Order parameter fluctuations at a buried quantum critical point
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.
Additional Information© 2012 National Academy of Sciences. Edited by Zachary Fisk, University of California, Irvine, CA, and approved March 9, 2012 (received for review February 9, 2012). Published ahead of print April 23, 2012. We thank X. Lin for help on sample growth. The work at the University of Chicago was supported by National Science Foundation (NSF) Grant DMR-0907025. Use of the Advanced Photon Source and the work at the Materials Science Division of Argonne National Laboratory were supported by the Department of Energy Basic Energy Sciences under Contract NE-AC02-06CH11357. The work at Zhejiang University was supported by NSF of China. Author contributions: Y.F., R.J. and T.F.R. designed research; Y.F., J.W., J.v.W., S.H., G.S., and P.B.L. performed research; Y.L. and Z.-A.X. contributed new reagents/analytic tools; Y.F., R.J., and T.F.R. analyzed data; and Y.F., R.J., J.v.W. and T.F.R. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission.
Published - PNAS-2012-Feng-7224-9.pdf