Condensed Matter Physics Commons^™

A Finite Difference Method For Studying Thermal Deformation In Three-Dimensional Thin Films Exposed To Ultrashort Pulsed Lasers, Suyang Zhang

Doctoral Dissertations

Thermal analysis related to ultrashort-pulsed lasers has been intensely studied in science and engineering communities in recent years, because the pulse duration of ultrashort-pulsed lasers is only the order of sub-picoseconds to femtoseconds, and the lasers have exclusive capabilities in limiting the undesirable spread of the thermal process zone in the heated sample. Studying the thermal deformation induced by ultrashort-pulsed lasers is essential for preventing thermal damage. For the ultrashort-pulsed laser, the thermal damage is different from that caused by the long pulsed lasers and cracks occur after heating.

This dissertation presents a new finite difference method for studying thermal …

Reflection High-Energy Electron Diffraction Studies Of Indium Phosphide (100) And Growth On Indium And Indium Nitride On Silicon (100), Mohamed Abd-Elsattar Hafez

Electrical & Computer Engineering Theses & Dissertations

Study of the effects of atomic hydrogen exposure on structure and morphology of semiconductor surfaces is important for fundamental properties and applications. In this dissertation, the electron yield of a hydrogen-cleaned indium phosphide (InP) surface was measured and correlated to the development of the surface morphology, which was monitored by in situ reflection high-energy electron diffraction (RHEED). Atomic hydrogen treatment produced a clean, well-ordered, and (2x4)-reconstructed InP(100) surface. The quantum efficiency, after activation to negative electron affinity, and the secondary electron emission were shown to increase with hydrogen cleaning time. RHEED patterns of low-index InP(100) surface were modified by the …

Structural Identification Of Cubic Iron-Oxide Nanocrystal Mixtures: X-Ray Powder Diffraction Versus Quasi-Kinematic Transmission Electron Microscopy, Peter Moeck

Physics Faculty Publications and Presentations

Two novel (and proprietary) strategies for the structural identification of a nanocrystal from either a single high-resolution (HR) transmission electron microscopy (TEM) image or a single precession electron diffraction pattern are proposed and their advantages discussed in comparison to structural fingerprinting from powder X-ray diffraction patterns. Simulations for cubic magnetite and maghemite nanocrystals are used as examples.

Photoconductivity In Bifeo3 Thin Films, S. R. Basu, L. W. Martin, Y. H. Chu, M. Gajek, R. Ramesh, R. C. Rai, X. S. Xu, J. L. Musfeldt

Xiaoshan Xu Papers

The optical properties of epitaxial BiFeO₃ thin films have been characterized in the visible range. Variable temperature spectra show an absorption onset near 2.17 eV, a direct gap (2.667±0.005 eV at 300 K), and charge transfer excitations at higher energy. Additionally, we report photoconductivity in BiFeO₃ films under illumination from a 100 mW/cm² white light source. A direct correlation is observed between the magnitude of the photoconductivity and postgrowth cooling pressure. Dark conductivities increased by an order of magnitude when comparing films cooled in 760 and 0.1 Torr. Large increases in photoconductivity are observed in light.

Optical Band Gap Of Bifeo3 Grown By Molecular-Beam Epitaxy, J. F. Ihlefeld, N. J. Podraza, Z. K. Liu, R. C. Rai, X. Xu, T. Heeg, Y. B. Chen, J. Li, R. W. Collins, J. L. Musfeldt, X. Q. Pan, J. Schubert, R. Ramesh, D. G. Schlom

Xiaoshan Xu Papers

BiFeO₃ thin films have been deposited on (001) SrTiO₃ substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO₃ films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with w rocking curve full width at half maximum values as narrow as 29 arc sec (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74 eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO₃ films.

Performance Comparison Of Pb(Zr0.52Ti0.48)O3-Only And Pb(Zr0.52Ti0.48)O3-On-Silicon Resonators, Hengky Chandrahalim, Sunil A. Bhave, Ronald G. Polcawich, Jeff Pulskamp, Dan Judy, Roger Kaul, Madan Dubey

Faculty Publications

This paper provides a quantitative comparison and explores the design space of lead zirconium titanate (PZT)–only and PZT-on-silicon length-extensional mode resonators for incorporation into radio frequency microelectromechanical system filters and oscillators. We experimentally measured the correlation of motional impedance (R_X) and quality factor (Q) with the resonators’ silicon layer thickness (t_Si). For identical lateral dimensions and PZT-layer thicknesses (t_PZT), the PZT-on-silicon resonator has higher resonant frequency (f_C), higher Q (5100 versus 140), lower R_X (51 Ω versus 205 Ω), and better linearity [third-order input intercept …

Nanocomposites For Novel Holographic Applications, Izabela Naydenova, Svetlana Mintova, Suzanne Martin, Vincent Toal

Articles

Nanozeolites incorporated into photopolymerizable holographic recording materials can be used for holographic sensing, data storage, and optical systems design.

Use Of Di- And Tripropionate Substrate Analogs To Probe The Active Site Of Human Recombinant Coproporphyrinogen Oxidase, Justin B. Morgenthaler, Reyna L. Barto, Timothy D. Lash, Marjorie A. Jones

Faculty Publications – Chemistry

Background: Defects in the enzyme coproporphyrinogen oxidase result in accumulation of porphyrins which may affect the severity of a subset of porphyrias. Thus evaluation of this enzyme for substrate selectivity is of value. Kinetic evaluations of recombinant human coproporphyrinogen oxidase have been undertaken using six di- and tripropionate analogs of the natural substrate coproporphyrinogen-III. These Substrate analogs were modified by having alkyl groups in place of one or both of the ring 13- or 17-propionate moieties. Material/Methods: Cloned human enzyme was incubated with analogs under apparent first order conditions and with various substrate concentrations. The kinetic values, K-m and V-max, …

Study Of Structural And Spectroscopic Properties Of Small Zns Clusters By Dft, Venkata Ramana Chaganti

Open Access Theses & Dissertations

ABSTRACT

The small clusters (aggregates of atoms containing a few tens of atoms) of semiconductors and metals often adopt very different shapes than the fragments of these materials in the bulk phase. Due to their large surface to volume ratio and unsaturated bonds, the small clusters often have properties that are very different from their bulk. Semiconductor clusters are extensively studied for their potential applications in a wide variety of systems from opto-electronic devices to spintronics. The present work is devoted to understanding the structural and electronic properties of small clusters of the zinc sulfide ZnnSn where n = 1 …

Fabrication Of Switchable Liquid Crystal Devices Using Surface Relief Gratings In Photopolymer, Pavani Kotakonda, Izabela Naydenova, Robert Howard, Suzanne Martin, Vincent Toal

Articles

Holographically recorded surface relief gratings in dry, self-developing acrylamide based photopolymer were used to fabricate two types of switchable liquid crystal (LC) device. One is an electrically switchable LC diffraction grating and the other is an electrically switchable polarization rotator. The electrically switchable diffraction grating was characterized by measuring the dependence of the intensity in the first diffracted order on the applied electric field. The polarization rotator was characterized by studying the influence of the applied electric field on the twist angle and the variation of intensity in the zero and the first orders of diffraction.

Charge Order, Dynamics, And Magnetostructural Transition In Multiferroic Lufe2o4, X. S. Xu, M. Angst, T. V. Brinzari, R. P. Hermann, J. L. Musfeldt, A. D. Christianson, D. Mandrus, B. C. Sales, S. Mcgill, J. -W. Kim, Z. Islam

Xiaoshan Xu Papers

We investigated the series of temperature and field-driven transitions in LuFe₂O₄ by optical and Mossbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the ‘‘order by fluctuation’’ mechanism for the development of charge order superstructure. Bragg splitting and large magneto-optical contrast suggest a low-temperature monoclinic distortion that can be driven by both temperature and magnetic field.

Oligo(Vinylidene Fluoride) Langmuir-Blodgett Films Studied By Spectroscopic, Rafal Korlacki, J. Travis Johnston, Jihee Kim, Stephen Ducharme, Daniel W. Thompson, Vladimir M. Fridkin, Zhongxin Ge, James M. Takacs

Stephen Ducharme Publications

Thin films of amphiphilic vinylidene fluoride oligomers prepared by Langmuir–Blodgett deposition on silicone substrates were investigated by comparing experimental and theoretical mid-infrared (IR) spectra. The experimental spectra were obtained using infrared spectroscopic ellipsometry. Theoretical spectra were calculated using density functional theory. Excellent correspondence of major IR bands in both data sets shows that the molecular backbone is oriented with the long axis normal to the substrate plane. This is in contrast to poly vinylidene fluoride[1] LB films, in which the polymer chains are parallel to the substrate.

Plasma Medicine, Mounir Laroussi, Alexander Fridman

Electrical & Computer Engineering Faculty Publications

Recent demonstrations of plasma technology in the treatment of living cells, tissues, and organs are creating a newfield at the intersection of plasma science and technology with biology and medicine - Plasma Medicine.

Charge Effects In Bilayer Electric Transport, Christopher Czech

All Theses

The mathematical isomorphism between the layer index in a bilayer system and the spin index in a spin-polarized system is used to draw an analogy between phenomena that occur in the two systems as a result of the electron-electron (e-e) interaction. First, the classical case of the Coulomb drag, a phenomenon resulting from momentum transfer through e-e collisions, is reviewed for both bilayer systems and spin-polarized systems. Then, the spin-backflow phenomenon, a result of the interaction-induced changes in the local quasiparticle energy that affects the charge transport in spin-polarized systems is presented. Finally, the iso-spin correlation is exploited to establish …

Spin Density Wave Phases In Semiconductor Superlattice, Liqiu Zheng

All Dissertations

We discuss the existence of spin instabilities and of possible ground states with broken spin symmetry in the presence of a tilted magnetic field for several semiconductor heterostructures. In each instance, the fundamental premise of our study is the existence of a spin degeneracy, controlled by tuning various experimentally controllable parameters, that permits the apparition of spin-flip processes driven entirely by the many-body Coulomb interaction. If in the case of a single quantum well, the spin instabilities trigger an abrupt paramagnetic to ferromagnetic phase transition, we demonstrate that in superlattices, at low temperatures, a stable spin density wave ($SDW$) ground …

Reversal Of Spin Polarization In Fe/Gaas (001) Driven By Resonant Surface States: First-Principles Calculations, Athanasios N. Chantis, Kirill D. Belashchenko, D. L. Smith, Evgeny Y. Tsymbal, Mark Van Schilfgaarde, R. C. Albers

Evgeny Tsymbal Publications

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with the voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin-density approximation, we calculate the spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of the applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions.

Bias Voltage Dependence Of Tunneling Anisotropic Magnetoresistance In Magnetic Tunnel Junctions With Mgo And Al2O3 Tunnel Barriers, Li Gao, Xin Jiang, See-Hun Yang, John D. Burton, Evgeny Y. Tsymbal, Stuart S. P. Parkin

Evgeny Tsymbal Publications

Tunneling anisotropic magnetoresistance (TAMR) is observed in tunnel junctions with transition metal electrodes as the moments are rotated from in-plane to out-of-plane in sufficiently large magnetic fields that the moments are nearly parallel to one another. A complex angular dependence of the tunneling resistance is found with twofold and fourfold components that vary strongly with bias voltage. Distinctly different TAMR behaviors are obtained for devices formed with highly textured crystalline MgO(001) and amorphous Al₂O₃ tunnel barriers. A tight-binding model shows that a fourfold angular dependence can be explained by the presence of an interface resonant state that …

Effect Of Tip Resonances On Tunnelling Anisotropic Magnetoresistance In Ferromagnetic Break Junctions: A First-Principles Study, John D. Burton, Renat F. Sabirianov, Julian P. Velev, O. N. Mryasov, Evgeny Y. Tsymbal

Materials Research Science and Engineering Center: Faculty Publications

First-principles calculations of electron tunneling transport in nanoscale Ni and Co break-junctions reveal strong dependence of the conductance on the magnetization direction, an effect known as tunneling anisotropic magnetoresistance TAMR. An important aspect of this phenomenon stems from resonant states localized in the electrodes near the junction break. The energy and broadening of these states is strongly affected by the magnetization orientation due to spin-orbit coupling, causing TAMR to be sensitive to bias voltage on a scale of a few millivolts. Our results bear a resemblance to recent experimental data and suggest that TAMR driven by resonant states is a …

The Thermoelectric Properties Investigation Of Ir3Ge7 Typed Representatives, Huqin Zhang

All Theses

Energy conversion is employed to power generation utilizing the Seebeck effect or cooling utilizing the Peltier effect. Herein, we study several Ir₃Ge₇ typed semiconductor materials for potential thermoelectric energy conversion. Mo₃Sb₇, crystallizing in Ir₃Ge₇ type, can be rendered semiconducting by a partial Sb-Te substitution without noticeable structure changes, resulting in Mo₃Sb₅Te₂ with 55 valence electrons per formula unit. Meanwhile, large cubic voids in the Mo₃Sb₅Te₂ crystal structure provide the possibility of filling the voids with small cations to decrease the …

Comparison Between Steady State And Laser Flash Techniques, Justine Andrews

All Theses

Thermoelectrics is a promising avenue of energy conservation. A material is deemed a good thermoelectric material when it portrays certain material characteristics. These characteristics include a high thermopower and electrical conductivity and a low thermal conductivity. This thesis focuses on the methods of measuring a material's thermal conductivity.
The steady state method is a measurement technique used in the low temperature thermal conductivity measurements. This technique assures that the system and sample are at equilibrium before a measurement point is taken. As a result the measurements are both precise and accurate.
The laser flash technique measures a sample's thermal conductivity …

Comparison Of Thermoelectric Properties Of Arc-Melted And Hot-Pressed Half-Heuslers, Brad Edwards

All Theses

Relatively low thermal conductivity and great electrical properties in combination with wide tuning parameters through doping make half-Heuslers high potential thermoelectric materials. In this study we plan to investigate the effects of hot-pressing and arc-melting on the thermoelectric properties of ZrNiSn_0.975Sb_0.025, and Hf_0.75Zr_0.25NiSn_0.975Sb_0.025 samples. Also, the effects of electron and phonon scattering due to the insertion of small 50 nm particles within the sample were investigated in ZrNiSn_0.975Sb_0.025 +4% Al₂O₃ and Hf_0.75Zr_0.25NiSn_0.975Sb_0.025 +4% Al₂O …

Electronic Structure Of Mos2 Nanotubes, Lingyun Xu

All Dissertations

First-principles methods enable one to study the electronic structure of solids, surfaces, or clusters as accurately as possible with moderate computational effort.
So we used a first-principles electronic structure method to calculate the electronic structure of free-standing layer of MoS₂ with ABA and ABC stacking. Our results suggest MoS₂ with ABA stacking which appears as an insulator has an energy gap of 1.64 eV. The covalent bonds between molybdenum and sulfur atoms are strong enough to form this gap. The ABC stacking will break the symmetry and becomes metallic. The valance and impurities calculations show the rigid-band picture …

Quantum Analysis Of A Linear Dc Squid Mechanical Displacement Detector, M. P. Blencowe, E. Buks

Dartmouth Scholarship

We provide a quantum analysis of a dc SQUID mechanical displacement detector within the subcritical Josephson current regime. A segment of the SQUID loop forms the mechanical resonator and motion of the latter is transduced inductively through changes in the flux threading the loop. Expressions are derived for the detector signal response and noise, which are used to evaluate the position and force detection sensitivity. We also investigate cooling of the mechanical resonator due to detector back reaction.

Defect-Mediated Properties Of Magnetic Tunnel Junctions, Julian P. Velev, Mikhail Ye. Zhuravlev, Kirill D. Belashchenko, Sitaram S. Jaswal, Evgeny Y. Tsymbal, T. Katayama, S. Yuasa

Evgeny Tsymbal Publications

Defects play an important role in the properties of metal oxides which are currently used as barrier layers in magnetic tunnel junctions (MTJs). We study the effect of O vacancies on the interlayer exchange coupling (IEC) and tunneling magnetoresistance (TMR) in Fe–MgO–Fe tunnel junctions. Measurements of IEC in fully epitaxial Fe–MgO–Fe(001) tunnel junctions show IEC is antiferromagnetic (AFM) for small MgO thickness but changes sign and then vanishes for large barrier thickness. First-principles calculations based on density functional theory demonstrate that the presence of neutral O vacancies (F-centers) in the MgO barrier can explain this behavior. Resonant tunneling …

Raman Spectroscopic Evidence For Anharmonic Phonon Lifetimes And Blueshifts In 1d Structures, Rahul Rao

All Dissertations

Anharmonic effects in two different quasi-1D systems were probed via micro-Raman spectroscopy. In the first system, we observed upshifts of peaks in the Raman spectra for b-Ga2O3 nanowires grown along the [110] growth direction compared those present in bulk b-Ga2O3. Contrary to our Raman studies on -Ga2O3 nanowires, downshifts in the Raman spectrum for b-Ga2O3 nanowires grown along [401] direction has also been reported by other research groups. We attribute these Raman shifts to the growth direction-induced lattice strains (compressive and tensile) present in the nanowires, and present a model based on the quasi-harmonic density functional theory to support our …

Nondiffusive Spin Dynamics In A Two-Dimensional Electron Gas, Christopher P. Weber, J. Orenstein, B. Andrei Bernevig, Shou-Cheng Zhang, Jason Stephens, D. D. Awschalom

Physics

We describe measurements of spin dynamics in the two-dimensional electron gas in GaAs=GaAlAs quantum wells. Optical techniques, including transient spin-grating spectroscopy, are used to probe the relaxation rates of spin polarization waves in the wave vector range from zero to 6 x 10⁴ cm^-1. We find that the spin polarization lifetime is maximal at a nonzero wave vector, in contrast with expectations based on ordinary spin diffusion, but in quantitative agreement with recent theories that treat diffusion in the presence of spin-orbit coupling

Interface Effects In Spin-Dependent Tunneling, Evgeny Y. Tsymbal, Kirill D. Belashchenko, Julian P. Velev, Sitaram S. Jaswal, Mark Van Schilfgaarde, Ivan I. Oleynik, Derek A. Stewart

Evgeny Tsymbal Publications

In the past few years the phenomenon of spin dependent tunneling (SDT) in magnetic tunnel junctions (MTJs) has aroused enormous interest and has developed into a vigorous field of research. The large tunneling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible application in random access memories and magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. One such question is the role of interfaces in MTJs and their effect on the spin polarization of the tunneling current and TMR. In this paper we consider different models which suggest that the …

Ferroelectric Properties Of Langmuir-Blodgett Copolymer Films At The Nanoscale, A. Tolstousov, R. V. Gaynutdinov, R. Tadros-Morgane, Herbert Kliem, Stephen Ducharme, Vladimir M. Fridkin

Stephen Ducharme Publications

Langmuir-Blodgett (LB) films are well known structures prepared as a result of successive transfer of monolayers (ML) from the gas-liquid interface onto solid substrates. One impressive possibility of LB method is the opportunity to vary the thickness of the film to an accuracy of one transfer, which could coincide with one ML. The ferroelectric properties of a copolymer of vinylidene fluoride and trifluorethylene P[VDF-TrFE] prepared by LB deposition are investigated in the region of a few monolayers.

Magnetic Enhancement In Cobalt-Manganese Alloy Clusters, Shuangye Yin, Ramiro Moro, Xiaoshan Xu, Walter A. De Heer

Xiaoshan Xu Papers

Magnetic moments of Co_NMn_M and Co_NV_M clusters (N ≤ 60; M ≤ N/3) are measured in molecular beams using the Stern-Gerlach deflection method. Surprisingly, the per atom average moments of Co_NMn_M clusters are found to increase with Mn concentration, in contrast to bulk CoMn. The enhancement with Mn doping is found to be independent of cluster size and composition in the size range studied. Meanwhile, Co_NV_M clusters show reduction of average moments with increasing V doping, consistent with what is expected in bulk CoV. The results are …

Investigation Of The Catalytic And Structural Roles Of Conserved Histidines Of Human Coproporphyrinogen Oxidase Using Site-Directed Mutagenesis, Shani J. Gitter, Christopher L. Cooper, Jon A. Frieson, Timothy D. Lash, Marjorie A. Jones

Faculty Publications – Chemistry

Background: The catalytic contribution of four conserved histidines of human coproporphyrinogen oxidase (CPO) has been investigated using site-directed mutagenesis to change histidine (H) into alanine (A). Material/Methods: The wild-type and mutant enzyme forms were analyzed for their ability to utilize coproporphyrinogen-III, mesoporphvrinogen-VI, and harderoporphyrinogen as substrates. Results: Wild-type CPO had specific activities of 4.9 +/- 0.9 nmole product/min/mg for coproporphyrinogen-III, 1.7 +/- 0.7 nmole ptoduct/min/mg for mesoporphyrinogen-VI, and 5.1 +/- 1.8 nmole product/min/mg for harderoporphyrinogen. The four mutant enzymes were catalytically competent With all three substrates, but to varying degrees. The most affected Mutant was the H158A enzyme which exhibited …