Cuprate pseudogap, charge order and Fermi-surface reconstruction

5 problems

• Origin of cuprate charge order and its relation to pairing

  Contested

  Below roughly 150 K, underdoped cuprates develop a weak, short-range periodic ripple in electron density (charge order) with a period of 3 to 4 lattice spacings; whether it competes with superconductivity, grows out of a modulated superconductor, or causes the pseudogap is unsettled.

• What is the pseudogap state of underdoped cuprates?

  Contested

  Above the superconducting temperature, underdoped cuprates lose many low-energy electron states, a partial gap called the pseudogap; it is unclear whether this is a separate phase of matter, pairs without long-range order, or a gradual crossover.

• What reconstructs the underdoped cuprate Fermi surface into small pockets?

  Partially resolved

  In strong magnetic fields, underdoped YBa2Cu3O6+x shows quantum oscillations, field-periodic wiggles in resistance that reveal a small closed pocket of electrons, whereas at zero field the Fermi surface is a single large hole-like contour; some order must cut the large surface into pieces.

• Is the pseudogap endpoint $p*$ a quantum critical point?

  Contested

  Near 19 percent hole doping the pseudogap vanishes, and there the electronic heat capacity peaks and resistance becomes linear in temperature, signs of a phase transition at zero temperature.

• Is the pseudogap temperature $T*$ a thermodynamic phase transition?

  Contested

  If the pseudogap is a true phase, crossing its boundary should leave a mark in heat capacity, as freezing does for water; no such mark has been clearly seen.