Open Access. Powered by Scholars. Published by Universities.®

Physics Commons

Open Access. Powered by Scholars. Published by Universities.®

1994

Iowa State University

Discipline
Keyword
Publication
Publication Type

Articles 1 - 30 of 38

Full-Text Articles in Physics

Critical Dynamics Of Nonconserved Ising-Like Systems, K. E. Bassler, Beate Schmittmann Dec 1994

Critical Dynamics Of Nonconserved Ising-Like Systems, K. E. Bassler, Beate Schmittmann

Beate Schmittmann

We show that the dynamical fixed point of Ising-like models, characterized by a single scalar, nonconserved ordering field, is stable near four dimensions with respect to all dynamic perturbations, including those of a nonequilibrium nature.


Low‐Temperature Phase Diagram Of Ybbipt, R. Movshovich, A. Lacerda, Paul C. Canfield, J. D. Thompson, Z. Fisk Nov 1994

Low‐Temperature Phase Diagram Of Ybbipt, R. Movshovich, A. Lacerda, Paul C. Canfield, J. D. Thompson, Z. Fisk

Paul C. Canfield

Resistivity measurements are reported on the cubic heavy‐fermion compound YbBiPt at ambient and hydrostatic pressures to ≊19 kbar and in magnetic fields to 1 T. The phase transition at T c =0.4 K is identified by a sharp rise in resistivity. That feature is used to build low‐temperature H‐T and P‐Tphase diagrams. The phase boundary in the H‐T plane follows the weak‐coupling BCS expression remarkably well from T c to T c /4, while small hydrostatic pressure of ≊1 kbar suppresses the low‐temperature phase entirely. These effects of hydrostatic pressure and magnetic ...


Recent Developments In Modeling Of The Stress Derivative Of Magnetization In Ferromagnetic Materials, David C. Jiles, Michael K. Devine Nov 1994

Recent Developments In Modeling Of The Stress Derivative Of Magnetization In Ferromagnetic Materials, David C. Jiles, Michael K. Devine

Ames Laboratory Publications

The effect of changing stress on the magnetization of ferromagnetic materials leads to behavior in which the magnetization may increase, or decrease, when exposed to the same stress under the same external conditions. A simple empirical law seems to govern the behavior when the magnetization begins from a major hysteresis loop. The application of the law of approach, in which the derivative of the magnetization with respect to the elastic energy supplied dM/dW is proportional to the magnetization displacement M anM, is discussed.


Frequency Dependence Of Hysteresis Curves In Conducting Magnetic Materials, David C. Jiles Nov 1994

Frequency Dependence Of Hysteresis Curves In Conducting Magnetic Materials, David C. Jiles

Ames Laboratory Publications

An extension of the hysteresis model has been developed that takes into account the effects on the hysteresis curves of eddycurrents in electrically conducting media. In the derivation presented it is assumed that the frequency of the applied field is low enough (or the thickness of the material medium small enough) that the skin effect can be ignored so that the magnetic field penetrates uniformly throughout the material. In this case, the dc hysteresis equation is extended by the addition of a classical eddy‐current‐loss term depending on (i) the rate of change of magnetization with time, (ii) the ...


Estimation Of The Free-Charge-Carrier Concentration In Fast-Ion Conducting Na2s-B2s3 Glasses From An Analysis Of The Frequency-Dependent, Ernest F. Hairetdinov, Nikolai F. Uvarov, Hitendra K. Patel, Steve W. Martin Nov 1994

Estimation Of The Free-Charge-Carrier Concentration In Fast-Ion Conducting Na2s-B2s3 Glasses From An Analysis Of The Frequency-Dependent, Ernest F. Hairetdinov, Nikolai F. Uvarov, Hitendra K. Patel, Steve W. Martin

Materials Science and Engineering Publications

ac and dc conductivities of fast-ion conducting xNa2S+(1-x)B2S3 glasses in the composition range 0.001≤x≤0.1 were analyzed using the Almond-West formula σ(ν)=σdc[1+(ν/νH)α]. The hopping frequency for free charge carriers νH, the dc conductivity σdc, and their respective activation energies were determined. The concentration of free charge carriers evaluated from the conductivities and estimated mobilities is in good agreement with the overall content of sodium cations in the glasses and it is proposed that this supports the concept of a strong and not weak electrolyte model for these ...


Spontaneous Structure Formation In Driven Systems With Two Species: Exact Solutions In A Mean-Field Theory, I. Vilfan, R. K. P. Zia, Beate Schmittmann Oct 1994

Spontaneous Structure Formation In Driven Systems With Two Species: Exact Solutions In A Mean-Field Theory, I. Vilfan, R. K. P. Zia, Beate Schmittmann

Beate Schmittmann

A stochastic lattice gas of particles, subject to an excluded volume constraint and to a uniform external driving field, is investigated. Using a mean-field theory for a system with equal number of oppositely charged particles, exact results are obtained. Focusing on the current-vs-density plot, we propose an explanation for the discontinuous transition found in earlier simulations. A critical value of the drive, below which this transition becomes continuous, is found. These results are supported by a bifurcation analysis, leading to an equation of motion for the amplitude of the soft mode.


Crystallography And Magnetism Of The Heavy-Fermion Compound Ybbipt, R. A. Robinson, A. Purwanto, M. Kohgi, Paul C. Canfield, T. Kamiyama, T. Ishigaki, J. W. Lynn, R. Erwin, E. Peterson, R. Movshovich Oct 1994

Crystallography And Magnetism Of The Heavy-Fermion Compound Ybbipt, R. A. Robinson, A. Purwanto, M. Kohgi, Paul C. Canfield, T. Kamiyama, T. Ishigaki, J. W. Lynn, R. Erwin, E. Peterson, R. Movshovich

Physics and Astronomy Publications

The super-heavy-fermion compound YbBiPt has the largest known linear specific-heat coefficient γ=8 J mol−1 K−2, and the source of this enormous ‘‘electronic’’ specific heat is of great current interest. Here we describe neutron-diffraction studies that indicate its previously reported crystallographic structure to be incorrect. We find that the Pt atom is on the unique site and can be thought of as an interstitial in a fictitious rock-salt structure YbBi, which can in turn be thought of as an ordered form of elemental bismuth. We find no evidence of disorder between sites, occupancy on the nominally vacant site ...


Band Dispersion In C60(111): An Angle-Resolved Photoemission Study, P. J. Benning, C. G. Olson, David W. Lynch, J. H. Weaver Oct 1994

Band Dispersion In C60(111): An Angle-Resolved Photoemission Study, P. J. Benning, C. G. Olson, David W. Lynch, J. H. Weaver

Physics and Astronomy Publications

Angle-resolved photoemission studies of single-crystal C60(111) films grown on GeS(001) reveal changes in valence feature line shape with emission angle and photon energy that are indicative of band dispersion. For an excitation energy (hν) of 10 eV, normal emission spectra show four sharp structures within the ∼1.1-eV-wide valence feature derived from the second highest molecular orbital (HOMO-1) of C60. For hν=8.1 eV, the 1-eV-wide HOMO-derived feature exhibits changes with emission angle mainly due to dispersion of 0.6 eV in the unoccupied bands. The distribution of electronic states underlying HOMO and HOMO-1 indicates ...


Magnetic Pair Breaking In Honi2b2c, Alan I. Goldman, C. Stassis, Paul C. Canfield, J. Zarestky, P. Dervenagas, B. K. Cho, David C. Johnston, B. Sternlieb Oct 1994

Magnetic Pair Breaking In Honi2b2c, Alan I. Goldman, C. Stassis, Paul C. Canfield, J. Zarestky, P. Dervenagas, B. K. Cho, David C. Johnston, B. Sternlieb

Physics and Astronomy Publications

Neutron-diffraction techniques have been used to study the interplay between superconductivity and magnetism in HoNi2B2C (Tc=8 K). The experimental results, obtained on single crystals, show that below approximately 4.7 K, this compound is in a simple antiferromagnetic state that coexists with superconductivity. Between approximately 4.7 and 6 K, an incommensurate modulated magnetic structure has been found. This observation strongly suggests that pair breaking associated with this incommensurate magnetic structure is responsible for the deep minimum in Hc2 and the near-reentrant behavior observed in this compound at approximately 5 K.


Method For Producing Strain Tolerant Multifilamentary Oxide Superconducting Wire, Douglas K. Finnemore, Theodore A. Miller, Jerome E. Ostenson, Louis A. Schwartzkopf, Steven C. Sanders Jul 1994

Method For Producing Strain Tolerant Multifilamentary Oxide Superconducting Wire, Douglas K. Finnemore, Theodore A. Miller, Jerome E. Ostenson, Louis A. Schwartzkopf, Steven C. Sanders

Iowa State University Patents

A strain tolerant multifilamentary wire capable of carrying superconducting currents is provided comprising a plurality of discontinuous filaments formed from a high temperature superconducting material. The discontinuous filaments have a length at least several orders of magnitude greater than the filament diameter and are sufficiently strong while in an amorphous state to withstand compaction. A normal metal is interposed between and binds the discontinuous filaments to form a normal metal matrix capable of withstanding heat treatment for converting the filaments to a superconducting state. The geometry of the filaments within the normal metal matrix provides substantial filament-to-filament overlap, and the ...


Compositional Short-Range Ordering In Metallic Alloys: Band-Filling, Charge-Transfer, And Size Effects From A First-Principles All-Electron Landau-Type Theory, J. B. Staunton, Duane D. Johnson, F. J. Pinski Jul 1994

Compositional Short-Range Ordering In Metallic Alloys: Band-Filling, Charge-Transfer, And Size Effects From A First-Principles All-Electron Landau-Type Theory, J. B. Staunton, Duane D. Johnson, F. J. Pinski

Duane D. Johnson

Using a mean-field statistical description, we derive a general formalism to investigate atomic short-range order in alloys based on a density-functional description of the finite-temperature, grand potential of the random alloy. This ‘‘first-principles,’’ Landau-type approach attempts to treat several contributions (electronic structure, Fermi surface, electrostatics, magnetism, etc.) to the electronic energy on an equal footing. An important ingredient for the statistical averaging is the replacement of the molecular mean fields (Weiss fields) with Onsager cavity fields, which forces the diagonal part of the fluctuation-dissipation theorem to be obeyed. To show its general applicability and usefulness, we apply the theory to ...


First-Principles All-Electron Theory Of Atomic Short-Range Ordering In Metallic Alloys: D022- Versus L12-Like Correlations, Duane D. Johnson, J. B. Staunton, F. J. Pinski Jul 1994

First-Principles All-Electron Theory Of Atomic Short-Range Ordering In Metallic Alloys: D022- Versus L12-Like Correlations, Duane D. Johnson, J. B. Staunton, F. J. Pinski

Duane D. Johnson

We use a ‘‘first-principles’’ concentration-wave approach based on a finite-temperature, electronic density-functional, mean-field, grand potential of the random alloy to investigate the high-temperature atomic short-range order (ASRO) in Ni75V25 and Pd75V25 solid solutions. Experimentally, these similar alloys both develop D022-type long-range order at low temperatures but different ASRO at high temperatures. Our calculations describe the measured ASRO well. We compare these results with those found for a hypothetical Co75Ti25 solid solution. Since this alloy orders directly from the melt into the L12 phase, it should exhibit strong L12-like ASRO, as we find in our calculations. We analyze the features in ...


Surface-Induced Ordering In Asymmetric Block Copolymers, Y. Liu, W. Zhao, X. Zheng, Alexander H. King, A. Sing, M. H. Rafailovich, J. Sokolov, K. H. Dai, E. J. Kramer, S. A. Schwarz, O. Gebizlioglu, S. K. Sinha Jul 1994

Surface-Induced Ordering In Asymmetric Block Copolymers, Y. Liu, W. Zhao, X. Zheng, Alexander H. King, A. Sing, M. H. Rafailovich, J. Sokolov, K. H. Dai, E. J. Kramer, S. A. Schwarz, O. Gebizlioglu, S. K. Sinha

Alexander H. King

The surface-induced ordering in thin films of asymmetric deuterated polystyrene (dPS)- poly(viny1pyridine) (PVP) diblock and triblock copolymers of comparable polymerization index and PVP volume fraction - 0.25) was studied using transmission electron microscopy, atomic force microscopy, secondary ion massspectrometry, and neutron reflectivity. The morphology of both di- and triblock copolymer films was found to be cylindrical except for the layer adjacent to the silicon oxide surface, which due to the strong interaction of silica with PVP, was lamellar. The spacing between adjacent cylindrical layers was found to be consistent with mean field theory predictions. In the triblock copolymer films ...


Estimation Of Fatigue Exposure From Magnetic Coercivity, Z. J. Chen, David C. Jiles, J. Kameda May 1994

Estimation Of Fatigue Exposure From Magnetic Coercivity, Z. J. Chen, David C. Jiles, J. Kameda

Ames Laboratory Publications

An investigation of the effects of fatigue on A533B steel under constant load amplitude is reported in this paper. It was found that the plastic strain of the sample accumulated logarithmically with the number of stress cycles after initial fatigue softening. Based on the fact that plastic strain is often linearly related to the coercivity of material, at least for small changes of H c , a phenomenological relationship has been developed and tested to correlate the number of stress cycles to this magnetic parameter. This result represents the first successful attempt to relate the fatigue exposure directly to a magnetic ...


Monitoring Neutron Embrittlement In Nuclear Pressure Vessel Steels Using Micromagnetic Barkhausen Emissions, Levent B. Sipahi, Madhav Rao Govindaraju, David C. Jiles May 1994

Monitoring Neutron Embrittlement In Nuclear Pressure Vessel Steels Using Micromagnetic Barkhausen Emissions, Levent B. Sipahi, Madhav Rao Govindaraju, David C. Jiles

Center for Nondestructive Evaluation Publications

In nuclear power plants, neutron embrittlement of pressure vessel steels has been one of the main concerns. The use of micromagnetic Barkhausen emissions is a promising method to monitor the variations in microstructural and subsurface stress states due to their influence on these emissions. Measurements of these emissions can reveal neutron irradiationdegradation in nuclear power plant components. Samples which were irradiated at differentneutron fluences and annealed at different temperatures were obtained from three reactor surveillance programs. The results of different neutron fluences and annealing procedures showed noticeable fractional changes in the magnetic Barkhausen effect signal parameter, ΔMBE/MBE, and in ...


Renormalization-Group Study Of A Hybrid Driven Diffusive System, K. E. Bassler, Beate Schmittmann May 1994

Renormalization-Group Study Of A Hybrid Driven Diffusive System, K. E. Bassler, Beate Schmittmann

Beate Schmittmann

We consider a d-dimensional stochastic lattice gas of interacting particles, diffusing under the influence of a short-ranged, attractive Ising Hamiltonian and a ‘‘hybrid’’ external field which is a superposition of a uniform and an annealed random drive, acting in orthogonal subspaces of dimensions one and m, respectively. Driven into a nonequilibrium steady state, the half-filled system phase segregates via a continuous transition at a field-dependent critical temperature. Using renormalization-group techniques, we show that its critical behavior falls into a new universality class with upper critical dimension dc=5-m, characterized by two distinct anisotropy exponents, which, like all other indices, are ...


Spin-Hole Doping In The Kondo Insulator Ce3bi4pt3 Studied By Neutron Scattering, A. Severing, T. Perring, J. D. Thompson, Paul C. Canfield, Z. Fisk Apr 1994

Spin-Hole Doping In The Kondo Insulator Ce3bi4pt3 Studied By Neutron Scattering, A. Severing, T. Perring, J. D. Thompson, Paul C. Canfield, Z. Fisk

Physics and Astronomy Publications

Inelastic neutron scattering from (Ce1−xLax)3Bi4Pt3, with x = 0, 0.1 and 0.25, shows that a gap in the spin excitation spectrum decreases monotonically with increasing x and remains well-defined even for x = 0.25.


Elastic Constants Of Bcc Cu-Al-Ni Alloys, Lluís Mañosa, M. Jurado, Antoni Planes, Jerel L. Zarestky, Thomas A. Lograsso, C. Stassis Apr 1994

Elastic Constants Of Bcc Cu-Al-Ni Alloys, Lluís Mañosa, M. Jurado, Antoni Planes, Jerel L. Zarestky, Thomas A. Lograsso, C. Stassis

Ames Laboratory Publications

We have measured the adiabatic elastic constants of two Cu-Al-Ni martensitic alloys using ultrasonic methods and we have compared the results to recent neutron-scattering experiments. It is shown that the elastic behavior of Cu-Al-Ni alloys follows the same trends exhibited by other Cu-based alloys; in particular, the TA2 long-wavelength acoustic modes are softer than all other modes.


Magnetic Resonance Spectroscopy Of Fullerenes And [Pi]-Conjugated Polymers , Paul Andrew Lane Jan 1994

Magnetic Resonance Spectroscopy Of Fullerenes And [Pi]-Conjugated Polymers , Paul Andrew Lane

Retrospective Theses and Dissertations

The photophysics of two novel [pi]-conjugated systems, buckminsterfullerene and [pi]-conjugated polymers, are studied by the technique of Photoluminescence-Detected Magnetic Resonance (PLDMR);The PLDMR of C60 and C70 dissolved in toluene/polystyrene glasses and of fullerene films is described. The PLDMR of the glasses is due to a triplet exciton delocalized over the entire molecule. That of 3C70 strikingly differs from 3C60 in both lineshape and dynamics, due to the much longer triplet lifetime of 3C70. The PLDMR of films indicates that the delocalized triplet is distorted by intermolecular interactions and reveals a half-field resonance due to triplet-triplet fusion ...


Nmr And Nqr Study Of The Thermodynamically Stable Quasicrystals , Ananda Shastri Jan 1994

Nmr And Nqr Study Of The Thermodynamically Stable Quasicrystals , Ananda Shastri

Retrospective Theses and Dissertations

[superscript]27Al and [superscript]63,65Cu NMR measurements are reported for powder samples of stable AlCuFe and AlCuRu icosahedral quasicrystals and their crystalline approximants, and for a AlPdMn single grain quasicrystal. Furthermore, [superscript]27Al NQR spectra at 4.2 K have been observed in the AlCuFe and AlCuRu samples. From the quadrupole perturbed NMR spectra at different magnetic fields, and from the zero field NQR spectra, a wide distribution of local electric field gradient (EFG) tensor components and principal axis system orientations was found at the Al site. A model EFG calculation based on a 1/1 AlCuFe approximant was ...


Optical Anisotropy Of (110) Surfaces Of Ag And Cu , Zai Chen Jan 1994

Optical Anisotropy Of (110) Surfaces Of Ag And Cu , Zai Chen

Retrospective Theses and Dissertations

A reflectance-difference spectroscopy (RD) system was designed and built. Automatically controlled by a computer (PC), the system was used to study the anisotropic optical response of the (110) surfaces of Ag and Cu. Four Ag(110) surfaces were prepared, one epitaxially-grown single-crystalline thin film surface and three chemically etched single crystal surfaces. The RD spectra show all surfaces have very similar anisotropic peaks around 3.8 eV and 3.9 eV. This anisotropy is produced by the surface layer of atoms and is sensitive to the details of the electronic band structure. The peak structures can be attributed to transitions ...


Theory Of Magneto-Optics And Circular Magnetic X-Ray Dichroism , Xindong Wang Jan 1994

Theory Of Magneto-Optics And Circular Magnetic X-Ray Dichroism , Xindong Wang

Retrospective Theses and Dissertations

This thesis contains three parts. In the first part, before getting into the discussion of magneto-optical properties for the systems under study, the author reviews some background subjects, such as the energy-band theory within the framework of density functional theory, the multiple scattering approach to the solution of band problem and its relation with the LMTO method, an account of the tight-binding LMTO method which we use exclusively in our calculations, the linear response theory and its application to the quantum theory of dielectric constants. In the second part, circular magnetic x-ray dichroism, as a powerful and newly developed experimental ...


A Closed-Form Integration Method For State-Space Time-Domain Analysis Of Electronic Circuits And Systems , Bruce Alan Alspaugh Jan 1994

A Closed-Form Integration Method For State-Space Time-Domain Analysis Of Electronic Circuits And Systems , Bruce Alan Alspaugh

Retrospective Theses and Dissertations

While most traditional circuit simulation programs such as SPICE rely upon numerical integration techniques to perform transient analysis, this dissertation presents an alternative approach that relies on closed-form integration. Traditional numerical approaches offer no hope of finding an analytical solution, even if it is known that the solutions will be in a simple functional form. By using closed-form methods, the computer can find the exact analytical solution to the network and completely avoid problems associated with numerical approximations, determining appropriate integration step sizes, stiff systems of differential equations, or accumulated global error. Computer run time is independent of the amount ...


Characterization And Analysis Of Viscoelastically Loaded Thin Film Piezoelectric Resonators Incorporated In An Oscillator Microsensing System , Ronald Patrick O'Toole Jan 1994

Characterization And Analysis Of Viscoelastically Loaded Thin Film Piezoelectric Resonators Incorporated In An Oscillator Microsensing System , Ronald Patrick O'Toole

Retrospective Theses and Dissertations

In the recent advancement of piezoelectric resonator technology, there has been a large growth in the application of these devices for chemical sensing. These sensors operate by detecting changes in their environment which perturb the electrical - acoustic operation and in turn can be harnessed by means of supporting electronics and signal processing to monitor various processes. Examples include remote environmental monitoring, chemical process control, and commercial gas phase detectors. In this dissertation, the chemical sensing theory and properties of piezoelectric resonators such as the bulk-acoustic wave thin-film resonator (TFR) and the quartz crystal microbalance (QCM) are developed. This analysis concentrates ...


Fabrication And Characterization Of Hydrogenated Amorphous Silicon Films, Cvd Diamond Films, And Their Devices , Hao Jia Jan 1994

Fabrication And Characterization Of Hydrogenated Amorphous Silicon Films, Cvd Diamond Films, And Their Devices , Hao Jia

Retrospective Theses and Dissertations

Hydrogenated amorphous silicon (a-Si:H) films have been prepared by radio frequency (rf) sputtering technique, and studied by infrared (IR) absorption, small angle x-ray scattering (SAXS), electron spin resonance (ESR), and secondary ion mass spectrometry (SIMS). The optoelectronic properties of a-Si:H films were improved dramatically by annealing at 250-330°C; strong correlation between hydrogen content and SAXS determined microvoid was found in a-Si:H film; and contradictory evidences to the widely invoked "multiple trapping" model for hydrogen diffusion in a-Si:H were also found for the first time and discussed in this work;As the second part of this ...


Classical Wave Propagation In Periodic And Random Media , Sreela Datta Jan 1994

Classical Wave Propagation In Periodic And Random Media , Sreela Datta

Retrospective Theses and Dissertations

This thesis presents the results of numerical studies of scalar and electromagnetic wave propagation in periodic and random dielectric media. The photonic energy band structure of periodic composite materials, consisting of regular spherical arrays or scaffoldings of one kind of dielectric material in a different background material, are studied using the plane wave expansion method in order to determine, among other things, the optimum conditions (crystal structure, dielectric constant ratio, filling ratio etc.) for the existence of photonic band gaps. The significance of these numerical studies lies in the potential for important applications of photonic band gap materials and also ...


Sub-Micron Scale Conduction Processes On Clean Surfaces , Kevin R. Kimberlin Jan 1994

Sub-Micron Scale Conduction Processes On Clean Surfaces , Kevin R. Kimberlin

Retrospective Theses and Dissertations

Fundamental electrical conductance measurement has been studied in-situ in two dimensions in the Ag/Si(111) system as a function of the incident adatom flux rate with a four probe technique. A conductance study in a three dimensional conical structure was also carried out with field emission techniques. Field emission is now well established and has been used in this case to characterize a novel geometry for use in flat panel displays to replace cathode ray tube monitors. For the two dimensional study on Ag/Si(111), the origin of conduction is still unclear, as both transport by means of ...


Kinetics Of Restricted Solid-On-Solid Models Of Film Growth, James W. Evans, M. C. Bartelt Jan 1994

Kinetics Of Restricted Solid-On-Solid Models Of Film Growth, James W. Evans, M. C. Bartelt

Physics and Astronomy Publications

We consider the kinetics of irreversible film growth in solid-on-solid models with various restrictions on the adsorption (or growth) sites. We show how the master equations for the probabilities of subconfigurations of filled sites can be analyzed exactly to obtain coverages and spatial correlations for the first several layers. These provide an efficient framework for analysis of the early-stage growth kinetics, and indicate rapid attainment of asymptotic behavior. We illustrate the (1+1)- and (2+1)-dimensional cases for the simplest restricted solid-on-solid condition, and various modifications.


Interface Propagation And Nucleation Phenomena For Discontinuous Poisoning Transitions In Surface-Reaction Models, James W. Evans, T. R. Ray Jan 1994

Interface Propagation And Nucleation Phenomena For Discontinuous Poisoning Transitions In Surface-Reaction Models, James W. Evans, T. R. Ray

Physics and Astronomy Publications

Here, we consider discontinuous nonequilibrium phase transitions to poisoned or ‘‘adsorbing’’ states in lattice-gas models of surface reactions. Specifically, we examine the monomer or CO-poisoning transition in the Ziff-Gulari-Barshad monomer-dimer reaction model for CO oxidation, modified to include adspecies diffusion. For CO pressures below the poisoning transition, we first characterize the propagation of and fluctuations at an interface between reactive and CO-poisoned states. Here, we utilize ideas from spatial contact models, reaction-diffusion theory, and kinetic roughening theory. Evolution is described by the Kardar-Parisi-Zhang equation, but with the nonlinearity and kinetic surface tension vanishing on approaching the transition. Next, again for ...


Anisotropy In Nucleation And Growth Of Two-Dimensional Islands During Homoepitaxy On "Hex" Reconstructed Au(100), S. Günther, E. Kopatzki, M. C. Bartelt, James W. Evans, R. J. Behm Jan 1994

Anisotropy In Nucleation And Growth Of Two-Dimensional Islands During Homoepitaxy On "Hex" Reconstructed Au(100), S. Günther, E. Kopatzki, M. C. Bartelt, James W. Evans, R. J. Behm

Physics and Astronomy Publications

We present results of a comprehensive scanning tunneling microscopy study of the nucleation and growth of Au islands on Au(100). It is shown that the reconstruction of the substrate produces strong anisotropic effects. Rate equation analysis of the experimental flux and temperature dependence of the island density suggests: (i) a critical size of i=3 for T=315−380 K, but i>3 above 400 K; and (ii) strongly anisotropic diffusion, preferentially parallel to the reconstruction rows (activation energy ∼0.2 eV). We comment on energetic and kinetic aspects of the observed island shape anisotropy.