Physics

Permanent URI for this communityhttp://hdl.handle.net/1903/2269

Browse

Search Results

Now showing 1 - 10 of 47
  • Thumbnail Image
    Item
    Electronic and Magnetic Properties of MnP-Type Binary Compounds
    (2019) Campbell, Daniel James; Paglione, Johnpierre; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The interactions between electrons, and the resulting impact on physical properties, are at the heart of present-day materials science. This thesis looks at this idea through the lens of several compounds from a single family: the MnP-type transition metal pnictides. FeAs and FeP show long range magnetic order with some similarities to the high temperature, unconventional iron-based superconductors. CoAs lies on the border of magnetism, with strong fluctuations but no stable ordered state. CoP, in contrast, shows no strong magnetic fluctuations but serves as a useful baseline in determining the origin (from composition, structure, or magnetic order) of behavior in the other materials. For this work, single crystals were grown with two different techniques: solvent flux and chemical vapor transport. In the case of FeAs the flux method resulted in the highest quality crystals yet produced. Extensive work was then performed on these samples at the University of Maryland and the National High Magnetic Field Laboratory. Quantum oscillations observed in high magnetic fields, in combination with density functional theory calculations, give insight into the Fermi surfaces of these materials. Large magnetoresistance in the phosphides, but not the arsenides, demonstrates differences in the choice of pnictogen atom that cannot be simply a product of electron count. Angle-dependent linear magnetoresistance in FeP is a sign of a possible Dirac dispersion and topological physics, as has been hinted at in other MnP-type materials. Ultimately, it is possible to examine results for all four compounds and draw conclusions on the role of each of the two elements in the formula, which can be extended to other members of this family.
  • Thumbnail Image
    Item
    Topological Rare Earth Half-Heusler HoPtBi
    (2019) Roncaioli, Connor Andrew; Paglione, Johnpierre; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Magnetic HoPtBi is created and characterized as a new half-Heusler Weyl-candidate. By analogy with the well-studied GdPtBi system we undertake measurements intended to understand the normal state of this material, before extending our study to search for characteristics of Weyl behavior. We find a material with semiconducting properties as well as a low temperature antiferromagnetic transition below 1.25K and a Curie-Weiss paramagnetic system above. Analysis of the magnetoresistance in HoPtBi finds multiple Weyl-like characteristics, including potential chiral anomaly and anomalous Hall angle components. Finally we found significant anisotropic magnetoresistance in HoPtBi dependent on field alignment relative to the crystalline axes of the material, which is unexpected for a paramagnetic compound. We will show that these behaviors indicate a material with a Fermi surface readily tuned by application of magnetic field.
  • Thumbnail Image
    Item
    ATOMICALLY PRECISE FABRICATION AND CHARACTERIZATION OF DONOR-BASED QUANTUM DEVICES IN SILICON
    (2019) Wang, Xiqiao; Silver, Richard M; Appelbaum, Ian; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Atomically precise donor-based quantum devices in silicon are a promising candidate for scalable solid-state quantum computing and analog quantum simulation. This thesis demonstrates success in fabricating state-of-the-art silicon-phosphorus (Si:P) quantum devices with atomic precision. We present critical advances towards fabricating high-fidelity qubit circuitry for scalable quantum information processing that demands unprecedented precision and reproducibility to control and characterize precisely placed donors, electrodes, and the quantum interactions between them. We present an optimized atomically precise fabrication scheme with improved process control strategies to encapsulate scanning tunneling microscope (STM)-patterned devices and technological advancements in device registration and electrical contact formation that drastically increase the yield of atomic-precision fabrication. We present an atomic-scale characterization of monolayer step edges on Si (100) surfaces using spatially resolved scanning tunneling spectroscopy and quantitatively determine the impact of step edge density of states on the local electrostatic environment. Utilizing local band bending corrections, we report a significant band gap narrowing behavior along rebonded SB step edges on a degenerately boron-doped Si substrate. We quantify and control atomic-scale dopant movement and electrical activation in silicon phosphorus (Si:P) monolayers using room-temperature grown locking layers (LL), sputter profiling simulation, and magnetotransport measurements. We explore the impact of LL growth conditions on dopant confinement and show that the dopant segregation length can be suppressed below one Si lattice constant while maintaining good epitaxy. We demonstrate weak-localization measurement as a high-resolution, high-throughput, and non-destructive method in determining the conducting layer thickness in the sub-nanometer thickness regime. Finally, we present atomic-scale control of tunnel coupling using STM-patterned Si:P single electron transistors (SET). We demonstrate the exponential scaling of tunnel coupling down to the atomic limit by utilizing the Si (100) 2×1 surface reconstruction lattice as a natural ruler with atomic-accuracy and varying the number of lattices counts in the tunnel gaps. We analyze resonant tunneling spectroscopy through atomically precise tunnel gaps as we scale the SET islands down to the few-donor quantum dot regime. Finally, by combining single/few-donor quantum dots with atomically defined single electron transistors as charge sensors, we demonstrate single electron charge sensing in few-donor quantum dots and characterize the tunnel coupling between few-donor quantum dots and precision-aligned single electron charge sensors.
  • Thumbnail Image
    Item
    Hybridization and enhancement processes in quasi-two dimensional superconductors
    (2019) Raines, Zachary Mark; Galitski, Victor M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Superconductivity is a field with a great many branches and applications. In this dissertation, we focus on two specific processes in superconductors -- light-induced enhancement and hybridization of collective modes -- in two types of quasi-two dimensional materials -- either the loosely coupled planes of a layered superconductor or a superconducting thin film. Motivated by experiments in the cuprates that have seen evidence of a transient superconducting state upon optical excitation we study the effects of inter-plane tunneling on the competition between superconductivity and charge order. We find that an optical pump can suppress the charge order and simultaneously enhance superconductivity, due to the inherent competition between the two. Taking into account that the charge order empirically shows a broad peak in c-axis momentum, we consider a model of randomly oriented charge ordering domains and study how interlayer coupling affects the competition of this order with superconductivity. Also in the cuprates, several groups have reported observations of collective modes of the charge order present in underdoped cuprates. Motivated by these experiments, we study theoretically the oscillations of the order parameters, both in the case of pure charge order, and for charge order coexisting with superconductivity. Using a hot-spot approximation we find in the coexistence regime two Higgs modes arising from hybridization of the amplitude oscillations of the different order parameters. We explore the damping channels of these hybrid modes. As another means of enhancing superconductivity we consider coupling a two-dimensional superconducting film to the quantized electromagnetic modes of a microwave resonator cavity. We find that when the photon and quasiparticle systems are out of thermal equilibrium, a redistribution of quasiparticles into a more favorable non-equilibrium steady-state occurs, thereby enhancing superconductivity in the sample, a fluctuation analog of a phenomenon known as the Eliashberg effect. Finally, following the recent success of realizing exciton-polariton condensates in cavities, we examine the hybridization of cavity photons with two types of collective modes in superconductors. Enabled by the recently predicted and observed supercurrent-induced linear coupling between these excitations and light, we find that significant hybridization between the superconductor's collective modes and resonant cavity photons can occur.
  • Thumbnail Image
    Item
    SUBMONOLAYER ADSORBATES: THEORETICAL STUDIES OF TRANSIENT MOBILITY AND SYMMETRY-BREAKING
    (2019) Morales Cifuentes, Josue Ricardo; Einstein, Theodore L.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Weakly bound submonolayer adsorbates provide important insight into fundamental descriptions of physics that would otherwise be masked, or even suppressed, by strong effects such as chemical binding. We focus on two surface effects: transient mobility at the microscopic scale, and symmetry-breaking at the atomic one. We present a novel island nucleation and growth model that explicitly includes, at the microscopic scale, the behavior of transient (ballistic) monomers. At a deposition rate F , monomers are assumed to be in a hot precursor state before thermalizing. In the limiting regimes of fast (diffusive) and slow (ballistic) thermalization, we recover the expected scaling of the island density, N : N ∝ F^α. We construct effective growth exponents, α eff , and activation energies to properly characterize the transitional regions between these limiting regimes. Through these constructs, we describe a rich and complex structure of metastable limiting regimes, asymptotic behavior and energetically driven transitions. Application to N (F, T ) ofrecent organic-molecule deposition experiments yields excellent fits. We have also studied, at the atomic scale, an effective potential mechanism that breaks the intrinsic two-fold sublattice (hexagonal) symmetry of (honeycomb) graphene using DFT calculations (VASP ver 5.3.3). We choose the specific system of CF3Cl adsorbates on single layer graphene, to benefit from experimental results obtained locally. Using ab initio van der Waals density functionals, we discover a physisorbed phase with binding energies of about 280 meV. For low coverages, sublattice symmetry-breaking effects are responsible for gap openings of 4 meV; contrastingly, in large coverages, it is the formation of ordered overlayers that opens gaps nearly 5 times as large, of roughly 18 meV. We discover that in both cases, differentiation of graphene’s two sublattices induces symmetry-breaking by means of adsorbate interactions that favor large ordered regions, coverage itself is insignificant. For CF3Cl adsorbates on bilayer graphene, symmetry-breaking effects caused by the formation of graphene-like overlayers, and not sublattice differentiation, opened gaps of 25 meV, the largest in our study.
  • Thumbnail Image
    Item
    Pressure Tuning the Topology of Quantum Materials
    (2019) Liu, I-Lin; Paglione, Johnpierre; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Topological materials have attracted great interest in condensed matter physics because of their potential applications for topological quantum computing. Transition metal dichalcogenides are very promising topological materials due to their novel topological properties. T$_d$-MoTe$_2$ has been highlighted as potential topological superconductor and type-II Weyl semimetal with Fermi arcs and Weyl nodes through density functional theory and angle-resolved photoemission spectroscopy studies. Recently, T'-MoTe$_2$ was proposed to support a higher-order topology via first principle calculations. Pressure plays a significant role in fine tuning the ground state between noncentrosymmetric T$_d$-MoTe$_2$ and T'-MoTe$_2$ preserved lattice inversion symmetry. The corresponding topology of their Fermi surfaces are thus associated with the structural transition, superconducting, and the band structure between T'-MoTe$_2$ and T$_d$-MoTe$_2$ under pressure. This dissertation presents an experimental study of Shubnikov-de Haas oscillations, neutron scattering and first-principles calculations, demonstrating how pressure tunes the band structure, superconducting transition temperature and the first-order structural transition in MoTe$_2$. Although results from angle-resolved photoemission spectroscopy and density functional theory have previously caused controversy, this work confirms the presence of nontrivial topology of higher-order topology in T'-MoTe$_2$ via the experimental determination of a nontrivial Berry's phase. Moreover, we discover a novel phase of topological matter, deemed a Topological Interface Network (TIN) that forms from a natural heterostructure of mixed T$_d$ and T' structural phases. This new electron structure exists at the interfaces between the domains of two topological structures. Such a novel state with superconductivity and its transition between breaking and conservation of lattice inversion symmetry raises the possibility of quantum phase transitions between different types of topological superconductors. This natural microstructure can be potentially useful in topological quantum computing.
  • Thumbnail Image
    Item
    Casimir and Optical Phenomena in Two-dimensional Systems
    (2019) Allocca, Andrew Anthony; Galitski, Victor; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The nature of the interaction of light with matter is a long-standing subject of great interest in condensed matter physics. Here I study the behavior of three electromagnetic effects arising from the coupling of light to two-dimensional electron systems: the Casimir effect, excitons in an insulator, and the formation of polaritons in a cavity. I begin by examining how the Casimir effect is affected by material properties. First we consider using the Casimir force as a probe of a change in the topology of a material's Fermi surface called a Lifshitz transition. Specifically, I study a spin-orbit coupled semiconducting system, which can be made to undergo this sort of transition with an external magnetic field, and find that the signature of this transition is a non-analyticity in the Casimir force at the transition point. I next consider how the phenomenon of weak localization can be used as a test of the role of disorder in the Casimir effect between metallic objects. I show how the sensitive dependence of the conductivity of a two-dimensional disordered metal on both temperature and magnetic field should translate into similar sensitivities of the Casimir force, assuming effects of disorder should be included at all. Next, I examine excitons formed in the bulk of an insulator as a system transitions between topological and trivial insulating phases, finding that the phases have different signatures in the exciton spectrum. This can be understood as an effect of the Berry curvature of the model, giving an indirect glimpse of topological properties. I construct a semiclassical model of the system to develop a qualitative intuition, then move to a numerical calculation in a full quantum model. Finally, I consider the formation of polaritons inside of a photonic cavity containing a two-dimensional superconducting layer. I show how a coupling can be engineered between cavity photons resonant with a collective mode of the superconductor called the Bardasis-Schrieffer mode, leading to hybridized superconductor polariton states. Motivated by exciton polariton condensates, I conjecture that a phase-coherent density of these objects could produce an exotic $s\pm id$ superconducting state.
  • Thumbnail Image
    Item
    SUPERCONDUCTORS THAT BREAK TIME-REVERSAL SYMMETRY
    (2019) Boyer, Lance L.; Yakovenko, Victor M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Since 2006 it has been discovered experimentally that the superconducting state spontaneously breaks time-reversal symmetry (TRS) in several materials, such as Sr2RuO4, UPt3, URu2Si2, PrOs4Sb12, and Bi/Ni bilayers. This dissertation studies three physical phenomena related to time-reversal symmetry breaking (TRSB) in these superconductors. The experimental evidence for TRSB comes from the magneto-optical polar Kerr effect, which is determined by the high frequency ac Hall conductivity. However, these superconductors are also expected to exhibit a spontaneous dc Hall effect in the absence of an applied magnetic field. In the first part of this dissertation we propose a method for measuring the low frequency Hall conductivity in superconductors with TRSB. The method is based on a Corbino disk geometry where an oscillating co-axial magnetic field induces circular electric field, which, in turn, induces radial charge oscillations due to the Hall conductivity. In the second part, we propose an explanation for the polar Kerr effect observed in the Hidden-Order phase of the heavy-fermion superconductor URu2Si2. Using a Ginzburg-Landau model for a complex order parameter, we show that the system can have a metastable ferromagnetic state, which produces the Kerr signal, even if the Hidden-Order state respects TRS. We predict that applying a reversed magnetic field should reset the system to the non-magnetic ground state, resulting in zero Kerr signal. In the third part of the dissertation, we investigate the conditions for the existence of a Majorana bound state on a vortex in a 2D d+id superconductor with strong spin-orbit coupling. This TRSB pairing was proposed earlier for the Ni/Bi bilayer. We find that the Majorana bound state can exist for a d+id pairing under conditions similar to those for s-wave pairing.
  • Thumbnail Image
    Item
    SIGNATURE OF MAJORANA MODES AND ASPECTS OF THEIR BRAIDING
    (2018) Nag, Amit; Sau, Jay Deep; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Majorana zero modes are emergent zero-energy quasiparticle excitations in certain superconducting systems that can be viewed as fractionalized or “half” electrons. These quasiparticles obey non-Abelian braiding statistics which is one manifestation of such half-electron character. Due to such non-Abelian braiding property, Majorana zero mode pairs hold promise as potential qubits for topological quantum computation. It is somewhat surprising that, at least theoretically, ordinary one-dimensional semiconductor systems can be induced to host such esoteric Majorana modes as edge states if some precise experimental conditions are satisfied. Because of the relative simplicity of material and experimental requirements to host Majorana modes, there has been a flurry of experimental effort to realize them in semiconductor nanowire systems. While experimental efforts have produced preliminary evidence for the presence of Majorana zero modes in these systems, a thorough confirmation is lacking. The experimental signature in question is the presence of a zero-bias conductance-peak that, while necessary, is not a sufficient criterion to establish the presence of underlying Majorana modes. Given the importance of Majorana braiding for topological quantum computation and skepticism over the presence of Majorana modes in these experimental systems, it would seem natural to attempt braiding these putative Majorana modes in the near future. In that case, an observation of non-Abelian statistics would provide the necessary and sufficient condition in favor of Majorana presence in the studied experimental systems. This thesis has three distinct parts. First, we assume perfect Majorana modes as given that can be successfully braided. In this case, we calculate the diabatic error due to the finite speed of braiding when the system is coupled to a Bosonic bath. Next, we grant that the mechanism for zero-bias conductance-peak is indeed topological, albeit the underlying Majorana modes may be imperfect (the modes are not precisely at zero energy). We study the interplay of dissipation and finite energy splitting of the Majorana modes and study its consequence regarding the probability of successful braiding. Lastly, we propose studying the correlation between independent left and right conductance measurements as a means to distinguish between a topological versus a non-topological the mechanism underlying the observed zero-bias conductance-peak.
  • Thumbnail Image
    Item
    MAJORANA AND ANDREEV BOUND STATES IN SEMICONDUCTOR-SUPERCONDUCTOR NANOSTRUCTURES
    (2018) Liu, Chun-Xiao; Sau, Jay D; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Majorana bound states have been a topic of active research over the last decades. In the perspective of theoretical physics, Majorana bound states, which are their own antiparticles, are zero-energy quasi-particle excitations in exotic superconductive systems. From a technological perspective, Majorana bound states can be utilized for the implementation of fault-tolerant quantum computation due to their topological properties. For example, two well-separated Majorana bound states can form a fermionic qubit state, the quantum information of its occupancy is stored in a nonlocal way, being robust against local decoherence. Also since Majorana bound states obey non-Abelian statistics, quantum gates can be implemented by braiding. Such gate operations are robust because small deviations in braiding trajectories do not affect the braiding results. So far the most promising platform for the realization of Majorana bound states is the one-dimensional semiconductor-superconductor nanostructures. The hallmark of the existence of Majorana bound states in such systems is a quantized zero-bias conductance peak in the tunneling spectroscopy for a normal-metal-superconductor junction. Although quantized zero-bias conductance peaks that resemble the theoretical prediction have been observed in several experimental measurements, confusing aspects of the data muddy the conclusion. One source of confusion results from the existence of another type of excitation in these systems, i.e., the topologically trivial near-zero-energy Andreev bound states. These excitations mimic many behaviors of the topological Majorana bound states. In this thesis, we first investigate the tunnel spectrsocopy signatures of both Majorana and Andreev bound states. Then we discuss multiple proposals for differentiating between Majorana and Andreev bound states. In Chapter 1, we give an overview for Majorana bound states in the context of both spinless p-wave superconductors and spin-orbit coupled nanowires in proximity with an s-wave superconductor. We also show how the existence of a zero-energy Majorana bound state leads to a quantized zero-bias conductance peak in tunneling spectroscopy at zero temperature. In Chapter 2, we discuss possible physical mechanisms responsible for the discrepancy between minimal theory of Majorana nanowire and real experimental observations. Specifically, we focus on the effect of dissipation inside the heterostructure. In Chapter 3, we show that a near-zero-energy Andreev bound state may arise quite generically in the semiconductor-superconductor nanowire in the presence of a smooth variation in chemical potential. Although such Andreev bound states are topologically trivial, they mimic the behaviors of the topological Majorana bound states in many aspects. In Chapter 4, we discuss multiple proposals for distinguishing between trivial Andreev bound states and topological Majorana bound states in the normal-metal-superconductor junction. In Chapter 5, we discuss a proposal for future experiments, i.e., a normal-superconductor-normal junction for a Coulomb blockaded superconductor. In this proposal, one can directly measure the topological invariant of the bulk superconductor. Finally Chapter 6 concludes the thesis.