Condensed Matter Physics Commons^™

Magnetic Properties Of Nb/Ni Superconducting / Ferromagnetic Multilayers, Sergiy A. Kryukov

Theses and Dissertations--Physics and Astronomy

Magnetic properties of Nb/Ni superconducting (SC) / ferromagnetic (FM) multilayers exhibit interesting properties near and below SC transition. A complex Field (H) – Temperature (T) phase boundary is observed in perpendicular and parallel orientation of ML with respect to DC field. We address the critical need to develop methods to make reliable magnetic measurements on SC thin films and ML, in spite of their extreme shape anisotropy and the strong diamagnetic response of the SC state.

Abrupt, highly reproducible “switching” of the SC state magnetization near the normal-state FM coercive fields has been observed in Nb/Ni ML. The SC penetration …

Two-Dimensional Electron Gas At Oxide Interfaces, Karolina Janicka

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Extraordinary phenomena can occur at the interface between two oxide materials. A spectacular example is a formation of a two-dimensional electron gas (2DEG) at the SrTiO₃/LaAlO₃ interface. In this dissertation the properties of the 2DEG are investigated from first principles.

The spatial extent of the 2DEG formed at the SrTiO₃/LaAlO₃ n-type interface is studied. It is shown that the confinement of the 2DEG is controlled by metal induced gap states formed in the band gap of SrTiO₃. The confinement width is then determined by the attenuation length of the metal induced gap …

Electrical Detection Of Mechanical Resonance In Nanotubes And Semiconducting Nanowires, Doyl Dickel

All Dissertations

In recent years, there has been substantial interest in the development of microelectro-mechanical systems (MEMS) and even nanoelectro-mechanical systems (NEMS) for use in a wide variety of applications both as experimental tools (refs) and in a continuing effort to decrease the size and cost and increase the efficiency of electrical components. In particular, cantilevered nanometer beams have been a recent focus due to a number of interesting properties, including enhanced field emission, high tensile strength, and piezoelectric properties. The ability to accurately determine the electrical and mechanical properties of these cantilevers is paramount in assessing their feasibility as MEMS and …

First-Principles Studies On Physical And Chemical Properties Of Nanostructures, Menghao Wu

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The physical and chemical properties of decorated graphene and graphene ribbons, single-layer III-V systems, three-dimensional carbon and BN foam, and transition-metal-molecular sandwich nanowires have been investigated by first-principle calculations and their potential applications have been predicted. First, it is shown that zigzag graphene nanoribbons (ZGNRs) can be converted into half metal when their edges are decorated by some chemical functional groups, and the half-metalicity is induced by chemical potential difference between two edges when one edge is decorated by electron-donating group like –OH and the other edge is decorated by electron-accepting group like –F, -NH₂, -N(CH₃) …

Spectroscopic Studies Of Melamine At High Pressure, Martin Donald Galley

UNLV Theses, Dissertations, Professional Papers, and Capstones

We have performed mid- and far- Infra Red (IR) absorption, Raman spectroscopy, and angular dispersive x-ray diffraction (XRD) studies on melamine under high pressure and room temperature. We have verified the presence of two prior reported phase transitions, the first between 1-2 GPa, and the second between 7-9 GPa. We have also found evidence of a third unreported phase transition between 14-16 GPa, during which, there was a sudden disappearance of all low energy peaks (^-1 ) in both the Raman and IR spectra. The far-IR peak movement experiences a discontinuity as the rate of peak movement suddenly changes. …

Thermodynamic Limit To Photonic-Plasmonic Light-Trapping In Thin Films On Metals, Eric A. Schiff

Physics - All Scholarship

We calculate the maximum optical absorptance enhancements in thin semiconductor films on metals due to structures that diffuse light and couple it to surface plasmon polaritons. The calculations can be used to estimate plasmonic effects on light-trapping in solar cells. The calculations are based on the statistical distribution of energy in the electromagnetic modes of the structure, which include surface plasmon polariton modes at the metal interface as well as the trapped waveguide modes in the film. The enhancement has the form 4n²+nλ/h (n – film refractive index, λ – optical wavelength, h …

Wave-Function Functionals For The Density, Marlina Slamet, Xiao-Yin Pan, Viraht Sahni

Publications and Research

We extend the idea of the constrained-search variational method for the construction of wave-function functionals psi[chi] of functions chi. The search is constrained to those functions chi such that psi[chi] reproduces the density rho(r) while simultaneously leading to an upper bound to the energy. The functionals are thereby normalized and automatically satisfy the electron-nucleus coalescence condition. The functionals psi[chi] are also constructed to satisfy the electron-electron coalescence condition. The method is applied to the ground state of the helium atom to construct functionals psi[chi] that reproduce the density as given by the Kinoshita correlated wave function. The expectation of single-particle …

Thermodynamics Of Magnetic Multilayers, Tathagata Mukherjee

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Our interest in thermodynamics of magnetic thin film heterostructure began by exploring the possibility to use magnetic nanostructures in the search for optimized magnetocaloric materials for potential room temperature refrigeration. In the present thesis magnetic thin film heterostructures are experimentally realized by Molecular Beam Epitaxy (MBE) and Pulsed Laser Deposition (PLD). Co/Cr and Fe/Cr superlattices were fabricated using mean-field theoretical concepts as guiding principles. The potential of artificial antiferromagnets for near room-temperature refrigeration is explored. Magnetocaloric properties are deduced from measurements of the temperature and field dependence of the magnetization of our samples. The effects of intra-plane and inter-plane exchange …

Self Assembly And Interface Chemistry Of Non-Metallated Tetraphenyl Porphyrin, Geoffrey Rojas

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The study of the electronic properties and geometrical arrangement of 5, 10, 15, 20-tetraphenyl-21H, 23H-porphine on metal is presented. The systems were analyzed using both scanning tunneling microscopy and photoelectron spectroscopy and compared across surfaces to determine how the interface chemistry between the metal and molecule affect the self-assembly and band structure of the adsorbed species. The molecules are found to self-assemble and grow on the Ag(111) surface in a manner described by similar models to weakly bound metal/metal surface systems. The CH-pi bonds between molecules are found to largely determine the relative inter-molecular arrangement, while the more isotropic van …

Functional Two-Dimensional Electronic Gases At Interfaces Of Oxide Heterostructures, Yong Wang

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

A quasi-two dimensional electron gas (2DEG) in oxide heterostructures such as LaAlO₃/SrTiO₃ has unique properties that are promising for applications in all-oxide electronic devices. In this dissertation, we focus on understanding and predicting novel properties of the 2DEG by performing first-principles electronic calculations within the frame work of density-functional theory (DFT).

The effects of polarization in all-oxide heterostructures incorporating different ferroelectric constituents, such as KNbO₃/ATiO₃ (A = Sr, Ba, Pb), are investigated. It is found that screening charge at the interface that counteracts the depolarizing electric field in the ferroelectric material significantly changes the …

Prba2(Cu1-Xmx)3o7 (X=0.2, M= Ga, Al, Fe, Co, Ni, And Zn) Epitaxial Thin Films, Hom Raj Kandel

Theses and Dissertations

Epitaxial thin film growth and characterization of cuprate oxide insulating materials has been motivated by scientific importance and technological need of these materials for various applications including the nanofabrication of high-Tc trilayer Josephson junction and electronics. For this purpose, we fabricated epitaxial and device-quality (110)-oriented PrBa2(Cu1-xMx)3O7 (x=0.2, M= Ga, Al, Fe, Co, Ni, and Zn) (PBCMO) thin films using excimer pulsed laser deposition technique and conducted systematic studies of the effects of various metals substitution for copper atoms at two different sites of the PrBa2Cu3O7 (PBCO) structure using various characterization techniques including the x-ray diffraction, atomic force microscopy, temperature dependent …

Domain Size And Structure In Exchange Coupled [Co/Pt]/Nio/[Co/Pt] Multilayers, Andrew G. Baruth, Shireen Adenwalla

Shireen Adenwalla Papers

We investigate the competing effects of interlayer exchange coupling and magnetostatic coupling in the magnetic heterostructure ([Co/Pt]/NiO/[Co/Pt]) with perpendicular magnetic anisotropy (PMA). This particular heterostructure is unique among coupled materials with PMA in directly exhibiting both ferromagnetic and antiferromagnetic coupling, oscillating between the two as a function of spacer layer thickness. By systematically tuning the coupling interactions via a wedge-shaped NiO spacer layer, we explore the energetics that dictate magnetic domain formation using high resolution magnetic force microscopy coupled with the magneto-optical Kerr effect. This technique probes the microscopic and macroscopic magnetic behavior as a continuous function of thickness and …

Effects Of Surface States, Defects And Dopants On The Optical And Magnetic Properties Of Low-Dimensional Materials, Ramakrishna Podila

All Dissertations

Nanomaterials have attracted the attention of researchers from various fields due to their unique features (that are otherwise absent in the bulk) such as quantum confinement, high surface to volume ratio, ability for surface modification etc. Since the discovery of fullerenes and carbon nanotubes, several synthesis techniques have been developed for nanomaterial growth. However, different control parameters in different synthesis techniques often result in nanostructures with varying defects that may alter their fundamental behavior. Such defects or disorder in the crystal lattice can lead to the disruption of lattice symmetry. The defect-induced symmetry lowering (or breaking) effects play a vital …

Raman Spectroscopic Study Of Solid Solution Spinel Oxides, Brian D. Hosterman

UNLV Theses, Dissertations, Professional Papers, and Capstones

Solid solution spinel oxides of composition MgxNi1−xCr2O4, NiFexCr2−xO4, and FexCr3−xO4 were synthesized and characterized using x-ray diffraction and Raman spectroscopy. Frequencies of the Raman-active modes are tracked as the metal cations within the spinel lattice are exchanged. This gives information about the dependence of the lattice vibrations on the tetrahedral and octahedral cations. The highest-frequency Raman-active mode, A1g, is unaffected by substitution of the divalent tetrahedral cation, whereas the lower frequency vibrations are more strongly affected by substitution of the tetrahedral cation. The change in wavenumber of many phonons is nonlinear upon cation exchange. All detected modes of MgxNi1−xCr2O4 and …

Decay Of Nuclear Hyperpolarization In Silicon Microparticles, M. Lee, M. C. Cassidy, C. Ramanathan, C. M. Marcus

Dartmouth Scholarship

We investigate the low-field relaxation of nuclear hyperpolarization in undoped and highly doped silicon microparticles at room temperature following removal from high field. For nominally undoped particles, two relaxation time scales are identified for ambient fields above 0.2 mT. The slower, T1,s, is roughly independent of ambient field; the faster, T1,f, decreases with increasing ambient field. A model in which nuclear spin relaxation occurs at the particle surface via a two-electron mechanism is shown to be in good agreement with the experimental data, particularly the field independence of T1,s. For boron-doped particles, a single relaxation time scale is observed. This …

Excitation-Induced Ge Quantum Dot Growth On Si(100)-2x1 By Pulsed Laser Deposition, Ali Oguz Er

Physics Theses & Dissertations

Self-assembled Ge quantum dots (QD) are grown on Si(100)-(2×1) with laser excitation during growth processes by pulsed laser deposition (PLD). In situ reflection-high energy electron diffraction (RHEED) and post-deposition atomic force microscopy (AFM) are used to study the growth dynamics and morphology of the QDs. A Q-switched Nd:YAG laser (λ = 1064 nm, 40 ns pulse width, 5 J/cm² fluence, and 10 Hz repetition rate) were used to ablate germanium and irradiate the silicon substrate. Ge QD formation on Si(100)-(2×1) with different substrate temperatures and excitation laser energy densities was studied. The excitation laser reduces the epitaxial growth temperature …

Measurement Of Spin Diffusion In Semi-Insulating Gaas, Christopher P. Weber, Craig A. Benko, Stanley C. Hiew

Physics

We use optical transient-grating spectroscopy to measure the spin diffusion of optically oriented electrons in bulk, semi-insulating GaAs(100). Trapping and recombination do not quickly deplete the photoexcited population. The spin diffusion coefficient of 88±12 cm²/s is roughly constant at temperatures from 15 K to 150 K, and the spin diffusion length is at least 450 nm. We show that it is possible to use spin diffusion to estimate the electron diffusion coefficient. Due to electron-electron interactions, the electron diffusion is 1.4 times larger than the spin diffusion.

The Interplay Between Symmetry And Static Dipoles With Adsorption On Molecular Substrates, Zhengzheng Zhang

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

This thesis presents evidence of preferential adsorption and the associated dipole-dipole interactions that can occur at molecule to molecule interfaces. The results are discussed in the context of the possibility of interactions caused by strong intrinsic dipoles when adsorbed on electrostatically biased substrates. Key is the discovery of lock and key adsorption chemistry by comparing the reversible absorption of the three isomers of di-iodobenzene (1,2-di-iodobenzene, 1,3-di-iodobenzene, and 1,4-di-iodobenzene) on molecular films of a quinonoid zwitterion. There is unequivocal evidence that the molecular adsorption and absorption of 1, 3-diiodobenzene is strongly favored at 150 K over the other isomers of di-iodobenzene. …

The Photofragmentation Processes Of The Closo-Carborane And The Local Structure Of Transition Metal Doped Semiconducting Boron Carbide Thin Films, Jing Liu

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

I investigated the photofragmentation processes of various closo-carboranes in an effort to understand the radical-induced polymerization of the closo-carboranes (i.e., semiconducting film growth), based on their partial dehydrogenation during plasma-enhanced chemical vapor deposition. The chemistry of vacuum ultraviolet VUV assisted dehydrogenation processes of both the closo-carboranes and related closo-phosphacarboranes were compared by photoionization mass spectrometry studies. The dominant ion pairs were identified and compared with the energetics constructed by theoretical modeling for the possible dissociation pathways.

Transition metal (Mn, Fe, Co) doped boron carbides thin films produced by plasma-enhanced chemical vapor deposition of orthocarborane (closo …

Electronic Structure Of Warm Dense Copper Studied By Ultrafast X-Ray Absorption Spectroscopy, B. I. Cho, K. Engelhorn, A. A. Correa, T. Ogitsu, Christopher P. Weber, H. J. Lee, J. Feng, P. A. Ni, Y. Ping, A. J. Nelson, D. Prendergast, R. W. Lee, R. W. Falcone, P. A. Heimann

Physics

We use time-resolved x-ray absorption spectroscopy to investigate the unoccupied electronic density of states of warm dense copper that is produced isochorically through the absorption of an ultrafast optical pulse. The temperature of the superheated electron-hole plasma, which ranges from 4000 to 10 000 K, was determined by comparing the measured x-ray absorption spectrum with a simulation. The electronic structure of warm dense copper is adequately described with the high temperature electronic density of state calculated by the density functional theory. The dynamics of the electron temperature is consistent with a two-temperature model, while a temperature-dependent electron-phonon coupling parameter is …