Optical spectra of the heavy fermion uniaxial ferromagnet UGe$_2$

TL;DR

This study investigates the optical properties of UGe$_2$, revealing how ferromagnetic transition affects electronic excitations and suggesting coupling to magnetic excitations relevant for superconductivity.

Contribution

It provides detailed optical spectra analysis of UGe$_2$ across magnetic transitions, highlighting changes in effective mass and scattering rate linked to magnetic excitations.

Findings

Significant changes in optical mass and scattering rate below $T_C$

No anomaly observed at the lower transition temperature $T_x$

Evidence of strong coupling to magnetic excitations in the ferromagnetic state

Abstract

We report a detailed study of UGe$_{2}$ single crystals using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low frequency interband transition and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp changes in the low frequency mass and scattering rate below the upper ferromagnetic transition $T_C = 53 K$. The characteristic changes are exhibited most strongly at an energy scale of around 12 meV (100 cm$^{-1}$). They recover their unrenormalized value above $T_C$ and for $\omega >$ 40 meV. In contrast no sign of an anomaly is seen at the lower transition temperature of unknown nature $T_x \sim$ 30 K, observed in transport and thermodynamic experiments. In the ferromagnetic state we find signatures of a strong coupling to the longitudinal magnetic excitations that have been proposed to mediate unconventional superconductivity in this compound.