Abstract

The cuprate superconductor hbox {YBa}_2hbox {Cu}_4hbox {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 hbox {YBa}_2hbox {Cu}_4hbox {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.

Highlights

  • The cuprate superconductor YBa2Cu4O8, 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

  • The electronic specific heat captures the entire spectrum of low-energy excitations and in principle can adjudicate in all these matters

  • We have previously investigated the 89 Y NMR Knight shift in Zn-doped Y­ 12415

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Summary

Introduction

The cuprate superconductor YBa2Cu4O8 , 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. From the field-dependent specific heat, we characterise the impact of fluctuations and impurity scattering on the thermodynamic properties. Both the ­pseudogap[1,2,3] and the origins of superconductivity in the cuprates remain enigmatic and a source of continuing dispute, especially the ­former[4]. The sample variability indicates unidentified impurities and these anomalies are ignored in the following)

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