Condensed Matter Physics Commons^™

Dielectric Function Tensor (1.5 Ev To 9.0 Ev), Anisotropy, And Band To Band Transitions Of Monoclinic Β-(AlXGa1–X)2O3 (X ≤ 0.21) Films, Matthew Hilfiker, Ufuk Kilic, Alyssa Mock, Vanya Darakchieva, Sean Knight, Rafal Korlacki, Akhil Mauze, Yuewei Zhang, James Speck, Mathias Schubert

Department of Electrical and Computer Engineering: Faculty Publications

A set of monoclinic β-(Al_xGa_1–x)₂O₃ films coherently grown by plasma-assisted molecular beam epitaxy onto (010)-oriented β-Ga₂O₃ substrates for compositions x ≤ 0.21 is investigated by generalized spectroscopic ellipsometry at room temperature in the spectral range of 1.5 eV–9.0 eV. We present the composition dependence of the excitonic and band to band transition energy parameters using a previously described eigendielectric summation approach for β-Ga₂O₃ from the study by Mock et al. All energies shift to a shorter wavelength with the increasing Al content in …

Voltage Controlled Magnetism In Cr2o3 Based All-Thin-Film Systems, Junlei Wang, Will Echtenkamp, Ather Mahmood, Christian Binek

Christian Binek Publications

Voltage-control of exchange biases through active selection of distinct domain states of the magnetoelectric and antiferromagnetic pinning layer is demonstrated for Cr2O3/CoPd heterostructures. Progress and obstacles towards an isothermal switching of exchange bias are discussed. An alternative approach avoiding exchange bias for voltage-controlled memory exploits boundary magnetization at the surface of Cr2O3 as voltage-controlled state variable. We demonstrate readout and switching of boundary magnetization in ultra-thin Cr2O3/Pt Hall bar devices where reversal of boundary magnetization is achieved via magnetoelectric annealing with simultaneously applied ±0.5 V and 400 mT electric and magnetic fields.

New Heusler Compounds In Ni-Mn-In And Ni-Mn-Sn Alloys, Xingzhong Li, W.-Y. Zhang, Shah R. Valloppilly, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Rapidly quenched ternary Ni-Mn-T (T = In, Sn) alloys exhibit features associated with magnetic skyrmions, so that XRD, TEM, EDS, SAED and HREM investigations were carried out for structural characterization on the two alloy systems. In this paper, we report a new type of Mn-rich Heusler compound with a cubic unit cell, a = 0.9150 nm in Ni-Mn-In and a = 0.9051 nm in Ni-Mn-Sn, which coexist with a Ni-rich full-Heusler compound with defects, a = 0.6094 nm in Ni-Mn-In and a = 0.6034 nm in Ni-Mn-Sn. A further analysis of the experimental results reveals a close structural relationship between …

Magnetization Dynamics In Coupled Thin Film Systems, Daniel J. Adams

LSU New Orleans Theses and Dissertations

A study is presented detailing experimental investigations of magnetization dynamics in nanostructured systems which are coupled magnetically. This work seeks to characterize the anisotropy of such systems through experimental techniques which probe microwave resonant absorption in the materials.

A custom-built experimental setup, designed and assembled in our labs, is explained in detail. This setup allows for angular-dependent ferromagnetic resonance (FMR) measurements in the sample plane through vector network analyzer spectroscopy and is adaptable to two different types of coplanar waveguides. This technique has proven effective for characterization of multiple types of magnetic systems, including multilayered structures as detailed here, with …

Space-Charge Limited Conduction In Epitaxial Chromia Films Grown On Elemental And Oxide-Based Metallic Substrates, C.-P. Kwan, Mike Street, Ather Mahmood, Will Echtenkamp, M. Randle, K. He, J. Nathawat, N. Arabchigavkani, B. Barut, S. Yin, R. Dixit, Uttam Singisetti, Christian Binek, J. P. Bird

Christian Binek Publications

We study temperature dependent (200 – 400 K) dielectric current leakage in high-quality, epitaxial chromia films, synthesized on various conductive substrates (Pd, Pt and V2O3). We find that trap-assisted space-charge limited conduction is the dominant source of electrical leakage in the films, and that the density and distribution of charge traps within them is strongly dependent upon the choice of the underlying substrate. Pd-based chromia is found to exhibit leakage consistent with the presence of deep, discrete traps, a characteristic that is related to the known properties of twinning defects in the material. The Pt- and V2O3-based films, in contrast, …

Nonlinear Optical Studies Of Bulk And Thin Film Complex Materials, Joel E. Taylor

LSU Doctoral Dissertations

Nonlinear optical studies of bulk and thin film materials provide a vast playground for physical and dynamical characterization. In this thesis, we have implemented experimental methods to probe novel phase transitions in single crystals using rotational anisotropic second harmonic generation (RASHG) and carrier dynamics in thin films with time-resolved pump-probe reflectivity. Furthermore, a novel low temperature ultra-high vacuum system coupled to nonlinear optics has been developed to extend lab capabilities. Doping (Bi_1-xSb_x)₂Se₃ with antimony, the surface electronic reconstruction near x=80% was identified with RASHG by deviations in the six-fold and three-fold polarization anisotropic …

Nuclear Magnetic Resonance Characterization Of Dynamics In Novel Electrochemical Materials, Christopher T. Mallia

Theses and Dissertations

As our daily use of electronics and electronic technology grows, so does the societal need for sustainable, renewable and portable electrical power. To this end, materials of interest in the electrochemical world are needed to advance the frontier of battery science and energy storage technologies so that a safer, more efficient and reliable electrical future can be realized. This work focuses on characterization of materials primarily of interest for use as electrolytes in rechargeable Lithium-Ion Batteries (LIBs). Despite their extraordinary power, LIB application in certain fields, such as in electric vehicles, has been limited due to performance and safety concerns. …

Toward Devices For Exploring Pt-Symmetry In Electronic Transport Of Graphene, Michael Carovillano

Senior Honors Papers / Undergraduate Theses

Parity-time symmetry, or PT -symmetry is the principle that in quantum mechanics a non- Hermitian Hamiltonian is capable of returning real eigenstates and real spectra.Recent research has demonstrated real world observation of PT -symmetry in electronics and optics. We aim to expand the regime of observed PT -symmetry through measurement of the electronic transport of graphene devices. Drawing from analogous experiments, we plan to use balanced ohmic resistance acting as both loss and relative gain to induce the required unbroken PT -symmetry regime. This paper analyzes techniques used in fabrication of such devices as well as the basis of the …

Electron Yield Measurements Of Multilayer Conductive Materials, Gregory Wilson, Matthew Robertson, Jordan Lee, Jr Dennison

Conference Proceedings

As energetic electrons interact with the surface of materials, they impart energy throughout the material. If the energy exchange is near the surface, secondary electrons within the material can be excited and emitted. It is also possible for the incident primary electron to undergo a quasi-elastic collision within the material, wherein the electron is backscattered and emitted from the surface. As the backscattered electron is leaving the material, it can continue to impart energy to the material, potentially exciting more secondary electrons as it approaches the surface on the way back out.

This process of imparting energy and charge to …

Strategies For Determining Electron Yield Material Parameters For Spacecraft Charge Modeling, Phil Lundgreen, Jr Dennison

Conference Proceedings

Modeling of space plasma environment-induced anomalies requires knowledge of:

• Environment and impinging fluxes during spacecraft orbits, which are mission specific and can be incorporated through environmental models and databases.
• Satellite geometry and orientation in the space environment, accomplished through such charging codes as NASCAP-2K, SPENVIS, or MUSCAT.
• Materials used in spacecraft construction, from the specific spacecraft design.
• Relevant materials properties characterizing the interaction of the materials with the environment and how these properties may change with exposure to the space environment.

A reliable, comprehensive database of spacecraft materials and the characterization of those materials is being created in the form …

Suppresion Of Electron Yield With Carbon Nanotube Forests: A Case Study, Brian Wood, Jordan Lee, Gregory Wilson, T.-C. Shen, Jr Dennison

Conference Proceedings

Electron emission of carbon nanotube (CNT) forests grown on silicon substrates was measured to investigate possible electron yield suppression due to the composition and morphology of CNT forests. CNT forests are vertically-oriented tubular formations of graphitic carbon grown on a substrate; these have been widely investigated for their extreme properties in optical, electrical, and mechanical aspects of physics and material sciences. CNT coatings are good candidates for yield reduction, in analogy with the near-ideal blackbody optical properties of CNT forests. Carbon with its low atomic number has an inherent low yield due to its low density of bulk electrons. Furthermore, …

Influence Of Vibrationally-Induced Structural Changes On Carbon Nanotube Forests Suppression Of Electron Yield, Jordan Lee, Brian Wood, Gregory Wilson, T.-C. Shen, Jr Dennison

Conference Proceedings

Carbon nanotube (CNT) forest coatings have been found to lower electron yield from material surfaces. The suppressed yields have been attributed to both the lower inherent yields of low-atomic number carbon and the enhanced electron recapture resulting from the morphology of the carbon layer. To explore the relative contributions of these two causes of yield suppression, tests have been made on CNT forest-coated conducting substrate samples subjected to vibrationally-induced changes of the coating structure. The extent of vibrationally-induced structural changes—due, for example, to shear-force conditions during space-vehicle transit—are of interest, as CNT have been a frequent topic of scientific curiosity …

Exploring The Electrical Properties Of Twisted Bilayer Graphene, William Shannon

Senior Theses

Two-dimensional materials exhibit properties unlike anything else seen in conventional substances. Electrons in these materials are confined to move only in the plane. In order to explore the effects of these materials, we have built apparatus and refined procedures with which to create two-dimensional structures. Two-dimensional devices have been made using exfoliated graphene and placed on gold contacts. Their topography has been observed using Atomic Force Microscopy (AFM) confirming samples with monolayer, bilayer, and twisted bilayer structure. Relative work functions of each have been measured using Kelvin Probe Force Microscopy (KPFM) showing that twisted bilayer graphene has a surface potential …

2d Materials Based Heterostructures : A Lithography Free Method., Adel Alruqi

Electronic Theses and Dissertations

Many properties of Two-dimensional (2D) materials are vastly different from those of their 3D counterparts. A large family of 2D materials ranging from gapless graphene to metallic NbSe2 (also superconducting), semiconducting MoS2, and insulating hexagonal boron nitride (h-BN) possessing a broad range of exciting new properties have emerged in recent years. Moreover, 2D materials provide the perfect platform to create laterally and vertically stacked heterostructures with intriguing properties. The physics of 2D materials based heterostructures is extremely interesting and novel 2D-heterostructured devices including tunneling diodes, tunneling transistors, photovoltaic cells, and light-emitting diodes have started to emerge. In this work, we …

Super Rapid Crystal Growth And Quench Of Monoclinic Bi-Ii* During Dynamic Compression, Zachary Allen Fussell

UNLV Theses, Dissertations, Professional Papers, and Capstones

We show that monoclinic Bi-II* forms during a dynamic compression regime with crystal growth rates from melt of ≈ 70 m/s. This extreme quench rate implies crystallization by non-diffusive processes and indicates that the liquid had a high degree of pre-ordering. Using ambient condition single crystal structure analysis we show for the first time that the monoclinic distorted phase of Bi (Bi-II) exists at ambient pressure, albeit bound to formation under dynamic compression. We review the pressure, temperature, and time conditions for formation and growth of this structure.

High-Pressure High-Temperature Exploration Of Phase Boundaries Using Raman Spectroscopy, Jasmine Kashmir Hinton

UNLV Theses, Dissertations, Professional Papers, and Capstones

Metastability of states can provide interesting properties that may not be readily accessible in a material’s ground state. Many materials show high levels of polymorphism, indicating a rich energy landscape and a potential for metastable states. Melt crystallization techniques provide a potential route to these states. We use a resistively heated diamond anvil cell (DAC) with fine control of a system’s pressure and temperature to explore these systems. Raman spectroscopy is used to track subtle structural changes across phase boundaries. Organic systems, such as glycine and aspirin, were our initial interest due to their high polymorphism and reported low melting …

Equation Of State Of H2o Ice Using Melt-Recrystallization, Zachary Michael Grande

UNLV Theses, Dissertations, Professional Papers, and Capstones

The recent surge in exoplanet discoveries due to advancements in astrophysical technology and analysis has brought the reliability of early equation of state measurements into question as they are the limiting factor when modeling composition of these planets. H2O content is among the most important for the search of habitable planets as well as in understanding planetary dynamics and atmosphere formation. Over the last three decades the equation of state of H2O has been investigated with various techniques but, has suffered from anisotropic strain and poor powder statistics resulting in a large discrepancy in equation of state fits. At pressures …

Specific Heat, Magnetic Susceptibility, And The Effect Of Pressure On Structural Properties And Valence Of Eumn2si2, Euco2si2, And Eu5in2sb6, Brian Edward Light

UNLV Theses, Dissertations, Professional Papers, and Capstones

Many intermetallic solids containing elements from the rare earth series show interesting and unusual behavior associated with 4f electrons. This behavior includes unusual magnetic order, strongly correlated electrons, intermediate valence, heavy fermions, the Kondo effect, superconductivity, and non-Fermi liquid (NFL) to name a few. When long range magnetic order is suppressed to T = 0 K by the application of an external tuning parameter such as pressure, magnetic field, or chemical doping, a quantum critical point (QCP) appears in which strong quantum fluctuations give rise to many of the mentioned unusual properties.

Most of the past studies on unusual 4f …

Lithium Molybdate-Sulfur Battery., Ruchira Ravinath Dharmasena

Electronic Theses and Dissertations

Rechargeable energy storage systems play a vital role in today’s automobile industry with the emergence of electric vehicles (EVs). In order to meet the targets set by the department of energy (DOE), there is an immediate need of new battery chemistries with higher energy density than the current Li- ion technology. Lithium–sulfur (Li–S) batteries have attracted enormous attention in the energy-storage, due to their high specific energy density of 2600 Wh kg^-1 and operational voltage of 2.0 V. Despite the promising electrochemical characteristics, Li-S batteries suffer from serious technical challenges such as dissolution of polysulfides Li₂S_{x …}

Topological Insulating States In Photonics And Acoustics, Xiang Ni

Dissertations, Theses, and Capstone Projects

Recent surge of interest in topological insulators, insulating in their interior but conducting at the surfaces or interfaces of different domains, has led to the discovery of a variety of new topological states, and their topological invariants are characterized by numerous approaches in the category of topological band theory. The common features shared by topological insulators include, the topological phase transition occurs if the bulk bandgap is formed due to the symmetries reduction, the topological invariants exist characterizing the global properties of the material and inherently robust to disorder and continuous perturbations irrespective of the local details. Most importantly, these …

Anomalous Hall Conductivity Of Noncollinear Magnetic Antiperovskites, Gautam Gurung, Ding-Fu Shao, Tula R. Paudel, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

The anomalous Hall effect (AHE) is a well-known fundamental property of ferromagnetic metals, commonly associated with the presence of a net magnetization. Recently, an AHE has been discovered in noncollinear antiferromagnetic (AFM) metals. Driven by nonvanishing Berry curvature of AFM materials with certain magnetic space-group symmetry, anomalous Hall conductivity (AHC) is very sensitive to the specific type of magnetic ordering. Here, we investigate the appearance of AHC in antiperovskite materials family ANMn₃ (A = Ga, Sn, Ni), where different types of noncollinear magnetic ordering can emerge. Using symmetry analyses and first-principles density-functional theory calculations, we show that with almost …

Origin Of Enhanced Anisotropy In Fept-C Granular Films Revealed By Xmcd, Robert Streubel, Alpha T. N'Diaye, Kumar Srinivasan, Antony Ajan, Peter Fischer

Robert Streubel Papers

We study the effect of carbon segregants on the spin and orbital moments of L10 FePt granular media using x-ray magnetic circular dichroism (XMCD) spectroscopy and report an effective decoupling of the structural film properties from the magnetic parameters of the grains. The carbon concentration reduces the grain size from (200 ± 160) nm2 down to (50 ± 20) nm2 for 40 mol. %C and improves sphericity and the order of grains, while preserving the crystalline order, spin and orbital moments, and perpendicular magnetocrystalline anisotropy. We identify the primary cause of enhanced saturation and coercive fields as the reduced demagnetization …

Improving The Readout Of Semiconducting Qubits, Matthew Jon Curry

Physics & Astronomy ETDs

Semiconducting qubits are a promising platform for quantum computers. In particular, silicon spin qubits have made a number of advancements recently including long coherence times, high-fidelity single-qubit gates, two-qubit gates, and high-fidelity readout. However, all operations likely require improvement in fidelity and speed, if possible, to realize a quantum computer.

Readout fidelity and speed, in general, are limited by circuit challenges centered on extracting low signal from a device in a dilution refrigerator connected to room temperature amplifiers by long coaxial cables with relatively high capacitance. Readout fidelity specifically is limited by the time it takes to reliably distinguish qubit …

Phase Evolution And Dynamic Behavior In Materials With Noncollinear Spin Textures, Eleanor M. Clements

USF Tampa Graduate Theses and Dissertations

Noncentrosymmetric magnetic materials have gained special attention due to their ability to stabilize topologically nontrivial magnetic states via the competition between symmetric exchange and the antisymmetric Dzyaloshinskii-Moriya (DM) interaction. The spin struc- tures in these materials have become a center of interest for spintronics applications due to their stable, particle-like properties, and high degree of tunability via control of external parameters, such as magnetic and/or electric field and temperature. Understanding how these robust magnetic structures stabilize, evolve, dynamically respond, and adhere to existing models, all in the presence of external stimuli, are topics of fundamental interest. In this dissertation, the …

3d Imaging Of A Binary Colloidal Fluid, Kate Livermore, Michael Korth

Undergraduate Research Symposium 2019

In order to understand the nature of colloidal fluids, we wanted to visualize a 3D representation of their thermodynamic distribution. Colloidal fluids are fluids that consist of two parts: colloidal particles and a dispensing medium. Some examples include toothpaste, mayonnaise, plaster, and muddy water. We could also think about colloidal fluids simply as a mixture of two or more parts. In this poster, we used the hard-sphere system to represent the structure of colloidal fluids in a 3D graph using Mathematica (MMA). In the hard-sphere system, we used two different sizes of impenetrable spheres that could not overlap to represent …

Magnetization Reversal And Local Switching Fields Of Ferromagnetic Co/Pd Microtubes With Radial Magnetization, Norbert Puwenberg, Christopher F. Reiche, Robert Streubel, Mishal Khan, Dipankar Mukherjee, Ivan V. Soldatov, Michael Melzer, Oliver G. Schmidt, Bernd Büchner, Thomas Mühl

Robert Streubel Papers

Three-dimensional nanomagnetism is a rapidly growing field of research covering both noncollinear spin textures and curved magnetic geometries including microtubular structures. We spatially resolve the field-induced magnetization reversal of free-standing ferromagnetic microtubes utilizing multifrequency magnetic force microscopy (MFM). The microtubes are composed of Co/Pd multilayer films with perpendicular magnetic anisotropy that translates to an anisotropy with radial easy axis upon rolling-up. Simultaneously mapping the topography and the perpendicular magnetostatic force derivative, the relation between surface angle and local magnetization configuration is evaluated for a large number of locations with slopes exceeding 45 degrees. The angle-dependence of the switching field is …

Valley-Dependent Lorentz Force And Aharonov-Bohm Phase In Strained Graphene P-N Junction, Sanjay Prabhakar, Rabindra Nepal, Roderick Melnik, Alexey Kovalev

Department of Physics and Astronomy: Faculty Publications

Veselago lens focusing in graphene p−n junction is promising for realizations of new generation electron optics devices. However, the effect of the strain-induced Aharonov-Bohm interference in a p−n junction has not been discussed before. We provide an experimentally feasible setup based on the Veselago lens in which the presence of strain can result in both the valley-dependent Lorentz force and Aharonov-Bohm interference. In particular, by employing the Green's function and tight-binding methods, we study the strain induced by dislocations and line defects in a p−n junction and show how the resulting Aharonov-Bohm phase and interference can be detected. Furthermore, for …

Ordered Growth Of Ferroelectric Diisopropylammonium-Bromide Microcrystals Through Slotted-Jar Growth And Lithographically Controlled Wetting, Andrew J. Fanning

Honors Program: Senior Projects (Public)

Organic molecular ferroelectrics show promise for industry applications because of their switchable high spontaneous polarization value, mechanical flexibility, and cost-effectiveness. Since these materials, namely diisopropylammonium bromide, exhibit ferroelectricity only in tandem with a high level of crystallinity, novel methods must be explored in order to ensure that high levels of crystallinity are achieved. This project seeked to perfect the methods of Slotted Jar Growth and Lithographically Controlled Wetting (LCW). Slotted Jar Growth uses temperature driven solution saturation to grow crystals on a desired substrate. LCW drives the growth of microscopic diisopropylammonium bromide crystals, in their ferroelectric phase, through the use …

X-Ray Ptychography On Low-Dimensional Hard-Condensed Matter Materials, Xiaowen Shi, Nicolas Burdet, Bo Chen, Gang Xiong, Robert Streubel, Ross Harder, Ian K. Robinson

Robert Streubel Papers

Tailoring structural, chemical, and electronic (dis-)order in heterogeneous media is one of the transformative opportunities to enable new functionalities and sciences in energy and quantum materials. This endeavor requires elemental, chemical, and magnetic sensitivities at the nano/atomic scale in two- and three-dimensional space. Soft X-ray radiation and hard X-ray radiation provided by synchrotron facilities have emerged as standard characterization probes owing to their inherent element-specificity and high intensity. One of the most promising methods in view of sensitivity and spatial resolution is coherent diffraction imaging, namely, X-ray ptychography, which is envisioned to take on the dominance of electron imaging techniques …

Dirac Nodal Line Metal For Topological Antiferromagnetic Spintronics, Ding-Fu Shao, Gautam Gurung, Shu-Hui Zhang, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which exploits the N´eel vector to control the topological electronic states and the associated spin-dependent transport properties. A recently discovered N´eel spin-orbit torque has been proposed to electrically manipulate Dirac band crossings in antiferromagnets; however, a reliable AFM material to realize these properties in practice is missing. In this Letter, we predict that room-temperature AFM metal MnPd₂ allows the electrical control of the Dirac nodal line by the N´eel spin-orbit torque. Based on first-principles density functional theory calculations, we show that reorientation of the N´eel vector leads to switching …