Change of electronic state in the noncentrosymmetric superconductor Th7Fe3

Abstract

57Fe Mössbauer spectroscopy and specific heat measurements were performed for the noncentrosymmetric superconductor Th7Fe3. An explicit increase in the center shift δCS(T), normalised spectral area A(T)/A(1.5 K) and electric quadrupole splitting ΔEQ(T) is observed below 60 K. The temperature dependencies of δCS(T) and A(T)/A(1.5 K) follow the Debye model for , whereas ΔEQ(T) can be described by an empirical T3/2 equation in both two temperature ranges 2 K–T* and T*–300 K. The behaviour of ΔEQ(T) can more willingly be interpreted in terms of the Fe3+ charge distributions on the singlet (dxy) ground state, the doublet (dxz, dzy) of T2g and doublet () of Eg excited states, characterised by respective splitting energies Δ1 and Δ2. It is shown that the excited energies deduced from the 57Fe Mössbauer data are in good agreement with those of Schottky specific heat. We argue that the anomaly detected in hyperfine parameters and specific heat substantially associates with change in the electronic state of the Fe atoms. We are convinced that a precursor crystal-electric field (CEF) effect has undergone before the onset of superconductivity sets in at lower temperature and the present results would spur new interest to researchers about the interplay between CEF and superconductivity in noncentrosymmetric materials.