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$_y$
  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. 
Y. Tanaka, A. Iyo, S. Itoh, K. Tokiwa, T. Nishio, T. Yanagisawa
cond-mat.supr-con