Titanium diselenide (TiSe₂) is a member of a vast family of transitional metal dichalcogenides, many of which show charge density wave (CDW) transitions at low temperatures. The CDW order can compete or entangle with other transitions such as superconductivity and antiferromagnetism, and it is a basic phenomenon of great interest in solid state physics. Specifically, TiSe₂, with a simple (2x2x2) CDW transition at T_C ~205 K in the bulk, remains a fascinating case. The transition has been attributed variably to excitonic interactions, band-type Jahn-Teller effects, etc. A detailed investigation of the electronic structure is complicated by the three-dimensional nature of the CDW order. The perpendicular electronic momentum is not necessarily conserved in angle-resolved photoemission spectroscopy (ARPES) measurements, making it difficult to pinpoint the gap locations in the Brillouin zone. A single layer of TiSe₂, by contrast, has a much simpler two-dimensional electronic band structure. Experimentally, it exhibits a (2×2) CDW transition at T_C ~232 K, which is, perhaps surprisingly, higher than the bulk T_C. The experiment reveals a small absolute band gap at room temperature, which grows wider with decreasing temperature T below T_C in accordance with a BCS-like mean-field behavior. The results are rationalized in terms of first-principles calculations, symmetry breaking, and phonon entropy effects. In light of these results, a careful re-examination of the bulk case reveals two transitions, one coming from the (2×2) ordering in individual layers and another coming from the anti-phase locking of the vertical stacking of layers in three dimensions. A further study of N -layer films, with N = 1-6, reveals how the CDW is affected by confinement effects and dimensional crossover. The results provide some detailed answers to long standing questions about CDW physics.
In collaboration with P. Chen, Y.-H. Chan, X.-Y. Fang, Y. Zhang, M. Y.Chou, S.-K. Mo, Z. Hussain, and A.-V. Fedorov

【講師紹介】
Tai Chang Chiang先生は表面、単原子層、超薄膜の光電子分光研究の第一人者でこれまでずっとフロンティアで研究を実施されています。数多くの賞を受賞され、またアメリカの放射光施設で施設長を務めておられました。

談話会委員　加藤 岳生 ・ 長谷川 幸雄