Quantum Criticality without Tuning in the Mixed Valence Compound beta-YbAlB4

TL;DR

This study demonstrates zero-field quantum criticality in the mixed-valence compound beta-YbAlB4 through high-precision magnetization measurements, challenging the conventional understanding that quantum critical points require tuning.

Contribution

It provides evidence of quantum criticality without tuning in a mixed-valence f-electron system, contrasting with traditional magnetic Kondo lattice systems.

Findings

Scaling of free energy indicates zero-field quantum criticality

Breakdown of Fermi-liquid behavior occurs in a mixed-valence state

Contrasts with integral valence quantum critical systems

Abstract

Fermi liquid theory, the standard theory of metals, has been challenged by a number of observations of anomalous metallic behavior found in the vicinity of a quantum phase transition. The breakdown of the Fermi liquid is accomplished by fine-tuning the material to a quantum critical point using a control parameter such as the magnetic field, pressure, or chemical composition. Our high precision magnetization measurements of the ultrapure f-electron based superconductor {\beta}-YbAlB4 demonstrate a scaling of its free energy indicative of zero-field quantum criticality without tuning in a metal. The breakdown of Fermi-liquid behavior takes place in a mixed-valence state, in sharp contrast with other known examples of quantum critical f-electron systems that are magnetic Kondo lattice systems with integral valence.