Bulletin of the American Physical Society
2008 Joint Fall Meeting of the Texas and Four Corners Sections of APS, AAPT, and Zones 13 and 16 of SPS, and the Societies of Hispanic & Black Physicists
Volume 53, Number 11
Friday–Saturday, October 17–18, 2008; El Paso, Texas
Session C3: General Theory I |
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Chair: Eric Hirschmann, Brigham Young University Room: Union East, 3rd Floor Smiley |
Friday, October 17, 2008 1:30PM - 1:42PM |
C3.00001: Time of Flight Curves For Charge Transport with Spatially Correlated Energetic Disorder David H. Dunlap, Paul E. Parris Among the ubiquitous characteristics of charge transport in amorphous semiconductors are the field dependent Poole-Frenkel (PF) mobility and the anomalously broad current-time transients observed in time of flight experiments. The Poole-Frenkel law has been attributed to the fact that the long-range electrostatic interactions in these uncompensated materials are only weakly screened. Consequently the energy landscape over which charges move is smooth and correlated. While the correlations obviously play a role in the general shape of the current-time curves, the relationship between the PF mobility and the shape of the transients has not yet been quantified. In this talk we present results from simulations and analytic calculations for a one-dimensional model. We report on the connection between PF behavior and the universality of the transient shapes seen in both dispersive and non-dispersive regimes. [Preview Abstract] |
Friday, October 17, 2008 1:42PM - 1:54PM |
C3.00002: Coupling of electrons to the electromagnetic field in a localized basis Roland Allen A simple formula is obtained for coupling electrons in a complex system to the electromagnetic field. It includes the effect of intra-atomic excitations and nuclear motion, and can be applied in, e.g., first-principles-based simulations of the coupled dynamics of electrons and nuclei in materials and molecules responding to ultrashort laser pulses. Some additional aspects of nonadiabatic dynamical simulations are also discussed, including the potential of ``reduced Ehrenfest'' simulations for treating problems where standard Ehrenfest simulations will fail. [Preview Abstract] |
Friday, October 17, 2008 1:54PM - 2:06PM |
C3.00003: Transient Conductivity of Kapton HN Justin Dekany, J.R. Dennison, Steve Hart The transient conductivity of the polymer Kapton HN has been determined by applying a constant voltage across the material and measuring the resulting current decay, collected over more than four orders of magnitude in time, using a custom vacuum system with a stable low-level electrometer. The model, used to describe conductivity as the material reaches a stable state, includes two distinct components. First, there is a polarization, with exponential time dependence, that results from the realignment of the molecular dipoles in the material that orient parallel to the applied electric field. The second component of the transient current flow, diffusive conductivity, with power law time dependence, results from a diffusion of charge injected into the material from the voltage plate. The model allows for more than one mechanism---and a corresponding decay term---for both polarization and diffusive conductivity. Results of the analysis are interpreted in terms of the polymer's complex molecular structure, blend of amorphous and nanocrystalline structure, and the nature of localized states used to determine the carrier density in insulating polymers. [Preview Abstract] |
Friday, October 17, 2008 2:06PM - 2:18PM |
C3.00004: Maximum Entropy Principle for the Microcanonical Ensemble Donald Kobe, Michele Campisi We use the Maximum Entropy Principle with a R\'{e}nyi entropy for a system in contact with a finite heat bath. The generalized distribution function we obtain is one previously proposed in a different context by C. Tsallis called the escort distribution with a parameter $q$. We show that this generalized distribution function describes a heat bath whose heat capacity can vary from zero to infinity depending on $q$. When \textit{q $\to $ 1} from below, the heat capacity of the bath becomes infinite and we obtain the canonical distribution. When \textit{q $\to -\infty $} the heat capacity of the bath reduces to zero and we obtain the microcanonical ensemble. When thermodynamic variables, like internal energy and pressure, are defined as statistical averages of mechanical quantities over the generalized distribution, they satisfy the combined first and second laws of thermodynamics, which Boltzmann called the ``heat theorem.'' [Preview Abstract] |
Friday, October 17, 2008 2:18PM - 2:30PM |
C3.00005: Convergence Study of the Path Integral Monte Carlo Technique for a Quantum Particle in a Supercritical Fluid Terrence Reese, Bruce Miller A quantum particle (qp) in a fluid near the liquid vapor-critical point creates a volume of altered density in which it becomes localized. In previous research we have used the Path Integral Monte Carlo (PIMC) technique to investigate the properties of the qp-fluid molecule system. The path integral formulation represents the quantum particle as a closed chain of P classical particles in which the quantum spread of the qp is manifest in the spread of the chain. This formulation allows classical Monte Carlo techniques to be used to compute quantum mechanical equilibrium values. In this work we will explore the convergence of the PIMC technique for a qp equilibrated in a Lennard-Jones fluid whose characteristics resemble Xenon. The computations were done for two densities of the fluid at 300 and 340K. The correlation function, the number of independent samples and the radius of gyration were computed. The autocorrelation function for the pick-off decay rate was computed for increments of 500 and 5000. It was discovered that for the lower density the computed values of the pick-off decay rate became independent within an increment of 500, while near the critical density an increment of more than 1000 was required for the values to become uncorrelated. [Preview Abstract] |
Friday, October 17, 2008 2:30PM - 2:42PM |
C3.00006: Parallelizing a Two-Species Direct Simulation Monte Carlo Calculation Daniel Wilcox, Ross Spencer The computer simulation of a rarified gas usually requires a particle-based simulation technique. Direct Simulation Monte Carlo (DSMC) is such a technique, and is a common choice for rarified gas dynamics. Very large problems are infeasible on a single computer, due to time and memory constraints. A method will be presented to parallelize a two-dimensional DSMC simulation that has two species of particles. Special considerations for the boundary conditions will be presented. Analysis will be provided of speedup and of capability for large problems. Computed images of the supersonic flow of a gas mixture through a small nozzle will also be shown. [Preview Abstract] |
Friday, October 17, 2008 2:42PM - 2:54PM |
C3.00007: A Diffusion Monte Carlo study of spin polarized Fermi gases Sumita Datta S-wave BCS super-fluid with unequal spin population and short-range interaction, phase separates into a BEC super-fluid and a fully polarized Fermi gas. In addition to these two states it has been suggested that there are some exotic states called FFLO states (Parish et al, Nature Physics, Vol. 3, Feb 2007). We apply Feynman Kac method to Li-6 to search for the signatures of the above states both at zero and finite temperature and to study the effect of non-perturbative treatment on its phase diagram. [Preview Abstract] |
Friday, October 17, 2008 2:54PM - 3:06PM |
C3.00008: Mesoscopic effects in Bose-Einstein condensate fluctuations of a weakly interacting gas in a box Konstantin Dorfman I study the quantum and thermal fluctuations of the Bose-Einstein condensate (BEC) in a box with the periodic boundary conditions under a particle-number constraint. I start with the particle-number conserving operator formalism of Girardeau and Arnowitt. I employ an expansion of the distribution function in terms of the multinomial coefficients. I present analytical formulas and numerical calculations for the central moments of the ground state occupation fluctuations in an weakly interacting Bose gas in a box with a mesoscopic number of particles in the framework of the Bogoliubov approximation. I discuss the mesoscopic effects in statistics of a weakly interacting gas versus statistics in the thermodynamic limit and statistics of an ideal gas. I emphasize the non-Gaussian nature of the BEC fluctuations. The crossover relations between the fluctuations of weakly interacting Bose gas and ideal Bose gas are obtained. In particular, high and low temperature asymptotics are presented. Suppression of the condensate fluctuations at the moderate temperatures and their enhancement at very low temperatures are described. [Preview Abstract] |
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