title: Unconventional Charge-Density-Wave Transition in Monolayer 1T-TiSe2.
abstract: Reducing the dimension in materials sometimes leads to unexpected discovery of exotic and/or pronounced physical properties such as quantum Hall effect in graphene and high-temperature superconductivity in iron-chalcogenide atomically thin films. Transition-metal dichalcogenides (TMDs) provide a fertile ground for studying the interplay between dimensionality and electronic properties, since they exhibit a variety of electronic phases like semiconducting, superconducting, and charge-density-wave (CDW) states. Among TMDs, bulk 1T-TiSe2 has been a target of intensive studies due to its unusual CDW properties with the periodic lattice distortions characterized by the three-dimensional (3D) commensurate wave vector. Clarifying the ground states of its two-dimensional (2D) counterpart is of great importance not only to pin down the origin of CDW, but also to find unconventional physical properties characteristic of atomic-layer materials. Here, we show the first experimental evidence for the realization of 2D CDW phase without Fermi-surface nesting in monolayer 1T-TiSe2. Our angle-resolved photoemission spectroscopy (ARPES) signifies an electron pocket at the Brillouin-zone corner above the CDW-transition temperature (TCDW ∼ 200 K), while, below TCDW, an additional electron pocket and replica bands appear at the Brillouin-zone center and corner, respectively, due to the back-folding of bands by the 2 × 2 superstructure potential. Similarity in the spectral signatures to bulk 1T-TiSe2 implies a common driving force of CDW, i.e., exciton condensation, whereas the larger energy gap below TCDW in monolayer 1T-TiSe2 suggests enhancement of electron-hole coupling upon reducing dimensionality. The present result lays the foundation for the electronic-structure engineering based with atomic-layer TMDs.
authors.affiliation: WPI Research Center, Advanced Institute for Materials Research, Tohoku University , Sendai 980-8577, Japan.
authors.forename: Katsuaki
authors.initials: K
authors.lastname: Sugawara
authors.affiliation: Department of Physics, Tohoku University , Sendai 980-8578, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Nakata
authors.affiliation: WPI Research Center, Advanced Institute for Materials Research, Tohoku University , Sendai 980-8577, Japan.
authors.forename: Ryota
authors.initials: R
authors.lastname: Shimizu
authors.affiliation: WPI Research Center, Advanced Institute for Materials Research, Tohoku University , Sendai 980-8577, Japan.
authors.forename: Patrick
authors.initials: P
authors.lastname: Han
authors.affiliation: WPI Research Center, Advanced Institute for Materials Research, Tohoku University , Sendai 980-8577, Japan.
authors.forename: Taro
authors.initials: T
authors.lastname: Hitosugi
authors.affiliation: Department of Physics, Tohoku University , Sendai 980-8578, Japan.
authors.forename: Takafumi
authors.initials: T
authors.lastname: Sato
authors.affiliation: WPI Research Center, Advanced Institute for Materials Research, Tohoku University , Sendai 980-8577, Japan.|Department of Physics, Tohoku University , Sendai 980-8578, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Takahashi
keywords: 1T-TiSe2; angle-resolved photoemission spectroscopy; charge density wave; electronic states; scanning tunneling microscopy; transition-metal dichalchogenides