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Title: Is the pseudogap endpoint p* a quantum critical point
Plain: 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.
Precise: With superconductivity suppressed by fields of several hundred kG, determine whether the endpoint $p*$ (0.19 to 0.23 depending on family) is a quantum critical point with an order parameter ($C/T \sim \operatorname{log}(1/T)$, Hall number change from $p\ \text{to}\ 1+p$) or a change of Fermi-surface topology without broken symmetry. An answer identifies the transition type and, if one exists, the order parameter whose fluctuations give the log divergence.
Settled by: Detection of the order parameter (or proof of its absence) by a symmetry-sensitive probe in the same high-field regime where C/T diverges.
Kind: well-posed
Answer type: classification
References: (empty)
Literature status: contested
Status note: High-field specific heat (2019) found $C/T \sim \log(1/T)$ at $p*$ in Nd- and Eu-substituted La2-xSrxCuO4, but the associated order parameter is unidentified.
Posed since: (empty)