What is the pseudogap state of underdoped cuprates
In plain words
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.
Precise statement
Identify the state below $T*(p)$ in hole-doped cuprates for $p < p* \sim 0.19$: a broken-symmetry phase (intra-unit-cell loop currents, nematic order, pair density wave, fluctuating antiferromagnet), a fractionalized metal with small Fermi pockets ($\mathrm{FL}*$), or a crossover. An answer specifies the order parameter or topological character and accounts for the Fermi arcs, the Knight-shift suppression, and the drop of Hall carrier density from $1+p$ to p near $p*$.
What would settle it
A symmetry-resolved or thermodynamic measurement establishing a phase transition at $T*$, plus a theory that reproduces the carrier-density change at $p*$ and the arc spectrum.
Related problems
- More general than Is the pseudogap endpoint p* a quantum critical point
- More general than Is the pseudogap temperature T* a thermodynamic phase transition