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.
@article{arxiv.1109.0260,
title = {Order parameter fluctuations at a buried quantum critical point},
author = {Yejun Feng and Jiyang Wang and R. Jaramillo and Jasper van Wezel and S. Haravifard and G. Srajer and Y. Liu and Z. -A. Xu and P. B. Littlewood and T. F. Rosenbaum},
journal= {arXiv preprint arXiv:1109.0260},
year = {2012}
}