Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C

TL;DR

This study uses point-contact spectroscopy to analyze electron-phonon interactions and superconducting states in HoNi2B2C, revealing detailed phonon spectra, crystal-electric-field effects, and two distinct superconducting phases separated by a critical temperature.

Contribution

First detailed spectral analysis of electron-phonon interactions and superconducting states in HoNi2B2C, identifying high-energy phonon modes and two separate superconducting regimes.

Findings

Phonon maxima at 16, 22, 34, 50, and 100 meV identified.

Superconducting gap exhibits BCS-like behavior below 5.6K.

Two superconducting states separated by T_c^* near the Neel temperature.

Abstract

The point-contact (PC) spectroscopy study of the electron-phonon(boson) interaction (EP(B)I) spectral function in HoNi2B2C reveals phonon maxima at 16 and 22meV and 34meV. For the first time the pronounced high energy maxima at about 50meV and 100meV were resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitation with a maximum near 10meV and a `magnetic` peak near 3meV are observed. The contribution of the CEF peak in EP(B)I constant \lambda_{PC} is evaluated as 20-30%, while contribution of the high frequency modes at 50 and 100meV amounts about 10% for each maxima. PC Andreev reflection measurements below the critical temperature T_c=8.5K reveals two different superconducting (SC) states separated by T_c^*=5.6K which is close to the Neel temperature T_N=5.3K. Below T_c^* the gap \Delta in HoNi2B2C exhibits a standard single-band BCS-like behavior with 2\Delta/kT_c^*=3.9. Above T_c^* the gap features in PC spectra are strongly suppressed pointing to a peculiar SC state between T_c^* and T_c, where incommensurate magnetic order develops.