Kinetic energy change with doping upon superfluid condensation in high-temperature superconductors
Kinetic energy change with doping upon superfluid condensation in high-temperature superconductors
In conventional BCS superconductors, the electronic kinetic energy increases upon superfluid condensation (the change $\ensuremath{\Delta}{E}_{\mathit{kin}}$ is positive). Here we show that in the high critical temperature superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$, $\ensuremath{\Delta}{E}_{\mathit{kin}}$ crosses over from a fully compatible conventional BCS behavior $(\ensuremath{\Delta}{E}_{\mathit{kin}}>0)$ to an unconventional behavior $(\ensuremath{\Delta}{E}_{\mathit{kin}}<0)$ as the free carrier density decreases. …