Marginal behavior of a two-dimensional fermionic system with saddle points

We calculated the self-energy atT = 0 for a two-dimensional fermionic system with hyperbolic dispersion. The existence of the saddle points in the energy gives rise to a marginal behavior, a result which has been obtained by numerical calculations.     

Marginal behavior of a layered two-dimensional electron gas interacting with localized spins

The self-energy of a layered two-dimensional (2D) electron gas interacting with localized spins has been calculated in the Fermi liquid approach. The (2D) electron-spin interaction gives rise to a spin gap in the energy spectrum, and if we include the plasmonic excitations a marginal behavior can occur for different values of the interlayer distance. The electron-spin interaction gives as the main effect the enhancement in the energy scale of the electronic excitations.     

Marginal fermi liquid behavior for a two-dimensional electronic system

The imaginary part of the self-energy of a two-dimensional Fermi system interacting with bosonic excitations has been calculated analytically and numerically. The result shows a marginal Fermi liquid behavior. The model can be a reasonable approach for studying the marginal behavior of a two-dimensional Millis-Monien-Pines phenomenological model.     

Toward a microscopic analysis of the marginal fermi liquid

We analyze the electron-boson interaction using many-body methods in the framework of the marginal Fermi liquid theory. Special attention is paid to the approximations necessary to obtain a marginal behavior. We show that the effective coupling constant between electrons is attractive on the entire energy scale. The renormalization of the bosonic spectrum due to the Coulomb interaction is also analyzed.     

Low-temperature thermodynamics of a one-dimensional interacting Fermi system

Using the exact solution of theSU(2) Thirring model, we derive an infinite set of coupled nonlinear integral equations describing the thermodynamic properties of a one-dimensional Fermi gas at a given temperature and magnetic field. The limiting case ofT=0 is considered, in which the magnetic field dependence of the ground state energy is calculated for both repulsive and attractive interactions.     

Feedback minimization of the first-passage failure of a hysteretic system under random excitations

The stochastic optimal bounded control of a hysteretic system for minimizing its first-passage failure is presented. The hysteretic system subjected to random excitation is firstly replaced by an equivalent nonlinear non-hysteretic system. The controlled non-hysteretic system is reduced to a one-dimensional controlled diffusion process by using the stochastic averaging of the energy envelope method. The dynamical programming equations and their associated boundary and final-time conditions for the problems of maximization of reliability and mean first-passage time are formulated. The optimal control law is derived from the dynamical programming equations and the control constraints. The dynamical programming equations for the maximum reliability problem and the mean first-passage time problem are finalized and solved numerically. Finally, numerical results are worked out to illustrate the application and effectiveness of the proposed method.     

Equivalent nonlinearization of nonlinear systems to random excitations

The broadest class of solvable reduced Fokker-Planck equation is given and a new equivalent nonlinear method is presented to obtain an approximate probability density function for the response of a nonlinear oscillator to Gaussian white noise excitations. The method is based on the least mean-square criterion and Euler equation. It is shown that this method, which is simpler and more reasonable, generalizes Caughey's method and gives the same results as Cai and Lin under purely additive excitations. Examples are given to show the applications of the method. In one of the examples, this method leads to a better approximation than that obtained from the energy dissipation criterion.     

A mechanism for pairing of RVB excitations

The fluctuations in the RVB order parameter over their meanfield values are considered in the BZA theory which mediate the interaction between the RVB excitations. The condition under which the effective interaction could be attractive and result in super-conductivity is investigated. The dependence of the transition temperature on the dopant concentration and the RVB order parameter is calculated.     

Plasmon-enhanced luminescence of Eu complex by using silver nanocubes for different excitations

The significant enhancement of luminescence of the europium (Eu) complex Eu(TTFA)₃ owing to the local surface plasmon resonance (LSPR) of the silver nanocubes was observed. The excitation and emission enhancements were both presented. The excitation enhancement mainly came from the balance of two aspects. Besides the spectra overlap between the LSPR spectrum and the excitation bands of Eu ions, the intrinsic intensity of the excitation of the Eu ions in absence of nanocubes also played an important role. The emission enhancement, which was the relative change in the radiative decay rate, non-radiative decay rate and quantum efficiency, of the Eu ions at 612 nm came from the change of the photonic mode density (PMD) and was concerned with concentrations of the nanocubes as well as the excitation wavelengths.     

Discretization of the Fermi surface in thin films

The thin film model in free-electron approximation is considered. A discrete Fermi surface is constructed depending on the film thickness and electron density. The Fermi wavevector, the mean energy per one electron and the density of states at the Fermi surface are calculated for the films with various thicknesses and different crystallographic orientation.     

Response analysis of nonlinear multi-degree-of-freedom systems with uncertain properties to non-Gaussian random excitations

The investigation reported in this paper is concerned with the development of an approach for response analysis of multi-degree-of-freedom (mdof) nonlinear systems with uncertain properties of large variations and under non-Gaussian nonstationary random excitations. The developed approach makes use of the stochastic central difference (SCD) method, time co-ordinate transformation (TCT), and adaptive time schemes (ATS). It is applicable to geometrically complicated systems idealized by the finite element method (FEM). For demonstration of its use and availability of results for direct comparison, a two-degree-of-freedom (tdof) nonlinear asymmetric system with uncertain natural frequencies and under Gaussian and non-Gaussian nonstationary random excitations is considered. Computed results obtained for the system with and without uncertain natural frequencies, and under Gaussian and non-Gaussian nonstationary random excitations are presented. It is concluded that the approach is relatively simple, accurate and efficient to apply.     

Superfluid Density in the BCS-BEC Crossover Regime of a Two-Component Fermi Gas at Finite Temperatures

We investigate the superfluid carrier density ρ _s in the BCS-BEC crossover regime of a superfluid Fermi gas at finite temperatures. Including pairing fluctuations within the gaussian fluctuation approximation, we determine the superfluid order parameter Δ, Fermi chemical potential μ, as well as ρ _s , in a consistent manner. In the weak-coupling regime, the normal fluid density ρ _n ≡ρ−ρ _s is dominated by quasi-particle excitations associated with the dissociation of Cooper pairs. As one approaches the BCS-BEC crossover regime, effects of pairing fluctuations on ρ _n become strong. In the strong-coupling BEC regime, ρ _n is shown to be dominated by Bogoliubov phonon in a molecular Bose superfluid.     

Seismic response analysis of a double-deck long-span cable-stayed bridge under multi-support excitations

In this article, the effect of earthquake ground motions, especially spatially varying motion, on the seismic response of a long-span double-deck cable-stayed bridge is investigated. Static and dynamic response analyses, with the focus on internal forces on web members, are carried out on the Shanghai Minpu Bridge which is the longest long-span double-deck cable-stayed highway bridge. Firstly, a program for the generation of multi-ground motions has been developed, based on the multivariate stochastic processes of the weighted amplitude wave superposition method in FORTRAN. Secondly, a refined finite element model composed of beam and shell elements and a simplified model using equivalent beam elements to model girders of the bridge are established in ABAQUS. Geometrical nonlinear static analysis is performed to get initial equilibrium configuration of the bridge for both the above models. Modal analysis is carried out on the initial equilibrium configuration and the natures of mode shapes for the refined model are discussed in detail. The dynamic response analyses under two groups of uniform seismic excitation indicate that the two models result in approximately identical results. Thirdly, the effect of nonlinear viscous dampers, located between the tower and the girder, on movement due to seismic excitation is investigated on the simplified model under uniform excitation. Results show that the dampers magnify the shear force and moment on lower tower columns in the longitudinal direction, but have relatively small effect on the upper tower columns. The shear force and moment of the tower base increase with the damping coefficient in a certain range. Finally, the characteristics of internal force for towers, subsidiary piers, and web members (vertical and oblique web members and side oblique chords) under multipoint excitation are examined. It is concluded that the abrupt changes on the shear envelope curves at the intersection of the tower column and the lower transverse beam are aroused due to the eight nonlinear fluid viscous dampers. The lateral moment of the subsidiary piers increases with the distance from the tower to subsidiary piers. The maximum axial forces of web members of side-span vary acutely, especially those of the vertical and oblique members close to the piers. For side oblique chords, the maximum axial force is uniformly distributed with the exception of members at the piers and the mid-span sections. Therefore, attention should be paid to the fluctuation forces of cables in seismic analysis of long-span cable-stayed bridges, since the uplift of girder ends may cause severe damage to bridges.     

Localization effects on magnetic excitations, Coulomb repulsion, and critical temperature in a quasi-3D superconductor

Using the Eliashberg equations for superconductivity in a quasi-3D high-temperature super-conductor, taking into consideration the¯q-dependent phonon-mediated electron-electron interaction, the effect of the disordered magnetic correlations, and Coulomb repulsion affected by disorder, we calculated the expression for the critical temperature. The phonon and the magnetic kernels have a logarithmic divergence and the critical temperature differs from the McMillan form. The Coulomb repulsion in the presence of disorder has been estimated. The disordered magnetic correlations can decrease the critical temperature or can suppress the superconductivity. The effect of various parameters on the decrease of the critical temperature has been analyzed.     

Quantum Phase Transition in the BCS-to-BEC Evolution of p-wave Fermi Gases

We discuss the possibility of a quantum phase transition in ultra-cold spin-polarized Fermi gases which exhibit a p-wave Feshbach resonance. We show that when fermionic atoms form a condensate that can be externally tuned between the BCS and BEC limits, the zero temperature compressibility and the spin susceptibility of the fermionic gas are non-analytic functions of the two-body bound state energy. This non-analyticity is due to a massive rearrangement of the momentum distribution in the ground state of the system. Furthermore, we show that the low temperature superfluid density is also non-analytic and exhibits a dramatic change in behavior when the critical value of the bound state energy is crossed.     

Shell Effects in the First Sound Velocity of an Ultracold Fermi Gas

We investigate the first sound of a normal dilute and ultracold two-component Fermi gas in a cylindrical trap with harmonic radial confinement. We show that the velocity of the sound that propagates along the axial direction strongly varies in the dimensional crossover of the system. In particular, we predict that the first-sound velocity exhibits shell effects: by increasing the density, that is by inducing the crossover from one to three-dimensions, the first-sound velocity shows jumps in correspondence with the filling of harmonic modes. The experimental achievability of these effects is discussed by considering ⁴⁰K atoms.      

Marginal effect of increasing ageing drivers on injury crashes

The safety effects of the ageing driving population have been a topic of research interests in health and transportation economics in recent years due to the ageing of the baby boomers. This study adds to the current knowledge by examining the marginal effects of changing the driver mix on injury crashes using data from the Canadian Province of Alberta between 1990 and 2004. Results from a Poisson regression model reveal that increasing the number of young and ageing drivers will result in an increase in the number of injury crashes whereas increasing the number of middle-aged drivers will result in a reduction. These results are in contrast to those obtained in a previous study on the marginal effects of changing the driver mix on fatal crashes in the Australian State of Queensland and some possible explanations for the differing results are provided.     

Flow and Critical Velocity of an Imbalanced Fermi Gas through an Optical Potential

Optical lattices offer the possibility to investigate the superfluid properties of both Bose condensates and Fermionic superfluid gases. When a population imbalance is present in a Fermi mixture, this leads to frustration of the pairing, and the superfluid properties will be affected. Here, we investigate how imbalance will influence the flow of a Fermi superfluid through an optical lattice. The flow through the lattice is analysed by taking into account coupling between neighboring layers of the optical lattice up to second order in the interlayer tunneling amplitude for single atoms. We find that the critical velocity of flow through the lattice decreases monotonically to zero as the imbalance is increased to 100%. Closed-form analytical expressions are given for the tunneling contribution to the action and for the critical velocity as a function of the binding energy of pairs in the (quasi) two-dimensional Fermi superfluid and as a function of the imbalance. These results are obtained in the mean-field approximation which is known to provide only a qualitative picture near unitarity. Nevertheless this mean-field result should provide a useful benchmark for theories that take into account fluctuations beyond the saddle-point.      

Persistent Currents in a Bose-Einstein Condensate in the Presence of Disorder

We examine bosonic atoms that are confined in a toroidal, quasi-one-dimensional trap, subjected to a random potential. The resulting inhomogeneous atomic density is smoothened for sufficiently strong, repulsive interatomic interactions. Statistical analysis of our simulations show that the gas supports persistent currents, which become more fragile due to the disorder.      

High Resolution Compton Scattering as a Probe of the Fermi Surface in the Iron-based Superconductor LaO1−x F x FeAs

We have carried out first-principles all-electron calculations of the (001)-projected 2D electron momentum density and the directional Compton profiles along the [100], [001] and [110] directions in the Fe-based superconductor LaOFeAs within the framework of the local density approximation. We identify Fermi surface features in the 2D electron momentum density and the directional Compton profiles, and discuss issues related to the observation of these features via Compton scattering experiments.