Is the pseudogap temperature $T*$ a thermodynamic phase transition?
In plain words
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.
Precise statement
In a clean hole-doped cuprate (YBa2Cu3O6+x or HgBa2CuO4+d), determine whether the free energy is nonanalytic along $T*(p)$: resolve a specific-heat or thermal-expansion anomaly at $T*$ matching the symmetry breaking claimed by polarized neutron scattering, torque magnetometry and polar Kerr rotation, or bound $\Delta C/T$ below the value required by a mean-field transition with the claimed order parameter. An answer is a detected singularity with its exponent, or such a bound.
What would settle it
High-resolution calorimetry or thermal expansion through $T*$ on crystals in which torque or neutron onsets are measured on the same piece.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Resonant ultrasound (2013) found an elastic anomaly along $T*(p)$, and torque magnetometry (2017) and polarized neutron data indicate symmetry breaking near $T*$, but no calorimetric anomaly at $T*$ has been resolved and the neutron results are disputed.
Parent problems
- Special case of What is the pseudogap state of underdoped cuprates? · open · from revision 1
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