Evidence for Uniform Coexistence of Ferromagnetism and Unconventional Superconductivity in UGe_2: A ^73Ge-NQR Study under Pressure

TL;DR

This study uses ^73Ge-NQR measurements under pressure to demonstrate the uniform coexistence of ferromagnetism and unconventional superconductivity in UGe_2, revealing a first-order transition and a line-node gap in the superconducting state.

Contribution

It provides direct evidence of uniform coexistence of ferromagnetism and unconventional superconductivity in UGe_2 under pressure, highlighting the role of a pressure-induced first-order transition.

Findings

Superconductivity peaks at P_x ~ 1.2 GPa with T_sc=0.7 K.

First-order transition from FM2 to FM1 around P_x ~ 1.2 GPa.

Coexistence of ferromagnetism and unconventional superconductivity with a line-node gap.

Abstract

We report on the itinerant ferromagnetic superconductor UGe_2 through ^73Ge-NQR measurements under pressure (P). The P dependence of the NQR spectrum signals a first-order transition from the low-temperature (T) and low-P ferromagnetic phase (FM2) to high-T and high-P one (FM1) around a critical pressure of P_x ~ 1.2 GPa. The superconductivity exhibiting a maximum value of T_sc=0.7 K at P_x ~ 1.2 GPa, was found to take place in connection with the P-induced first-order transition. The nuclear spin-lattice relaxation rate 1/T_1 has probed the ferromagnetic transition, exhibiting a peak at the Curie temperature as well as a decrease without the coherence peak below T_sc. These results reveal the uniformly coexistent phase of ferromagnetism and unconventional superconductivity with a line-node gap. We remark on an intimate interplay between the onset of superconductivity and the underlying electronic state for the ferromagnetic phases.