Pressure dependence of the magnetization in the ferromagnetic superconductor UGe_2

TL;DR

This study investigates how pressure affects magnetization in the ferromagnetic superconductor UGe_2, revealing first order magnetic transitions and suggesting superconductivity is linked to electronic density of states rather than quantum criticality.

Contribution

It provides the first detailed pressure-dependent magnetization measurements in UGe_2, challenging the quantum criticality explanation for superconductivity in this material.

Findings

Both magnetic transitions are first order, not quantum critical points.

Pressure dependence of superconductivity does not match static susceptibility changes.

Superconductivity may be related to a spike in electronic density of states.

Abstract

The recent discovery that superconductivity occurs in several clean itinerant ferromagnets close to low temperature magnetic instabilities naturally invites an interpretation based on a proximity to quantum criticality. Here we report measurements of the pressure dependence of the low temperature magnetisation in one of these materials, UGe_2. Our results show that both of the magnetic transitions observed in this material as a function of pressure are first order transitions and do not therefore correspond to quantum critical points. Further we find that the known pressure dependence of the superconducting transition is not reflected in the pressure dependence of the static susceptibility. This demonstrates that the spectrum of excitations giving superconductivity is not that normally associated with a proximity to quantum criticality in weak itinerant ferromagnets. In contrast our data suggest that instead the pairing spectrum might be related to a sharp spike in the electronic density of states that also drives one of the magnetic transitions.