Experimental observation of a first-order phase transition below the superconducting transition temperature in the multilayer cuprate superconductor HgBa$_2$Ca$_4$Cu$_5$O$_y$

TL;DR

This study reports the observation of a first-order phase transition below the superconducting transition temperature in the multilayer cuprate superconductor HgBa$_2$Ca$_4$Cu$_5$O$_y$, characterized by a hysteretic specific heat jump at 41 K.

Contribution

It provides experimental evidence of a first-order phase transition related to phase difference fluctuations in a multilayer cuprate superconductor, revealing residual entropy in BCS pairs.

Findings

Hysteretic specific heat jump at 41 K

Residual entropy due to phase fluctuations

First-order transition below Tc

Abstract

A first-order phase transition is found in the multilayer cuprate superconductor, HgBa$_2$Ca$_4$Cu$_5$O$_y$ (Hg-1245), with a superconducting transition temperature of 108 K, under zero magnetic field. We observed a hysteretic specific heat jump around 41 K. We conclude that the Bardeen-Cooper-Schrieffer pairs have a residual entropy due to fluctuations in the phase difference between the five CuO$_2$ planes in a unit cell of Hg-1245, and that this fluctuation freezes below the first-order phase transition temperature.