Field-dependent specific heat of the canonical underdoped cuprate superconductor YBa$_2$Cu$_4$O$_8$

TL;DR

This study uses high-temperature, field-dependent specific heat measurements on YBa$_2$Cu$_4$O$_8$ to demonstrate that the pseudogap persists well above the traditionally assumed closing temperature, challenging existing pseudogap theories.

Contribution

It provides direct thermodynamic evidence that the pseudogap in YBa$_2$Cu$_4$O$_8$ does not close near $T^*$, contrary to common assumptions, and explores the effects of fluctuations and impurities.

Findings

Pseudogap remains open at least up to 400 K.

Wilson ratio is consistent with a weakly-interacting Fermion system.

Field-dependent specific heat reveals fluctuation and impurity effects.

Abstract

The cuprate superconductor YBa$_2$Cu$_4$O$_8$, in comparison with most other cuprates, has a stable stoichiometry, is largely free of defects and may be regarded as the canonical underdoped cuprate, displaying marked pseudogap behaviour and an associated distinct weakening of superconducting properties. This cuprate `pseudogap' manifests as a partial gap in the electronic density of states at the Fermi level and is observed in most spectroscopic properties. After several decades of intensive study it is widely believed that the pseudogap closes, mean-field like, near a characteristic temperature, $T^*$, which rises with decreasing hole concentration, $p$. Here, we report extensive field-dependent electronic specific heat studies on YBa$_2$Cu$_4$O$_8$ up to an unprecedented 400 K and show unequivocally that the pseudogap never closes, remaining open to at least 400 K where $T^*$ is typically presumed to be about 150 K. We show from the NMR Knight shift and the electronic entropy that the Wilson ratio is numerically consistent with a weakly-interacting Fermion system for the near-nodal states. And, from the field-dependent specific heat, we characterise the impact of fluctuations and impurity scattering on the thermodynamic properties.