Quantum Criticality in the Ferromagnetic Superconductor UCoGe under Pressure and Magnetic Field

TL;DR

This study maps the pressure-temperature phase diagram of UCoGe, revealing quantum critical behavior, non-Fermi liquid properties, and the effects of magnetic field on its ferromagnetic superconductivity.

Contribution

It provides detailed resistivity measurements under pressure and magnetic field, unveiling the evolution from ferromagnetic to paramagnetic states and the associated quantum critical phenomena in UCoGe.

Findings

Suppression of Curie temperature to zero at critical pressure p_c

Observation of non-Fermi liquid behavior near p_c

Different magnetic field effects on superconductivity depending on field direction

Abstract

The pressure-temperature phase diagram of the orthorhombic ferromagnetic superconductor UCoGe was determined by resistivity measurements up to $10.5~$GPa. The Curie temperature $T_C$ is suppressed with pressure and vanishes at the critical pressure $p_c\approx1~$GPa. Superconductivity is observed in both the ferromagnetic state at low pressure, and in the paramagnetic state above $p_c$ up to about 4$~$GPa. Non-Fermi liquid behavior appears in a large pressure range. The resistivity varies linearly with temperature around $p_c$ and evolves continuously with pressure to a $T^2$ Fermi-liquid behavior for $p \gtrapprox 5$~GPa. The residual resistivity as a function of pressure shows a maximum far above $p_c$ at $p^\star=7.2~$GPa and the amplitude of the inelastic scattering term of the resistivity decreases by more than one order in magnitude at $p^\star$, which appears to mark the entrance into a weakly correlated regime. The pressure dependence of the upper critical field for magnetic field applied along the $b$ and $c$ axis illustrates the drastic difference in the field dependence of the ferromagnetic superconducting pairing. While for $H\parallel b$ axis $H_{c2}(T)$ is driven by the suppression of the ferromagnetic order, it is dominated by the strong initial suppression of the ferromagnetic fluctuations for a field applied in the easy magnetization axis $c$.