title: Structural Phase Transition and Material Properties of Few-Layer Monochalcogenides.
abstract: GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a critical temperature T_{c} well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above T_{c}. As the in-plane lattice transforms from a rectangle into a square at T_{c}, the electronic, spin, optical, and piezoelectric properties dramatically depart from earlier predictions. Indeed, the Y and X points in the Brillouin zone become effectively equivalent at T_{c}, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at T_{c}. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement is drawn by theoretical predictions of giant piezoelectricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about 3×10^{-12}  C/K m here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides.
authors.affiliation: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: Mehrshad
authors.initials: M
authors.lastname: Mehboudi
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Benjamin M
authors.initials: BM
authors.lastname: Fregoso
authors.affiliation: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: Yurong
authors.initials: Y
authors.lastname: Yang
authors.affiliation: Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors.forename: Wenjuan
authors.initials: W
authors.lastname: Zhu
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors.forename: Arend
authors.initials: A
authors.lastname: van der Zande
authors.affiliation: Departamento de Física, Universidad de Oviedo, 33007 Oviedo, Spain.
authors.forename: Jaime
authors.initials: J
authors.lastname: Ferrer
authors.affiliation: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: L
authors.initials: L
authors.lastname: Bellaiche
authors.affiliation: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: Pradeep
authors.initials: P
authors.lastname: Kumar
authors.affiliation: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: Salvador
authors.initials: S
authors.lastname: Barraza-Lopez