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Random organization and plastic depinning

Description: We provide evidence that the general phenomenon of plastic depinning can be described as an absorbing phase transition, and shows the same features as the random organization which was recently studied in periodically driven particle systems [L. Corte, Nature Phys. 4, 420 (2008)]. In the plastic flow system, the pinned regime corresponds to the absorbing state and the moving state corresponds to the fluctuating state. When an external force is suddenly applied, the system eventually organizes into one of these two states with a time scale that diverges as a power law at a nonequilibrium transition. We propose a simple experiment to test for this transition in systems with random disorder.
Date: January 1, 2008
Creator: Reichhardt, Charles & Reichhardt, Cynthia
Partner: UNT Libraries Government Documents Department

Universal versus Materials-Dependent Two-Gap Behaviour of the High-Tc Cuprate Superconductors: Angle-Resolved Photoemission Study of La2-xSrxCuO4

Description: We have investigated the doping and temperature dependences of the pseudogap and superconducting gap in the single-layer cuprate La{sub 2-x}Sr{sub x}CuO{sub 4} by angle-resolved photoemission spectroscopy. The results clearly exhibit two distinct energy and temperature scales, namely, the gap around ({pi}, 0) of magnitude {Delta}* and the gap around the node characterized by the d-wave order parameter {Delta}{sub 0}. In comparison with Bi2212 having higher T{sub c}'s, {Delta}{sub 0} is smaller, while {Delta}* and T* are similar. This result suggests that {Delta}* and T* are approximately material-independent properties of a single CuO{sub 2} plane, in contrast to the material-dependent {Delta}0, representing the pairing strength.
Date: November 30, 2008
Creator: Yoshida, T.; Hashimoto, Makoto; Ideta, S.; Fujimori, Atsushi; Tanaka, K.; Mannella, Norman et al.
Partner: UNT Libraries Government Documents Department

Comment on"Elucidating the Mechanism of Nucleation near the Gas-Liquid Spinodal"

Description: In a recent Letter [1], Bhimalapuram, Chakrabarty and Bagchi (BCB) study the phase transformation mechanism of the Lennard-Jones fluid and the non-conserved Ising model. They compute the free energy as a function of the size of the largest droplet of the stable phase. In apparent contradiction to classical nucleation theory (CNT), they find that in both systems the free energy develops a minimum at subcritical cluster sizes. In this Comment we argue that this minimum is specific to the chosen order parameter, and that the observed behavior is in fact consistent with CNT. CNT states that the free energy F(N) of a single cluster of size N is a concave function with a maximum at the critical nucleus size N{sub c}. BCB, on the other hand, calculate the probability distribution of N*, the size of the largest cluster in the system, and compute the free energy {beta}F*(N*) = -ln P(N*), where {beta} = 1/k{sub B}T. This order parameter does not measure the size of a single cluster. Instead, when sampling small values of N*, one measures the statistical weight of configurations in which all clusters are at most N* in size. Hence a free energy penalty is incurred when one constrains N* to values smaller than the largest average cluster in the simulation volume V. It is this penalty that causes the sudden increase of F* as N* {yields} 0 and the minimum at intermediate values of N*. We now illustrate how F(N) can be calculated from simulations. Our argument is intuitive but not exact, a formal derivation that yields an equivalent result can be found in Ref. 2. We choose the Ising model for concreteness. We aim to compute the probability that a given cluster has size N, where we imagine the center of the cluster to be fixed ...
Date: June 18, 2008
Creator: Maibaum, Lutz & Maibaum, Lutz
Partner: UNT Libraries Government Documents Department

Coexistence of antiferromagnetic order and superconductivity in CeRh[sub 1[minus]x]M[sub x]In[sub 5](M=Ir and Co)

Description: We report a systematic neutron diffraction study on the coexistence of long-range magnetic order and superconductivity in heavy fermion compounds CeRhl-,M,Ins (M=Ir,Co). In addition to the incommensurate antiferromagnetic component in pure CeRhIn5, new type of antiferromagnetic component is found to concur with appearance of superconductivity in the Ir and Co alloy series. There is no detectable effect of the superconducting transition on magnetic order parameters. We compare those results with similar studies we performed on CeRhIn:, under pressure. We also discuss possible theoretical scenarios.
Date: December 4, 2002
Creator: Llobet-Megias, A. (Anna); Christianson, A. D. (Andrew D.); Bao, W. (Wei); Gardner, J. S.; Pagliuso, P. J. (Pascoal J.); Moreno, N. O. et al.
Partner: UNT Libraries Government Documents Department

Tail states in clean superconductors with magnetic impurities.

Description: We analyse the behavior of the density of states in a singlet s-wave superconductor with weak magnetic impurities in the clean limit. By using the method of optimal fluctuation and treating the order parameter self-consistently we show that the density of states is finite everywhere in the superconducting gap, and that it varies as in N(E) {proportional_to} -|E-{Delta}{sub 0}|{sup (7-d)/4} the mean field gap edge {Delta}{sub 0} in a d-dimensional superconductor. In contrast to most studied cases the optimal fluctuation is strongly anisotropic.
Date: January 1, 2002
Creator: Vekhter, I. (Ilya); Shytov, A. V.; Gruzberg, I. A. & Balatsky, A. V. (Alexander V.)
Partner: UNT Libraries Government Documents Department

Bulk Superconductivity and Disorder in Single Crystals of LaFePO

Description: We have studied the intrinsic normal and superconducting properties of the oxypnictide LaFePO. These samples exhibit bulk superconductivity and the evidence suggests that stoichiometric LaFePO is indeed superconducting, in contrast to other reports. We find that superconductivity is independent of the interplane residual resistivity {rho}{sub 0} and discuss the implications of this on the nature of the superconducting order parameter. Finally we find that, unlike T{sub c}, other properties in single-crystal LaFePO including the resistivity and magnetoresistance, can be very sensitive to disorder.
Date: February 15, 2010
Creator: Analytis, James G.; Chu, Jiun-Haw; Erickson, Ann S.; Kucharczyk, Chris; /Stanford U., Appl. Phys. Dept.; Serafin, Alessandro et al.
Partner: UNT Libraries Government Documents Department

A Phase-Field Model for Grain Growth

Description: A phase-field model for grain growth is briefly described. In this model, a poly-crystalline microstructure is represented by multiple structural order parameter fields whose temporal and spatial evolutions follow the time-dependent Ginzburg-Landau (TDGL) equations. Results from phase-field simulations of two-dimensional (2D) grain growth will be summarized and preliminary results on three-dimensional (3D) grain growth will be presented. The physical interpretation of the structural order parameter fields and the efficient and accurate semi-implicit Fourier spectral method for solving the TDGL equations will be briefly discussed.
Date: December 23, 1998
Creator: Chen, L.Q.; Fan, D.N. & Tikare, V.
Partner: UNT Libraries Government Documents Department

Phase Transitions with Semi-Diffuse Interfaces

Description: In this paper we examine new ''phase-field'' models with semi-diffuse interfaces. These models have the property that the -1/+1 planar phase transitions take place over a finite interval. The models also support multiple interface solutions with interfaces centered at arbitrary points L{sub 1} < L{sub 2} < ... < L{sub N}. These solutions correspond to local minima of an entropy functional rather than saddle points and are dynamically stable. The classical models have no such exact solutions but they do support solutions with N equally spaced transition points where the order parameter transitions between valves p{sub min}(N) and p{sub max}(N) satisfying -1 < p{sub min}(N) < 0 < p{sub max} (N) < 1. These solutions of the classical model are saddle points of the entropy functional associated with those models and are not dynamically stable.
Date: July 16, 2003
Creator: Greenberg, James M.
Partner: UNT Libraries Government Documents Department

Density of topological defects after a quench

Description: We present results of numerical studies of the Landau-Ginzburg dynamics of the order parameter in one-dimensional models inspired by the condensed matter analogues of cosmological phase transitions. The main goal of our work is to show that, as proposed by one of us, the density of the frozen-out topological defects is set by the competition between the quench rate - the rate at which the phase transition is taking place - and the relaxation rate of the order parameter. In other words, the characteristic domain size, which determines the typical separation of topological defects in the new broken symmetry phase, is of the order of the correlation length at the instant at which the relaxation timescale of the order parameter equals the time remaining to the phase transition. In estimating the size of topological domains, this scenario shares with the original Kibble mechanism the idea that topological defects will form along the boundaries of independently selected regions of the new broken symmetry vacuum. However, it derives the size of such domains from non-equilibrium aspects of the transition (quench rate), as opposed to Kibble`s original proposal in which their size was estimated from the Ginzburg temperature above which thermally activated symmetry restoration can occur.
Date: May 13, 1997
Creator: Laguna, P. & Zurek, W.H.
Partner: UNT Libraries Government Documents Department

MESON2000 Conference Summary

Description: This short contribution is a lite MESON2000 conference summary. As appropriate for the 600th anniversary of the Jagellonian University, it begins with a brief summary of the last 600 years of European history and its place in hadron physics. Next a ''physicist chirality'' order parameter PC is introduced. When applied to MESON2000 plenary speakers this order parameter illustrates the separation of hadron physicists into disjoint communities. The individual plenary talks in MESON2000 are next sorted according to the subconference associated with each of the 36 plenary speakers. Finally, I conclude with a previously unreported Feynman story regarding the use of models in hadron physics.
Date: April 26, 2001
Creator: Barnes, T.
Partner: UNT Libraries Government Documents Department

Increased medium-range order in amorphous silicon with increased substrate temperature

Description: Using fluctuation electron microscopy, the authors have measured the medium-range order of magnetron sputtered silicon thin films as a function of substrate temperature from the amorphous to polycrystalline regimes. They find a smooth increase in the medium-range order of the samples, which they interpret in the context of the paracrystalline structural model as an increase in the size of and/or volume fraction occupied by the paracrystalline grains. These data are counter to the long-standing belief that there is a sharp transition between amorphous and polycrystalline structures as a function of substrate temperature.
Date: August 15, 2000
Creator: Voyles, P. M.; Gerbi, J. E.; Treacy, M. M. J.; Gibson, J. M. & Aberlson, J. R.
Partner: UNT Libraries Government Documents Department

Medium-range order in hydrogenated amorphous silicon measured by fluctuation microscopy

Description: The authors have characterized with fluctuation electron microscopy the medium-range order of hydrogenated amorphous silicon thin films deposited by a variety of methods. Films were deposited by reactive magnetron sputtering, hot-wire chemical vapor deposition, and plasma enhanced chemical vapor deposition with and without H{sub 2} dilution of the SiH{sub 4} precursor gas. All of the films show the signature of the paracrystalline structure typical of amorphous Si. There are small variations in the degree of medium-range order with deposition methods and H{sub 2} content. The PECVD film grown with high H{sub 2} dilution contains Si crystals {approximately} 5 nm in diameter at a density of {approximately} 10{sup 9} cm{sup 2}. The amorphous matrix surrounding these crystals shows no difference in medium-range order from the standard PECVD film.
Date: April 17, 2000
Creator: Voyles, P. M.; Treacy, M. M. J.; Jin, H.-C.; Abelson, J. R.; Gibson, J. M.; Guha, S. et al.
Partner: UNT Libraries Government Documents Department

Disorder-driven nonequilibrium melting studied by electron diffraction, brillouis scattering, and molecular dynamics

Description: In the present paper, a brief overview of the electron diffraction, Brillouin scattering and molecular dynamics studies of radiation-induced amorphization of ordered intermetallic compounds is presented. In these studies, measured changes in the velocity of surface acoustic phonons, lattice constant, and the Bragg-Williams long-range order parameter induced by irradiation were compared with the results of computer simulations of defect-induced amorphization. The results indicate that progressive chemical disordering of the superlattice structure during irradiation is accompanied by an expansion of the lattice and a large change in sound velocity corresponding to a {approximately} 50% decrease in the average shear modulus. The onset of amorphization occurs when the average shear modulus of the crystalline compound becomes equal to that of the amorphous phase. This elastic softening criterion for the onset of amorphization and the dependence of the average shear modulus on the long-range-order parameter are in excellent agreement with molecular dynamics simulations. Both the experimental observations and computer simulations confirm the predictions of the generalized Lindemann melting criterion which stipulates that thermodynamic melting of a defective crystal occurs when the sum of the dynamic and static mean-square atomic displacements reaches a critical value identical to that for melting of the defect-free crystal. In this broader view of melting, the crystal-to-glass transformation is a disorder-driven nonequilibrium melting process occurring at temperatures below the Kauzmann isentropic glass-transition temperature.
Date: December 21, 1999
Creator: Okamoto, P. R.; Lam, N. Q. & Grimsditch, M.
Partner: UNT Libraries Government Documents Department

Ordering Phenomena in Undercooled Alloys

Description: Much of the work performed under this grant was devoted to using modern ideas in kinetics to understand atom movements in metallic alloys far from thermodynamic equilibrium. Kinetics arguments were based explicitly on the vacancy mechanism for atom movements. The emphasis was on how individual atom movements are influenced by the local chemical environment of the moving atom, and how atom movements cause changes in the local chemical environments. The author formulated a kinetic master equation method to treat atom movements on a crystal lattice with a vacancy mechanism. Some of these analyses [3,10,16] are as detailed as any treatment of the statistical kinetics of atom movements in crystalline alloys. Three results came from this work. Chronologically they were (1) A recognition that tracking time dependencies is not necessarily the best way to study kinetic phenomena. If multiple order parameters can be measured in a material, the ''kinetic path'' through the space spanned by these order parameters maybe just as informative about the chemical factors that affect atom movements [2,3,5-7,9-11,14-16,18,19,21,23,24,26,36,37]. (2) Kinetic paths need not follow the steepest gradient of the free energy function (this should be well-known), and for alloys far from equilibrium the free energy function can be almost useless in describing kinetic behavior. This is why the third result surprised me. (3) In cluster approximations with multiple order parameters, saddle points are common features of free energy functions. Interestingly, kinetic processes stall or change time scale when the kinetic path approaches a state at a saddle point in the free energy function, even though these states exist far from thermodynamic equilibrium. The author calls such a state a ''pseudostable'' (falsely stable) state [6,21,26]. I have also studied these phenomena by more ''exact'' Monte Carlo simulations. The kinetic paths showed features similar to those found in analytical theories. ...
Date: July 17, 1997
Creator: Fultz, Brent
Partner: UNT Libraries Government Documents Department

EXELFS of Metallic Glasses

Description: The feasibility of using extended energy-loss fine structure (EXELFS) obtained from {approximately}1 nm regions of metallic glasses to study their short-range order has been examined. Ionization edges of the metallic glasses in the electron energy-loss spectrum (EELS) have been obtained from PdNiP bulk metallic glass and Ni{sub 2}P polycrystalline powder in a transmission electron microscope. The complexity of EXELFS analysis of L- and M-ionization edges of heavy elements (Z>22, i.e. Ni and Pd) is addressed by theoretical calculations using an ab initio computer code, and its results are compared with the experimental data.
Date: November 30, 1999
Creator: Ito, Y.; Alamgir, F.M.; Schwarz, R.B.; Jain, H. & Williams, D.B.
Partner: UNT Libraries Government Documents Department

Intermediate-range order in lead metasilicate glass

Description: The complementarity of anomalous X-ray scattering (AXS) and neutron diffraction (ND) techniques is used here to investigate the nature and origin of intermediate-range order in lead metasilicate glass. Both X-ray and neutron source factors reveal small peaks at low wave vector which are shown to be associated with intermediate-range order of the Pb-O network. The combination of AXS and ND is shown to be a powerful tool to correlate contributions from the different atom pairs to such a peak. The information thus derived is compared with results from alkali germanate glasses and with structural data on corresponding crystalline compounds.
Date: March 1, 1997
Creator: Suzuya, Kentaro; Price, D.L.; Saboungi, M.L. & Ohno, Hideo
Partner: UNT Libraries Government Documents Department

Coexistence of Haldane Gap Excitations and Long-Range Order in R{sub 2}BaNiO{sub 5} (R=Rare Earth)

Description: R2BaNiO5 (R = rare earth) quasi-1-D antiferromagnets are structurally equivalent to the well-studied 1-D S = 1 Haldane-gap compound Y2BaNiO5. Unlike the Y-nickelate though, these materials undergo 3-D magnetic ordering at finite temperatures. Recent inelastic neutron scattering studies of Pr2BaNiO5 and (Nd(x)Y(1-x))2BaNiO5 revealed purely one-dimensional gap excitations that propagate exclusively on the Ni-chains and are strikingly similar to Haldane gap modes in Y2BaNiO5. In the ordered phase these excitations survive and actually coexist with conventional spin waves. The results suggest that the Haldane singlet ground state of the Ni-chains is not fully destroyed by Neel ordering.
Date: June 19, 1997
Creator: Zheludev, A.
Partner: UNT Libraries Government Documents Department

Ordering phenomena in undercooled alloys. Final report, 1992--1996

Description: Most of the work performed under this grant was on the topic of kinetics of ordering in undercooled alloys. Especially in the last year, however, the direction changed to a study of vibrational entropy of alloy phases. Kinetics arguments were based explicitly on the vacancy mechanism for atom movements. The emphasis was on how individual atom movements are influenced by the local chemical environment of the moving atom, and how atom movements cause changes in the local chemical environments. The author formulated a kinetic master equation method to treat atom movements on a crystal lattice with a vacancy mechanism. Some of these analyses are as detailed as any treatment of the statistical kinetics of atom movements in crystalline alloys. Three results came from this work and are given here.
Date: July 1, 1997
Creator: Fultz, B.
Partner: UNT Libraries Government Documents Department

Topological signatures of medium range order in amorphous semiconductor models

Description: The topological local cluster (or Schlaefli cluster) concept of Marians and Hobbs is used to detect topologically crystalline regions in models of disordered tetrahedral semiconductors. The authors present simple algorithms for detecting both Wells-type shortest circuits and O'Keeffe-type rings, which can be used to delineate alternative forms of the Schlaefli cluster in models.
Date: May 23, 2000
Creator: Treacy, M. M. J.; Voyles, P. M. & Gibson, J. M.
Partner: UNT Libraries Government Documents Department

Experimental methods and data analysis for fluctuation microscopy

Description: The authors have developed a new electron microscopy technique called fluctuation microscopy which is sensitive to medium-range order in disordered materials. The technique relies on quantitative statistical analysis of low-resolution dark-field electron micrographs. Extracting useful information from such micrographs involves correcting for the effects of the imaging system, incoherent image contrast caused by large scale structure in the sample, and the effects of the foil thickness.
Date: February 16, 2000
Creator: Voyles, P. M.; Treacy, M. M. J.; Gibson, J. M.; Jin, H.-C. & Abelson, J. R.
Partner: UNT Libraries Government Documents Department

Disorder-driven hysteresis-loop criticality in Co/CoO-films.

Description: The effect of magnetic disorder on the magnetization reversal process in thin Co/CoO-films has been investigated. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation. For temperatures above a critical temperature T{sub c}, we observe a discontinuous magnetization reversal, whereas smooth magnetization loops occur for T < T{sub c}. Our measurements establish the existence of a disorder-driven critical point in the non-equilibrium phase diagram. In addition, we observe scaling behavior in the vicinity of the critical point and determine the critical exponents to {beta} = 0.022 {+-} 0.006 and {beta}{delta} = 0.30 {+-} 0.03.
Date: November 1, 2000
Creator: Berger, A.; Inomata, A.; Jiang, J. S.; Pearson, J. E. & Bader, S. D.
Partner: UNT Libraries Government Documents Department


Description: The authors show that spin-orbit scattering off an isolated magnetic impurity in a singlet d{sub x{sup 2}-y{sup 2}} superconductor generates a d{sub xy} order parameter with locally broken time-reversal symmetry. A consequence of the induced d{sub xy} component are orbital charge currents and a magnetic field in the vicinity of the magnetic impurity.
Date: April 1, 1999
Creator: GRAF, M. & AL, ET
Partner: UNT Libraries Government Documents Department