Ferromagnetic Quantum Critical Endpoint in UCoAl

TL;DR

This study investigates the quantum critical behavior in UCoAl through resistivity and magnetostriction measurements, revealing a ferromagnetic quantum critical endpoint and associated effective mass enhancements under pressure.

Contribution

It provides the first detailed experimental characterization of the ferromagnetic quantum critical endpoint in UCoAl, including pressure-dependent phase transitions and effective mass behavior.

Findings

Identification of the quantum critical endpoint at P_QCEP ~ 1.5 GPa

Observation of a sharp effective mass enhancement near the QCEP

Discovery of a new high-field anomaly H* above P_QCEP

Abstract

Resistivity and magnetostriction measurements were performed at high magnetic fields and under pressure on UCoAl. At ambient pressure, the 1st order metamagnetic transition at H_m ~ 0.7 T from the paramagnetic ground state to the field-induced ferromagnetic state changes to a crossover at finite temperature T_0 ~11 K. With increasing pressure, H_m linearly increases, while T_0 decreases and is suppressed at the quantum critical endpoint (QCEP, P_QCEP ~ 1.5 GPa, H_m ~ 7 T). At higher pressure, the value of H_m identified as a crossover continuously increases, while a new anomaly appears above P_QCEP at higher field H* in resistivity measurements. The field dependence of the effective mass (m*) obtained by resistivity and specific heat measurements exhibits a step-like drop at H_m at ambient pressure. With increasing pressure, it gradually changes into a peak structure and a sharp enhancement of m* is observed near the QCEP. Above P_QCEP, the enhancement of m* is reduced, and a broad plateau is found between H_m and H*. We compare our results on UCoAl with those of the ferromagnetic superconductor UGe2 and the itinerant metamagnetic ruthenate Sr3Ru2O7.