Charge order and superconductivity in kagome metals

5 problems

• Is the kagome charge-density wave electronic or phonon driven?

  Contested

  The charge order could arise from electrons near special points of the band structure interacting with each other, or from coupling to lattice vibrations that soften.

• Does the charge order in $\mathrm{CsV_3Sb_5}$ break time-reversal symmetry?

  Contested

  Some experiments suggest that the charge order in $\mathrm{CsV_3Sb_5}$ comes with tiny circulating electric currents (loop currents) that break time-reversal symmetry, while sensitive optical tests see nothing.

• Density-wave order and pairing in pressurized $\mathrm{CsCr_3Sb_5}$

  Open

  $\mathrm{CsCr_3Sb_5}$, a chromium kagome metal, forms a density wave at normal pressure and becomes a superconductor near 6 K when squeezed, next to a metal whose resistance is linear in temperature; what the density wave is and whether its fluctuations pair the electrons is unknown.

• Is there a pair density wave in superconducting $\mathrm{CsV_3Sb_5}$?

  Contested

  Tunneling microscopy saw the superconducting gap of $\mathrm{CsV_3Sb_5}$ oscillate in space with a new period, which could be a pair density wave or a side effect of the charge order.

• Superconducting gap structure of AV3Sb5

  Contested

  Whether the superconducting gap of these kagome metals vanishes somewhere on the Fermi surface, and whether it changes sign, is disputed.