4f Electron effects in high Tc RBa 2Cu 3O 7 − δ (R = rare earth) superconductors

Abstract

Experiments on high T c RBa 2Cu 3O 7 − δ (R = rare earth; δ ≈ 0.1) compounds have revealed a variety of interesting effects that involve the 4f electrons of the R ions. Schottky anomalies due to the partial lifting of the degeneracy of the Hund's rules ground state multiplets of the R ions by the crystalline electric field (CEF) are found in the low temperature specific heat. With some exceptions, the anisotropy of the room temperature paramagnetic susceptibility correlates with the sign of the second order Stevens factor of the CEF hamiltonian. Specific heat anomalies due to the antiferromagnetic ordering of the R 3+ ions in RBa 2Cu 3O 7 − σ compounds with R  Nd, Sm, Dy and Er can be described well by the anisotropic twodimensional Ising model with an exchange interaction parameter ratio that ranges from approximately 50 for neodymium to approximately 4 for dysprosium. The magnetic ordering temperatures and shapes of the magnetic specific heat anomalies change markedly when the superconductivity of the compounds is quenched by increasing the oxygen vacancy concentration to δ ≳ 0.5, indicating that RKKY and/or superexchange interactions are involved in the magnetic ordering of the R 3+ ions in these materials. The anomalous pressure dependence of T c and the normal state electrical resistivity in the (Y 1 − xPr x x )Ba 2Cu 3O 7 − δ system suggest that the praseodymium 4f states are hybridized with valence band states, which may be responsible for the lack of metallic behavior and superconductivity in PrBa 2Cu 3O 7 − δ. The compound PrBa 2Cu 3O 7 − δ appears to exhibit some form of complex antiferromagnetic order at approximately 16 K and has a γT contribution to the specific heat at low temperatures with a large γ value of approximately 169 mJ (mol praseodymium) −1 K −2, reminiscent of heavy fermion behavior.