Ferromagnetic quantum critical point in a locally noncentrosymmetric and nonsymmorphic Kondo metal

TL;DR

This paper reports the discovery of a ferromagnetic quantum critical point in a centrosymmetric Kondo metal, where local noncentrosymmetric and nonsymmorphic symmetries, combined with spin-orbit coupling, protect the critical point.

Contribution

It introduces a new mechanism for ferromagnetic QCP protection involving local noncentrosymmetric and nonsymmorphic symmetries in centrosymmetric materials.

Findings

Ferromagnetic QCP observed in CeSi$_{2- ext{delta}}$ with Ag doping.

Strange-metal behaviour associated with the QCP.

Spin-orbit coupling and nonsymmorphic symmetry protect the QCP.

Abstract

Quantum critical points (QCPs), zero-temperature phase transitions, are windows to fundamental quantum-mechanical phenomena associated with universal behaviour and can provide parallels to the physics of black holes. Magnetic QCPs have been extensively investigated in the vicinity of antiferromagnetic order. However, QCPs are rare in metallic ferromagnets due to the coupling of the order parameter to electronic soft modes [1,2]. Recently, antisymmetric spin-orbit coupling in noncentrosymmetric systems was suggested to protect ferromagnetic QCPs [3]. Nonetheless, multiple centrosymmetric materials host FM QCPs, suggesting a more general mechanism behind their protection. In this context, CeSi$_{2-\delta}$, a dense Kondo lattice crystallising in a centrosymmetric structure, exhibits ferromagnetic order when Si is replaced with Ag. We report that the Ag-substitution controls the strength of the Kondo coupling, leading to a transition between paramagnetic and ferromagnetic Kondo phases. Remarkably, a ferromagnetic QCP accompanied by concurrent strange-metal behaviour emerges. Herein, we suggest that, despite the centrosymmetric structure, spin-orbit coupling arising from the local noncentrosymmetric structure, in combination with nonsymmorphic symmetry, can protect ferromagnetic QCPs. Our findings present a unique example of Kondo coupling-driven ferromagnetic QCP through chemical doping and offer a general guideline for discovering new ferromagnetic QCPs.