Upper critical field and heat capacity in the reentrant superconducting system (Lu1-xErx)RuB2

Low-temperature heat capacity, upper-critical-field measurements, and ac magnetic susceptibility are utilized to study the interaction between superconductivity and magnetic order in the orthorhombic compounds (Lu1-xErx)RuB2. Reentrant superconductivity is observed in a concentration range just below a critical value (xcr 0.5), where the superconducting and magnetic phase boundaries meet. For samples with concentrations greater than xcr, two magnetic transitions are observed, opening the possibility of complex magnetic states. The strength of the effective exchange field, Hex, is derived from the conduction-electron rare-earth spin-exchange interaction. Superconductivity is quenched and reentrance occurs when Hex becomes comparable to the Pauli paramagnetic limiting field. Heat-capacity data for LuRuB2 yield a relatively high Debye temperature of 487 K and an electronic specific-heat coefficient of 7.05 mJ/mol K2.