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Band-structure calculations of Fermi-surface pockets in<i>ortho</i>-II<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>YBa</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Cu</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>6.5</mml:mn></mml:msub></mml:mrow></mml:math>
We explore whether the quantum oscillation signals recently observed in ortho-II ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ (OII-Y123) may be explained by conventional density-functional band-structure theory. Our calculations show that the Fermi surface of OII-Y123 is extremely sensitive to small shifts in the relative positions of the bands. With rigid band shifts of around $\ifmmode\pm\else\textpm\fi{}30\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ …