Showing papers in "Physical Review in 2006"

Ambipolar charge transport in organic field-effect transistors

Abstract: A model describing charge transport in disordered ambipolar organic field-effect transistors is presented. The basis of this model is the variable-range hopping in an exponential density of states developed for disordered unipolar organic transistors. We show that the model can be used to calculate all regimes in unipolar as well as ambipolar organic transistors, by applying it to experimental data obtained from ambipolar organic transistors based on a narrow-gap organic molecule. The threshold voltage was determined independently from metal insulator semiconductor diode measurements. An excellent agreement between theory and experiment is observed over a wide range of biasing regimes and temperatures.

Violent folding of a flame front in a flame-acoustic resonance

Abstract: The first direct numerical simulations of violent flame folding because of the flame-acoustic resonance are performed. Flame propagates in a tube from an open end to a closed one. Acoustic amplitude becomes extremely large when the acoustic mode between the flame and the closed tube end comes in resonance with intrinsic flame oscillations. The acoustic oscillations produce an effective acceleration field at the flame front leading to a strong Rayleigh-Taylor instability during every second half period of the oscillations. The Rayleigh-Taylor instability makes the flame front strongly corrugated with elongated jets of heavy fuel mixture penetrating the burnt gas and even with pockets of unburned matter separated from the flame front.

Aging and intermittency in a p-spin model of a glass

Abstract: We numerically analyze the statistics of the heat flow between an aging system and its thermal bath, following a method proposed and tested for a spin-glass model in a recent paper [P. Sibani and H. J. Jensen, Europhys. Lett. 69, 563 (2005)]. The present system, which lacks quenched randomness, consists of Ising spins located on a cubic lattice, with each plaquette contributing to the total energy the product of the four spins located at its corners. Similarly to our previous findings, energy leaves the system in rare but large, so-called intermittent, bursts which are embedded in reversible and equilibriumlike fluctuations of zero average. The intermittent bursts, or quakes, dissipate the excess energy trapped in the initial state at a rate which falls off with the inverse of the age. This strongly heterogeneous dynamical picture is explained using the idea that quakes are triggered by energy fluctuations of record size, which occur independently within a number of thermalized domains. From the temperature dependence of the width of the reversible heat fluctuations we surmise that these domains have an exponential density of states. Finally, we show that the heat flow consists of a temperature independent term and a term with an Arrhenius temperature dependence. Microscopic dynamical and structural information can thus be extracted from numerical intermittency data. This type of analysis seems now within the reach of time resolved microcalorimetry techniques.

Ab initio calculations of mean free paths and stopping powers

Abstract: A method is presented for first-principles calculations of inelastic mean free paths and stopping powers in condensed matter over a broad energy range. The method is based on ab initio calculations of the dielectric function in the long wavelength limit using a real-space Green’s function formalism, together with extensions to finite momentum transfer. From these results we obtain the loss function and related quantities such as optical-oscillator strengths and mean excitation energies. From a many-pole representation of the dielectric function we then obtain the electron self-energy and inelastic mean free paths (IMFP). Finally using our calculated dielectric function and the optical-data model of Fernandez-Varea et al., we obtain collision stopping powers (CSP) and penetration ranges. The results are consistent with semi-empirical approaches and with experiment.

Absolute photoionization cross sections for Xe4+, Xe5+, Xe6+ near13.5 nm: Experiment and Theory

Abstract: Absolute photoionization cross-section measurements for amixture of ground and metastable states of Xe4+, Xe5+, and Xe6+ arereported in the photon energy range of 4d->nf transitions, which occurwithin or adjacent to the 13.5 nm window for extreme ultravioletlithography light source development. The reported values allow thequantification of opacity effects in xenon plasmas due to these 4d nfautoionizing states. The oscillator strengths for the 4d->4f and4d->5f transitions in Xeq+ (q=1-6) ions are calculated usingnonrelativistic Hartree-Fock and random phase approximations. These arecompared with published experimental values for Xe+ to Xe3+ and with thevalues obtained from the present experimental cross-section measurementsfor Xe4+ to Xe6+. The calculations assisted in the determination of themetastable content in the ion beams for Xe5+ and Xe6+. The experimentswere performed by merging a synchrotron photon beam generated by anundulator beamline of the Advanced Light Source with an ion beam producedby an electron cyclotron resonance ion source.

Hydrodynamic description of the adiabatic piston

Abstract: A closed macroscopic equation for the motion of the two-dimensional adiabatic piston is derived from standard hydrodynamics. It predicts a damped oscillatory motion of the piston towards a final rest position, which depends on the initial state. In the limit of large piston mass, the solution of this equation is in quantitative agreement with the results obtained from both hard disk molecular dynamics and hydrodynamics. The explicit forms of the basic characteristics of the piston's dynamics, such as the period of oscillations and the relaxation time, are derived. The limitations of the theory's validity, in terms of the main system parameters, are established.

Low energy effective gravitational equations on a Gauss-Bonnet brane

Abstract: We present effective gravitational equations at low energies in a Z{sub 2}-symmetric braneworld with the Gauss-Bonnet term. Our derivation is based on the geometrical projection approach, and we solve iteratively the bulk geometry using the gradient expansion scheme. Although the original field equations are quite complicated due to the presence of the Gauss-Bonnet term, our final result clearly has the form of the Einstein equations plus correction terms, which is simple enough to handle. As an application, we consider homogeneous and isotropic cosmology on the brane. We also comment on the holographic interpretation of bulk gravity in the Gauss-Bonnet braneworld.