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Title: What is the pseudogap state of underdoped cuprates
Plain: 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: 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 $p*$.
Settled by: 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.
Kind: phenomenon
Answer type: classification
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: 1989