First-Principles Study of Charge Density Waves and Electron-Phonon Coupling in Transition Metal Dichalcogenides, and Magnetism of Surface Adatoms
Liu, Amy Y
Recently, low-dimensional materials have attracted great attention, not only because of novel physics, but also because of potential applications in electronic devices, where performance drives innovation. This thesis presents theoretical and computational studies of magnetic adatoms on surfaces and of charge density wave (CDW) phases in two-dimensional materials.We investigate the structural, electronic, and magnetic properties of Ni adatoms on an MgO/Ag(100) surface. Using density functional theory, we find that strong bonding is detrimental to the magnetic moment of an atom adsorbed on a surface. Previously, it was shown that Co retains its full gas-phase magnetic moment on the MgO/Ag(100) surface. For Ni we find the total value of spin depends strongly on the binding site, and there are two sites close in energy.Next, we study the 1T polymorph of the transition metal dichalcogenide TaS2. Bulk 1T-TaS2 is metallic at high temperatures, but adopts an insulating commensurate CDW below ~180 K. The nature of the transition is under debate, and it is unclear whether the transition persists down to a monolayer. Transport and Raman measurements suggest that the commensurate CDW is absent in samples thinner than ~10 layers. Our theoretical and experimental study of the vibrational properties of 1T-TaS2 demonstrates that the commensurate CDW is robust upon thinning -- even down to a single layer.Bulk 2H-TaS2 also has a CDW instability at low temperatures. A recent study of a single layer of 1H-TaS2 grown on Au(111) revealed that the CDW phase is suppressed down to temperatures well below the bulk transition temperature (75 K), possibly due to electron doping from the substrate. We present a first-principles study of the effects of electron doping on the CDW instability in monolayer 1H-TaS2, finding that electron doping: 1) shifts the electronic bands by ~0.1 eV; 2) impacts the strength and wave vector dependence of electron-phonon coupling in the system, suppressing the CDW instability. Furthermore, our work identifies electron-phonon coupling -- not peaks in the real or imaginary parts of the electronic susceptibility -- as the main cause of CDW instabilities in real 2D systems.
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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 ...