Multiple energy scales at a quantum critical point

TL;DR

This study investigates a heavy fermion metal near a quantum critical point, revealing an additional energy scale in the excitation spectrum that approaches zero at criticality, indicating a new class of quantum critical behavior.

Contribution

It provides experimental evidence for an extra energy scale in quantum criticality, beyond the traditional order parameter fluctuations, in a heavy fermion system.

Findings

Identification of an additional energy scale in the excitation spectrum.

Both energy scales vanish at the quantum critical point.

Evidence for a new class of quantum criticality.

Abstract

We report thermodynamic measurements in a magnetic-field-driven quantum critical point of a heavy fermion metal, YbRh$_2$Si$_2$. The data provide evidence for an energy scale in the equilibrium excitation spectrum, that is in addition to the one expected from the slow fluctuations of the order parameter. Both energy scales approach zero as the quantum critical point is reached, thereby providing evidence for a new class of quantum criticality.