Heat capacities have been measured for the high-Tc superconductors LBa2Cu3O7-y (L=Gd,Ho,Er,Y) between 0.15 and 100 K. While the superconducting phase-transition temperatures of these compounds are nearly the same at Tc=92 K, the heat-capacity data indicate magnetic ordering in the system of rare earths at Tm=2.21 K for GdBa2Cu3O7-y and at Tm=0.60 K for ErBa2Cu3O7-y. In both cases, the cooperative heat-capacity peak resmbles that of the two-dimensional (2D) Ising model. In addition, broad heat-capacity anomalies, which cannot be attributed to crystalline field splittings, are found. A semiquantitative fit can be obtained by assuming that two-thirds of the rare earths constitute a 2D Ising system and the rest of them form a 1D Ising-like system. The heat capacity of the HoBa2Cu3O7-y shows a very flat maximum at 5 K. The separation of the two lowest-lying singlet levels of Ho3+ ions is estimated to be 7 K.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics