No abstract

The specific heat, C p , has been measured with high precision in a mono-crystal of YBa 2 Cu 3 O 7−δ (YBCO) superconductor in the temperature range 55K-300K. The measurements provide data with a relative resolution Δ C p / C p of approximately 5 · 10−4. The main conclusions are: 1) Close to the superconducting transition the specific heat shows influence of fluctuations. 2) A reversible second order or weakly first order phase transition is found to occur at T o = 229 K .

Ultrasonic, vibrating-reed, and specific-heat measurements in the new high-T, superconductor YBCO are analysed showing glasslike dynamics over 7 orders of magnitude in frequency, from 1 Hz to 10 MHz. The relaxation time seems to diverge towards To = 210 K with an approximate value of the effective dynamic exponent v x = 8. This value is typical for glass systems. We suggest that the observed behaviour is caused by an intrinsic structural instability related to the oxygen vacancies, with dynamics dominated by random fields or glasslike behaviour.The new class of high-T, superconducting copper-oxide perovskites discovered by Bednorz and Muller [l] soon led to the synthesis of other related compounds, notably the oxygen-deficient perovskite high-T, superconductor Y B ~& U ~O ~-~ (YBCO) 121. The new superconductors have proven to possess interesting elastic [3-51 and thermodynamic [6,7] properties. It has been suggested [3] that the elastic properties of Lal.s5Bao.I&u04 are similar to a structural glass, and that these characteristics are related to observations of glasslike superconducting behaviour [3,8,91. The tetragonal-to-orthorhombic structural phase transition [lo-121 and the oxygen content [13] have decisive influence on the superconducting behaviour of YBCO, implying that the degree of order in the oxygen system is an important parameter for the material properties.Recently specific-heat measurements in YBCO have provided evidence for an ordering phenomenon near 220 K [61. Ordering related to the oxygen system was suggested as a possible explanation. Still, many important structural aspects, especially related to the phase diagram with varying oxygen content, remain to be clarified in all of these materials.Structural instabilities may, as seems to be the case in the A-15 compounds, be of importance for the extraordinary superconducting properties of these materials [14,151. In this letter we present possible evidence for glasslike dynamics in YBCO over 7 orders of magnitude in frequency, based on the interpretation of elastic [4,5,16-191 and thermodynamic data [6,7]. First, a brief summary of results from our ultrasonic, vibrating-reed, and specific-heat measurements is given, followed by a short description of the other