title: Emergence of coherence in the charge-density wave state of 2H-NbSe2.
abstract: A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature T(cdw). Here we investigate, using photoemission, X-ray scattering and scanning tunnelling microscopy, the canonical CDW compound 2H-NbSe2 intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude, which has impacts on the electronic dispersion, giving rise to an energy gap. The phase transition at T(cdw) marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in 'pseudogap' states.
authors: 1] Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA.
authors: U
authors: U
authors: Chatterjee
authors: 1] Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA [2] Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, USA.
authors: J
authors: J
authors: Zhao
authors: Department of Physics, Temple University, Philadelphia, Pennsylvania 19122, USA.
authors: M
authors: M
authors: Iavarone
authors: Department of Physics, Temple University, Philadelphia, Pennsylvania 19122, USA.
authors: R
authors: R
authors: Di Capua
authors: 1] Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] Institute of Solid State Physics, Karlsruhe Institute of Technology, PO Box 3640, D-76021 Karlsruhe, Germany.
authors: J P
authors: JP
authors: Castellan
authors: Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104, USA.
authors: G
authors: G
authors: Karapetrov
authors: 1] Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
authors: C D
authors: CD
authors: Malliakas
authors: 1] Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
authors: M G
authors: MG
authors: Kanatzidis
authors: Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors: H
authors: H
authors: Claus
authors: 1] Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] CHESS, Cornell University, Ithaca, New York 14853, USA.
authors: J P C
authors: JP
authors: Ruff
authors: 1] Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] Institute of Solid State Physics, Karlsruhe Institute of Technology, PO Box 3640, D-76021 Karlsruhe, Germany.
authors: F
authors: F
authors: Weber
authors: 1] Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] Institute for Theoretical Physics, University of Amsterdam, Tyndall Avenue, 1090 GL Amsterdam, The Netherlands.
authors: J
authors: 000000029378008X
authors: J
authors: van Wezel
authors: 1] Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA [2] Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, USA.
authors: J C
authors: JC
authors: Campuzano
authors: Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors: R
authors: 0000000195653140
authors: R
authors: Osborn
authors: Department of Physics, Ohio State University, Columbus, Ohio 43210, USA.
authors: M
authors: M
authors: Randeria
authors: Department of Physics, Ohio State University, Columbus, Ohio 43210, USA.
authors: N
authors: N
authors: Trivedi
authors: Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors: M R
authors: MR
authors: Norman
authors: Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors: S
authors: 0000000256590383
authors: S
authors: Rosenkranz