Specific heat anomaly at 220 K connected with superconductivity at 90 K in ceramic YBa2Cu3O7–δ

Abstract

Extraordinary efforts have recently been put into trying to understand the new class of high-temperature superconducting copper oxides discovered by Bednorz and Muller1. Here, we present high-resolution specific heat measurements on four different samples of YBa2Cu3O7–δ (YBCO) which reveal a new anomaly at 220 K (see also ref. 2). The strength of the anomaly as measured by its height is related to the size of the jump at the superconducting transition at 90 K. Specific heat measurements are sensitive and reliable volume probes of ordering phenomena in the bulk of solids; our results therefore establish that there are ordering phenomena taking place in YBCO at ∼220 K. These findings may be of importance for understanding reports of oxygen ordering3–5 and of various unstable superconducting transitions in the range from 150 K (ref. 6) to well above 200 K (ref. 7).