Order parameter fluctuations at a buried quantum critical point

TL;DR

This study uses x-ray diffraction to directly observe quantum critical fluctuations of charge density waves in 2H-NbSe2 under high pressure, revealing a continuous quantum phase transition within a superconducting phase.

Contribution

It provides direct experimental evidence of order parameter fluctuations near a quantum critical point using x-ray diffraction, even inside a superconducting phase.

Findings

Observation of broad quantum critical fluctuation regime

Detection of a continuous quantum phase transition

Confirmation of enhanced quantum fluctuations in low-dimensional systems

Abstract

Quantum criticality is a central concept in condensed matter physics, but the direct observation of quantum critical fluctuations has remained elusive. Here we present an x-ray diffraction study of the charge density wave (CDW) in 2H-NbSe2 at high pressure and low temperature, where we observe a broad regime of order parameter fluctuations that are controlled by proximity to a quantum critical point. X-rays can track the CDW despite the fact that the quantum critical regime is shrouded inside a superconducting phase, and, in contrast to transport probes, allow direct measurement of the critical fluctuations of the charge order. Concurrent measurements of the crystal lattice point to a critical transition that is continuous in nature. Our results confirm the longstanding expectations of enhanced quantum fluctuations in low dimensional systems, and may help to constrain theories of the quantum critical Fermi surface.