title: Strongly enhanced charge-density-wave order in monolayer NbSe2.
abstract: Two-dimensional materials possess very different properties from their bulk counterparts. While changes in single-particle electronic properties have been investigated extensively, modifications in the many-body collective phenomena in the exact two-dimensional limit remain relatively unexplored. Here, we report a combined optical and electrical transport study on the many-body collective-order phase diagram of NbSe2 down to a thickness of one monolayer. Both the charge density wave and the superconducting phase have been observed down to the monolayer limit. The superconducting transition temperature decreases on lowering the layer thickness, but the newly observed charge-density-wave transition temperature increases from 33 K in the bulk to 145 K in the monolayer. Such highly unusual enhancement of charge density waves in atomically thin samples can be understood to be a result of significantly enhanced electron-phonon interactions in two-dimensional NbSe2 (ref. 4) and is supported by the large blueshift of the collective amplitude vibration observed in our experiment. Our results open up a new window for search and control of collective phases of two-dimensional matter, as well as expanding the functionalities of these materials for electronic applications.
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, USA.
authors.forename: Xiaoxiang
authors.initials: X
authors.lastname: Xi
authors.affiliation: Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079, USA.
authors.forename: Liang
authors.initials: L
authors.lastname: Zhao
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, USA.
authors.forename: Zefang
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland.
authors.forename: Helmuth
authors.initials: H
authors.lastname: Berger
authors.affiliation: Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland.
authors.forename: László
authors.initials: L
authors.lastname: Forró
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, USA.|Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079, USA.
authors.forename: Jie
authors.initials: J
authors.lastname: Shan
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, USA.
authors.forename: Kin Fai
authors.initials: KF
authors.lastname: Mak