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Effective determination of coexistence curves using reversible-scaling molecular dynamics simulations

Description: We present a simulation technique that allows the calculation of a phase coexistence curve from a single nonequilibrium molecular dynamics (MD) simulation. The approach is based on the simultaneous simulation of two coexisting phases, each in its own computational cell, and the integration of the relevant Clausius-Clapeyron equation starting from a known coexistence point. As an illustration of the effectiveness of our approach we apply the method to explore the melting curve in the Lennard-Jones phase diagram.
Date: December 18, 2000
Creator: de Koning, M; Antonelli, A & Yip, S
Partner: UNT Libraries Government Documents Department

Magnesium-Uranium Alloy System

Description: Analytical, x-ray, thermal, and metallographic data have been obtained in the study of the magnesium-uranium system, and a proposed phase diagram has been constructed.
Date: 1954
Creator: Tracy, George A.; Chiotti, P. & Wilhelm, Harley A.
Partner: UNT Libraries Government Documents Department

Density-functional calculations of a, b, g, d, dp, and e plutonium

Description: Total energies for the six known polymorphs of plutonium metal have been calculated within spin and orbital polarized density-functional theory as a function of lattice constant. Theoretical equilibrium volumes and bulk moduli correspond well with experimental data and the calculated total energies are consistent with the known phase diagram of Pu. It is shown that a preference for formation of magnetic moments, increasing through the {alpha} {yields} {beta} {yields} {gamma} phases, explain their position in the ambient pressure phase diagram and their anomalous variation of atomic density. A simple model is presented that establishes a relationship between atomic density, crystal symmetry, and magnetic moments which is universally valid for all Pu phases.
Date: October 22, 2003
Creator: Soderlind, P & Sadigh, B
Partner: UNT Libraries Government Documents Department

The f-spin physics of rare-earth iron pnictides: influence of d-electron antiferromagnetic order on heavy fermion phase diagram

Description: Some of the high {Tc} iron pnictides contain rare-earth elements, raising the question of how the existence and tunability of a d-electron antiferromagnetic order influences the heavy fermion behavior of the f-moments. With CeOFeP and CeOFeAs in mind as prototypes, we derive an extended Anderson lattice model appropriate for these quaternary systems. We show that the Kondo screening of the f-moments are efficiently suppressed by the d-electron ordering. We also argue that, inside the d-electron ordered state (as in CeOFeAs), the f-moments provide a rare realization of a quantum frustrated magnet with competing J{sub 1}-J{sub 2}-J{sub 3} interactions in an effective square lattice. Implications ofr the heavy fermion physics in broader contexts are also discussed.
Date: January 1, 2009
Creator: Zhu, Jian-xin; Dai, Jianhui & Si, Qimiao
Partner: UNT Libraries Government Documents Department

Effect of salt identity on the phase diagram for a globularprotein in aqueous electrolyte solution

Description: Monte Carlo simulations are used to establish the potential of mean force between two globular proteins in an aqueous electrolyte solution. This potential includes nonelectrostatic contributions arising from dispersion forces first, between the globular proteins, and second, between ions in solution and between each ion and the globular protein. These latter contributions are missing from standard models. The potential of mean force, obtained from simulation, is fitted to an analytic equation. Using our analytic potential of mean force and Barker-Henderson perturbation theory, we obtain phase diagrams for lysozyme solutions that include stable and metastable fluid-fluid and solid-fluid phases when the electrolyte is 0.2 M NaSCN or NaI or NaCl. The nature of the electrolyte has a significant effect on the phase diagram.
Date: February 22, 2006
Creator: Bostrom, Mathias; Tavares, Frederico W.; Ninham, Barry W. & Prausnitz, John M.
Partner: UNT Libraries Government Documents Department

Hadronic Correlations and Fluctuations

Description: We will provide a review of some of the physics which can be addressed by studying fluctuations and correlations in heavy ion collisions. We will discuss Lattice QCD results on fluctuations and correlations and will put them into context with observables which have been measured in heavy-ion collisions. Special attention will be given to the QCD critical point and the first order co-existence region, and we will discuss how the measurement of fluctuations and correlations can help in an experimental search for non-trivial structures in the QCD phase diagram.
Date: October 9, 2008
Creator: Koch, Volker
Partner: UNT Libraries Government Documents Department

Nature of the Triple Point in Chromium Alloys: Mode-Softening of the Incommensurate Spin Density Wave

Description: The magnetic stiffness parameter, A2, associated with the critical fluctuations in the paramagnetic phase close to the incommensurate spin-density-wave (SDW) transition of a Cr+0.18at.%Re single crystal, when compared with that of in pure chromium and dilute alloys of vanadium in chromium, indicates that the singularity responsible for the onset of the commensurate SDW phase occurs when A2 approaches zero at a finite value of the incommensurability parameter, delta 0.017 rlu, instead of zero as usually assumed. The mode of the incommensurate SDW softens as delta approaches this critical value with rhenium doping, and conversely hardens as delta increases when vanadium is added to chromium. Also, the amplitude of the critical scattering increases as delta approaches its end point, by a factor of about two going from 0.2 and 0.5at.%V to pure chromium, and by another factor near two in going from pure Cr to Cr+0.18at.%Re.
Date: December 31, 1997
Creator: Noakes, D.R.; Fawcett, E.; Sternlieb, B.L.; Shirane, G. & Jankowska, J.
Partner: UNT Libraries Government Documents Department

Estimated D$sub 2$-DT-T$sub 2$ phase diagram in the three-phase region

Description: A composite of experimental eH$sub 2$-D$sub 2$ phase-diagram data at the three-phase line is assembled from the literature. The phase diagram is a smooth cigar shape without a eutectic point, indicating complete miscibility of liquid and solid phases. Additional data are used to estimate the D$sub 2$-T$sub 2$, D$sub 2$-DT, and DT-T$sub 2$ binary phase diagrams. These are assembled into the ternary D$sub 2$-DT-T$sub 2$ phase diagram. A surface representing the chemical equilibrium of the three species is added to the phase diagram. At chemical equilibrium, it is estimated that 50-50 liquid D-T at 19.7 K is in equilibrium with 42 mole percent T vapor and 54 percent T solid. Infrared spectroscopy is suggested as a means of component analysis of liquid and solid mixtures. (auth)
Date: August 26, 1975
Creator: Souers, P.C.; Hickman, R.G. & Tsugawa, R.T.
Partner: UNT Libraries Government Documents Department

Phase Diagram of Iron, Revised-Core Temperatures

Description: Shock-wave experiments on iron preheated to 1,573 K conducted from 14 to 73 GPa, yield new data for sound velocities of the {gamma}- and liquid-phases. Melting was observed in the highest pressure ({approximately} 71 {+-} 2 GPa) experiments at calculated shock temperatures of 2,775 {+-} 160 K. This single crossing of the {gamma}-liquid boundary measured here agrees closely with the {gamma}-iron melting line determined by Boehler [1993], Saxena et al. [1993], and Jephcoat and Besedin [1997]. This {gamma}-iron melting curve is {approximately} 300 C lower than that of Shen et al. [1998b] at 80 GPa.
Date: January 27, 1999
Creator: Ahrens, T.J.; Chen, G.Q. & Holland, K.G.
Partner: UNT Libraries Government Documents Department

The energetics and electronic origins for atomic long- and short-range order in Ni-Fe invar alloys

Description: States of magnetic and compositional order are strongly coupled in many magnetic alloys, with Ni-Fe Invar being the most celebrated example. Results of an electronic-based method that addresses compositional and magnetic disorder, as well as atomic short-range order and energetics, are discussed. This allows a system-dependent microscopic understanding of the interplay of chemical, magnetic, and displacive effects, and a direct comparison to diffuse scattering experiments. Discussion is in context of total-energy calculations for various magnetic states in chemically disordered and ordered Ni- Fe alloys, emphasizing the importance of exchange-splitting and the implication for phase stability in Ni-Fe system.
Date: December 31, 1996
Creator: Johnson, D.D. & Shelton, W.A.
Partner: UNT Libraries Government Documents Department

The Dielectric Properties and Phase Transitions of [Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3}]{sub 0.905} (PbTiO{sub 3}){sub 0.095}: Influence of Pressure

Description: Studies of the influences of temperature, hydrostatic pressure, dc biasing field and frequency on the dielectric constant ({epsilon}{prime}) and loss (tan {delta}) of single crystal [pb (Zn{sub 1/3}Nb{sub 2/3})O{sub 3}]{sub 0.905} (PbTiO{sub 3}){sub 0.095}, or PZN-9.5PT for short, have provided a detailed view of the ferroelectric (FE) response and phase transitions of this technologically important material. While at 1 bar, the crystal exhibits on cooling a cubic-to-tetragonal FE transition followed by a second transition to a rhombohedral phase, pressure induces a FE-to-relaxer crossover, the relaxer phase becoming the ground state at pressures {ge}5 kbar. Analogy with earlier results suggests that this crossover is a common feature of compositionally-disordered soft mode ferroelectrics and can be understood in terms of a decrease in the correlation length among polar domains with increasing pressure. Application of a dc biasing electric field at 1 bar strengthens FE correlations, and can at high pressure re-stabilize the FE response. The pressure-temperature-electric field phase diagram was established. In the absence of dc bias the tetragonal phase vanishes at high pressure, the crystal exhibiting classic relaxor behavior. The dynamics of dipolar motion and the strong deviation from Curie-Weiss behavior of the susceptibility in the high temperature cubic phase are discussed.
Date: August 1, 2000
Partner: UNT Libraries Government Documents Department

Importance of interlayer effects for T{sub c} in {kappa}-(ET){sub 2}X superconductors.

Description: We report high-resolution measurements of the linear thermal-expansion coefficient perpendicular to the conducting planes, {alpha}{perpendicular}(T), on single crystals of the superconductors {kappa}-(ET){sub 2}X with X=I{sub 3}, CU(NCS){sub 2} and Cu[N(CN){sub 2}]Br. For all three compounds we find pronounced negative phase-transition anomalies at {Tc}. Using the Ehrenfest relation, the uniaxial-pressure coefficients of {Tc} can be determined. We find that interlayer effects are of crucial importance for {Tc} in this class of materials. Regarding also interlayer effects we compare our results with recent theories addressing the p-T phase diagram.
Date: August 22, 2000
Creator: Muller, J.; Lang, M.; Schlueter, J. A.; Geiser, U. & Schweitzer, D.
Partner: UNT Libraries Government Documents Department

Structure--properties relationships for manganese perovskites

Description: By combining the results of dc magnetization, ac susceptibility, magnetoresistivity, magnetostriction, and x-ray and neutron powder diffraction data for stoichiometric La{sub 1{minus}x}Sr{sub x}MnO{sub 3} the authors have constructed a phase diagram that describes the magnetic, transport, and structural properties and the relationships among them as a function of composition and temperature. Correlations among physical and structural properties have been observed that are consistent with a competition between ferromagnetism and JT distortion. A metallic state occurs below the Curie temperature when both coherent and incoherent JT distortions are suppressed.
Date: October 14, 1999
Creator: Dabrowski, B.; Xiong, X.; Chmaissem, O.; Bukowski, Z. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

C15 intermetallic compounds HfV{sub 2}+Nb

Description: Phase fields and equilibria in the Hf-V-Nb system were determined using a combination-of SEM, EDS and x-ray diffraction. The structural stability of the C15 HfV{sub 2+}Nb was studied by x-ray diffraction and specific heat measurements. The elastic constants of C15 HfV{sub 2+}Nb were measured by the resonant ultrasound spectroscopy technique. First-principle quantum mechanical calculations based on the local-density-functional theory have been employed to study the total energy and electronic structure of C15 HfV{sub 2}, which can be used to understand the physical and metallurgical properties of the C15 intermetallics HfV{sub 2+}Nb.
Date: December 31, 1994
Creator: Chu, F.; Chen, S.P.; Mitchell, T.E.; Pope, D.P. & Liu, C.T.
Partner: UNT Libraries Government Documents Department

Integrated Quantum/Classical Modeling of Hydrogenic Materials

Description: Path integral Monte Carlo simulations and calculations were performed on molecular hydrogen liquids. The equation-of-state, internal energies, and vapor liquid phase diagrams from simulation were found to be in quantitative agreement with experiments. Analytical calculations were performed on,H2 liquids using integral equation methods to study the degree of localization of the hydrogen molecules. Very little self-trapping or localization was found as a function of temperature and density. Good qualitative agreement was found between the integral equation calculations and the quantum Monte Carlo simulations for the radius of gyration of the hydrogen molecules. Path integral simulations were also performed on molecular hydrogen on graphite surfaces, slit pores, and in carbon nanotubes. Significant quantum effects on the adsorption of hydrogen were observed.
Date: November 1, 1999
Partner: UNT Libraries Government Documents Department

Magnetism in single-crystalline CePtSn.

Description: CePtSn exhibits two antiferromagnetic transitions at low temperatures. We report on magnetoresistance and in magnetization studies of single-crystalline CePtSn in magnetic fields up to 18 T. The data were taken to establish the magnetic phase diagrams for CePtSn in fields applied along the principal directions.
Date: August 4, 1999
Creator: Bordallo, H. N.; Chang, S.; Lacerda, A. H.; Nakotte, H.; Takabatake, T. & Torikachvili, M. S.
Partner: UNT Libraries Government Documents Department