Condensed Matter Physics Commons^™

Investigation Of The Diffusion Processes In Self-Processing Acrylamide-Based Photopolymer System, Izabela Naydenova, Raghavendra Jallapuram, Robert Howard, Suzanne Martin, Vincent Toal

Articles

Results from the investigation of the diffusion processes in a dry acrylamide-based photopolymer system are presented. The investigation is carried out in the context of experimental work on optimization of the high spatial frequency response of the photopolymer. Tracing the transmission holographic grating dynamics at short times of exposure is utilized to measure diffusion coefficients. The results reveal that two different diffusion processes contribute with opposite sign to the refractive index modulation responsible for the diffraction grating build up. Monomer diffusion from dark to bright fringe areas increases the refractive index modulation. It is characterized with diffusion constant D0=1.6E-7 cm2/s. …

Anisotropy Of Exchange Stiffness And Its Effect On The Properties Of Magnets, K. D. Belashchenko

Kirill Belashchenko Publications

Using the spin-spiral formulation of the tight-binding linear muffin-tin orbital method, the principal components of the exchange stiffness tensor are calculated for typical hard magnets including tetragonal CoPt-type and hexagonal YCo₅ alloys. The exchange stiffness is strongly anisotropic in all studied alloys. This anisotropy makes the domain wall surface tension anisotropic. Competition between this anisotropic surface tension and magnetostatic energy controls the formation and dynamics of nanoscale domain structures in hard magnets. Anisotropic domain wall bending is described in detail from the general point of view and with application to cellular Sm–Co magnets. It is shown that the repulsive …

Role Of Nanophase Oxides In Short-Term Atmospheric Corrosion Of Structural Steels, Rama Balasubramanian

Physics Theses & Dissertations

Systematic studies on the development of nanophase iron oxides in the corrosion products of carbon and weathering steel were performed to understand the role of nanophase oxides in short-term atmospheric corrosion. Similarities and/or differences between short-term and long-term atmospheric corrosion were established by studying carbon steel and weathering steel coupons exposed in mild marine environments for short-term and comparing it with previously established long-term data. Influence of substitutional elements, in particular chromium, in forming nanophase goethite was investigated. Crystallographic, magnetic and morphological properties of nanophase chromium substituted goethite have been characterized in order to understand the protective nature of chromium-substituted …

Mapping Surface Polarization In Thin Films Of The Ferroelectric Polymer P(Vdf-Trfe)., Bradley W. Peterson, Stephen Ducharme, Vladimir M. Fridkin, Timothy J. Reece

Stephen Ducharme Publications

Pyroelectric Scanning Microscopy (PSM) has been developed to enable mapping of surface polarization in ferroelectric thin films, in particular the copolymer polyvinylidene fluoride trifluororethylene, or P(VDF-TrFE). The Chynoweth method for dynamically measuring pyroelectric current is employed in conjunction with a micropositioning system to construct two-dimensional images of the film polarization. These images have revealed enhancement of the polarization near the edges of the film below the average coercive field, with the center's polarization increasing thereafter to meet the edge value at saturation.

Extended Beg Model Of Halogenated Methanes Physisorbed On Ionic Crystals, T. E. Burns, Jr Dennison, Jason Kite

Journal Articles

The 2D dielectric phases and phase transitions of adsorbed dipolar molecules are modeled using a dilute spin-one Ising model. This model is studied in the Blume-Emery-Griffiths formalism, using a mean-field approximation, where the interaction parameters are related to system interaction energies using a unique averaging procedure. The model is applied to four halogenated methane species physisorbed on MgO(100) and NaCl(100) surfaces using previous experimental and theoretical studies to estimate the interaction energy parameters. We find that temperature- and coverage-dependent antiferroelectric to ferroelectric, coverage-dependant ferroelectric up to ferroelectric down, reentrant ferroelectric to ferroelectric, and order-disorder dipole phase transitions can occur. Phase …

Tem Study Of Crystalline Structures Of Cr–N Thin Films, Xingzhong Li, J. Zhang, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Cr–N films were grown on Si (001) substrates by reactive magnetron sputtering under an N2/Ar atmosphere at room temperature. The composition of the films, expressed as Cr_1–xN_x, can be varied by changing the N₂/Ar pressure ratio during the synthesis process. Crystalline states of Cr–N films have been studied using electron diffraction. It is well known that two intermediate phases, Cr₂N (hexagonal) and CrN (cubic), exist in the Cr–N system, and small variations around the ideal stoichiometry are tolerated. The present study shows that cubic CrN with vacancies rather than hexagonal Cr_{2 …}

Investigation Of The Photoinduced Surface Relief Modulation In Acrylamide-Based Photopolymer, Izabela Naydenova, Emilia Mihaylova, Suzanne Martin, Vincent Toal

Conference Papers

A surface relief diffraction grating is inscribed in acrylamide photopolymer by holographic recording with spatial frequency below 300 l/mm. The periodic surface modulation appears in addition to a volume phase holographic grating. Due to the nature of the presented photopolymerisable material the gratings are induced by light only and no post-processing is required. An investigation of the dependance of the amplitude of the photoinduced relief modulation on spatial frequency of recording and on sample thickness has been carried out utilising white light interferometry. A model of the mechanism of surface relief formation is proposed on the basis of the measured …

Spin Uncoupling In Free Nb Clusters: Support For Nascent Superconductivity, Ramiro Moro, Shuangye Yin, Xiaoshan Xu, Walt A. De Heer

Xiaoshan Xu Papers

Molecular beam Stern-Gerlach deflection measurements on Nb clusters (Nb_N, N <100) show that at very low temperatures the odd-N clusters deflect due to a single unpaired spin that is uncoupled from the cluster. At higher temperatures the spin is coupled and no deflections are observed. Spin uncoupling occurs concurrently with the transition to the recently found ferroelectric state, which has superconductor characteristics [Science 300, 1265 (2003)]. Spin uncoupling (also seen in V, Ta, and Al clusters) is analogous to the reduction of spin-relaxation rates observed in bulk superconductors below T_c.

Electron Irradiation Effects On Ferroelectric Copolymer Langmuir-Blodgett Films, Christina M. Othon, Stephen Ducharme

Stephen Ducharme Publications

The effect of irradiation on the ferroelectric properties of Langmuir-Blodgett films of the copolymer poly(vinylidene fluoride-trifluorethelene) is investigating using 1.26 MeV electrons with dosages from 16 to 110 Mrad. Irradiation causes a systematic decrease in the phase transition temperature, coercive field and polarization of these thin films.

Ferroelectricity At Molecular Level, L. M. Blinov, A. V. Bune, Peter A. Dowben, Stephen Ducharme, Vladimir M. Fridkin, S. P. Palto, K. A. Verkhovskaya, G. V. Vizdrik, S. G. Yudin

Stephen Ducharme Publications

he synthesis of ultrathin ferroelectric nanostructures by the Langmuir-Blodgett method and their properties are reviewed. It is shown that ferroelectricity exists in one monolayer of the ferroelectric P(VDF-TrFE) copolymer, i.e., at the molecular level. The specific characteristics of switching of ultrathin ferroelectric films are established.

Measurement Of Conductivity And Charge Storage In Insulators Related To Spacecraft Charging, A Robb Frederickson, Jr Dennison

Journal Articles

Improved experimental methods are discussed for laboratory measurement of conductivity and electric field in insulating spacecraft material intended for space radiation and plasma environments. These measurement techniques investigate the following features: 1) Measurements of conductivity are up to four orders of magnitude smaller than those determined by existing standard methods. 2) Conductivity is altered as radiation accumulates and trapping states fill with electrons. 3) With intense keV electron irradiation, electrons are continually emitted for hours from the irradiated surface after the irradiation ceases. 4) Charging induced by electron irradiation is strongly modified by the electron-hole pairs that the irradiation generates …

Theoretical Investigation Of The Surface Vibrational Modes In Germanium Nanocrystals, Shang-Fen Ren, Wei Cheng, Peter Y. Yu

Faculty publications – Physics

We have used a microscopic lattice dynamical model to study phonon modes in germanium (Ge) NC with size varying between 47 to 7289 atoms (diametersimilar to6.8 nm). By separating these atoms into bulk and surface atoms we have found that surface modes can exist in Ge NC both at low frequencies (<50>cm(-1)) and at high frequency (similar to260 cm(-1)). The latter mode is a resonant mode which occurs in the "pseudogap" between the acoustic and optical phonon branches in bulk Ge. From the low frequency surface modes we have been able to reconstruct the spheroidal and torsional Lamb modes …

Measurement Of Conductivity And Charge Storage In Insulators Related To Spacecraft Charging, A. R. Frederickson, Jr Dennison

Conference Proceedings

Several methods have been combined to measure conductivity and charge storage in insulating spacecraft materials. In order to avoid insulator problems, the motions of conducting electrons and holes must prevent the development of large electric fields exceeding 1E5 V/cm, where problems occur in spacecraft insulators. Approximate knowledge of the electric fields is important. One must consider generation of mobile electrons and holes, their trapping, thermal de-trapping, mobility and recombination.

Classical methods to measure thin film insulator conductivity apply a constant voltage to two electrodes on the sample and measure the resulting current for tens of minutes. Under constant voltage, a …

Refrigerator With Phonon Filters: An Application Of The Phonon Deficit Effect In Superconducting Tunnel Junctions, G. G. Melkonyan, Armen Gulian

Mathematics, Physics, and Computer Science Faculty Articles and Research

Nonequilibrium phenomena in thin solid films can result in cooling effects. These types of effects were predicted theoretically a while ago, and only recently were demonstrated experimentally in superconductor-insulator-normal metal (SIN) tunnel junctions. Since then, there is a growing interest in tunneling effects for the purpose to develop on-chip refrigerators. Thin film devices have the advantage of being extremely compact, operate in a continuous mode, dissipate little power, and can easily be integrated in cryogenic detectors. Currently these refrigerators can generate cooling in the order of 100 mK in an environment of 0.3–0.5 K. There are reasons to expect that …

Triplet Superconductors From The Viewpoint Of Basic Elements For Quantum Computers, Armen M. Gulian, Kent S. Wood

Mathematics, Physics, and Computer Science Faculty Articles and Research

We discuss possibilities of utilizing superconductors with Cooper condensates in triplet pairing states (where the spin of condensate pairs is S=1) for practical realization of quantum computers. Superconductors with triplet pairing condensates have features that are unique and cannot be found in the usual (singlet pairing, S=0) superconductors. The symmetry of the order parameter in some triplet superconductors (e.g., ruthenates) corresponds to doubly-degenerate chiral states. These states can serve as qubit base states for quantum computing.

Comment On: 'Depolarization Corrections To The Coercive Field In Thin-Film Ferroelectrics', Stephen Ducharme, Vladimir Fridkin

Stephen Ducharme Publications

The Letter by Dawber et al. [J. Phys.: Condens. Matter 15 L393 (2003)] notes that incomplete screening in the electrodes of a ferroelectric capacitor can result in an underestimate for the true coercive field in films of nanometer thickness. We show that their estimate of the magnitude of this correction it too large in the case of ferroelectric copolymer Langmuir- Blodgett films and, as a result, invalidates the claim that finite-size scaling of the ferroelectric coercive field is evident in films thinner than 15 nm.

Dynamics Of Phase Transitions On Low-Index Metal Surfaces, Bo Lin

Electrical & Computer Engineering Theses & Dissertations

The surface superheating and phase transitions at the low-index surface of metal were investigated using conventional continuous and 100-ps time-resolved reflection high-energy electron diffraction. Three metal surfaces, In(111), Au(110) and Pb(111), have been investigated in this work. The high temperature behavior of the In(111) surface was investigated using reflection high-energy electron diffraction with 100-ps temporal resolution. The change of surface vacancy density on In(111) is observed from 300 K to near the bulk melting point. The vacancy-formation energy of In(111) surface is estimated from experimental results. The surface vacancy density is observed to increase with temperature; however, the average random …

Ferroelectricity In Free Niobium Clusters, Ramiro Moro, Xiaoshan Xu, Shuangye Yin, Walt A. De Heer

Xiaoshan Xu Papers

Electric deflections of gas-phase, cryogenically cooled, neutral niobium clusters [Nb_N; number of atoms (N) = 2 to 150, temperature (T) = 20 to 300 kelvin], measured in molecular beams, show that cold clusters may attain an anomalous component with very large electric dipole moments. In contrast, room-temperature measurements show normal metallic polarizabilities. Characteristic energies k_BT_G(N) [Boltzmann constant k_B times a transition temperature T_G(N)] are identified, below which the ferroelectric-like state develops. Generally, T_G decreases [110 > T_G(N) > 10K] as …

Spin-Dependent Tunneling In Magnetic Tunnel Junctions, Evgeny Y. Tsymbal, O. N. Mryasov, Patrick R. Leclair

Evgeny Tsymbal Publications

The phenomenon of electron tunneling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fields of physics, as well as creating sub-fields on its own. Spin-dependent tunneling in magnetic tunnel junctions (MTJs) has recently aroused enormous interest and has developed in a vigorous field of research. The large tunneling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible applications in non-volatile random access memories and next-generation magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of spindependent tunneling. In this review article we present an …

Thermoelectric Cooling At Cryogenic Temperatures, S. R. Harutyunyan, V. H. Vardanyan, A. S. Kuzanyan, V. R. Nikoghosyan, S. Kunii, K. S. Wood, Armen Gulian

Mathematics, Physics, and Computer Science Faculty Articles and Research

Experimental results demonstrating Peltier cooling below 10 K are reported, using crystals of the thermoelectric cerium hexaboride (CeB₆). Direct measurements of the Peltier cooling showed δT up to ∼0.2 K in magnitude at T∼4–5 K. All three kinetic parameters: resistivity (ρ), heat conductivity (k), and Seebeck coefficient (S), characterizing the thermoelectric figure of merit ZT=S²T/ρk, were measured, giving high-confidence results.

Calculations Of Surface Effects On Phonon Modes And Raman Intensities Of Ge Quantum Dots, Shang-Fen Ren, Wei Cheng

Faculty publications – Physics

Phonon modes and Raman intensities of Ge quantum dots (QDs) with two different types of surfaces, a free standing surface or a fixed surface, in a size range from five atoms to 7 nm in diameter, are calculated by using a microscopic valence force field model. The results are compared, and the effects of surfaces on phonon properties of QDs are investigated. It is found that phonon modes and Raman intensities of QDs with these two different types of surfaces have obvious differences which clearly reveal the effects of the surfaces of QDs. The calculated results agree with existing experimental …

Dielectric Properties Of A Ferroelectric Copolymer Langmuir–Blodgett Film, Mahantappa S. Jogad, Stephen Ducharme

Stephen Ducharme Publications

We report measurements of the real (epsilon prime) and imaginary (epsilon double-prime) parts of the relative complex permittivity of a Langmuir–Blodgett film of ferroelectric copolymer of vinylidene fluoride (70%) with trifluoroethylene (30%). The measurements were made in the temperature range of 35 to 125° C, and frequency range of 19 Hz to 5 MHz. The results indicate low frequency loss due to conduction and dielectric loss peak near the ferroelectric–paraelectric phase transition.

Spin Injection Into Amorphous Semiconductors, Evgeny Y. Tsymbal, V. M. Burlakov, Ivan I. Oleinik

Evgeny Tsymbal Publications

Using a realistic model for the atomic and electronic structure of amorphous silicon, we explore spin injection into amorphous semiconductors. We calculate the spin-dependent conductance of magnetoresistive devices within the Landauer-Büttiker formalism including inelastic scattering. We find that reducing the density of injected carriers and increasing the spin polarization of the electrodes are favorable for spin injection, whereas inelastic scattering is detrimental, and show that the upper limit for magnetoresistance is given by Julliere's formula.

Competition Between Ferromagnetic Metallic And Paramagnetic Insulating Phases In Manganites, G. Li, H. D. Zhou, S. J. Feng, Xiaojuan Fan, X. G. Li, Z. D. Wang

Physics Faculty Research

La_0.67Ca_0.33Mn_1−xCu_xO₃(x=0 and 0.15) epitaxial thin films were grown on the (100) LaAlO₃ substrates, and the temperature dependence of their resistivity was measured in magnetic fields up to 12 T by a four-probe technique. We found that the competition between the ferromagnetic metallic (FM) and paramagnetic insulating (PI) phases plays an important role in the observed colossal magnetoresistance(CMR) effect. Based on a scenario that the doped manganites approximately consist of phase-separated FM and PI regions, a simple phenomenological model was proposed to describe the CMR effect. Using this model, we …

Calculations On The Size Effects Of Raman Intensities Of Silicon Quantum Dots, Wei Cheng, Shang-Fen Ren

Faculty publications – Physics

Raman intensities of Si quantum dots (QD's) with up to 11489 atoms (about 7.6 nm in diameter) for different scattering configurations are calculated. First, phonon modes in these QD's, including all vibration frequencies and vibration amplitudes, are calculated directly from the lattice-dynamic matrix by using a microscopic valence force field model combined with the group theory. Then the Raman intensities of these quantum dots are calculated by using a bond-polarizability approximation. The size effects of the Raman intensity in these QD's are discussed in detail based on these calculations. The calculations are compared with the available experimental observations. We are …

Sensor Development For Single-Photon Thermoelectric Detectors, Armen Gulian, K. S. Wood, G. G. Fritz, D. Van Vechten, H.-D. Wu, J. S. Horwitz, G. R. Badalyantz, S. R. Harutyunyan, V. H. Vartanyan, S. A. Petrosyan, A. S. Kuzanyan

Mathematics, Physics, and Computer Science Faculty Articles and Research

As we reported earlier [1], thermoelectric detectors can be competitive as nondispersive energy resolving focal-plane instruments in X-ray/UV spectrum. The first generations of prototype devices demonstrated the viability of detector design and provided good agreement between theoretical expectations and experimental data. These devices exploited sensors made of gold with a small fraction of iron impurity. To get the projected high resolution one needs another type of material, namely, lanthanum-cerium hexaborides. We report on the first experimental tests of the feasibility of lanthanum-cerium films as sensor materials. Progress with thin films of these materials argues for the success of these thermoelectric …

Specification Of Jecp/Sp: Stereographic Projection With An Application For Specimen Orientation Adjustment Using Tem Holders, Xingzhong Li

Nebraska Center for Materials and Nanoscience: Faculty Publications

1. Purpose of the program 2. Graphic user interface and program design 3. Crystallographic principle and implementation 4. System requirement 5. Installation and user instruction 6. How to contact the author 7. References

1. Purpose of the program JECP/SP is a computer program in Java Electron Crystallography Package. JECP is developed for quantitative electron diffraction and image processing, the package is designed and written by Dr. XingZhong Li. JECP/SP provides the all necessary functions of stereographic projection for regular application and furthermore it can be used to minimized the difficulties encountered when tilting highly beam-sensitive, or small-grain-size specimens with known …

Oxidation-Induced Stacking Fault In (100) And (111) Silicon Wafers, Mei Keat Liang

Student Works (2000-2009)

Oxidation-induced stacking fault (OISF) dimension and density change on (100) and (111) wafers, as a result of repeated preferential etching were studied using AFM. optical microscope and angular polishing techniques. Density on (100) and (111) wafers is independent of surface removal up to 3.0µm. This indicates the single layer nature of OISF formed from the sand blasted wafer. Besides having higher density, OISFs from (100) wafer were longer than OISF from (111) wafer. OISF width increases linearly with surface removal however the length does not change significantly with surface removal. ln the OISF normal depth studies using angular polishing technique, …

Jecp/Ed Manual, Xingzhong Li

Nebraska Center for Materials and Nanoscience: Faculty Publications

JECP/ED is a Java program with graphic interface for simulating electron diffraction pattern. It is designed and written by Dr. Xingzhong in University of Nebraska- Lincoln. This is verision 12-2002. It can be used for simulating electron diffraction zone pattern(ZOLZ with/without FOLZ), it allows to select zone axis, high voltage, sample thickness in the simulation, optional to show index, intensity and Laue center, featureed with manuniplating the pattern with slightly tilting, 360 degree rotation, zoom. The diffraction intensity is calculated on the basis of the kinematical theory. The program is extended to simulate electron diffraction with a precession of incident …

Effect Of Cr3+ And So42- On The Structure Of Rust Layer Formed On Steels By Atmospheric Corrosion, Masato Yamashita, Hitoshi Uchida,, Desmond C. Cook .

Physics Faculty Publications

The effect of chromium ion (Cr³⁺) and sulfate ion (SO₄^2-) on the structure of the rust layer formed by thin electrolyte-film corrosion of low-alloy steels has been examined. By using X-ray diffraction spectroscopy, it was found that coexistence of Cr³⁺ and SO₄^2- in the electrolyte film covering the steel surface quickly forms the Cr-goethite (α-(Fe _1-X, Cr_X)OOH) layer which has been known as the final protective rust layer. Scanning vibrating electrode measurements showed that the rust layer formed under the electrolyte film containing Cr³⁺ and SO₄^{2- …}