Diffusion in channeled structures: xenon in a crystalline sodalite

Diffusion in a soft confining environment: dynamic effects of thermal fluctuations

A dynamical model of a soft, thermally fluctuating two-dimensional tube is used to study the effect of thermal fluctuations of a confining environment on diffusive transport. The tube fluctuations in both space and time are driven by Brownian motion and suppressed by surface tension and the rigidity of the surrounding environment. The dynamical fluctuations modify the concentration profile boundary condition at the tube surface. They decrease the diffusive transport rate through the tube for two important cases: uniform tube fluctuations (wave vector, q=0 mode) for finite tube lengths and fluctuations of any wave vector for infinitely long tubes.

Phase-space algorithm for simulating quantum nonlinear response functions of bosons using stochastic classical trajectories

Using the positive P-representation of the density matrix, we develop an algorithm for calculating the quantum many-body nonlinear response functions of a system of bosons driven impulsively by external fields. The formalism maps the quantum time evolution of N boson degrees of freedom into a stochastic dynamics of 4N classical degrees of freedom. The first- and the third-order response functions are calculated by propagating the parameters of the P-representation using a set of coupled Langevin equations with multiplicative noise. These parameters serve as classical variables. Two classical ways for computing the response functions are presented. In the nonequilibrium method, an observable is calculated for weak impulsive pulses, and the response functions are obtained by taking its derivatives with respect to the pulse amplitudes. In the alternative, equilibrium simulation, the response functions are expressed in terms of time-correlation functions involving the P-representation parameters and stability matrices representing the perturbation of the trajectories. The stability matrices can be propagated simultaneously with the Langevin equations for the parameters. The formalism is generalized for a many-body boson system coupled to a harmonic bath.

Diffusion in channeled structures. III. Quantum corrections induced by lattice vibrations

For large energy barrier systems the lattice vibrations can play an important role in the diffusion of a guest component inside channeled structures. Here, quantum corrections to the guest diffusion are studied within a semiclassical framework in which the guest behaves classically and the lattice quantum mechanically. The permeability is expressed in terms of correlation functions that are calculated using path integrals. Forward-backward path integrals for propagators are combined, and, using the Martin-Siggia-Rose formalism [Phys. Rev. A. 8, 423 (1973)], are transformed into a set of generalized Langevin equations that reduce to the classical equation of motion at high temperatures. The random initial conditions to these equations are specified by the density matrix from which an approximate expression for the quantum mechanical potential of mean force is derived. The quantum potential of mean force and activation energy obtained for the guest inside the lattice is slightly higher than the one for the fully classical system, and the diffusion of the guest in the quantum lattice is slower compared to its fully classical counterpart. The macroscopic intrinsic permeability of alpha -quartz towards neon is reported, P'qtm(300K)=9.91x10(8) s/(mkg) within the semiclassical approximation and is lower than P'class (300K)=1.26x10(9) s/(mkg) obtained in the classical case at 300K for the same potential model.