Copper-oxide superconductors: pairing and superconducting state

5 problems

• Does the charge-transfer gap set the maximum $T_{c}$ across cuprate families?

  Partially resolved

  The energy needed to move an electron from oxygen to copper, the charge-transfer gap, differs between cuprate families; superexchange theories predict that a smaller gap gives stronger pairing and a higher transition temperature.

• Why the superfluid density collapses in overdoped cuprates

  Contested

  Adding holes beyond the optimum lowers the superconducting temperature, and the density of paired electrons falls in step even though the metal looks more ordinary; disorder and new physics are both proposed as the cause.

• What interaction binds electrons into pairs in the cuprates?

  Contested

  In a superconductor electrons form pairs (Cooper pairs) that move without resistance; in ordinary metals lattice vibrations supply the attraction, but in cuprates the source of the attraction is not agreed upon.

• Is there a bulk pair density wave in any cuprate?

  Contested

  A pair density wave is a superconductor whose pair amplitude oscillates in space like a standing wave; hints of it appear in tunneling maps, around magnetic vortices and in one stripe-ordered cuprate.

• Does 45-degree twisted $\mathrm{Bi2Sr2CaCu2O8}+x$ break time-reversal symmetry?

  Contested

  Stacking two thin flakes of the cuprate Bi2Sr2CaCu2O8+x rotated by 45 degrees is predicted to create a superconducting state whose pairs circulate in one sense, breaking time-reversal symmetry; Josephson experiments by different groups disagree.