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Seminar:Novel Properties of Single Layers of Transition Metal Dichalcogenides

Prof. T.-C. Chiang

University of Illinois at Urbana-Champaign


Time: 15:00-16:00pm, Monday, April 23, 2018

Location: Room 242, East 4, Zijingang campus, Zhejiang University




Films as thin as a single molecular layer can exhibit novel properties that are very different from the bulk counterparts. This talk focuses on two model systems: titanium diselenide (TiSe2) and titanium ditelluride (TiTe2) that exhibit related but very different behaviors. Both materials belong to 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 it is a basic phenomenon of great interest in solid state physics. Specifically, TiSe2, with a (2x2x2) CDW transition at TC = 205 K in the bulk, remains a fascinating case; the transition has been attributed variably to excitonic interactions, band-type Jahn-Teller effects, electron-phonon coupling, etc. Our angle-resolved photoemission spectroscopy (ARPES) and x-ray diffraction measurements of the single layer show a (2×2) CDW transition at TC = 232 K, which is surprisingly higher than the bulk TC. The question is, why? Our measurements of the single layer reveal a small absolute band gap at room temperature, which grows wider with decreasing temperature T below TC in accordance with a BCS-like mean-field behavior; the results are well described in terms of band-structure and fluctuation effects. TiTe2, by contrast, is a metal in both the bulk and single-layer forms. Bulk and N-layer TiTe2, with N  > 1, show no CDW transitions. Interesting, the single layer (N = 1) shows a (2×2) transition at TC = 92 K; it also exhibits a pseudogap that cannot be explained by any existing model or theory. The singular behavior of single-layer TiTe2 appears to exemplify the emergence of new physics in the 2D limit.

In collaboration with P. Chen, C.-Z. Xu, Y. Zhang, S.-K. Mo, Z. Hussain, and A.-V. Fedorov (MBE and photoemission); X.-Y. Fang and Howoong Hong (MBE and x-ray diffraction); Y.-H. Chan and M. Y. Chou (theory); Woei Wu Pai and A. Karn (STM/STS); A. Takayama and S. Hasegawa (4-point probe)



Introduction of Prof. Tai-Chang Chiang

Professor Chiang received his Ph.D. in physics from UC Berkeley in 1978. He joined the Department of Physics at the University of Illinois in 1980. Professor Chiang has done seminal research on the electronic properties, lattice structure, and dynamic behavior of surfaces, interfaces, and ultrathin films. He is credited for being the first one to create atomically uniform films of thicknesses ranging from a single layer to well over a hundred layers. Such films function as miniature electron interferometers in which electrons bounce back and forth
between the two boundaries to form standing waves, also known as quantum well states. Professor Chiang also did pioneering work on the application of angle-resolved and core-level photoemission to surface, thin film, and superlattice research. He was one of the first to demonstrate that atoms of single-crystal surfaces have core level binding energies different from the bulk atoms; this work led to the development of quantitative methods for surface structure analysis. He developed systematic methods for three-dimensional band structure mapping, clarified the photoemission processes in terms of bulk and surface effects. His research on x-ray thermal diffusem scattering for phonon mapping is now a topic in textbooks. He received the Davisson-Germer Prize from American Physical Society and was elected Academician of Academia Sinica (Taiwan).