First-principles Study of Charge Density Waves, Electron-phonon Coupling, and Superconductivity in Transition-metal Dichalcogenides
In this thesis we investigate the electronic and vibrational properties of several transition-metal dichalcogenide materials through first-principles calculations. First, the charge-density-wave (CDW) instability in 1T-TaSe2 is studied as a function of pressure. Density-functional calculations accurately capture the instability at ambient pressures and predict the suppression of the CDW distortion under pressure. The instability is shown to be driven by softening of selected phonon modes due to enhanced electron-phonon matrix elements, rather than by nesting of the Fermi surface or other electronic mechanisms. We also discuss the possibility of electron-phonon superconductivity in compressed 1T-TaSe2.Another polymorph of TaSe2 is then investigated. We focus on the origin of the CDW instability in bulk and single-layer 2H-TaSe2. The role of interlayer interactions and the effect of spin-orbit coupling are examined. The results show that the CDW instability has weak dependence on interlayer interactions and spin-orbit coupling, which is in contrast to the closely related 2H-NbSe2 material, where the CDW ordering vector is predicted to depend on dimensionality.The electron-phonon interaction in electron-doped single-layer MoS2 is also studied. The calculation predicts a weak coupling at low doping levels. It then grows rapidly to a maximum of $lambda ~1.7, after which it begins to decrease with additional doping. The superconducting transition temperature is expected to follow the same trends. This behavior is explained by the appearance, disappearance, growth, and shrinkage of Fermi sheets with different orbital character. These results, which are similar to the experimentally observed superconducting dome in gate-tuned thin flakes of MoS2, reveal the importance of having the right mix of states at the Fermi level to enhance the electron-phonon interaction in this material.
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First-principles Study of Charge Density Waves, Electron-phonon Coupling, and Superconductivity in Transition-metal Dichalcogenides GE, YIZHI (Georgetown University, 2013)In this thesis we investigate the electronic and vibrational properties of several transition-metal dichalcogenide materials through first-principles calculations. First, the charge-density-wave (CDW) instability in 1T-TaSe2 ...
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