Sr3Ru2O7: Thermodynamics of Phase Formation in a Quantum Critical Metal

TL;DR

This study investigates the thermodynamics of phase formation in Sr3Ru2O7 near a quantum critical point, revealing a nematic phase that forms directly from quantum criticality and contains unexpected entropy.

Contribution

It provides experimental evidence of a nematic phase emerging from a quantum critical state, highlighting the role of entropy and Fermi surface topology changes in such transitions.

Findings

Nematic phase forms directly from quantum criticality.

The phase contains more entropy than predicted by mean-field theory.

The quantum critical point is hidden by the nematic phase, separating two Fermi liquids.

Abstract

The behaviour of matter near zero temperature continuous phase transitions, or 'quantum critical points' (QCPs) is a central topic of study in condensed matter physics. In fermionic systems, fundamental questions remain unanswered: the nature of the quantum critical regime is unclear because of the apparent breakdown of the concept of the quasiparticle, a cornerstone of existing theories of strongly interacting metals. Even less is known experimentally about the formation of ordered phases from such a quantum critical 'soup'. Here, we report a study of the specific heat across the phase diagram of the model system Sr3Ru2O7, which features an anomalous phase whose transport properties are consistent with those of an electronic nematic. We show that this phase, which exists at low temperatures in a narrow range of magnetic fields, forms directly from a quantum critical state, and contains more entropy than mean-field calculations predict. Our results suggest that this extra entropy is due to remnant degrees of freedom from the highly entropic state above T_c. The associated quantum critical point, which is 'concealed' by the nematic phase, separates two Fermi liquids, neither of which has an identifiable spontaneously broken symmetry, but which likely differ in the topology of their Fermi surfaces.