Specific Heat of RBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-x}$ and RBaSrCu$\text{}_{3}$O$\text{}_{7-x}$ Compounds (R = Sm, Dy, Er)

The low-temperature specific-heat C(T) data of high-T$\text{}_{c}$ superconductors RBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-x}$(R123O$\text{}_{7-x}$) were reviewed. A discussion of the C(T) data obtained on single-crystalline samples of Er123O$\text{}_{7-x}$ and Sm123O$\text{}_{7-x}$ compounds in applied magnetic fields for better evaluation of the crystal-electric-field parameters is given. The C(T) measurements of Sr-substituted compounds RBaSrCu$\text{}_{3}$O$\text{}_{7}$ revealed the λ-type of peak related to the antiferromagnetic order at 1.2 K and 0.6 K respectively for Dy- and Er-compounds. The magnetic singlet ground state in these compounds is produced by weak exchange interactions of -0.05 T/μ$\text{}_{B}$ and -0.035 T/μ$\text{}_{B}$, respectively. For Nd- and Sm-compounds, only a broad anomaly around 0.6 K was observed which is well fitted with a Schottky-type anomaly with an energy splitting of 1.4 K.