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$_y$

摘要

A first-order phase transition is found in the multilayer cupratesuperconductor, HgBa$_2$Ca$_4$Cu$_5$O$_y$ (Hg-1245), with a superconductingtransition temperature of 108 K, under zero magnetic field. We observed ahysteretic specific heat jump around 41 K. We conclude that theBardeen-Cooper-Schrieffer pairs have a residual entropy due to fluctuations inthe 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 transitiontemperature.