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Articles 32011 - 32040 of 36559

Full-Text Articles in Physical Sciences and Mathematics

Nanosubharmonics: The Dynamics Of Small Nonlinear Contacts, Nancy Burnham, A Kulik, G Gremaud, Gad Briggs Jun 1995

Nanosubharmonics: The Dynamics Of Small Nonlinear Contacts, Nancy Burnham, A Kulik, G Gremaud, Gad Briggs

Nancy A. Burnham

We observed the generation of subharmonics and chaos in a nanometer-sized mechanical contact. To first order, the behavior matches that of macroscopic systems, with some intriguing secondary differences. As the occurrence of periodic behavior (subharmonics) is related to the coefficient of restitution, it may be possible to image local energy dissipation with nanometer resolution.

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Simultaneous Determination Of Fe 3p Spin-Orbit And Exchange Splittings In Photoemission, D. J. Huang, D. Mark Riffe, J. L. Erskine Jun 1995

Simultaneous Determination Of Fe 3p Spin-Orbit And Exchange Splittings In Photoemission, D. J. Huang, D. Mark Riffe, J. L. Erskine

All Physics Faculty Publications

Spin-resolved core-level photoemission data from the 3p level of ultrathin Fe films [1.4–5.1 monolayers (ML)] epitaxially grown on W(110) have been obtained. A nonlinear least-squares analysis, based on a one-particle Hamiltonian that simultaneously includes core-valence exchange and core-hole spin-orbit interactions, is developed. It is first tested on Fe 2p magnetic circular dichroism (MCD) photoemission spectra and shown to successfully describe the MCD asymmetry data. The model is then used to analyze our thin-film 3p data. With increasing film thickness the spin-orbit splitting (0.67±0.02 eV) remains constant (as expected), the exchange splitting increases from 0±0.12 eV to 0.41±0.05 eV, the average …

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Evidence For A Supercritical ‘Transition’ To The Isotropic Phase In A Highly Chiral Liquid Crystal, Z. Kutnjak, C. W. Garland, John Loren Passmore , '94, Peter J. Collings Jun 1995

Evidence For A Supercritical ‘Transition’ To The Isotropic Phase In A Highly Chiral Liquid Crystal, Z. Kutnjak, C. W. Garland, John Loren Passmore , '94, Peter J. Collings

Physics & Astronomy Faculty Works

The results of two independent experiments in the vicinity of the “transition” from the third blue phase (BPIII) to isotropic phase (I) are reported for a highly chiral liquid crystal. Heat capacity measurements using a high-resolution calorimeter and dynamic light-scattering measurements using circularly polarized light have been performed. The data show a continuous evolution of BPIII into I with no critical fluctuations. This is strong evidence that the BPIII-I transition in this compound is supercritical, indicating that the BPIII and I phases possess the same macroscopic symmetry.

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Supercritical Conversion Of The 3rd Blue Phase To The Isotropic-Phase In A Highly Chiral Liquid-Crystal, Z. Kutnjak, C. W. Garland, J. Loren Passmore, Peter J. Collings Jun 1995

Supercritical Conversion Of The 3rd Blue Phase To The Isotropic-Phase In A Highly Chiral Liquid-Crystal, Z. Kutnjak, C. W. Garland, J. Loren Passmore, Peter J. Collings

Physics & Astronomy Faculty Works

The results of two independent experiments in the vicinity of the “transition” from the third blue phase ( BPIII) to isotropic phase ( I) are reported for a highly chiral liquid crystal. Heat capacity measurements using a high-resolution calorimeter and dynamic light-scattering measurements using circularly polarized light have been performed. The data show a continuous evolution of BPIII into I with no critical fluctuations. This is strong evidence that the BPIII-I transition in this compound is supercritical, indicating that the BPIII and I phases possess the same macroscopic symmetry.

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Modeling Of Decay Rate For Molecules At An Island Surface, Ting Xiong Jun 1995

Modeling Of Decay Rate For Molecules At An Island Surface, Ting Xiong

Dissertations and Theses

The decay rates for molecules at rough surfaces are studied via an island surface model, with particular emphasis on the effect due to the distribution of surface roughness. Two extreme cases are studied when the surface islands distribute themselves evenly and when they coalesce to form local clusters at the molecule-substrate interface. The optical properties of the interfacial layer in these two cases are described by the Maxwell-Garnett and the fractal-cluster models, respectively. Among other results, it is found that both enhancement and suppression of the surface-induced decay rates are possible due to the presence of roughness, with more dramatic …

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Molecular Hydrogen Formation On Astrophysically Relevant Surfaces, Gianfranco Vidali, N. Katz, Itay Furman, Ofer Biham, Valerio Pirronello Jun 1995

Molecular Hydrogen Formation On Astrophysically Relevant Surfaces, Gianfranco Vidali, N. Katz, Itay Furman, Ofer Biham, Valerio Pirronello

Physics - All Scholarship

Recent experimental results about the formation of molecular hydrogen on astrophysically relevant surfaces under conditions close to those encountered in the interstellar medium are analyzed using rate equations. The parameters of the rate equation model are fitted to temperature-programmed desorption curves obtained in the laboratory. These parameters are the activation energy barriers for atomic hydrogen diffusion and desorption, the barrier for molecular hydrogen desorption, and the probability of spontaneous desorption of a hydrogen molecule upon recombination. The model is a generalization of the Polanyi-Wigner equation and provides a description of both first and second order kinetic processes within a single …

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Scaling Of Thin-Film Nd1.85ce0.15cuo4-Y Resistivity-Current Isotherms At Low Fields: Implications For Vortex Phase Transitions And Universality, Brandon R. Brown Jun 1995

Scaling Of Thin-Film Nd1.85ce0.15cuo4-Y Resistivity-Current Isotherms At Low Fields: Implications For Vortex Phase Transitions And Universality, Brandon R. Brown

Physics and Astronomy

We have investigated a second-order phase transition of the vortex lattice in thin-film Nd1.85Ce0.15CuO4-y from 1 mT to 1 T applied magnetic fields. The dc resistivity-current density data for each field exhibit critical scaling consistent with a second-order vortex-liquid to vortex-solid phase transition. The high-field data support the vortex-glass model and have the same critical exponents as thin-film YBa2Cu3O7-δ. At low fields we observe a change in the critical exponents and universal functions. The critical regime diminishes as the field is decreased but remains nonzero even at 1 mT. …

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Electron Transport In Magnetic Inhomogeneous Media, Horacio E. Camblong Jun 1995

Electron Transport In Magnetic Inhomogeneous Media, Horacio E. Camblong

Physics and Astronomy

Giant magnetoresistance has been observed in both magnetic multilayers and magnetic granular solids. We develop a framework for unifying these particular realizations of inhomogeneous magnetic media, based on the real-space Kubo formula. It constitutes a spin-dependent form of linear response theory, associated with internal spin-dependent fields arising from spin accumulation; moreover, we discuss the physical meaning of these spin dependences. For magnetic multilayers we discuss the particular cases of collinear and noncollinear configurations, and we consider limiting cases of the elastic mean-free path to inhomogeneity-scale ratio for granular solids. Furthermore, we introduce the concept of magnetically self-averaging systems, which include …

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Low Dose, Robert Katz, Francis A. Cucinotta Jun 1995

Low Dose, Robert Katz, Francis A. Cucinotta

Robert Katz Publications

Low doses of radiation are frequently inferred to mean doses of the order of 10-100 mGy, regardless of the end point or the nature of the irradiation, as if the energy were deposited uniformly through all targets. Yet, the fluence in a charged particle beam at which there is an average of 1 charged particle transit per target leaves 37% of the targets untouched. At an average of 3 transits per target, 5% of the targets experience no particle transits. Only at an average of 5 transits per target do 99% of the targets achieve one or more transits. One …

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Vibrational-State-Dependent Decay Of The Co C(1s) Excitation, N. Saito, Franz Heiser, Oliver Hemmers, A. Hempelmann, K. Wieliczek, J. Viefhaus, U. Becker Jun 1995

Vibrational-State-Dependent Decay Of The Co C(1s) Excitation, N. Saito, Franz Heiser, Oliver Hemmers, A. Hempelmann, K. Wieliczek, J. Viefhaus, U. Becker

Environmental Studies Faculty Publications

Molecular ionization and ionic fragmentation of core excited CO valence and Rydberg states are studied by partial-ion-yield spectroscopy at vibrational resolution. The vibrational intensities of the Rydberg excitations differ significantly depending on the ionization and/or fragmentation channel. The observed intensity variations are explained by nondissociative excited final ionic states of the subsequent molecular Auger decay. The vibrational levels of these final states become selectively depopulated due to interaction with dissociative decay channels leading to ionic fragmentation.

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Phonon Replicas In The Photoluminescence Emission Of Alxga1-Xas Alloys, D. C. Reynolds, David C. Look, R. Kaspi, D. N. Talwar Jun 1995

Phonon Replicas In The Photoluminescence Emission Of Alxga1-Xas Alloys, D. C. Reynolds, David C. Look, R. Kaspi, D. N. Talwar

Physics Faculty Publications

Phonon replicas in the photoluminescence spectra of a direct gap AlxGa1−xAs alloy have been observed. The GaAs‐like transverse optical and longitudinal optical as well as AlAs‐like longitudinal optical modes were observed at the Γ‐point. We also observe what we believe to be the longitudinal acoustical phonons at the L‐point in the Brillouin zone.

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Computer Simulation Study Of The Permeability Of Driven Polymers Through Porous Media, Grace M. Foo, Ras B. Pandey Jun 1995

Computer Simulation Study Of The Permeability Of Driven Polymers Through Porous Media, Grace M. Foo, Ras B. Pandey

Faculty Publications

A computer simulation model is used to study the permeability of polymer chains driven by a biased flow field through a porous medium in two dimensions. The chains are modeled by constrained self-avoiding walks, which reptate through the heterogeneous medium with a biased probability imposed by the driven field. A linear response description is used to evaluate an effective permeability. The permeability σ shows an unusual decay behavior on reducing the porosity ps. We find that the permeability decreases on increasing the bias above a characteristic value Bc. This characteristic bias shows a logarithmic decay on …

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The Mandelstam-Leibbrandt Prescription In Light Cone Quantization Gauge Theories, Gary Mccartor, David G. Robertson Jun 1995

The Mandelstam-Leibbrandt Prescription In Light Cone Quantization Gauge Theories, Gary Mccartor, David G. Robertson

Physics Faculty Scholarship

Quantization of gauge theories on characteristic surfaces and in the light-cone gauge is discussed. Implementation of the Mandelstam-Leibbrandt prescription for the spurious singularity is shown to require two distinct null planes, with independent degrees of freedom initialized on each. The relation of this theory to the usual light-cone formulation of gauge field theory, using a single null plane, is described. A connection is established between this formalism and a recently given operator solution to the Schwinger model in the light-cone gauge.

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Optimal Wavefront Reconstruction For A Coherent Diffracted Field, William W. Arrasmith Jun 1995

Optimal Wavefront Reconstruction For A Coherent Diffracted Field, William W. Arrasmith

Theses and Dissertations

In sheared coherent beam interferometric imaging, an estimate of the average reflectivity profile of the object can be computed from measurements of point to point phase differences in the far field interference pattern and a suitable phase reconstruction technique. The phase difference information is encoded in the irradiance of three identical, shifted and superimposed speckled laser beam patterns. A minimum variance phase reconstruction technique is presented to estimate the phase of the field in the measurement plane from the phase differences and evaluate its performance. Prior knowledge of the phase covariance is used in the minimum variance reconstructor. Analytic calculations …

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Vibrational Properties Of A Loaded String, Samantha Parmley, Tom Zobrist, Terry Clough, Anthony Perez-Miller, Mark Makela Jun 1995

Vibrational Properties Of A Loaded String, Samantha Parmley, Tom Zobrist, Terry Clough, Anthony Perez-Miller, Mark Makela

All Faculty Scholarship for the College of the Sciences

In this paper we discuss our study of a string–mass chain and its anology to quantum mechanical systems. Theoretical predictions are made based upon the numerical solution to the wave equation. These predictions are tested experimentally using both normal mode analysis and pulse analysis. The frequency band structures for periodic and disordered string mass chains are studied as well as their corresponding eigenfunctions. The theoretical and experimental results are in accord. This experiment, suitable for advanced physics majors, demonstrates many important features of quantum mechanics: eigenvalues, superposition principle, band structure, gap modes, and Anderson localization.

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Finite Thermal Diffusivity At Onset Of Convection In Autocatalytic Systems: Discontinuous Fluid Density, D. A. Vasquez, Boyd F. Edwards, J. W. Wilder Jun 1995

Finite Thermal Diffusivity At Onset Of Convection In Autocatalytic Systems: Discontinuous Fluid Density, D. A. Vasquez, Boyd F. Edwards, J. W. Wilder

All Physics Faculty Publications

A linear convective stability analysis for propagating autocatalytic reaction fronts includes density differences due to both thermal and chemical gradients. Critical parameters for the onset of convection are calculated for an unbounded geometry, a vertical slab, and a vertical cylinder. Thermal effects are important at unstable wavelengths well above the critical wavelength for the onset of convection.

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A Stochastic Theory Of Inhomogeneously Broadened Linewidths In Solids, Ulrich Zürcher Jun 1995

A Stochastic Theory Of Inhomogeneously Broadened Linewidths In Solids, Ulrich Zürcher

Physics Faculty Publications

We investigate spectral diffusion decay using a model for solids that consists of two-level-systems (TLSs) interacting via strain fields. For the case when the rate of TLS flips vanishes, we find algebraic decay of correlation functions of the local field. We show that properties of equilibrium fluctuations are in agreement with the hierarchical picture proposed by Basché and Moerner: TLSs far away produce fast fluctuations that are small in magnitude, and close TLSs produce large fluctuations that are less frequent.

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A High-Bandwidth Frequency-Domain Photon Migration Instrument For Clinical Use, Steen J. Madsen, Eric R. Anderson, Richard C. Haskell, Bruce J. Tromberg May 1995

A High-Bandwidth Frequency-Domain Photon Migration Instrument For Clinical Use, Steen J. Madsen, Eric R. Anderson, Richard C. Haskell, Bruce J. Tromberg

All HMC Faculty Publications and Research

We have developed a high-bandwidth frequency-domain photon migration (FDPM) instrument which is capable of noninvasively determining the optical properties of biological tissues in near-real-time. This portable, inexpensive, diode-based instrument is unique in the sense that we employ direct diode laser modulation and avalanche photodiode detection. Diffusion models were used to extract the optical properties (absorption and transport scattering coefficients)of tissue-simulating solutions.from the 300 kHz to I GHz photon density wave data.

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Nanoscale Visualization And Control Of Ferroelectric Domains By Atomic Force Microscopy, Oleg Kolosov, Alexei Gruverman, Jun Hatano, Koichiro Takahashi, Hiroshi Tokumoto May 1995

Nanoscale Visualization And Control Of Ferroelectric Domains By Atomic Force Microscopy, Oleg Kolosov, Alexei Gruverman, Jun Hatano, Koichiro Takahashi, Hiroshi Tokumoto

Alexei Gruverman Publications

The nanoscale visualization and control of domain structure with atomic force microscopy (AFM) in the ferroelectric crystal guanidinium aluminum sulfate hexahydrate is reported. The origin of the domain contrast in the topographic of AFM images is explained by the piezoelectric deformation of the crystal surface in the internal electric field. The domain structure was modified by applying a voltage to the conductive AFM tip. The dynamics of domain growth has been directly observed for the first time with a resolution of 10 nm.

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State-Selected Ion-Molecule Reactions: Statistical Calculations With Constraints, Cornelis J. Uiterwaal, J. Van Eck, A. Niehaus May 1995

State-Selected Ion-Molecule Reactions: Statistical Calculations With Constraints, Cornelis J. Uiterwaal, J. Van Eck, A. Niehaus

C.J.G.J. Uiterwaal Publications

For the two reactive systems, NH3+(Eint)+N H3→NH4++NH2 and H2+ (Eint)+H2→H3+ + H, for which the relative cross sections were measured earlier in our group for Ec.m.≈40 meV we calculated the relative cross section as a function of internal energy using the statistical Rice– Ramsperger–Kassel–Marcus (RRKM) theory that implicitly conserves total energy and total angular momentum. We found satisfactory agreement between theory and experiment by imposing rather mild constraints upon the loose transition state configuration. These constraints involve …

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Infrared Emission And Dynamics Of Outlfows In Late-Type Stars, Željko Ivezić, Moshe Elitzur May 1995

Infrared Emission And Dynamics Of Outlfows In Late-Type Stars, Željko Ivezić, Moshe Elitzur

Physics and Astronomy Faculty Publications

The dynamical structure and infrared emission of winds around late-type stars are studied in a self-consistent model that couples the equations of motion and radiative transfer. Thanks to its scaling properties, both the dynamics and IR spectrum of the solution are fully characterized by τF, the flux-averaged optical depth of the wind. Five types of dust grains are considered: astronomical silicate, crystalline olivine, graphite, amorphous carbon and SiC, as well as mixtures. Analysis of infrared signatures provides constraints on the grain chemical composition and indications for the simultaneous existence of silicate and carbon grains. The abundances of crystalline …

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Spin-Polarized Photoemission Studies Of The Exchange Splitting Of The Gd 5d Electrons Near The Curie Temperature, Dongqi Li, J. Pearson, S.D. Bader, D.N. Mcilroy, C. Waldfried, Peter A. Dowben May 1995

Spin-Polarized Photoemission Studies Of The Exchange Splitting Of The Gd 5d Electrons Near The Curie Temperature, Dongqi Li, J. Pearson, S.D. Bader, D.N. Mcilroy, C. Waldfried, Peter A. Dowben

Peter Dowben Publications

Spin-polarized photoemission was employed to investigate the temperature dependence of the exchange splitting of Gd(0001) for both the surface state near the Fermi level EF and the 5d bulk bands. The bulk bands at 1–2 eV below E F show Stoner-like behavior where two peaks with opposite spin polarization shift toward each other as the temperature increases. In contrast, the temperature dependence of the surface state indicates spin-mixing behavior due to fluctuating local moments. These differences are attributed to the degree of itinerancy of the 5d electrons in the two cases.

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Cosmological Histories For The New Variables Of Ashtekar, Seth Major, L. Smolin May 1995

Cosmological Histories For The New Variables Of Ashtekar, Seth Major, L. Smolin

Articles

Histories and measures for quantum cosmology are investigated through a quantization of the Bianchi type IX cosmology using path integral techniques. The result, derived in the context of Ashtekar variables, is compared with earlier work.

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Magnetic Force Microscopy Of The Submicron Magnetic Assembly In A Magnetotactic Bacterium, R. B. Proksch, T. E. Schäffer, B. M. Moskowitz, E. D. Dahlberg, Dennis A. Bazylinski, Richard B. Frankel May 1995

Magnetic Force Microscopy Of The Submicron Magnetic Assembly In A Magnetotactic Bacterium, R. B. Proksch, T. E. Schäffer, B. M. Moskowitz, E. D. Dahlberg, Dennis A. Bazylinski, Richard B. Frankel

Physics

A magnetic force microscope (MFM) was used to image topography and magnetic forces from a chain of submicron single magnetic domain particles produced by and contained in isolated magnetotactic bacteria. The noncontact magnetic force microscope data were used to determine a value for the magnetic moment of an individual bacterial cell, of order 10−13 emu, consistent with the average magnetic moment of bacteria from the same sample, obtained by superconducting quantum interference device magnetometry. The results represent the most sensitive quantification of a magnetic force microscope image to date.

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Bandwidth And Additive Bandwidth Of Graphs, Maria Kujawski May 1995

Bandwidth And Additive Bandwidth Of Graphs, Maria Kujawski

Honors Capstone Projects and Theses

No abstract provided.

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Disorder And Noncollinear Magnetism In Permanent-Magnet Materials With The Thmn12 Structure, R. Lorenz, J. Hafner, Sitaram Jaswal, David J. Sellmyer May 1995

Disorder And Noncollinear Magnetism In Permanent-Magnet Materials With The Thmn12 Structure, R. Lorenz, J. Hafner, Sitaram Jaswal, David J. Sellmyer

David Sellmyer Publications

We report calculations of the noncollinear magnetic structures of YFe12-xMox permanent-magnet materials, using a novel variant of a spin-polarized tight-binding–linear-muffin-tin-orbital technique allowing for local spin-quantization axes on each site and considering spin-orbit coupling. The ternary YFe12-xMox compounds crystallize in the tetragonal ThMn12 structure which can be stabilized only by the partial substitution of Fe by an early transition metal like Mo. We show that the substitutional disorder leads to canted spin structures at low Mo content ( x~1) and to spin-glass-like behavior at higher Mo content (x~3).

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Electron Scattering From 10b, A Cichocki, J Dubach, R S. Hicks, Gerald Alvin Peterson May 1995

Electron Scattering From 10b, A Cichocki, J Dubach, R S. Hicks, Gerald Alvin Peterson

Gerald Alvin Peterson

Electron scattering measurements have been made in order to determine the longitudinal and transverse form factors of low-lying level in 10B. With the exception of the broad 5.18 MeV level, results were obtained for all known levels up to 6.7 MeV. The measurements span the momentum transfer range q=0.48–2.58 fm-1. The primary objective of this work was to improve the data on the pure isovector M3 form factor of the 1.740 MeV excitation, the transform of which yields the 1p3/2 single-nucleon wave function. A Woods-Saxon potential was found to provide a much better representation of the data than the harmonic …

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Evidence Of Dilution-Induced Griffiths Instabilities In K2Cu1-XZnXF4 And Fe1-XZnXF2, Christian Binek, Wolfgang Kleeman May 1995

Evidence Of Dilution-Induced Griffiths Instabilities In K2Cu1-XZnXF4 And Fe1-XZnXF2, Christian Binek, Wolfgang Kleeman

Christian Binek Publications

Weak quasistatic singularities are evidenced within the Griffiths regime of temperatures Tc(x)≤T≤Tc(x=0) in the diluted antiferromagnet Fe1-xZnxF2, x=0.53, and in the diluted ferromagnet K2Cu1-xZnxF4, x=0.2, by measurements of the magnetic susceptibility, χ’ vs T, in zero external field. Significant deviations from classical (γ=1, for Fe0.47Zn0.53F2) and nonclassical (γ≳1, for K2Cu0.8Zn0.2F4) Curie-Weiss-type behavior, χ’∝[T-Tc(x)] -Y, are found just below Tc(x=0) by using linear-regression data …

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Distortion And Evolution Of A Localized Vortex In An Irrotational Flow, Joseph F. Lingevitch, Andrew J. Bernoff May 1995

Distortion And Evolution Of A Localized Vortex In An Irrotational Flow, Joseph F. Lingevitch, Andrew J. Bernoff

All HMC Faculty Publications and Research

This paper examines the interaction of an axisymmetric vortex monopole, such as a Lamb vortex, with a background irrotational flow. At leading order, the monopole is advected with the background flow velocity at the center of vorticity. However, inhomogeneities of the flow will cause the monopole to distort. It is shown that a shear‐diffusion mechanism, familiar from the study of mixing of passive scalars, plays an important role in the evolution of the vorticity distribution. Through this mechanism, nonaxisymmetric vorticity perturbations which do not shift the center of vorticity are homogenized along streamlines on a Re1/3 time scale, much faster …

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Resonant Photoemission Studies Of The Thickness Dependence Of The Unoccupied Gd 5D Bands, Peter A. Dowben, Dongqi Li, Jiandi Zhang, M. Onellion May 1995

Resonant Photoemission Studies Of The Thickness Dependence Of The Unoccupied Gd 5D Bands, Peter A. Dowben, Dongqi Li, Jiandi Zhang, M. Onellion

Peter Dowben Publications

Ultrathin Gd films have been studied with constant initial-state spectroscopy (CIS) of photoemission utilizing linearly polarized light of synchrotron radiation. The photoemission cross section of the 5d surface state near EF shows strong photon energy dependence, i.e., sharp peaks in CIS spectra near the Gd 5p1/2 absorption edge. These peaks originate from resonant photoemission processes involving the occupied 5p and unoccupied 5d levels of the surface atoms. The symmetry of the unoccupied states were determined experimentally by changing the polarization of light, where pp-polarized light excites only to the unoccupied dz2 …

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