Peculiarities of the superconducting gaps and the electron-boson interaction in TmNi2B2C as seen by point-contact spectroscopy

TL;DR

This study uses point-contact spectroscopy to explore the multiband superconducting gaps and electron-boson interactions in TmNi2B2C, revealing a second larger gap, reentrant spectral features, and phonon and CEF-related excitations.

Contribution

It provides the first evidence of a second superconducting gap and reentrant features in TmNi2B2C, expanding understanding of multiband superconductivity in nickel borocarbides.

Findings

Presence of two superconducting gaps, with the second being twice as large.

Observation of reentrant features in Andreev-reflection spectra under magnetic field.

Identification of phonon maxima at 9.5 meV and 15 meV, and CEF-related excitations.

Abstract

Point-contact (PC) investigations on the title compound in the normal and superconducting (SC) state (Tc=10,6 K) are presented. The temperature dependence of the SC gap of TmNi2B2C determined from Andreev-reflection (AR) spectra using the standard single-gap approximation (SGA) deviates from the BCS behavior in displaying a maximum at about Tc/2. A refined analysis within the two-gap approximation provides evidence for the presence of a second gap twice as large as the main gap (the first one), while the latter is close to that within the SGA. This way, TmNi2B2C expands the number of nickel borocarbide superconductors which exhibit a clear multiband character. Additionally, for the first time "reentrant" features were found in the AR spectra for some PCs measured in a magnetic field. The PC spectroscopy of the electron-boson interaction in TmNi2B2C in the normal state reveals a pronounced phonon maximum at 9.5meV and a more smeared one around 15 meV, while at higher energies the PC spectra are almost featureless. Additionally, the most intense peak slightly above 3meV observed in the PC spectra of TmNi2B2C is presumably caused by crystalline-electric-field (CEF) excitations. The peak near 1meV detected for some PC spectra is connected with a modification of the CEF probably due to boron or carbon vacancies, allowing to probe the local stoichiometry by PC spectroscopy.