Unconventional Charge-Density-Wave Transition in Monolayer 1T-TiSe2

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• 作者：Katsuaki Sugawara ; Yuki Nakata ; Ryota Shimizu ; Patrick Han ; Taro Hitosugi ; Takafumi Sato ; Takashi Takahashi
• 刊名：ACS Nano
• 出版年：2016
• 出版时间：January 26, 2016
• 年：2016
• 卷：10
• 期：1
• 页码：1341-1345
• 全文大小：415K
• ISSN：1936-086X

文摘

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 (T_CDW ～ 200 K), while, below T_CDW, 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 T_CDW 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.