Browsing by Subject "Topological insulator"
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Item Ab-initio electronic structure and quantum transport calculations on quasi-two-dimensional materials for beyond Si-CMOS devices(2013-05) Chang, Jiwon, active 2013; Banerjee, Sanjay; Register, Leonard F.Atomically two-dimensional (2-D) graphene, as well as the hexagonal boron nitride dielectric have been and are continuing to be widely investigated for the next generation nanoelectronic devices. More recently, other 2-D materials and electronic systems including the surface states of topological insulators (TIs) and monolayers of transition metal dichalcogenides (TMDs) have also attracted considerable interest. In this work I have focused on these latter two material systems on possible device applications. TIs are characterized by an insulating bulk band gap and metallic Dirac surface states which are spin-polarized. Here, the electronic structures of bulk and thin film TIs are studied using ab-initio density functional theory (DFT). Band inversion, an essential characteristic of TIs, is shown in the bulk band structures. Properties of TI surface bands in thin film such as the critical film thickness to induce a gap, the thickness dependent gap size, and the localization length of surface states are reported. Effects of crystalline dielectric materials on TI surface states are also addressed by ab-initio calculations. I discuss the sensitivity of Dirac point degeneracy and linear band dispersion of TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and TI. Additionally, this work presents research on exciton condensation in TI using a tight-binding model combined with self-consistent non-local Hartree-Fock mean-field theory. Possibility of exciton condensation in the TI Bi₂Se₃ thin film is assessed. Non-equilibrium Green's function (NEGF) simulations with the atomistic tight-binding (TB) Hamiltonian are carried out to explore the performance of metal-oxide-semiconductor field-effect-transistor (MOSFET) and tunnel field-effect-transistor (TFET) based on the Bi₂Se₃ TI thin film. How the high dielectric constant of Bi₂Se₃ affects the performance of MOSFET and TFET is presented. Bulk TMDs such as MoS₂, WS₂ and others are the van der Waals-bonded layered material, much like graphite, except monolayer (and Bulk) TMDs have a large band gap in-contrast to graphene (and graphite). Here, the performance of nanoscale monolayer MoS₂ n-channel MOSFETs are examined through NEGF simulations using an atomistic TB Hamiltonian. N- and p-channel MOSFETs of various monolayer TMDs are also compared by the same approach. I correlate the performance differences with the band structure differences. Finally, ab-initio calculations of adatom doping effects on the monolayer MoS₂ is shown. I discuss the most stable atomic configurations, the bonding type and the amount of charge transfer from adatom to the monolayer MoS₂.Item Engineering exotic linear and nonlinear electromagnetic responses using spatial and spatiotemporal modulation(2019-05) Tymchenko, Mykhailo; Alù, Andrea; Belkin, Mikhail; Bank, Seth R.; Wasserman, Daniel; Khanikaev, Alexander B.Periodicity and modulation lie at the heart of modern electromagnetic, acoustic and mechanical engineering, dramatically altering the way in which waves interact with periodically structured media. The main idea driving the intense research into periodic systems is the fact that periodicity breaks the dependence on natural properties of constituent media and instead allows one to blend the responses of various materials and leverage their geometric shapes to obtain collective responses on demand. In the realm of electromagnetics, over the past two decades there has been an explosive surge of interest to artificially engineered time-invariant periodic structures thanks to numerous fascinating linear and nonlinear effects they enable. In this dissertation, I will present some transformative developments in the area of efficient nonlinear generation and wave mixing in thin 2D periodic structures based on multi-quantum-wells, as well as show the possibility to engineer to the great extent the dispersion topology of surface waves propagating along ideally thin conducting sheets with 1D spatial periodicity such as graphene ribbons. In parallel with the progress in obtaining desired responses in time-invariant periodic structures, significant progress is being made in applying temporal and synchronous spatial and temporal modulation to engage new degrees of freedom and extend the spectrum of achievable electromagnetic phenomena even further. In this dissertation, I will also show that spatiotemporal modulation applied to electronic networks holds a key to obtain ultrawideband and extremely compact delays far beyond those achievable in time-invariant systems. Spatiotemporal modulation also allows for all kinds of nonreciprocal devices to be seamlessly integrated in an electronic chip by overcoming the size and magnetic material incompatibility constraints. This fact holds a truly groundbreaking potential for future electronic devices and wireless systems by enabling their simultaneous transmit-and-receive operation. Finally, I will show that spatiotemporal modulation enables a direct translation of some of the most advanced and intricate concepts of condensed matter physics – topological insulators – to the realm of classical electronic circuits. Compared to standalone nonreciprocal devices, topologically-nontrivial electronic circuits provide an even larger toolbox to obtain various nonreciprocal functionalities by enforcing a wideband unidirectional transmission robust to defects and imperfectionsItem Interaction effects in topological insulators(2012-05) Wen, Jun, doctor of physics; Fiete, Gregory A; MacDonald, Allan H; Niu, Qian; Yao, Zhen; Chelikowsky, James RIn this thesis we employ various mean-field approaches to study the shortrange interaction effects in topological insulators. We start with the Kane-Mele model on the decorated honeycomb lattice and study the stability of topological insulator phase against different perturbations. We establish an adiabatic connection between a noninteracting topological insulator and a strongly interacting spin liquid in its Majorana fermion representation. We use the Hartree-Fock mean-field approach, slave-rotor approach and slave-boson approach to study correlation effects related to topological insulators. With the spontaneous symmetry breaking mechanism, we can have an interaction driven topological insulator with extended Hubbard models on the kagome lattice and decorated honeycomb lattice. For the interplay among spin-orbit coupling, distortion and correlation effect in transition metal oxides, we use the slave-rotor mean-field approach to study its phase transition. We identify regimes where a strong topological Mott insulator and a weak topological insulator reside due to the strong Coulomb interaction and distortion. This is relevant to experiments with the transition metal oxides as they hold promise to realize topological insulators. To study the doping effects and a possible spin liquid in Kane-Mele-Hubbard model on the honeycomb lattice, we employ the slave-boson mean-field approach which is appropriate for the intermediate interaction strength. We compare our results with those obtained from other methods.Item Molecular beam epitaxy of topological insulator Bi₂Se₃(2012-05) Chen, Yuxuan, 1986-; Shih, Chih-Kang; de Lozane, Alejandro L.In this thesis, I show my effort in growing atomically flat Bi₂Se₃ thin films using molecular beam epitaxy (MBE) method. Bi₂Se₃ is a kind of topological insulator, whose exotic surface states have been found in the samples that I grew.Item Optoelectronic, structural, and topological properties of van der Waals layered materials under extreme conditions(2018-08) Kim, Joonseok; Akinwande, Deji; Lin, Jung-Fu; Banerjee, Sanjay K; Dodabalapur, Ananth; Wang, YaguoThe concept of Internet of Things (IoT) has been discussed extensively in the recent years, where billions of smart devices and sensors communicate with each other and provide ubiquitous service. Two-dimensional (2D) materials for such application could be exposed to extreme conditions that IoT devices may experience, such as mechanically stressing, chemically reactive, high-temperature, and/or radiative environment. Therefore, it is crucial to understand the materials' properties under extreme conditions, and further engineer the properties from the acquired knowledge. In this dissertation, we focus on the effects of oxygen/moisture condition on air-sensitive 2D materials, and effects of hydrostatic pressure on 2D and other layered materials. In Chapter 2, we report detailed study on air-degradation of few-layer phosphorene films and field effect transistors, as well as an effective encapsulation method that enhances the stability of devices up to several months. In the later parts we explore the effects of hydrostatic pressure on layered materials, where the anisotropic van der Waals structure exhibit remarkably large pressure-modulation of material properties. In Chapter 3, pressure effects on Raman modes in bulk Mo₀.₅W₀.₅S₂ alloy are examined to discover strengthening of inter-layer interactions under pressure. In Chapter 4, pressure-induced structural transition of bulk WTe₂ is discussed, where layer sliding introduces inversion symmetry, similar to the case in monolayer WTe₂. In Chapter 5, evolution of optical band gaps of monolayer WS₂ and Mo₀.₅W₀.₅S₂ are studied, where we show different pressure-behaviors of band edges according to the composition. In Chapter 6, structural, vibrational, and topological electronic properties of Bi₁.₅Sb₀.₅Te₁.₈Se₁.₂ topological insulator alloy is explored, to show that the topological states could be modulated by pressure, without transitions in the crystal structure.Item Perpendicular And Parallel Field Magnetoresistance In Molecular Beam Epitaxy Grown Bi2Te3(2014-08) Dey, Rik; Banerjee, SanjayThe topological insulator Bi2Te3 has been grown on Si(111)-(7 × 7) surface by molecular beam epitaxy. Reflection high energy electron diffraction, in situ scanning tunnelling microscopy, x-ray photoelectron spectroscopy and ex situ x-ray diffraction studies have been performed to analyze the quality of the growth. These analyses suggest a very good layer-by-layer epitaxial growth of Bi2Te3 on the atomically at Si surface. The magnetoresistance of the samples has been studied with magnetic field perpendicular and parallel to the sample surface, up to 9 T, over a temperature range of 2 K to 20 K. A sharp dip at low fields (0 T - 1 T) and near-linear behavior for high fields (> 4 T) have been observed in the perpendicular field magnetoresistance. The low field dip is due to weak antilocalization that agrees well with the simplified Hikami-Larkin-Nagaoka model. It has been demonstrated that both the low field dip and the high field near-linear behavior can be explained by the original Hikami-Larkin-Nagaoka formula alone in a system with strong spin-orbit coupling. From the fitting of the perpendicular field magnetoresistance the phase coherence length, the mean free path and the spin-orbit relaxation time have been estimated. The phase coherence length shows power law dependence with temperature indicating two dimensional nature of the transport. The power law also suggests electron electron interaction as the prominent dephasing mechanism. The out-of-plane spin-orbit relaxation time is determined to be small and the in-plane spin-orbit relaxation time is found to be comparable to the momentum relaxation time. The estimation of these charge and spin transport parameters is useful for topological insulator based magneto electric device applications. It also has been shown that the strong spin-orbit coupling suppresses the Zeeman contribution in perpendicular field magnetoresistance. The logarithmic divergence of perpendicular field magnetoresistance with temperature for low temperature range (2 K - 20 K) at high fields shows the presence of Coulomb interaction in the spin singlet channel. For magnetoresistance with the field parallel to the sample surface, the observed magnetoresistance has parabolic dependence for small fields (0 T - 0.6 T) and logarithmic dependence for large fields (> 3 T), which is due to the Zeeman effect. It is found that the data are inconsistent with only the Maekawa and Fukuyama theory of non interacting electrons with Zeeman contributions to the transport, but are consistent with theory if one also takes into account the electron electron interaction and the Zeeman splitting term in the electron electron interaction theory of Lee and Ramakrishnan. The Zeeman g-factor and the strength of Coulomb scattering due to electron electron interaction have been estimated from fitting of the parallel field magnetoresistance. The magnetoresistance also shows anisotropy with respect to the field directions. The angle dependent anisotropic magnetoresistance can be fitted well by the original HLN theory alone. The anisotropy can have potential application in anisotropic magnetic sensors.Item Scanning tunneling microscope studies of 2D superconductor and 3D intrinsic topological insulator(2015-05) Nam, Hyoungdo; Shih, Chih-Kang; de Lozanne, Alex; Markert, John T; Fiete, Gregory A; Shi, LiElectrons show unusual and interesting behaviors both in low dimensions and on material surfaces, distinct from what they display in bulk materials. These intriguing properties have been studied in order to understand their origins. One area where this can be seen is in the case of superconductivity, where superconducting phase fluctuation in a thin superconductor is supposed to substantially suppress the superconductivity of the material as the film thickness decreases. To test this, we prepared epitaxially grown and globally flat lead (Pb) films; here, the thinnest film was 1.4 nm thick. Four different length scale measurements, ranging from the nm to the mm scale, gave consistent superconducting transition temperatures. Our results proved that the film of 1.4 nm still has strong superconducting phase stiffness; namely, the superfluid phase is rigid even in 1.4 nm thin superconductor film. Moreover, the parallel critical magnetic field is remarkably strong so that superconductivity is still observed in Zeeman fields, exceeding the Pauli limit. In addition, the surface of 3D topological insulator has a novel quantum state induced by strong spin-orbit interaction. A number of material studies were conducted to find a surface dominated conduction topological insulator that has a large energy gap and a single Dirac cone. Moreover, it is necessary for the material to be stable against aging unlike most 3D topological insulators, such as Bi₂Se₃. Here, Bi₂Te₂Se and BiSbTeSe₂ were studied in terms of their structures, electronic properties, and aging effects on them. Scanning tunneling microscopy analysis attested that Bi₂Te₂Se is an order alloy, which has a slight randomness of 15 %, whereas BiSbTeSe₂ is a random alloy. Scanning tunneling spectroscopy on BiSbTeSe₂ confirmed that the Dirac point tends to stay around the Fermi level under the strong band structure change, induced by random structure. The most surprising observation was that BiSbTeSe₂ showed remarkable stability despite the rich composition of selenium (Se). Even after aging for seven days, the Fermi level and the Dirac point remained at almost the same level in bulk band gap. Both observations are very important for applications to utilize the exotic topological surface state.Item Surface wave manipulation with polar dielectric thin films and topological photonic system(2018-06-18) Lai, Kueifu; Shih, Chih-Kang; Shvets, G.; Florin, Ernst-Ludwig; Fiete, Gregory; Belkin, MikhailDifferent from the well-studied wave propagation in the bulk where a plane wave extending to infinite is often conceived, the surface wave exists in the boundary defined by domains of distinct medium is well-confined with exponentially decaying tails away from this interface. This tightly-localized nature grants us enormous capability to manipulate the wave transport by tailoring the property of the interfaces and further enables various functionality for practical application. In this dissertation, a charged particle accelerator based on the surface phonon polaritons on the polar dielectric (silicon carbide) thin film is demonstrated to withstand high energy laser power and holds the promise of ultra-high accelerating gradiant in future experimental realization. The framework of wave propagation is then expanded beyond the homogenized medium to crystals with discrete periodicity which is referred as photonic crystals. The Bloch wave construct predicts that the topological insulator, a novel phenomenon in solid state physics, can be emulated by exquisite design of photonic system. Consequently, the surface waves (or edge states) between two topologically distinct photonic crystals exhibit robust and defect-immune wave transport which facilitates wide variety of applications. In particular, the RF delay line, polarized wave sorting, and two-beam accelerator based on the photonic topological insulator are investigated.Item Theoretical and experimental studies on topological insulators and topological insulator based spintronic devices(2019-08-16) Dey, Rik; Banerjee, Sanjay; Register, Leonard F.; Tsoi, Maxim; Incorvia, Jean; Yu, Edward TThree dimensional (3D) topological insulators (TIs) are unique materials with insulating bulk and two dimensional (2D) metallic surface states having spin-momentum locked Dirac-band dispersion. The remarkable property of spin-momentum locking of the 2D surface states provides an opportunity for manipulating the coupled spin and charge degrees of freedom of electrons on the surface of a 3D TI by controlling one or the other. The charge current-induced spin polarization of the 2D surface states of a 3D TI and subsequent diffusion or tunneling of spin current in an adjacent material, or conversion of spin current to charge current on the surface of a 3D TI, are a few among many effects of this spin-momentum locking, which renders TIs as promising candidates for spintronic applications. In this dissertation, we provide a theoretical description of the electronic transport of the TI surface states in proximity to a non-magnetic (NM) or a ferromagnetic (FM) material, and derive the transport equations based on quantum kinetic equation of non-equilibrium Green’s function. The transport equations are solved for appropriate boundary conditions to obtain the efficiency of the spin current-to-charge current conversion in TI/NM/FM or TI/FM heterostructure, or to calculate the efficiency of the detection of charge current induced spin polarization on the surface of a TI with FM tunnel contacts. We find that these efficiencies strongly depend on the tunnel conductance of the interface and decreases with increasing tunnel conductance, implying the necessity of design optimization of the tunnel interface in actual devices. Here, we also describe low-temperature magnetotransport measurements on an epitaxial Bi2Se3 thin film, and identify the contribution of the surface states and the quasi-2D bulk states to the transport from localization and interaction effects. We present two-terminal resistance measurement with FM and NM contacts on the surface of epitaxial and exfoliated Bi2Te3 films, and find change of resistance with reversal of the FM magnetization direction. We also measure magnetic hysteresis properties of sputtered Bi2Te3-Fe heterostructure and obtain enhancement of coercive field of Fe in the heterostructure, which could be due to strong spin-orbit coupling proximity effect arising from the Bi2Te3 film.Item Towards high-density low-power spin-transfer-torque random access memory(2015-08) Roy, Urmimala; Banerjee, Sanjay; Register, Leonard F.; Tutuc, Emanuel; Sreenivasan, S. V.; Tsoi, MaximIn this work, we investigate the prospects for spin-transfer-torque random access memory (STTRAM) as the new generation low-power high-density non-volatile memory. Possible means to lower the switching current and increase the packing density of STTRAM are proposed. In an STTRAM cell, the logical value of the memory bit is stored as orientation of magnetic moment in its ferromagnetic ``free'' layer. The bit typically consists of two thin film ferromagnets (FM) separated by an insulating tunnel barrier as in a magnetic tunnel junction (MTJ) structure. One of the two FM layers has fixed magnetization direction, while the other layer is free to be switched. We first study STT-assisted switching in spin valve structures with in-plane, perpendicular, and canted magnetizations in free and (or) reference layers using point contact measurements to explore the use of non-collinear magnetizations in free and fixed FM to reduce both switching current and time. Next, we consider the possibility of storing two memory bits within a single MTJ with a cross-shaped free layer that could still be addressed by one selection transistor. We provide a detailed discussion of the switching dynamics and associated regions of reliable switching currents, in addition to illustrating the effects of varying device geometry on the latter. Moving on from the standard MTJ structure, we then consider the possible use of topological insulators, as opposed to the fixed FM, as the spin-polarizer layer. It has been established that spin and momentum are locked helically in the surface states of a three-dimensional topological insulator (TI). Suggestions of possible use of the TI spin-polarized surface states in spintronic devices to induce reversal of a magnet have been made using theoretical and experimental studies. Here, we simulate magnetization reversal of a metallic nanomagnet by an underlying TI. The TI, thanks to the spin-momentum helical locking of the surface states, causes a spin-polarized current injection to the FM above it. We study the efficiency of the spin injection as a function of varying transparency of the TI-FM interface. The transport in the TI and the FM is assumed to be diffusive at room temperature for the assumed resistance values. Finally, we take into account random thermal fluctuations leading to write error rate (WER) in STTRAM write operation and use Fokker-Planck method and stochastic Landau-Lifshitz-Gilbert-Slonczewski equation to model WER in STTRAM. We conclude with possible future research directions.Item Transport behavior and weak adhesion of quantum confined epitaxial Bi and Bi1-xSbx films(2018-05) Muschinske, Sarah E.; Bank, Seth RobertNovel devices, such as those based on principles of spin and magnetization instead of traditional electronic transport, necessitate the development of new materials systems. Bismuth (Bi) is a promising materials systems for these applications due to its high mobility, large spin-orbit interaction and metallic surface states which exhibit Rashba spin-splitting. In addition, Bi exhibits a thickness dependent band gap due to quantum size effects occurring at uniquely long length scales (>100nm) due to its large de Broglie wavelength. This allows the metallic surface states to be isolated from the bulk in the band gap. Alloying Bi with antimony (Sb) to create Bi1-xSbx allows access to topologically non-trivial surface states which have the aforementioned qualities associated with Bi, as well as added protection against backscattering and non-magnetic perturbation. In addition, the non-trivial topology of Bi1-xSbx makes it a contender for topological quantum computing devices using Majorana fermions and braided states. In this thesis, we explore the transport properties of Bi and Bi1-xSbx as the film thicknesses and alloy compositions are varied, providing a basis for custom design of these materials for specific applications. In addition, we report advances in the understanding of Bi and Bi1-xSbx growth on Si (111) through the measurement of their weak adhesion to the Si (111) substrate as well as the identification of (012) oriented crystal growth of Bi1-xSbx and the effect of this crystal structure transition on the transport properties of the films. Finally, we report the development of a dry transfer method which can be used to transfer high quality epitaxial Bi and Bi1-xSbx films to arbitrary substrates which may facilitate their integration into novel spin-based devices