Anomalies in heat capacity of YBa_2Cu_3O_{6+x} in normal state

Abstract

The temperature dependence of heat capacity for the samples of 90 K phase of YBa_2Cu_3O_{6+x} with x = 0.85, 0.90 and 0.95 was investigated above T_c. For separating the heat capacity into regular and anomalous contributions the special technique was used. For all the samples the anomalies in intervals 100-200 K (T_{low}), 205-230 K (T_m) and 260-290 K (T_h) were detected. At T = 3T_c the temperature "echo" from superconducting phase transition was discovered. This "echo" is connected with the anomaly T_h, which is believed to mark the rise of pairing carriers of charge without phase coherence. We connect the anomaly T_m with the Neel point of magnetic subsystem which has antiferromagnetism coexisting with superconductivity. The x - dependence of entropy S_h points out that T_h - process probably exists only in the frameworks of 90 K phase. The curves S_m(x) and S_h(x) intersect each other at the optimal doping. The discovered anomalies mark new lines on the phase diagram of YBa_2Cu_3O_{6+x} - compounds.