Multiband superconductivity in HoNi2B2C

Abstract

HoNi2B2C single crystals prepared by the floating zone method were investigated by resistivity, magnetization, specific heat and point-contact measurements. The coexistence of superconductivity (SC) and antiferromagnetism (AFM) in HoNi2B2C is considered experimentally and theoretically for commensurate (C), incommensurate (IC) and metamagnetic magnetic structures within the multiband picture of Fermi surface sheets (FSS) obtained within full potential FPLO–LDA calculations. Special attention is paid to the admixture of Ho 5d states which mediate the pair breaking exchange between the Ho 4f derived local moments and the conduction electrons bearing the superconductivity. Within the paramagnetic and the uniform IC spiral phases, anisotropic multiband SC coexists with magnetism. Within the C-phase (below TN=5.3K) single band isotropic SC survives at a single FSS free of Ho 5d states in accord with isotropic standard WHH shaped upper critical fields Hc2(T). The superconducting energy gap obtained from point-contact data was found to disappear at a slightly higher temperature T∗=5.6K which is also observed in specific heat measurements. Using the experimental values for the energy gap Δ(0) and Hc2(0) at zero temperature, the characteristic phonon frequency, the electron–phonon coupling constant and the Fermi velocity of the residual superconducting phase were estimated from the standard isotropic single band model.