Browsing by Subject "spin"
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Item 4d and 5d compounds as the new frontier of the anisotropic spin physics(2017-07) Sizyuk, YuriyI perform a series of studies of the magnetism of 4d and 5d transition metal compounds. In particular I concentrate on the realization of anisotropic magnetic Hamiltonians by use of the spin-orbit coupling to tie together the real space geometry and spin space magnetism. In the first part, I derive the magnetic Hamiltonians of Sr2IrO4 and Na2IrO3 from microscopic parameters. The difficulty of these calculation arises from the fact that many microscopic parameters, such as Hund's coupling, spin-orbit coupling, and crystal field distortions are all of the same order and thus have to be treated on an equal footing. The competition and cooperation of these interactions leads to a rich magnetic Hamiltonians with many different anisotropic interactions. My calculations provide a clear dependence of these interactions on the microscopic parameters. This in turn can be used experimentally to single out and enhance given anisotropies by changing the microscopic parameters. In the second part I propose experimental measurements for the anisotropic interactions. In particular I study how different anisotropic interactions contribute to the anisotropy in the Curie-Weiss temperatures of these compounds. I show that the difference of Curie-Weiss temperatures along particular axes gives a way to measure the strength of the anisotropic interactions in the compounds. In the last part, I study how the multitude of the magnetic anisotropies determine the magnetic ground state in 4d and 5d compounds. We have developed a new method to calculate the fluctuational contribution to the free energy in anisotropic Hamiltonians at any temperature within the magnetically ordered phase. The calculation can be done for both classical (which includes only thermal fluctuations) and quantum (quantum and thermal fluctuations) systems. I also study the effects of external magnetic field applied to the nearest neighbor Kitaev-Heisenberg model, a model of particular interest for alpha-RuCl3.Item Modeling of Transport Phenomena in Two-Dimensional Semiconductors(2016-12) Liu, YueRecently, transition metal dichalcogenides and black phosphorus (BP) emerged as new 2D semiconductors due to the advantages of moderate energy band gap, high carrier mobility, ultra thin film and high anisotropy. Together with graphene, 2D materials have been utilized in the development of biomedical devices, touch screen and display technologies, and flexible applications such as wearable electronics and IoT devices. They also open up new opportunities in research fields including spintronics, optoelectronics and next generation post-silicon transistor. In this dissertation, we present theoretical modeling for several topics related to 2D materials. Starting with the fundamental tight-binding theory of graphene, we review electronic properties for graphene including massless 2x2 Dirac Hamiltonian and pseudo-spin wave function. Followed by discussion of ballistic transport, a detailed analysis on graphene diffusive transport is provided. Ionized impurity scattering and carrier screening effect is considered in the model. The momentum relaxation time and mobility for graphene is modeled. A non-linear Thomas Fermi screening is introduced to improve the simulation accuracy. Taking the real spin into account, the new Hamiltonian is a 4x4 matrix. An external field perpendicular to the graphene breaks the reflection symmetry and introduces a Rashba spin-orbit interaction, which couples pseudo-spin and real spin. The relevant charge carrier states are no longer spin eigenstates. Rashba interaction is found to be quite small compared to Coulomb impurity scattering. To characterize the spin-polarized electrons tunneling from electrodes and transport in graphene, a spin valve device modeling and magnetoresistance calculation is developed. Black phosphorus possesses excellent properties like other 2D materials for high performance nanoelectronic applications. Moreover, there is a uniquely high in-plane anisotropy in BP due to its puckered crystal structure. To model the anisotropic transport, a model based on the BTE is developed, considering the full anisotropic electronic structure. For zero temperature calculation with ionized impurity limited scattering, anisotropy ratio 3-4 can be obtained from the model. Due to the dominating effect of screening, mobility is found to decrease weakly with increasing temperature. For , a smaller anisotropy ratio of 1.8-3.5 matching experimental measurements indicates that impurity scattering is an important mechanism for black phosphorus.