Chemically-Mediated quantum criticality in NbFe_2

TL;DR

This paper investigates the origin of quantum criticality in NbFe_2, revealing that an unconventional band critical point explains magnetic behavior and how alloying tunes the Fermi level to access this critical point.

Contribution

It demonstrates that alloying in NbFe_2 shifts the Fermi level to an unconventional band critical point, providing a new understanding of quantum criticality in this compound.

Findings

Unconventional band critical point explains quantum criticality.

Fermi level shifts with alloying, accessing the critical point.

Resistivity remains constant near critical composition.

Abstract

Laves-phase Nb{1+c}Fe_{2-c} is a rare itinerant intermetallic compound exhibiting magnetic quantum criticality at c_{cr}=1.5%Nb excess; its origin, and how alloying mediates it, remains an enigma. For NbFe_2, we show that an unconventional band critical point (uBCP) above the Fermi level E_F explains most observations, and that chemical alloying mediates access to this uBCP by an increase in E_F with decreasing electrons (increasing %Nb), counter to rigid-band concepts. We calculate that E_F enters the uBCP region for c_{cr} > 1.5%Nb and by 1.74%Nb there is no Nb site-occupation preference between symmetry-distinct Fe sites, i.e., no electron-hopping disorder, making resistivity near constant as observed. At larger Nb (Fe) excess, the ferromagnetic Stoner criterion is satisfied.