corrections to the Fermi surface become substantial. Physically, these arise from the magnetic susceptibility‐induced modifications in the Free energy, resulting in the generalization of the semiclassical quantization rule for LLs. The magnetic field‐induced Fermi surface expansion is corroborated via the measurements of nonlinear Landau fan diagrams, deviations in the Lifshitz–Kosevich (LK) fit of longitudinal magnetoresistivity, and the magnetic field dependence of the effective cyclotron mass extracted from transport measurements. These findings provide a new perspective and fresh insights into the intriguing physics of magnetic field‐based probes of the Fermi surface.

Observation of topological Hall effect and Nernst effect in the canted antiferromagnetic phase of MnBi<sub>2</sub>Te<sub>4</sub>

2025-06-16
Haiyang Yang , Junwu Huang , Shangjie Tian +7 more | Chinese Physics Letters
Abstract In magnetic topological materials, the interplay between magnetism and nontrivial topology gives rise to exotic quantum transport phenomena, including the anomalous Hall effect (AHE) and anomalous Nernst effect (ANE). 
 Abstract In magnetic topological materials, the interplay between magnetism and nontrivial topology gives rise to exotic quantum transport phenomena, including the anomalous Hall effect (AHE) and anomalous Nernst effect (ANE). Here, we report the observation of intrinsic topological Hall and topological Nernst effects below the NĂ©el temperature ( T N = 25 K) in the antiferromagnetic (AFM) topological insulator MnBi 2 Te 4 . The maximum of topological Hall resistivity reaches approximately 9 ” Ω·cm at 2 K, while the topological Nernst signal attains a peak value of 0:1 ” V=K near 10 K. These anomalous transport behaviors originate from the net Berry curvature induced by the non-collinear spin structure in the canted AFM state. Our results suggest a close connection between the topological thermoelectric effect and non-collinear antiferromagnetic order in AFM topological insulators.

Local potential distribution generates edge currents in a magnetic topological insulator

2025-06-16
NULL AUTHOR_ID | Physical review. B./Physical review. B

Warping Effect-Induced Spin Current Absorption at Various Timescales in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>Fe</mml:mi><mml:mo>/</mml:mo><mml:msub><mml:mrow><mml:mi>Bi</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Te</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> Heterostructures

2025-06-13
J.H. Liu , Hao‐Pu Xue , Zhuo Deng +7 more | Physical Review Letters

Effects of chemical disorder and spin-orbit coupling on electronic-structure and Fermi-surface topology of YbSb-based monopnictides

2025-06-13
NULL AUTHOR_ID | Physical review. B./Physical review. B

Weyl points in ball-and-spring mechanical systems

2025-06-13
Zoltån Guba , György Frank , G. Pintér +1 more | Physical review. B./Physical review. B

Non-Majorana origin of anomalous current-phase relation and Josephson diode effect in Bi <sub>2</sub> SE <sub>3</sub> /NbSe <sub>2</sub> Josephson junctions

2025-06-13
Andrei Kudriashov , Xiong Zhou , Razmik A. Hovhannisyan +7 more | Science Advances
Josephson junctions (JJs) are key to superconducting quantum technologies and the search for self-conjugate quasiparticles potentially useful for fault-tolerant quantum computing. In topological insulator (TI)–based JJs, measuring the current-phase relation 
 Josephson junctions (JJs) are key to superconducting quantum technologies and the search for self-conjugate quasiparticles potentially useful for fault-tolerant quantum computing. In topological insulator (TI)–based JJs, measuring the current-phase relation (CPR) can reveal unconventional effects such as Majorana bound states (MBS) and nonreciprocal transport. However, reconstructing CPR as a function of magnetic field has not been attempted. Here, we present a platform for field-dependent CPR measurements in planar JJs made of NbSe 2 and few-layer Bi 2 Se 3 . When a flux quantum <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">Ω</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:mrow> </mml:math> threads the junction, we observe anomalous peak-dip CPR structure and nonreciprocal supercurrent flow. We show that these arise from a nonuniform supercurrent distribution that also leads to a robust and tunable Josephson diode effect. Furthermore, despite numerous previous studies, we find no evidence of MBS. Our results establish magnetic field–dependent CPR as a powerful probe of TI-based superconducting devices and offer design strategies for nonreciprocal superconducting electronics.