Quantum Tricritical Points in NbFe$_2$

TL;DR

This paper investigates quantum tricritical points in NbFe$_2$, revealing how a buried ferromagnetic quantum critical point within a spin-density-wave phase can lead to diverging susceptibilities, advancing understanding of quantum phase transitions in metals.

Contribution

It introduces the first experimental evidence of quantum tricritical points in NbFe$_2$, modeled by a two-order-parameter theory, highlighting their potential universality in metals with competing magnetic orders.

Findings

Identification of quantum tricritical points in NbFe$_2$

Modeling of the phase diagram with a two-order-parameter theory

Divergence of both uniform and finite q susceptibilities at QTCPs

Abstract

Quantum critical points (QCPs) emerge when a 2nd order phase transition is suppressed to zero temperature. In metals the quantum fluctuations at such a QCP can give rise to new phases including unconventional superconductivity. Whereas antiferromagnetic QCPs have been studied in considerable detail ferromagnetic (FM) QCPs are much harder to access. In almost all metals FM QCPs are avoided through either a change to 1st order transitions or through an intervening spin-density-wave (SDW) phase. Here, we study the prototype of the second case, NbFe$_2$. We demonstrate that the phase diagram can be modelled using a two-order-parameter theory in which the putative FM QCP is buried within a SDW phase. We establish the presence of quantum tricritical points (QTCPs) at which both the uniform and finite $q$ susceptibility diverge. The universal nature of our model suggests that such QTCPs arise naturally from the interplay between SDW and FM order and exist generally near a buried FM QCP of this type. Our results promote NbFe$_2$ as the first example of a QTCP, which has been proposed as a key concept in a range of narrow-band metals, including the prominent heavy-fermion compound YbRh$_2$Si$_2$.