Time-varying External Field Research Articles

Monte-carlo simulation of the P-E hysteresis and domain switching in ferroelectric Potts lattice

A Monte-Carlo approach based on the Q-state Potts model is developed to simulate dynamic hysteresis of ferroelectric Potts lattice against periodic time-varying external field E. The dynamic responses of the hysteresis dependent of frequency ω of applied field and domain size R are studied. The P-E loop exhibits very different patterns in various ranges of frequency. Its area can be scaled with Ω-ω1/3 at low ω The remnant polarization Pr shows a maximal as a function of ω At small domain size, the dynamic hysteresis is significantly compressed, predicting ferroelectricity weakening effect at small domain size.

ANTIOPE, a program for computer experiments on spin dynamics

ANTIOPE is a self-contained program, running on personal computers, for simulating a variety of real and hypothetical NMR experiments. Systems of up to five spins with fixed or anisotropic truncated Hamiltonians can be subjected to time-varying external fields of arbitrary amplitudes, frequencies, and phases and to the effects of random motions and sample rotations. The resulting 1D and 2D data sets can be manipulated and plotted interactively. No conventional programming skills are required: the program interacts with the user in NMR terms and provides extensive prompting, error trapping, warning facilities and context-sensitive help.

Transient magnetic field and temperature modeling in large magnet applications

A coupled magnetic/thermal model for studying heat and magnetic field diffusion in conducting materials subject to time-varying external fields has been developed, using the General Dynamics Thermal Analyzer code. Applications include energy storage devices, pulsed power transformers, and electromagnetic launchers, and the time scales of interest range from a magnetic field pulse of a microsecond to hundreds of seconds. The results of this model, showing the time and spatial variation of the magnetic field and temperature, are discussed for the projectile of an electromagnetic launcher. >

Numerical simulations of multiphoton ionization and above-threshold electron spectra.

We study above-threshold ionization (ATI) of a one-dimensional model atom in a time-varying external laser field. The time-dependent Schr\odinger equation is integrated in space and time using the Crank-Nicholson method, and the photoelectron energy spectrum is then computed by projecting the wave function onto the energy eigenstates of the time-independent zero-field Hamiltonian. We demonstrate an intensity-dependent ponderomotive shift of the ionization threshold, find a free-electron scaling of the number of ATI peaks with intensity and frequency of the field, and contrast the numerical simulations with two simple Keldysh-type models. For certain field parameters we encounter turn-on transients in the form of ``energy-nonconserving'' substructure within each ATI peak. Effects on the results of the physical parameters such as length and shape of the laser pulse on one hand, and of the iteration parameters on the other, are discussed in detail.

Quantum systems with external electromagnetic fields: The large mass asymptotics

The large mass asymptotics of the quantum evolution problem for a system of charged particles that mutually interact through scalar fields and couple to an arbitrary time-varying external electromagnetic field is rigorously described. If K(x,t; y,s;m) denotes the coordinate space propagator (time evolution kernel) of this system, the singular perturbation behavior of K as mass m→∞ is expressed in terms of a gauge invariant asymptotic expansion. In terms of the external fields and interparticle interactions, this expansion provides a nonperturbative approximation for the propagator K that is valid for all particle coordinates x, y and for finite time displacements t−s. For the class of analytic scalar and vector fields that are defined as Fourier transforms of time-dependent measures, the existence of this asymptotic series for K in powers of (m)−1 is established for both real and complex masses. Explicit bounds for the error term are obtained and a manifestly gauge invariant transport recurrence relation is derived that uniquely determines all the coefficient functions of the asymptotic series. The small time asymptotic expansion of K is shown to be embedded within the large mass expansion.

Volterra equation formalism for response theory

A basis for a Volterra equation formalism is established in the framework of the projection operator for the study of the response of a closed system to time-varying external fields. The strange evolution exponentials contain only the unperturbed Liouvillian. The total response tensor and the first kind fluctuation-dissipation theorem are expressed in terms of solutions of Volterra equations. The properties of the Volterra equation ensure the validity of the results derived in the present approach.

Time-dependent perturbation theory with permanent effects

The presence of time-varying external fields invalidates usual perturbation theories as expressed by Feynman diagrams. We derive the correct Feynman graphs, which automatically ensure unitary and causality. The relevance of these considerations for cosmological problems is pointed out.

An experiment clarifying the distribution of transport currents in superconducting wires

The distribution of the transport current in a twisted multifilament superconductor is studied experimentally and theoretically for a sample configuration that simulates the windings of superconducting magnets. Theoretical analysis shows that the terminal voltage during a change of the current is directly related to the current distribution inside the composite conductor. A strongly non-uniform distribution due to the self-field effect has been found when the transport current is changed in a steady field. A time-varying transverse field tends to smear out this non-uniformity. The experimental results are in good agreement with calculations except for the high current region where the appearance of an anomalous resistive state peculiar to the filamented conductor must be taken into account. Time-varying external fields have been used to improve substantially the field stability of a small superconducting solenoid operating in the persistent mode.

Electric Field Effect on Critical Opalescence of Dipolar Binary Mixture

A modified van der Waals model is used to describe the effect of a time-varying external electric field on the critical mixing-point properties of a dipolar binary mixture. Assuming that the behavior of the concentration fluctuations about equilibrium is governed by linearized hydrodynamic equations and that the diffusion coefficient is determined by the instantaneous equilibrium state, the decay of the fluctuations is described. The relaxation time characterizing the electric field effect upon the light scattering by critical opalescence is related to the free relaxation of the concentration fluctuations.