title: Order parameter fluctuations at a buried quantum critical point.
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-NbSe(2) 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 long-standing expectations of enhanced quantum fluctuations in low-dimensional systems, and may help to constrain theories of the quantum critical Fermi surface.
authors.affiliation: The Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Yejun
authors.initials: Y
authors.lastname: Feng
authors.forename: Jiyang
authors.initials: J
authors.lastname: Wang
authors.forename: R
authors.initials: R
authors.lastname: Jaramillo
authors.forename: Jasper
authors.initials: J
authors.lastname: van Wezel
authors.forename: S
authors.initials: S
authors.lastname: Haravifard
authors.forename: G
authors.initials: G
authors.lastname: Srajer
authors.forename: Y
authors.initials: Y
authors.lastname: Liu
authors.forename: Z-A
authors.initials: ZA
authors.lastname: Xu
authors.forename: P B
authors.initials: PB
authors.lastname: Littlewood
authors.forename: T F
authors.initials: TF
authors.lastname: Rosenbaum
mesh_terms: D003460:Crystallization; D008956:Models, Chemical; D009556:Niobium / Q000737:chemistry*; D044367:Phase Transition; D011312:Pressure; D011789:Quantum Theory; D012643:Selenium / Q000737:chemistry*; D018036:Selenium Compounds / Q000737:chemistry; D044366:Transition Temperature; D014961:X-Ray Diffraction / Q000379:methods*
chemical_list: D018036:Selenium Compounds; D009556:Niobium; D012643:Selenium