Browsing by Author "Park, Kyungwha"

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    Decoherence of Transverse Electronic Spin Current in Magnetic Metals
    Lim, Youngmin (Virginia Tech, 2022-05-31)
    Transport of spin angular momentum (spin currents) in magnetic thin films is important for non-volatile spin-based memory devices and other emerging information technology applications. It is especially important to understand how a spin current propagates across interfaces and how a spin current interacts with magnetic moments. The great interest in devices based on ferromagnetic metals generated intensive theoretical and experimental studies on the basic physics of spin currents for the last few decades. Of particular interest recently is the so-called "pure" electronic spin current, which is carried by electrons and yet unaccompanied by net charge flow, in part because of the prospect of transporting spin with minimal Joule heating. However, in contrast to ferromagnetic metals, spin transport in antiferromagnetic metals, which are promising materials for next-generation magnetic information technology, is not well understood yet. This dissertation addresses the mechanisms of transport by pure spin current in thin-film multilayers incorporating metals with antiferromagnetic order. We focus on two specific materials: (1) CoGd alloys with ferrimagnetic sublattices, which resemble antiferromagnets near the compensation composition, and (2) elemental antiferromagnetic Cr, which can be grown as epitaxial films and hence serve as a model system material. For both the CoGd and Cr studies, spin-valve-like structures of NiFe/Cu/CoGd and NiFe/Cu/Cr/CoFe are prepared to conduct ferromagnetic resonance spin pumping experiments. Precessing magnetization in the NiFe "spin source" pumps a transverse spin current to the adjacent layers. We measure the loss of the spin angular momentum in the "spin sink" layer by measuring the broadening of the resonance linewidth, i.e., the non-local damping enhancement, of the spin source. The antiparallel magnetic moments of Co and Gd sublattices partially cancel out the dephasing of a transverse spin current, thereby resulting in a long spin dephasing length of ≈ 5-6 nm near the magnetic compensation point. We find evidence that the spin current interacts somewhat more strongly with the itinerant transition-metal Co magnetism than the localized rare-earth-metal Gd magnetism in the CoGd alloy. We also examine spin transport via structurally clean antiferromagnetic Cr, epitaxially grown with BCC crystal order. We observe strong spin reflection at the Cu/Cr interface, which is surprising considering that thin layers of Cu and Cr individually are transparent to spin currents carried by electrons. Further, our results indicate other combinations of electrically conductive elemental metals (e.g., Cu/V) can form effective spin-reflecting interfaces. Overall, this thesis advances the basic understanding of spin transport in metallic thin films with and without magnetic order, which can aid the development of next generations of efficient spintronic devices. This work was supported in part by the National Science Foundation, Grant No. DMR-2003914.
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    Driven Magnetic Flux Lines in Type-II Superconductors: Nonequilibrium Steady States and Relaxation Properties
    Klongcheongsan, Thananart (Virginia Tech, 2009-03-31)
    We investigate the nonequilibrium steady state of driven magnetic flux lines in type-II superconductors subject to strong point or columnar pinning centers and the aging dynamics of nonequilibrium relaxation process in the presence of weak point pinning centers. We employ a three-dimensional elastic line model and Metropolis Monte Carlo simulations. For the first part, we characterize the system by means of the force-velocity / current-voltage curve, static structure factor, mean vortex radius of gyration, number of double-kink and half-loop excitations, and velocity / voltage noise features. We compare the results for the above quantities for randomly distributed point and columnar defects. Most of both numerical works have been done in two-dimensional systems such as thin film in which the structure of flux lines is treated as a point-like particle. Our main point of investigation in this paper is to demonstrate that the vortex structure and its other transport properties may exhibit a remarkable variety of complex phenomena in three-dimensional or bulk superconductors. The second part devotes to the study of aging phenomena in the absence of a driving force in disordered superconductors with much weaker point disorder. By investigating the density autocorrelation function, we observe all three crucial properties of the aging phenomena; slow power-law relaxation, breaking of time-translation invariance, and the presence of the dynamical scaling. We measure the dynamical exponents b and lambda_c/z and compare to other work. We find exponent values increase for increasing pinning strength, smaller interaction range, lower temperature, and denser defect density while the exponents measured in other approach tend to decrease.
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    Dynamically Corrected Quantum Control: A Geometrical Framework
    Zeng, Junkai (Virginia Tech, 2019-10-22)
    Implementing high-fidelity quantum control and suppressing the unwanted environmental noise has been one of the essential challenges in developing quantum information technologies. In the past, driving pulse sequences based on Dirac delta functions or square wave functions, such as Hahn spin echo or CPMG, have been developed to dynamically correcting the noise effects. However, implementing these ideal pulses with high fidelity is a challenging task in real experiments. In this thesis, we provide a new and simple method to explore the entire solution space of driving pulse shapes that suppress environmental noise in the evolution of the system. In this method, any single-qubit phase gate that is first-order robust against quasi-static transversal noise corresponds to a closed curve on a two-dimensional plane, and more general first-order robust single-qubit gates correspond to closed three-dimensional space curves. Second-order robust gates correspond to closed curves having the property that their projection onto any two-dimensional planes shall enclose a zero net area. The driving pulse shapes that implement the gates can be determined by the curvature, torsion, and the length of the curve. By utilizing the framework it is possible to obtain globally optimal solutions in pulse shaping in respect of experimental constraints by mapping them into geometrical optimization problems. One such problem we solved is to prove that the fastest possible single-qubit phase gates that are second-order noise-resistant shall be implemented using sign-flipping square functions. Since square waves are not experimentally feasible, we provide a method to smooth these pulses with minimal loss in gate speed while maintaining the robustness, based on the geometrical framework. This framework can also be useful in diagnosing the noise-cancellation properties of pulse shapes generated from numerical methods such as GRAPE. We show that this method for pulse shaping can significantly improve the fidelity of single-qubit gates through numerical simulation.
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    Effects of Electron-Vibron Coupling in Single-Molecule Magnet Transport Junctions Using a Hybrid Density Functional Theory and Model Hamiltonian Approach
    Mccaskey, Alexander Joseph (Virginia Tech, 2014-05-14)
    Recent experiments have shown that junctions consisting of individual single-molecule magnets (SMMs) bridged between two electrodes can be fabricated in three-terminal devices, and that the characteristic magnetic anisotropy of the SMMs can be affected by electrons tunneling through the molecule. Vibrational modes of the SMM can couple to electronic charge and spin degrees of freedom, and this coupling also influences the magnetic and transport properties of the SMM. The effect of electron-vibron coupling on transport has been extensively studied in small molecules, but not yet for junctions of SMMs. The goals of this thesis will be two-fold: to present a novel approach for studying the effects of this electron-vibron coupling on transport through SMMs that utilizes both density functional theory calculations and model Hamiltonian construction and analysis, and to present a software framework based on this hybrid approach for the simulation of transport across user-defined SMMs. The results of these simulations will indicate a characteristic suppression of the current at low energies that is strongly dependent on the overall electron-vibron coupling strength and number of molecular vibrational modes considered.
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    Electrical injection and detection of spin polarization in InSb/ferromagnet nanostructures
    Kim, Yong-Jae (Virginia Tech, 2012-07-30)
    We present studies of the electical detection of spin injection and transport in InSb/CoFe heterostructures. As a narrow gap semiconductor, InSb has a high mobility and strong spin-orbit interaction. Using ferromagnetic CoFe, lateral InSb/CoFe devices are fabricated by semiconductor processing techniques. The saturation magnetizations of various CoFe electrodes with different widths are calculated from Hall measurements in which the fringing fields of the CoFe electrodes are detected. A magnetic model provides reasonable estimation of the saturation magnetization for micrometer scale geometries. The interface magnetoresistance measurements of InSb/CoFe thin film layered structures present a unique peak at low field, having a symmetric behavior in magnetic field with a critical field Hc and a strong temperature dependence. We attribute our signal to a ferromagnetic phase in the InSb induced by spin injection. In a non-local lateral spin valve measurement, we observed the following. Firstly, Hc of the lateral spin valve signals is identical to Hc of interface magnetoresistance signals. Secondly, the non-local lateral spin valve signals are strongly dependent on temperature, which is also a unique characteristic magnetoresistance. Thirdly, the signals are tunable in response to an applied injector bias. Lastly, the signals are dependent on the exact interfaces. Based on these observations, the detected signals may be considered as spin current signals. The Hall and magnetoresistance signals are measured locally and non-locally in InSb/CoFe Hall devices. The non-local magnetoresistance signals exhibit asymmetric behavior in applied magnetic field which are considered as signatures of spin phenomena. The non-local Hall signals present switching behavior with the CoFe magnetization switching at the coercive field. The non-local Hall signals in a perpendicular field show Hc, similarly seen in non-local lateral spin valves. Inverse spin Hall effect measurements with tilted magnetic fields show an in-plane magnetic field dependence in non-local type Hall signal and a perpendicular magnetic field dependence in the local Hall measurement. We have found that the signal can have its origin in a spin current from our observation of Hc and hysteresis in the magnetization traces. As yet, the spin current transport mechanism is unknown.
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    Electron Transport via Single Molecule Magnets with Magnetic Anisotropy
    Luo, Guangpu (Virginia Tech, 2019-02-07)
    Single molecule magnets (SMMs) are molecules of mesoscopic scale which exhibit quantum properties such as quantum tunneling of magnetization, quantum interference, spin filtering effects, strong spin-phonon coupling and strong hyperfine Stark effects. These effects allow applications of SMMs to high-density information storage, molecular spintronics, and quantum information science. Therefore, SMMs are of interest to physicists, chemists, and engineers. Recently, experimental fabrication of individual SMMs within transistor set-ups have been achieved, offering a new method to examine magnetic properties of individual SMMs. In this thesis, two types of SMMs, specifically Eu2(C8H8)3 and Ni9Te6(PEt3)8, are theoretically investigated by simulating their electron transport properties within three-terminal transistor set-ups. An extended metal atom chain (EMAC) consists of a string of metallic atoms with organic ligands surrounding the string. EMACs are an important research field for nanoelectronics. Homometallic iron-based EMACs are especially attractive due to the high spin and large magnetic anisotropy of iron(II). We explore the exchange coupling of iron atoms in two EMACs: [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Chapter 1 provides an introduction to SMMs, electron transport experiments via SMMs and an introduction to density functional theory (DFT). Chapter 2 presents a theoretical study of electron transport via Eu2(C8H8)3. This type of molecule is interesting since its magnetic anisotropy type changes with oxidation state. The unique magnetic properties lead to spin blockade effects at zero and low bias. In other words, the current through this molecule is completely suppressed until the bias voltage exceeds a certain value. Chapter 3 discusses a theoretical study of electron transport via Ni9Te6(PEt3)8. The magnetic anisotropy of this magnetic cluster has cubic symmetry, which is higher than most SMMs. With appropriate magnetic anisotropy parameters, in the presence of an external magnetic field, uncommon phenomena such as low-bias blockade effects, negative conductance and discontinuous conductance lines, are observed. In Chapter 2 and 3 DFT-calculated magnetic anisotropy parameters are used and electron transport properties are calculated by solving master equations at low temperature. Chapter 4 examines the exchange coupling between iron ions in EMACs [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. The exchange coupling constants are calculated by using the least-squares fitting method, based on the DFT-calculated energies from different spin configurations.
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    Exact Diagonalization Studies of Strongly Correlated Systems
    Raum, Peter Thomas (Virginia Tech, 2020-01-14)
    In this dissertation, we use exact diagonalization to study a few strongly correlated systems, ranging from the Fermi-Hubbard model to the fractional quantum Hall effect (FQHE). The discussion starts with an overview of strongly correlated systems and what is meant by strongly correlated. Then, we extend cluster perturbation theory (CPT), an economic method for computing the momentum and energy resolved Green's function for Hubbard models to higher order correlation functions, specifically the spin susceptibility. We benchmark our results for the one-dimensional Fermi-Hubbard model at half-filling. In addition we study the FQHE at fillings $nu = 5/2$ for fermions and $nu = 1/2$ for bosons. For the $nu = 5/2$ system we investigate a two-body model that effectively captures the three-body model that generates the Moore-Read Pfaffian state. The Moore-Read Pfaffian wave function pairs composite fermions and is believed to cause the FQHE at $nu = 5/2$. For the $nu = 1/2$ system we estimate the entropy needed to observe Laughlin correlations with cold atoms via an ansatz partition function. We find entropies achieved with conventional cooling techniques are adequate.
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    Examining Topological Insulators and Topological Semimetals Using First Principles Calculations
    Villanova, John William (Virginia Tech, 2018-04-30)
    The importance and promise that topological materials hold has been recently underscored by the award of the Nobel Prize in Physics in 2016 ``for theoretical discoveries of topological phase transitions and topological phases of matter." This dissertation explores the novel qualities and useful topologically protected surface states of topological insulators and semimetals. Topological materials have protected qualities which are not removed by weak perturbations. The manifestations of these qualities in topological insulators are spin-momentum-locked surface states, and in Weyl and Dirac semimetals they are unconventional open surface states (Fermi arcs) with anomalous electrical transport properties. There is great promise in utilizing the topologically protected surface states in electronics of the future, including spintronics, quantum computers, and highly sensitive devices. Physicists and chemists are also interested in the fundamental physics and exotic fermions exhibited in topological materials and in heterostructures including them. Chapter 1 provides an introduction to the concepts and methods of topological band theory. Chapter 2 investigates the spin and spin-orbital texture and electronic structures of the surface states at side surfaces of a topological insulator, Bi2Se3, by using slab models within density functional theory. Two representative, experimentally achieved surfaces are examined, and it is shown that careful consideration of the crystal symmetry is necessary to understand the physics of the surface state Dirac cones at these surfaces. This advances the existing literature by properly taking into account surface relaxation and symmetry beyond what is contained in effective bulk model Hamiltonians. Chapter 3 examines the Fermi arcs of a topological Dirac semimetal (DSM) in the presence of asymmetric charge transfer, of the kind which would be present in heterostructures. Asymmetric charge transfer allows one to accurately identify the projections of Dirac nodes despite the existence of a band gap and to engineer the properties of the Fermi arcs, including spin texture. Chapter 4 investigates the effect of an external magnetic field applied to a DSM. The breaking of time reversal symmetry splits the Dirac nodes into topologically charged Weyl nodes which exhibit Fermi arcs as well as conventionally-closed surface states as one varies the chemical potential.
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    Experimental Measurements by Antilocalization of the Interactions between Two-Dimensional Electron Systems and Magnetic Surface Species
    Zhang, Yao (Virginia Tech, 2014-06-18)
    Low-temperature weak-localization (WL) and antilocalization (AL) magnetotransport measurements are sensitive to electron interference, and thus can be used as a probe of quantum states. The spin-dependent interactions between controllable surface magnetism and itinerant electrons in a non-magnetic host provide insight for spin-based technologies, magnetic data storage and quantum information processing. This dissertation studies two different host systems, an In$_{0.53}$Ga$_{0.47}$As quantum well at a distance from the surface of a heterostructure, and an accumulation layer on an InAs surface. Both the systems are two-dimensional electron systems (2DESs), and possess prominent Rashba spin-orbit interaction caused by structural inversion asymmetry, which meets the prerequisites for AL. The surface local moments influence the surrounding electrons in two ways, increasing their spin-orbit scattering, and inducing magnetic spin-flip scattering, which carries information about magnetic interactions. The two effects modify the AL signals in opposing directions: the spin-flip scattering of electrons shrinks the signal, and requires a close proximity to the species, whereas the increase of spin-orbit scattering broadens and increases the signal. Accordingly, we only observe an increase in spin-orbit scattering in the study of the interactions between ferromagnetic Co$_{0.6}$Fe$_{0.4}$ nanopillars and the relatively distant InGaAs quantum well. With these CoFe nanopillars, a decrease in spin decoherence time is observed, attributed to the spatially varying magnetic field from the local moments. A good agreement between the data and a theoretical calculation suggests that the CoFe nanopillars also generate an appreciable average magnetic field normal to the surface, of value $\sim$ 35 G. We also performed a series of comparative AL measurements to experimentally investigate the interactions and spin-exchange between InAs surface accumulation electrons and local magnetic moments of rare earth ions Sm$^{3+}$, Gd$^{3+}$, Ho$^{3+}$, of transition metal ions Ni$^{2+}$, Co$^{2+}$, and Fe$^{3+}$, and of Ni$^{2+}$-, Co$^{2+}$-, and Fe$^{3+}$-phthalocyanines deposited on the surface. The deposited species generate magnetic scattering with magnitude dependent on their electron configurations and effective moments. Particularly for Fe$^{3+}$, the significant spin-flip scattering due to the outermost 3d shell and the fairly high magnetic moments modifies the AL signal into a WL signal. Experiments indicate a temperature-independent magnetic spin-flip scattering for most of the species except for Ho$^{3+}$ and Co$^{2+}$. Ho$^{3+}$ yields electron spin-flip rates proportional to the square root of temperature, resulting from transitions between closely spaced energy levels of spin-orbit multiplets. In the case of Co$^{2+}$, either a spin crossover or a spin-glass system forms, and hence spin-flip rates transit between two saturation regions as temperature varies. Concerning the spin-orbit scattering rate, we observe an increase for all the species, and the increase is correlated with the effective electric fields produced by the species. In both 2DESs, the inelastic time is inversely proportional to temperature, consistent with phase decoherence via the Nyquist mechanism. Our method provides a controlled way to probe the quantum spin interactions of 2DESs, either in a quantum well, or on the surface of InAs.
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    Hydrodynamic and ballistic transport in high-mobility GaAs/AlGaAs heterostructures
    Gupta, Adbhut (Virginia Tech, 2021-09-24)
    The understanding and study of electron transport in semiconductor systems has been the instigation behind the growth of semiconductor electronics industry which has enabled technological developments that are part of our everyday lives. However, most materials exhibit diffusive electron transport where electrons scatter off disorder (impurities, phonons, defects, etc.) inevitably present in the system, and lose their momentum. Advances in material science have led to the discovery of materials which are essentially disorder-free and exhibit exceptionally high mobilities, enabling transport physics beyond diffusive transport. In this work, we explore non-diffusive transport regimes, namely, the ballistic and hydrodynamic regimes in a high-mobility two-dimensional electron system in a GaAs quantum well in a GaAs/AlGaAs heterostructure. The hydrodynamic regime exhibits collective fluid-like behavior of electrons which leads to the formation of current vortices, attributable to the dominance of electron-electron interactions in this regime. The ballistic regime occurs at low temperatures, where electron-electron interactions are weak, constraining the electrons to scatter predominantly against the device boundaries. To study these non-diffusive regimes, we fabricate mesoscopic devices with multiple point contacts on the heterostructure, and perform variable-temperature (4.1 K to 40 K) zero-field nonlocal resistance measurements at various locations in the device to map the movement of electrons. The experiments, along with interpretation using kinetic simulations, demarcate hydrodynamic and ballistic regimes and establish the dominant role of electron-electron interactions in the hydrodynamic regime. To further understand the role of electron-electron interactions, we perform nonlocal resistance measurements in the presence of magnetic field in transverse magnetic focusing geometries under variable temperature (0.39 K to 36 K). Using our experimental results and insights from the kinetic simulations, we quantify electron-electron scattering length, while also highlighting the importance of electron-electron interactions even in ballistic transport. At a more fundamental level, we reveal the presence of current vortices in both hydrodynamic and surprisingly, ballistic regimes both in the presence and absence of magnetic field. We demonstrate that even the ballistic regime can manifest negative nonlocal resistances which should not be considered as the hallmark signature of hydrodynamic regime. The work sheds a new light on both hydrodynamic and ballistic transport in high-mobility solid-state systems, highlighting the similarities between these non-diffusive regimes and at the same time providing a way of effectively demarcating them using innovative device design, measurement schemes and one-to-one modeling. The similarities stem from total electron system momentum conservation in both the hydrodynamic and ballistic regimes. The work also presents a sensitive and precise experimental technique for measuring electron-electron scattering length, which is a fundamental quantity in solid-state physics.
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    The interaction between a monolayer of single-molecule magnets and a metal surface
    Barraza-Lopez, Salvador; Avery, Michael C.; Park, Kyungwha (American Institute of Physics, 2008-04-01)
    We calculate within density functional theory (DFT) and the LSDA+U formalism the electronic properties of a nanostructure in which single-molecule magnets Mn(12) are adsorbed via thiol groups onto the Au(111) surface. Our DFT calculation shows 1.23 electrons being transferred from the surface to the Mn(12) molecule, dominated by the tail on the electronic charge distribution from the gold slab. LSDA+U calculations reveal that the on-site Coulomb repulsion U does not alter the direction of the electronic charge transfer obtained from DFT, because the gold Fermi level still lies above the lowest unoccupied molecular orbital (LUMO). The U term opens up the energy gap between the highest occupied molecular orbital (HOMO) and the LUMO for an isolated standard Mn(12) but it minimally affects the gap for a sulfur-terminated Mn(12). (c) 2008 American Institute of Physics.
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    Metastability of Magnetic Nanoparticles in Magnetization Relaxation with Different Dynamics and Distributions of Magnetic Anisotropy
    Yamamoto, Yoh (Virginia Tech, 2013-06-11)
    We study the metastability of magnetic nanoparticles with size distributions. We simulate an array of magnetic nanoparticles with a spin S = 1 ferromagnetic Blume-Capel model on a square lattice. Studying decays of the metastable state in the Blume-Capel model at low temperatures requires an extremely long computational time in kinetic Monte Carlo simulations. Therefore, we use an advanced algorithm adapted from the Monte Carlo with absorbing Markov chain algorithm for the Ising model in order to study the Blume-Capel model with size distributions. We modeled the particle size distributions as distributions of magnetic anisotropy. We compute the low-temperature average lifetime of the magnetization relaxation using kinetic Monte Carlo simulations with the advanced algorithms. We also calculate the lifetime using the absorbing Markov chains method for analytical results. Our results show that the lifetime of the metastable state follows a modified-Arrhenius law where the energy barrier has a dependency on temperature and standard deviation of the distributions in addition to magnetic field and magnetic anisotropy. The magnetic anisotropy barrier is determined by the smallest particle within a given distribution. We also study magnetization relaxation in different single critical droplet regions using different dynamics: Glauber and phonon-assisted dynamics. We find that the lifetime follows the modified-Arrhenius law for both dynamics, and an explicit form of the lifetime differs in different regions for different dynamics. For the Glauber dynamics, the Arrhenius prefactor does not depend on the standard deviation of the distribution of the magnetic anisotropy. For the phonon-assisted dynamics, however, even the prefactor of the lifetime depends on the standard deviation and is significantly reduced for a wide distribution of magnetic anisotropy. Furthermore, the phonon-assisted dynamics forbids transitions between degenerate energy states and results in an increase of the energy barrier at the single critical droplet region boundary compared to that for the Glauber dynamics. We find that the spin system with a distribution of magnetic anisotropy finds lower-energy relaxation pathways to avoid degenerate state, and the energy barrier becomes the same for both dynamics.
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    Modeling of nucleation-based stochastic processes in cellular systems
    Xu, Xiaohua (Virginia Tech, 2010-08-11)
    Molecular cell biology has been an intensively studied interdisciplinary field with the rapid development of experimental techniques and fast upgrade of computational hardware and numerical tools. Recent technological developments have led to single-cell experiments which allow us to probe the role of stochasticity in cellular processes. Stochastic modeling of the corresponding processes is thus an essential ingredient for the understanding and interpretation of cellular systems of interest. In this thesis, we explore several nucleation-based stochastic cellular processes, i.e. Min protein oscillation in Escherichia coli, pausing phenomena in DNA transcription, and single-molecule enzyme kinetics. We focus on the key experimental results and build up stochastic models accordingly to provide quantitative insights to the underlying physical mechanisms for the corresponding biological processes. We utilize specific mathematical methods and computational algorithms to gain a better understanding and make predictions for further experimental explorations in the relevant fields.
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    Modern Electronic Structure Theory using Tensor Product States
    Abraham, Vibin (Virginia Tech, 2022-01-11)
    Strongly correlated systems have been a major challenge for a long time in the field of theoretical chemistry. For such systems, the relevant portion of the Hilbert space scales exponentially, preventing efficient simulation on large systems. However, in many cases, the Hilbert space can be partitioned into clusters on the basis of strong and weak interactions. In this work, we mainly focus on an approach where we partition the system into smaller orbital clusters in which we can define many-particle cluster states and use traditional many-body methods to capture the rest of the inter-cluster correlations. This dissertation can be mainly divided into two parts. In the first part of this dissertation, the clustered ansatz, termed as tensor product states (TPS), is used to study large strongly correlated systems. In the second part, we study a particular type of strongly correlated system, correlated triplet pair states that arise in singlet fission. The many-body expansion (MBE) is an efficient tool that has a long history of use for calculating interaction energies, binding energies, lattice energies, and so on. We extend the incremental full configuration interaction originally proposed for a Slater determinant to a tensor product state (TPS) based wavefunction. By partitioning the active space into smaller orbital clusters, our approach starts from a cluster mean-field reference TPS configuration and includes the correlation contribution of the excited TPSs using a many-body expansion. This method, named cluster many-body expansion (cMBE), improves the convergence of MBE at lower orders compared to directly doing a block-based MBE from an RHF reference. The performance of the cMBE method is also tested on a graphene nano-sheet with a very large active space of 114 electrons in 114 orbitals, which would require 1066 determinants for the exact FCI solution. Selected CI (SCI) using determinants becomes intractable for large systems with strong correlation. We introduce a method for SCI algorithms using tensor product states which exploits local molecular structure to significantly reduce the number of SCI variables. We demonstrate the potential of this method, called tensor product selected configuration interaction (TPSCI), using a few model Hamiltonians and molecular examples. These numerical results show that TPSCI can be used to significantly reduce the number of SCI variables in the variational space, and thus paving a path for extending these deterministic and variational SCI approaches to a wider range of physical systems. The extension of the TPSCI algorithm for excited states is also investigated. TPSCI with perturbative corrections provides accurate excitation energies for low-lying triplet states with respect to extrapolated results. In the case of traditional SCI methods, accurate excitation energies are obtained only after extrapolating calculations with large variational dimensions compared to TPSCI. We provide an intuitive connection between lower triplet energy mani- folds with Hückel molecular orbital theory, providing a many-body version of Hückel theory for excited triplet states. The n-body Tucker ansatz (which is a truncated TPS wavefunction) developed in our group provides a good approximation to the low-lying states of a clusterable spin system. In this approach, a Tucker decomposition is used to obtain local cluster states which can be truncated to prune the full Hilbert space of the system. As a truncated variational approach, it has been observed that the self-consistently optimized n-body Tucker method is not size- extensive, a property important for many-body methods. We explore the use of perturbation theory and linearized coupled-cluster methods to obtain a robust yet efficient approximation. Perturbative corrections to the n-body Tucker method have been implemented for the Heisenberg Hamiltonian and numerical data for various lattices and molecular systems has been presented to show the applicability of the method. In the second part of this dissertation, we focus on studying a particular type of strongly correlated states that occurs in singlet fission material. The correlated triplet pair state 1(TT) is a key intermediate in the singlet fission process, and understanding the mechanism by which it separates into two independent triplet states is critical for leveraging singlet fission for improving solar cell efficiency. This separation mechanism is dominated by two key interactions: (i) the exchange interaction (K) between the triplets which leads to the spin splitting of the biexciton state into 1(TT),3(TT) and 5(TT) states, and (ii) the triplet-triplet energy transfer integral (t) which enables the formation of the spatially separated (but still spin entangled) state 1(T...T). We develop a simple ab initio technique to compute both the triplet-triplet exchange (K) and triplet-triplet energy transfer coupling (t). Our key findings reveal new conditions for successful correlated triplet pair state dissociation. The biexciton exchange interaction needs to be ferromagnetic or negligible compared to the triplet energy transfer for favorable dissociation. We also explore the effect of chromophore packing to reveal geometries where these conditions are achieved for tetracene. We also provide a simple connectivity rule to predict whether the through-bond coupling will be stabilizing or destabilizing for the (TT) state in covalently linked singlet fission chromophores. By drawing an analogy between the chemical system and a simple spin-lattice, one is able to determine the ordering of the multi-exciton spin state via a generalized usage of Ovchinnikov's rule. In the case of meta connectivity, we predict 5(TT) to be formed and this is later confirmed by experimental techniques like time-resolved electron spin resonance (TR-ESR).
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    Moire superlattices at the topological insulator Bi2Te3
    Schouteden, Koen; Li, Zhe; Chen, Taishi; Song, Fengqi; Partoens, Bart; Van Haesendonck, Chris; Park, Kyungwha (Springer Nature, 2016-02-08)
    We report on the observation of complex superlattices at the surface of the topological insulator Bi2Te3. Scanning tunneling microscopy reveals the existence of two different periodic structures in addition to the Bi2Te3 atomic lattice, which is found to strongly affect the local electronic structure. These three different periodicities are interpreted to result from a single small in-plane rotation of the topmost quintuple layer only. Density functional theory calculations support the observed increase in the DOS near the Fermi level, and exclude the possibility that strain is at the origin of the observed Moire pattern. Exploration of Moire superlattices formed by the quintuple layers of topological insulators holds great potential for further tuning of the properties of topological insulators.
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    Molecular Simulation Of Nanoscale Transport Phenomena
    Banerjee, Soumik (Virginia Tech, 2008-06-16)
    Interest in nanoscale heat and mass transport has been augmented through current trends in nanotechnology research. The theme of this dissertation is to characterize electric charge, mass and thermal transport at the nanoscale using a fundamental molecular simulation method, namely molecular dynamics. This dissertation reports simulations of (1) ion intake by carbon nanotubes, (2) hydrogen storage in carbon nanotubes, (3) carbon nanotube growth and (4) nanoscale heat transfer. Ion transport is investigated in the context of desalination of a polar solution using charged carbon nanotubes. Simulations demonstrate that when either a spatially or temporally alternating charge distribution is applied, ion intake into the nanotubes is minimal. Thus, the charge distribution can either be maintained constant (for ion encapsulation) or varied (for water intake) in order to achieve different effects. Next, as an application of mass transport, the hydrogen storage characteristics of carbon nanotubes under modified conditions is reported. The findings presented in this dissertation suggest a significant increment in storage in the presence of alkali metals. The dependence of storage on the external thermodynamic conditions is analyzed and the optimal range of operating conditions is identified. Another application of mass transport is the growth mode of carbon nanostructures (viz. tip growth and base growth). A correct prediction of the dominant growth mode depends on the energy gain due to the addition of C-atoms from the carbon-metal catalyst solution to the graphene sheets forming the carbon nanostructures. This energy gain is evaluated through molecular dynamics simulations. The results suggest tip growth for Ni and base growth for Fe catalysts. Finally, unsteady nanoscale thermal transport at solid-fluid interfaces is simulated using non-equilibrium molecular dynamics simulations. It is found that the simulated temperature evolution deviates from an analytical continuum solution due to the overall system heterogeneity. Temperature discontinuities are observed between the solid-like interfaces and their neighboring fluid molecules. With an increase in the temperature of the solid wall the interfacial thermal resistance decreases.
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    Optimal Control Protocols for Quantum Memory Network Applications
    Takou, Evangelia (Virginia Tech, 2024-06-25)
    Quantum networks play an indispensable role in quantum information tasks such as secure communications, enhanced quantum sensing, and distributed computing. In recent years several platforms are being developed for such tasks, witnessing breakthrough technological advancement in terms of fabrication techniques, precise control methods, and information transfer. Among the most mature and promising platforms are color centers in solids. These systems provide an optically active electronic spin and long-lived nuclear spins for information storage. The first part of this dissertation is concerned with error mechanisms in the control of electronic and nuclear spins. First, I will focus on control protocols for improved electron-spin rotations tailored to specific color centers in diamond. I will then discuss how to manipulate the entanglement between the electron and the always-coupled nuclear spin register. I will describe a general formalism to quantify and control the generation of en- tanglement in an arbitrarily large nuclear spin register. This formalism incorporates exactly the dynamics with unwanted nuclei, and quantifies the performance of entangling gates in the presence of unwanted residual entanglement links. Using experimental parameters from a well-characterized multinuclear spin register, I will show that preparation of multipartite entanglement in a single-shot is possible, which drastically reduces the total gate time of conventional protocols. Then, I will present a new formalism for describing all-way entanglement and show how to design gates that prepare GHZM states. I will show how to incorporate errors such as unwanted correlations, electronic dephasing errors or pulse control errors. The second part of this thesis focuses on the preparation of all-photonic graph states from a few quantum emitters. I will introduce heuristic algorithms that exploit graph theory concepts in order to reduce the entangling gate counts, and also discuss the role of locally equivalent graphs in the optimization of the generation circuits.
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    Physical, electrical and electrochemical characterizations of transition metal compounds for electrochemical energy storage
    Yuan, Qifan (Virginia Tech, 2015-02-03)
    Electrochemical energy storage has been widely used in various areas, including new energy sources, auto industry, and information technology. However, the performance of current electrochemical energy storage devices does not meet the requirements of these areas that include both high energy and power density, fast recharge time, and long lifetime. One solution to meet consumer demands is to discover new materials that can substantially enhance the performance of electrochemical energy storage devices. In this dissertation we report four transition metal materials systems with potential applications in electrochemical energy storage. Nanoscale and nanostructured materials are expected to play important roles in energy storage devices because of their enhanced and sometimes unique physical and chemical properties. Studied here is the comparative electrochemical cation insertion into a nanostructured vanadium oxide, a promising electrode material candidate, for the alkali metal ions Li+, Na+ and K+ and the organic ammonium ion, in aqueous electrolyte solutions. Observed are the distinctive insertion processes of the different ions, which yield a correlation between physical degradation of the material and a reduction of the calculated specific charge. The results reveal the potential of this nanostructured vanadium oxide material for energy storage. Vanadium based electrochemical systems are of general interest, and as models for vanadium based solid-state electrochemical processes, the solution state and the solid-state electrochemical properties of two cryolite-type compounds, (NH4)3VxGa1-xF6, and Na3VF6, are studied. The electrochemical behavior of (NH4)3VxGa1-xF6 explored the possibility of using this material as an electrolyte for solid state energy storage systems. Zeolite-like materials have large surface to volume ratios, with ions and neutral species located in the nanometer sized pores of the 3-dimensional framework, potentially yielding high energy density storage capabilities. Yet the insulating nature of known zeolite-like materials has limited their use for electrical energy storage. Studied here are two vanadium based zeolite-like structures, the oxo-vanadium arsenate [(As6V15O51)-9]∞, and the oxo-vanadium phosphate [(P6V15O51)-9]∞, where the former shows electronic conduction in the 3-dimensional framework. Mixed electronic and ionic conductivity, from the framework and from the cations located within the framework, respectively, is measured in the oxo-vanadium arsenate, and allows the use of this material in electrochemical double-layer capacitor configuration for energy storage. By contrast, the oxo-vanadium phosphate shows ionic conduction only. Lastly, a new strontium manganese vanadate with a layered structure exhibiting mixed protonic and electronic conductivity is studied. The various transition metal compounds and materials systems experimentally studied in this thesis showcase the importance of novel materials in future energy storage schemes.
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    Plasmonic Enhancement of Nonlinear Optical Responses by Gold Nanorods
    Lee, Jeong-Ah (Virginia Tech, 2017-01-09)
    The increase in the magnitude of local electric fields through resonances of plasmonic excitations in metallic nanoparticles is a major area of current optical research. This dissertation is focused on plasmon-enhanced second harmonic generation of organic ionic self-assembled films via localized surface plasmon resonance of gold nanorods. By matching the plasmon resonance of the gold nanorods to the wavelength of the fundamental light, it is possible to greatly enhance the SHG efficiency. To demonstrate this, the surface of the gold nanorods was functionalized with a nonlinear-optical (NLO) polymer, PCBS, via the layer-by-layer method and deposited on a polymer thin film created on a glass substrate using the ionic self-assembled multilayer (ISAM) method. The sample fabrication is divided into two parts: gold nanorod synthesis and functionalization. The gold nanorods were synthesized by the seed-mediated method with varying amounts of silver ions to control their LSPR wavelengths. The functionalization started by replacing the original thick CTAB bilayer on the surface of the gold nanorods by a thin PAH-DTC layer via dialysis. The nanorods were then alternately coated with PAH (polycation) and PCBS (NLO polyanion) up to three bilayers of PAH/PCBS. The number of polymer layers on the nanorods was chosen in consideration of the LSPR decay length (a few nm). The functionalized gold nanorods were then deposited on either PAH/PCBS or PAH/PSS ISAM films. Characterization was performed via optical spectral measurement, zeta potential measurement, and field-emission scanning electron microscopy (FESEM). The LSPR wavelength shifted when the surrounding medium changed. It was red-shifted for each added polymer layer on the nanorod surface. However, when the functionalized nanorods were deposited on the ISAM film, the resonance peak blue-shifted. The zeta potential confirmed the proper electric charge of each polymer layer coated on the nanorods. Finally, FESEM was performed on the samples for visual inspection of the nanorod deposition and distribution after the SHG measurement was complete. The SHG from the functionalized gold nanorods was measured using a Maker-like fringe method. In this method, second harmonic waves generated from the front and rear sides of the substrate interfere constructively and destructively when the sample is rotated with respect to the incoming pump wave. Electrical noise reduction techniques were implemented to improve the SHG signal readings. Signal processing was implemented using LabVIEW software in order to read a reliable SHG signal from the setup. The maximum tolerable fluence of the gold nanorods was determined in order to prevent optical damage. The interference fringe pattern was observed from the functionalized gold nanorods and compared with that from the conventional ISAM film. The enhancement from the gold nanorods was as high as 600 times compared to the bare films. Polarization dependent SHG measurements were conducted to ascertain the effect of coupling between p- or s-polarized fundamental incident light to the SH light. To further improve the SHG enhancement, the self-assembly method herein can be extended from a monolayer to multilayers of functionalized gold nanorods.
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    Properties of Nanoscale Biomaterials for Cancer Detection and Other Applications
    Geist, Brian Lee (Virginia Tech, 2009-04-24)
    The first thermal cycling experiments of ionic self-assembled multilayer (ISAM) films have been reported examining their survivability through repeated thermal cycles from -20° C to 120° C in ambient atmospheric conditions. The films were constructed from alternating layers of Nile Blue A and gold nanoparticles which provided a strong absorbance in the optical wavelength range. No degradation of the optical characteristics of the ISAM films was observed [1]. Techniques for measuring the capacitance and resistivity of various ISAM films have also been developed allowing for a more complete electrical characterization of ISAM films. Capacitance measurements enabled a calculation of the dielectric function and breakdown field strength of the ISAM films. The capacitance measurement technique was verified by measuring the dielectric function of a spin-coated thin film PMMA, which has a well characterized dielectric function [2]. Surface-enhanced Raman spectroscopy (SERS) has been studied as a possible detection method for malignant melanoma revealing spectral differences in blood sera from healthy horses and horses with malignant melanoma. A SERS microscope system was constructed with the capability of resolving the Raman signal from biologically important molecules such as beta-carotene and blood sera. The resulting Raman signals from sera collected from horses with malignant melanoma were found to have additional peaks not found in the Raman signals obtained from sera collected from healthy horses. A systematic analysis of the combination of absorbance and fluorescence signals of blood sera collected from populations of healthy dogs and dogs with cancer has resulted in a rapid and cost-effective method for monitoring protein concentrations that could possibly be used as part of a cancer screening process. This method was developed using the absorbance and fluorescence signals from known serum proteins, the combinations of which were used to match the absorbance and fluorescence signals of blood sera allowing for an accurate determination of protein concentrations in blood sera [3]. Finally, a novel method for measuring the melting point of DNA in solution using capacitance measurements is presented. This method allows for the determination of the melting temperature as well as the melting entropy and melting enthalpy of DNA strands. Two different short strands of DNA, 5'-CAAAATAGACGCTTACGCAACGAAAAC-3' along with its complement and 5'-GGAAGAGACGGAGGA-3' along with its complement were used to validate the technique as the characteristics of these strands could be modeled using theoretical methods. This experimental technique allows for the precise determination of the melting characteristics of DNA strands and can be used to evaluate the usefulness of theoretical models in calculating the melting point for particular strands of DNA. Additionally, a micro-fluidic device has been proposed that will allow for a rapid and cost-effective determination of the melting characteristics of DNA [4].