Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C in the normal and superconducting state
Abstract
Point-contact (PC) spectroscopy measurements on antiferromagnetic (AF) (TN=5.2K) HoNi2B2C single crystals in the normal and two different superconducting (SC) states (Tc=8.5K and Tc*=5.6K) are reported. The PC study of the electron-boson(phonon) interaction (EB(P)I) spectral function reveals pronounced phonon maxima at 16, 22 and 34meV. For the first time the high energy maxima at about 50meV and 100meV are resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitations with a maximum near 10meV and a `magnetic` peak near 3meV are observed. The contribution of the 10-meV peak in PC EPI constant λPC is evaluated as 20-30%, while contribution of the high energy modes at 50 and 100meV amounts about 10% for each maxima, so the superconductivity might be affected by CEF excitations. The SC gap in HoNi2B2C exhibits a standard single-band BCS-like dependence, but vanishes at Tc*=5.6K<Tc, with 2/kTc*=3.9. The strong coupling Eliashberg analysis of the low-temperature SC phase with Tc*=5.6K =TN, coexisting with the commensurate AF structure, suggests a sizable value of the EPI constant λs=0.93. We also provide strong support for the recently proposed by us ''Fermi surface (FS) separation'' scenario for the coexistence of magnetism and superconductivity in magnetic borocarbides, namely, that the superconductivity in the commensurate AF phase survives at a special (nearly isotropic) FS sheet without an admixture of Ho 5d states. Above Tc* the SC features in the PC characteristics are strongly suppressed pointing to a specific weakened SC state between Tc* and Tc.