Evolution of ferromagnetic and non-Fermi liquid state with doping:the case of Ru doped UCoGe

TL;DR

This study explores how Ru doping in UCoGe affects its ferromagnetic and non-Fermi liquid properties, revealing a ferromagnetic dome and a quantum critical point with non-Fermi liquid behavior near the transition.

Contribution

It provides a detailed phase diagram and insights into the evolution of ferromagnetism and non-Fermi liquid behavior with Ru doping in UCoGe, supported by experimental and electronic structure calculations.

Findings

Maximum Curie temperature of 8.6 K at x≈0.1

Superconductivity disappears at x≈0.03

Quantum critical point at x=0.31 with non-Fermi liquid behavior

Abstract

We have investigated the impact of Ru substitution for Co on the behavior of the ferromagnetic superconductor UCoGe by performing x-ray diffraction, magnetization, specific heat and electrical resistivity measurements on polycrystalline samples of the $\mathrm{UCo}_{1-x}\mathrm{Ru}_{x}\mathrm{Ge}$ series ($0\geq x\leq0.9$). The initial Ru substitution up to $x\approx0.1$ leads to a simultaneous sharp increase of the Curie temperature and spontaneous magnetization up to maximum values of $T_{\mathrm{C}}=8.6 K$ and $M_{\mathrm{S}}=0.1 \mu_{\mathrm{B}}$ per formula unit, respectively, whereas superconductivity vanishes already for $x\approx0.03$. Further increase of the Ru content beyond $x\approx0.1$ leads to a precipitous decrease of both, $T_{\mathrm{C}}$ and $M_{\mathrm{S}}$ towards a ferromagnetic quantum critical point (QCP) at $x_{\mathrm{cr}}=0.31$. Consequently the $T-x$ magnetic phase diagram consists of a well-developed ferromagnetic dome. We discuss the evolution of ferromagnetism with $x$ on the basis of band structure changes due to varying 5$f$-ligand hybridization. This scenario is supported by the results of electronic structure calculations and consideration of the simplified periodic Anderson model. The analysis of the temperature dependencies of the electrical resistivity and heat capacity at low temperatures of the samples in the vicinity of the QCP reveals a non-Fermi liquid behavior and assigns the ferromagnetic quantum phase transition to be most likely of a continuous Hertz-Millis type.