Thesis advisor: Vidya Madhavan / We present Scanning Tunneling Microscopy (STM) studies on several systems in which spin-orbit coupling leads to new and interesting physics, and where tuning by doping and/or strain can significantly modify the electronic properties, either inducing a phase transition or by sharply influencing the electronic structure locally. In the perovskite Iridate insulator Sr3Ir2O7, we investigate the parent compound, determining the band gap and its evolution in response to point defects which we identify as apical oxygen vacancies. We investigate the effects of doping the parent compound with La (in place of Sr) and Ru (in place of Ir). In both cases a metal-insulator transition (MIT) results: at x ~ 38% with Ru, and x ~ 5% with La. In the La-doped samples we find nanoscale phase separation at dopings just below the MIT, with metallic spectra associated with clusters of La atoms. Further, we find resonances near the Fermi energy associated with individual La atoms, suggesting an uneven distribution of dopants among the layers of the parent compound. Bi2Se3 is a topological insulator which hosts linearly dispersing Dirac surface states. Doping with In (in place of Bismuth) brings about topological phase transition, achieving a trivial insulator at x ~ 4%. We use high-magnetic field Landau level spectroscopy to study the surface state’s properties approaching the phase transition and find, by a careful analysis of the peak positions find behavior consistent with strong surface-state Zeeman effects: g~50. This interpretation implies, however, a relabeling of the Landau levels previously observed in pristine Bi2Se3, which we justify through ab initio calculations. The overall picture is of a g-factor which steadily decreases as In is added up to the topological phase transition. Finally, we examine the effects of strain on the surface states of (001) thin films of the topological crystalline insulator SnTe. When these films are grown on closely-related substrates—in this case PbSe(001)—a rich pattern of surface strain emerges. We use phase-sensitive analysis of atomic-resolution STM topographs to measure the strain locally, and spatially-resolved quasiparticle interference imaging to compare the Dirac point positions in regions with different types of strain, quantifying for the first time the effect of anisotropic strain on the surface states of a topological crystalline insulator. / Thesis (PhD) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Since the discovery of giant magnetoresistance in 1980s, Spintronics became an exciting field which studies numerous phenomena including the spin transport in magnetic heterostructures, magnetization dynamics and the interplay between them. I have investigated different topics during my graduate research. In this dissertation, I summarize all my projects including spin pumping, spin convertance and spin injection into ballistic medium. First, we develop a linear response formalism for spin pumping effect. Spin pumping refers that a precessing emits a spin current into its adjacent nonmagnetic surroundings, which was originally proposed using scattering theory. The newly developed formalism is demonstrated to be identical the early theory in limiting case. While our formalism is convenient to include the effects of disorders and spin-orbit coupling which can resolve the quantitative controversies between early theory and experiments. Second, the spin pumping experiments indicates a much smaller spin Hall angle compared with the results obtained via the spin transfer torque measurements. We found that such issues can be resolved when taking into consideration the effects of non-local conductivity. And we conclude neither of the two methods measures the real spin Hall angle while the spin pumping methods provides much accurate estimations. Third, we developed the spin transport equations in weak scattering medium in the presence of spin-orbit coupling. Before this, all spin dependent electron transport has been modeled by the conventional spin diffusion equation. While recent spin injection experiments have seen the failure of spin diffusion equation. As the experimental fitting using spin diffusion models led to unrealistic conclusions. At last, we study the spin convertance in anti-ferromagnetic multilayers, where the spin information can be mutually transferred between ferromagnetic/anti-ferromagnetic and conduction electrons. Our theory successfully explained the experiment results that the insertion of thin NiO film between YIG/Pt largely enhances the spin Seebeck currents.

Electrons not only have charges but also have spin. By utilizing the electron spin, the energy consumption of electronic devices can be reduced, their size can be scaled down and the efficiency of `read' and `write' in memory devices can be significantly improved. Hence, the manipulation of electron spin in electronic devices becomes more and more appealing for the advancement of microelectronics. In spin-based devices, the manipulation of ferromagnetic order parameter using electrical currents is a very useful means for current-driven operation. Nowadays, most of magnetic memory devices are based on the so-called spin transfer torque, which stems from the spin angular momentum transfer between a spin-polarized current and the magnetic order parameter. Recently, a novel spin torque effect, exploiting spin-orbit coupling in non-centrosymmetric magnets, has attracted a massive amount of attention. This thesis addresses the nature of spin-orbit coupled transport and torques in non-centrosymmetric magnetic semiconductors. We start with the theoretical study of spin orbit torque in three dimensional ferromagnetic GaMnAs. Using the Kubo formula, we calculate both the current-driven field-like torque and anti-damping-like torque. We compare the numerical results with the analytical expressions in the model case of a magnetic Rashba two-dimensional electron gas. Parametric dependencies of the different torque components and similarities to the analytical results of the Rashba two-dimensional electron gas in the weak disorder limit are described. Subsequently we study spin-orbit torques in two dimensional hexagonal crystals such as graphene, silicene, germanene and stanene. In the presence of staggered potential and exchange field, the valley degeneracy can be lifted and we obtain a valley-dependent Berry curvature, leading to a tunable antidamping torque by controlling the valley degree of freedom. This thesis then addresses the influence of the quantum spin Hall effect on spin orbit torque in nanoribbons with a hexagonal lattice. We find a dramatic modification of the nature of the torque (field like and damping-like component) when crossing the topological phase transition. The relative agnitude of the two torque components can be significantly modifies by changing the magnetization direction. Finally, motivated by recent experimental results, we conclude by investigating the features of spin-orbit torque in magnetic transition metal dichalcogenides. We find the torque is associated with the valley polarization. By changing the magnetization direction, the torque can be changed from a finite value to zero when the valley polarization decreases from a finite value to zero.

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This thesis analyses the effect spin-orbit coupling has on the dispersion of Langmuir waves in magnetized plasmas, using recently developed kinetic theories of plasmas including quantummechanical and relativistic effects. Two new wave modes appearclose to the resonance <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5CDelta%20%5Comega_%7Bc%7D" /> = ( g/2 − 1)<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Comega_%7Bc%7D" /> , where <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Comega_%7Bc%7D" /> is the cyclotron frequency and g is the electron gyromagnetic ratio. Forconsidered long wave lengths the deviation from this resonanceis very small. The wave modes are also very weakly damped.

In this Thesis we discussed ultracold Fermi gas with an s-wave interaction and synthetic spin-orbit coupling under a variety of conditions. We considered the system in both three and two spatial dimensions, with equal-Rashba-Dresselhaus type or Rashba-only type of spin-orbit-coupling, and with or without an artificial Zeeman field. We found competing effects on Fermionic superfluidity from spin-orbit coupling and Zeeman fields, and topologically non-trivial states in the presence of both fields. We gave an outlook on the many-body physics in the last.

We study the spin dynamics in a high-mobility two dimensional electron gas (2DEG) system with generic spin-orbit interactions (SOIs). We derive a set of spin dynamic equations which capture the purely exponential to the damped oscillatory spin evolution modes observed in different regimes of SOI strength. Hence we provide a full treatment of the D'yakonov-Perel's mechanism by using the microscopic linear response theory from the weak to the strong SOI limit. We show that the damped oscillatory modes appear when the electron scattering time is larger than half of the spin precession time due to the SOI, in agreement with recent observations. We propose a new way to measure the scattering time and the relative strength of Rashba and linear Dresselhaus SOIs based on these modes and optical grating experiments. We discuss the physical interpretation of each of these modes in the context of Rabi oscillation. In the finite temperature, We study the spin dynamics in the presence of impurity and electron-electron (e-e) scattering in a III-V semiconductor quantum well. Starting from the Keldysh formalism, we develop the spin-charge dynamic equation at finite temperature in the presence of inelastic scattering which provide a new approach to describe the spin relaxation from the weak to the strong spin-orbit coupling (SOC) regime. In the weak SOC regime, our theory shows that when the system is near the SU(2) symmetry point, because the spin relaxation due to DP mechanism is suppressed dramatically, the spin relaxation is dominated by the Elliott-Yafet (EY) mechanism in a wide temperature regime. The non-monotonic temperature dependence of enhanced-lifetime of spin helix mode is due to the competition between the DP and EY mechanisms. In the strong SOC regime, the our theory is consistent to the previous theoretical results at zero temperature.

In the search for low power operation of microelectronic devices, spin-based solutions have attracted undeniable increasing interest due to their intrinsic magnetic nonvolatility. The ability to electrically manipulate the magnetic order using spin-orbit interaction, associated with the recent emergence of topological spintronics with its promise of highly efficient charge-to-spin conversion in solid state, offer alluring opportunities in terms of system design. Although the related technology is still at its infancy, this thesis intends to contribute to this engaging field by investigating the nature of the charge and spin transport in spin-orbit coupled and topological systems using quantum transport methods. We identified three promising building blocks for next-generation technology, three classes of systems that possibly enhance the spin and charge transport efficiency: (i)- topological insulators, (ii)- spin-orbit coupled magnonic systems, (iii)- topological magnetic textures (skyrmions and 3Q magnetic state). Chapter 2 reviews the basics and essential concepts used throughout the thesis: the spin-orbit coupling, the mathematical notion of topology and its importance in condensed matter physics, then topological magnetism and a zest of magnonics. In Chapter 3, we study the spin-orbit torques at the magnetized interfaces of 3D topological insulators. We demonstrated that their peculiar form, compared to other spin-orbit torques, have important repercussions in terms of magnetization reversal, charge pumping and anisotropic damping. In Chapter 4, we showed that the interplay between magnon current jm and magnetization m in homogeneous ferromagnets with Dzyaloshinskii-Moriya (DM) interaction, produces a field-like torque as well as a damping-like torque. These DM torques mediated by spin wave can tilt the imeaveraged magnetization direction and are similar to Rashba torques for electronic systems. Moreover, the DM torque is more efficient when magnons are thermally driven. Chapters 5 and 6 carry throughout tight-binding studies on the topological charge-spin transport in two-dimensional lattices with ferromagnetic skyrmions and 3Q magnetic structure. We use the Landauer-Buttiker formalism and evaluate the robustness of the topological signals. For the 3Q state, a spin-polarized quantum anomalous Hall state with chiral edge modes, unaffected by deformation and disorder, is reachable in zero net magnetization. We finish with concluding remarks and perspectives.

Systems with strong spin-orbit coupling are of particular interest in solid state physics as an avenue for observing and manipulating spin physics using standard electrical techniques. This dissertation focuses on the characteristics of elemental bismuth (Bi), which exhibits some of the strongest intrinsic spin-orbit coupling of all elements, and InSb, which exhibits some of the strongest intrinsic spin-orbit coupling of all compound semiconductors. The experiments performed study the quantum transport signatures of nano- and micron-scale lithographically defined devices as well as spin-orbit coupled material/ferromagnet interfaces. All Bi structures are fabricated from Bi thin "films, and hence a detailed analysis of<br />the characteristics of Bi "film growth by thermal evaporation is provided. Morphologically and electrically high quality "films are grown using a two stage deposition procedure. The phase and spin coherence of Bi geometries constrained in one, two, and three dimensions are systematically studied by analysis of the weak antilocalization transport signature, a quantum interference phenomenon sensitive to spin-orbit coupling. The "findings indicate that the phase coherence scales proportionally to the limiting dimension of the structure for sizes less than 500 nm. Specifically, in Bi wires, the phase coherence length is approximately as long as the wire width. Dephasing due to quantum confinement e"ffects limit the phase coherence in small Bi structures, impairing the observation of controlled interference phenomena in nano-scale Bi rings. The spin coherence length is independent of dimensional constraint by the film thickness, but increases significantly as the lateral dimensions, such as wire width, are constrained. This is a consequence of the quantum transport contribution from the strongly spin-orbit coupled Bi(001) surface state. To probe the Bi surface state further, Bi/CoFe junctions are fabricated. The anisotropic magnetoresistance of the CoFe is modifi"ed when carriers tunnel into the CoFe from Bi, possibly due to a spin dependent tunneling process or an interaction between the spin polarized density of states in CoFe and the anisotropic spin-orbit coupled density of states in Bi. InSb/CoFe junctions are studied as InSb "films are a simpler spin-orbit coupled system compared to Bi "films. For temperatures below 3.5 K, a large, symmetric, and abrupt negative magnetoresistance is observed. The low-"field high resistance state has similar temperature and magnetic "field dependences as the superconducting phase, but a superconducting component in the device measurements seems absent. A differential conductance measurement of the InSb/CoFe interface during spin injection indicates a quasiparticle gap present at the Fermi energy, coinciding with the large magnetoresistance. / Ph. D.

Neste trabalho apresentamos a determinação autoconsistente da constante de interação spin-órbita em heteroestruturas com duas sub-bandas. Como recentemente proposto, ao obter o hamiltoneano de um sistema com duas sub-bandas na aproximação de massa efetiva, constata-se a presença de um acoplamento inter-subbanda que não se anula mesmo em heteroestruturas simétricas. Apresentamos aqui as deduções teóricas que levaram à proposição desse novo acoplamento e mostramos o cálculo autoconsistente da intensidade do acoplamento e a comparamos com a intensidade do acoplamento Rashba, já amplamente estudado. Discutimos o método k.p e a Aproximação da Função Envelope e mostramos a obtenção do modelo de Kane 8x8 para semicondutores com estrutura zincblende. Aplicamos o método do \"folding down\'\' ao hamiltoneano de Kane isolando o setor correspondente à banda de condução. Escrevemos dessa forma um hamiltoneano efetivo para a banda de condução no contexto de um poço quântico com uma barreira. Através da projeção desse hamiltoneano nos dois primeiros estados da parte orbital verifica-se o surgimento de um acoplamento inter-subbanda. Finalmente escrevemos o hamiltoneano efetivo 4x4 que descreve as duas primeiras subbandas de um poço quântico e obtivemos seus autoestados e autoenergias. Finalmente fizemos o cálculo autoconsistente das funções de onda e energias de um gás de elétrons em poços quânticos simples e duplos através da aproximação de Hartree e a partir desses resultados determinamos o valor da constante de acoplamento Rashba e da nova constante inter-subbanda. Entre os resultados obtidos destacam-se o controle elétrico da constante de acoplamento inter-subbanda através de um eletrodo externo e um efeito de renormalização da massa efetiva que pode chegar até 5% em algumas estruturas. / In this work we present the self-consistent determination of the spin-orbit coupling constant in heterostructure with two subbands.As recently proposed, the effective hamiltonian for the conduction band in the effective mass approximation contains an inter-subband spin-orbit coupling which is non-zero even for symmetric heterostructures. We present the theoretical derivation which leads to this proposal and show a selfconsistent determination of the coupling constant. We also compare the magnitude of the new coupling constant with the usual Rashba coupling. Starting with a discussion of the k.p method and the Envelope Function Approximation (EFA) we show the derivation of the 8x8 Kane model for semiconductors with zincblende structure. We then apply the \"folding down\'\' method, isolating the conduction band sector of the EFA hamiltonian. By projecting this hamiltonian in the first two states of the orbital part, we find an effective 4x4 hamiltonian that contains an inter-subband spin orbit coupling. The eingenvalues and eigenvectors of this hamiltonian are shown and, specializing the model for single and double quantum wells, we self-consistently determine the inter-subband and Rashba coupling constants in the Hartree approximation. The results indicate the possibility of electrical control of the coupling constant and show an effective mass renormalization effect that can be up to 5% in some cases.