Phase diagram of UGe2. Whether there are quantum phase transitions ?

TL;DR

This paper investigates the phase diagram of UGe2, revealing that the transition from paramagnetic to ferromagnetic state shifts from second order to first order under pressure, challenging the expectation of quantum critical behavior.

Contribution

It introduces a magneto-elastic mechanism explaining the transition's change from second to first order in UGe2, emphasizing the role of anisotropy and weak first order transitions.

Findings

Transition becomes discontinuous at high pressures

Magneto-elastic effects induce first order transition

Weak first order transition can be amplified to observable levels

Abstract

The phase diagram of several itinerant ferromagnets reveals the common feature. The phase transition temperature decreases with pressure increase and reaches zero value at some critical pressure $P_c$ such that at low enough temperatures one can expect critical behavior specific for quantum phase transition. It is not the case, however. Being the second order at ambient pressure the transition from paramagnetic to ferromagnetic state at high pressures - low temperatures is transformed to the discontinuous jump. We discuss the magneto-elastic mechanism of development of the first order type instability at the phase transition to the ferromagnet state in strongly anisotropic ferromagnet UGe2. Using the parameters characterizing the properties of UGe$_2$ we argue the effectiveness of this mechanism transforming the very weak first order type transition to the really observable one.