Unraveling pairon excitations and the antiferromagnetic contributions in the cuprate specific heat

Abstract

Thermal measurements, such as entropy and specific heat, reveal key elementary excitations for understanding the cuprates. In this paper, we study the specific heat measurements on three different compounds La${}_{2-x}$Sr${}_x$CuO${}_4$, Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ and YBa${}_2$Cu${}_3$O${}_{7-\delta }$ and show that the data are compatible with ‘pairons’ and their excitations. However, the precise fits require the contribution of the antiferromagnetic entropy deduced from the magnetic susceptibility $\chi \left(T\right)$.

Two temperature scales are involved in the excitations above the critical temperature $T_c$: the pseudogap $T^{\ast }$, related to pairon excitations, and the magnetic correlation temperature, $T_{max}$, having very different dependencies on the carrier density ($p$). In agreement with our previous analysis of $\chi \left(T\right)$, the $T_{max}\left(p\right)$ line is not the signature of a gap in the electronic density of states, but is rather the temperature scale of strong local antiferromagnetic correlations which dominate for low carrier concentration. These correlations progressively evolve into paramagnetic fluctuations in the overdoped limit.

Our results are in striking contradiction with the model of Tallon and Storey (2023), who reaffirm the idea of a $T$-independent gap $E_g$, whose temperature scale $T_g=E_g/k_B$ decreases linearly with $p$ and vanishes at a critical value $p_c\sim 0.19$.

Finally, we discuss the unconventional fluctuation regime above $T_c$, which is associated with a mini-gap $\delta \sim$ 2 meV in the pairon excitation spectrum. This energy scale is fundamental to the condensation mechanism.