Studies of charge density waves in reduced dimensions by surface x-ray scattering

Charge Density Wave, Thin Film Growth, Surface X-ray Scattering

Fang, Xinyue

关键词: Charge Density Wave, Thin Film Growth, Surface X-ray Scattering;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/99189/FANG-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Single-layer TiSe2 is a small-gap semiconductor of interest for two-dimensional electronic applications. This thesis reports synchrotron x-ray measurements of the Ti and Se atomic displacements as a function of temperature, with a high sensitivity of 0.001 Å. The results reveal a (2×2) CDW structure at temperatures below a critical temperature of TC1 = 233 K. The onset of atomic displacements at TC1 is abrupt with an infinite slope, and the overall temperature dependence follows a BCS-like second-order mean-field behavior. A bilayer TiSe2 film also exhibit a CDW transition of the same character at TC2 = 229 K. A five-layer TiSe2 film is essentially bulk-like with a lower transition temperature of TC5 = 204 K. Our study demonstrate that, contrary to many existing models and theories, modulation of the electronic structure alone cannot fully account for the CDW transition. Lattice modulation is an integral part of the physics of CDWs. The CDW transition is emanated from a subtle interplay between the electronic and lattice degrees of freedom, which are coupled together and cannot be separated.