Interface superconductivity: FeSe/$\mathrm{SrTiO_3}$ and $\mathrm{KTaO_3}$

5 problems

• Why does $\mathrm{SrTiO_3}$ raise the transition temperature of monolayer FeSe?

  Contested

  Electrons donated by the substrate explain part of the rise, since FeSe doped with electrons in other ways reaches about 40 to 48 K. Whether vibrations of the substrate supply the rest is disputed.

• Why does $\mathrm{KTaO_3}$ interface superconductivity depend on crystal orientation?

  Open

  Electron gases at the surface of $\mathrm{KTaO_3}$ superconduct at about $2\,\mathrm{K}$ on one crystal face and at much lower temperature, or not at all, on another, which no simple theory explains.

• Pairing symmetry of monolayer FeSe on $\mathrm{SrTiO_3}$

  Contested

  Monolayer FeSe lacks the hole pocket that standard theories of iron-based superconductors rely on, so it is unclear whether the superconducting gap changes sign between its electron pockets.

• Are photoemission replica bands evidence of interfacial forward scattering?

  Contested

  Photoemission shows faint copies of the FeSe bands shifted by the energy of a substrate vibration, read as strong electron-phonon coupling. Others argue the copies form when emitted electrons lose energy after leaving the sample.

• What is the zero-resistance transition temperature of monolayer FeSe on $\mathrm{SrTiO_3}$?

  Contested

  Different methods give very different transition temperatures, from about 40 K in transport on protected films and about 65 K in magnetic measurements to a single report above 100 K, so even the basic number is uncertain.