Tubular structures of GaS

In this Brief Report we demonstrate, using density-functional tight-binding theory, that gallium sulfide (GaS) tubular nanostructures are stable and energetically viable. The GaS-based nanotubes have a semiconducting direct gap which grows towards the value of two-dimensional hexagonal GaS sheet and is in contrast to carbon nanotubes largely independent of chirality. We further report on the mechanical properties of the GaS-based nanotubes

Ab initio simulations of liquid systems: Concentration dependence of the electric conductivity of NaSn alloys

Liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80% sodium) are studied using density functional calculations combined with molecular dynamics(Car-Parrinello method). The frequency-dependent electric conductivities for the systems are calculated by means of the Kubo-Greenwood formula. The extrapolated DC conductivities are in good agreement with the experimental data and reproduce the strong variation with the concentration. The maximum of conductivity is obtained, in agreement with experiment, near the equimolar composition. The strong variation of conductivity, ranging from almost semiconducting up to metallic behaviour, can be understood by an analysis of the densities-of-states.Comment: LaTex 6 pages and 2 figures, to appear in J.Phys. Cond. Ma

Thermodynamics of Thermoelectric Phenomena and Applications

Fifty years ago, the optimization of thermoelectric devices was analyzed by considering the relation between optimal performances and local entropy production. Entropy is produced by the irreversible processes in thermoelectric devices. If these processes could be eliminated, entropy production would be reduced to zero, and the limiting Carnot efficiency or coefficient of performance would be obtained. In the present review, we start with some fundamental thermodynamic considerations relevant for thermoelectrics. Based on a historical overview, we reconsider the interrelation between optimal performances and local entropy production by using the compatibility approach together with the thermodynamic arguments. Using the relative current density and the thermoelectric potential, we show that minimum entropy production can be obtained when the thermoelectric potential is a specific, optimal value

Dynamical Evolution of Galaxies in Clusters

Tidal forces acting on galaxies in clusters lead to a strong dynamical evolution. In order to quantify the amount of evolution, I run self-consistent N-body simulations of disk galaxies for a variety of models in the hierarchically forming clusters. The tidal field along the galactic orbits is extracted from the simulations of cluster formation in the Omega_0=1; Omega_0=0.4; and Omega_0=0.4, Omega_Lambda=0.6 cosmological scenarios. For large spiral galaxies with the rotation speed of 250 km/s, tidal interactions truncate massive dark matter halos at 30 +- 6 kpc, and thicken stellar disks by a factor 2 to 3, increasing Toomre's parameter to Q > 2 and halting star formation. Low density galaxies, such as the dwarf spheroidals with the circular velocity of 20 km/s and the extended low surface brightness galaxies with the scale length of 10-15 kpc, are completely disrupted by tidal shocks. Their debris contribute to the diffuse intracluster light. The tidal effects are significant not only in the core but throughout the cluster and can be parametrized by the critical tidal density. The tidally-induced evolution results in the transformation of the infalling spirals into S0 galaxies and in the depletion of the LSB population. In the low Omega_0 cosmological models, clusters form earlier and produce stronger evolution of galaxies.Comment: accepted to Ap

Giant vesicles at the prolate-oblate transition: A macroscopic bistable system

Giant phospholipid vesicles are shown to exhibit thermally activated transitions between a prolate and an oblate shape on a time scale of several seconds. From the fluctuating contour of such a vesicle we extract ellipticity as an effective reaction coordinate whose temporal probability distribution is bimodal. We then reconstruct the effective potential from which we derive an activation energy of the order of $k_BT$ in agreement with theoretical calculations. The dynamics of this transition is well described within a Kramers model of overdamped diffusion in a bistable potential. Thus, this system can serve as a model for macroscopic bistability.Comment: 10 pages, LaTeX, epsfig, 4 eps figures included, to appear in Europhys. Let

Ab initio simulations of liquid NaSn alloys: Zintl anions and network formation

Using the Car-Parrinello technique, ab initio molecular dynamics simulations are performed for liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80 % sodium). The obtained structure factors agree well with the data from neutron scattering experiments. The measured prepeak in the structure factor is reproduced qualitatively for most compositions. The calculated and measured positions of all peaks show the same trend as function of the composition.\\ The dynamic simulations also yield information about the formation and stability of Sn$_4$ clusters (Zintl anions) in the liquid. In our simulations of compositions with 50 and 57 % sodium we observe the formation of networks of tin atoms. Thus, isolated tin clusters are not stable in such liquids. For the composition with 20 % tin only isolated atoms or dimers of tin appear, ``octet compounds'' of one Sn atom surrounded by 4 Na atoms are not observed.Comment: 12 pages, Latex, 3 Figures on reques

A general variational principle for spherically symmetric perturbations in diffeomorphism covariant theories

We present a general method for the analysis of the stability of static, spherically symmetric solutions to spherically symmetric perturbations in an arbitrary diffeomorphism covariant Lagrangian field theory. Our method involves fixing the gauge and solving the linearized gravitational field equations to eliminate the metric perturbation variable in terms of the matter variables. In a wide class of cases--which include f(R) gravity, the Einstein-aether theory of Jacobson and Mattingly, and Bekenstein's TeVeS theory--the remaining perturbation equations for the matter fields are second order in time. We show how the symplectic current arising from the original Lagrangian gives rise to a symmetric bilinear form on the variables of the reduced theory. If this bilinear form is positive definite, it provides an inner product that puts the equations of motion of the reduced theory into a self-adjoint form. A variational principle can then be written down immediately, from which stability can be tested readily. We illustrate our method in the case of Einstein's equation with perfect fluid matter, thereby re-deriving, in a systematic manner, Chandrasekhar's variational principle for radial oscillations of spherically symmetric stars. In a subsequent paper, we will apply our analysis to f(R) gravity, the Einstein-aether theory, and Bekenstein's TeVeS theory.Comment: 13 pages; submitted to Phys. Rev. D. v2: changed formatting, added conclusion, corrected sign convention

Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm−1^{-1} from a metallic spintronic emitter

To explore the capabilities of metallic spintronic thin-film stacks as a source of intense and broadband terahertz electromagnetic fields, we excite a W/CoFeB/Pt trilayer on a large-area glass substrate (diameter of 7.5 cm) by a femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, wavelength 800 nm). After focusing, the emitted terahertz pulse is measured to have a duration of 230 fs, a peak field of 300 kV cm$^{-1}$ and an energy of 5 nJ. In particular, the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of amplitude maximum, thereby facilitating nonlinear control over matter in this difficult-to-reach frequency range and on the sub-picosecond time scale.Comment: 7 pages, 4 figure

Stochastic thermodynamics of chemical reaction networks

For chemical reaction networks described by a master equation, we define energy and entropy on a stochastic trajectory and develop a consistent nonequilibrium thermodynamic description along a single stochastic trajectory of reaction events. A first-law like energy balance relates internal energy, applied (chemical) work and dissipated heat for every single reaction. Entropy production along a single trajectory involves a sum over changes in the entropy of the network itself and the entropy of the medium. The latter is given by the exchanged heat identified through the first law. Total entropy production is constrained by an integral fluctuation theorem for networks arbitrarily driven by time-dependent rates and a detailed fluctuation theorem for networks in the steady state. Further exact relations like a generalized Jarzynski relation and a generalized Clausius inequality are discussed. We illustrate these results for a three-species cyclic reaction network which exhibits nonequilibrium steady states as well as transitions between different steady states.Comment: 14 pages, 2 figures, accepted for publication in J. Chem. Phy

Mobility and Diffusion of a Tagged Particle in a Driven Colloidal Suspension

We study numerically the influence of density and strain rate on the diffusion and mobility of a single tagged particle in a sheared colloidal suspension. We determine independently the time-dependent velocity autocorrelation functions and, through a novel method, the response functions with respect to a small force. While both the diffusion coefficient and the mobility depend on the strain rate the latter exhibits a rather weak dependency. Somewhat surprisingly, we find that the initial decay of response and correlation functions coincide, allowing for an interpretation in terms of an 'effective temperature'. Such a phenomenological effective temperature recovers the Einstein relation in nonequilibrium. We show that our data is well described by two expansions to lowest order in the strain rate.Comment: submitted to EP