G. M. Wysin

Optical bistability with surface plasmons

We theoretically examine bistable operation in reflection with simultaneous excitation of the surface-plasmon mode at the interface with a nonlinear Kerr medium. Bistability may occur for an incident power an order of magnitude below that reported previously for a grazing-incidence geometry.

MAGNON MODES AND MAGNON-VORTEX SCATTERING IN TWO-DIMENSIONAL EASY-PLANE FERROMAGNETS

We calculate the magnon modes in the presence of a vortex in a circular system, combining analytical calculations in the continuum limit with a numerical diagonalization of the discrete system. The magnon modes are expressed by the S-matrix for magnon-vortex scattering, as a function of the parameters and the size of the system and for different boundary conditions. Certain quasi-local translational modes are identified with the frequencies which appear in the trajectory X(t) of the vortex center in recent Molecular Dynamics simulations of the full many-spin model. Using these quasi-local modes we calculate the two parameters of a 3rd-order quation of motion for X(t). This equation was recently derived by a collective variable theory and describes very well the trajectories observed in the simulations. Both parameters, the vortex mass and a factor on the third time derivative of X(t), depend strongly on the boundary conditions.

Overshoot in the response of a photoconductor excited by subpicosecond pulses

Experiments designed to observe velocity overshoot in GaAs photoconductors excited by 2.0 eV photons are discussed. Monte Carlo transient velocity computations are presented which indicate that, for 620 nm (2.0 eV) excitation of GaAs, velocity overshoot will not occur for fields less than 20 kV/cm. Photoconductive device model equations are solved numerically for the case of subpicosecond‐pulse excitation. The computed photoconductor response is observed to have an overshoot on the picosecond time scale resulting from charge‐separation effects. The overshoot behavior is very similar to that observed in measurements of subpicosecond photoconductor response and previously interpreted in terms of velocity overshoot. We conclude that experimentally observed overshoot response at 620 nm is the result of charge‐separation effects and not the result of velocity overshoot.

Soliton dynamics on an easy-plane ferromagnetic chain

It is now generally accepted that the description of solitons in a classical easy-plane ferromagnetic chain in terms of a sine-Gordon theory is inadequate. The structural and dynamic properties of these solitons are not very clear. The authors present here results of a numerical simulation of the dynamics of a single soliton as well as collisions between a soliton-antisoliton pair. The dynamics of a single soliton appears to be consistent with variational method calculations. The energy dispersion E(u), where u is the propagation velocity, consists of three continuously connected branches. Only the first branch is sine-Gordon-like with an effective soliton mass. A soliton-antisoliton pair collision leads to a variety of final states. As a function of magnetic field B, there are four major regimes. At very low fields, the pair transmit through each other similar to a pair collision for true sine-Gordon solitons. For somewhat higher fields, the pair forms a bound state (breather mode) on collision. Further increase in magnetic field leads to reflection of the soliton-antisoliton pair. As a function of increasing collision velocity usG for an initial sine-Gordon pair, the various critical fields in general decrease. Furthermore, there are details in the final state diagram (in the usG-B plane) that correspond to resonance scattering (for breather modes) and branch transfer (in the pair collision leading to reflection). Implications of these results for quasi-one-dimensional ferromagnets such as CsNiF3 and CHAB ((C6H11NH3)CuBr3) are suggested. In particular, they suggest that non-linear elementary excitations in these chains may be breathers rather than isolated solitons.

Dynamics and hysteresis in square lattice artificial spin ice

Dynamical effects under geometrical frustration are considered in a model for artificial spin ice on a square lattice in two dimensions. Each island of the spin ice has a three-component Heisenberg-like dipole moment subject to shape anisotropies that influence its direction. The model has real dynamics, including rotation of the magnetic degrees of freedom, going beyond the Ising- type models of spin ice. The dynamics is studied using a Langevin equation solved via a second-order Heun algorithm. Thermodynamic properties such as the specific heat are presented for different couplings. A peak in specific heat is related to a type of melting-like phase transition present in the model. Hysteresis in an applied magnetic field is calculated for model parameters where the system is able to reach thermodynamic equilibrium.

Vortex-in-nanodot potentials in thin circular magnetic dots

Vortex states in thin circular magnetic nanodots are studied using auxiliary constraining fields as a way to map out the potential energy space of a vortex, while avoiding a rigid vortex approximation. In the model, isotropic Heisenberg exchange competes with the demagnetization field caused by both surface and volume magnetization charge densities. The system energy is minimized while applying a constraint on the vortex core position, using Lagranges method of undetermined multipliers. The undetermined multiplier is seen to be the external field needed to hold the vortex core in place at a desired radial distance r from the dot center. This auxiliary field is applied only in the core region of the vortex. For a uniform nanodot, the potential energy is found to be very close to parabolic with r, as in the rigid vortex approximation, while the constraining field increases linearly with r. Effects of nonmagnetic impurities and holes in the medium can also be estimated. An impurity or hole in the dot can lead to bistable operation between the two minima that result under the application of a transverse applied magnetic field.

LOW-TEMPERATURE STATIC AND DYNAMIC BEHAVIOR OF THE TWO-DIMENSIONAL EASY-AXIS HEISENBERG MODEL

We apply the self-consistent harmonic approximation (SCHA) to study static and dynamic properties of the two-dimensional classical Heisenberg model with easy-axis anisotropy. The static properties obtained are magnetization and spin wave energy as functions of temperature, and the critical temperature as a function of the easy-axis anisotropy. We also calculate the dynamic correlation functions using the SCHA renormalized spin wave energy. Our analytical results, for both static properties and dynamic correlation functions, are compared to numerical simulation data combining cluster\char21{}Monte Carlo algorithms and spin dynamics. The comparison allows us to conclude that far below the transition temperature, where the SCHA is valid, spin waves are responsible for all relevant features observed in the numerical simulation data; topological excitations do not seem to contribute appreciably. For temperatures closer to the transition temperature, there are differences between the dynamic correlation functions from SCHA theory and spin dynamics; these may be due to the presence of domain walls and solitons.

Single-kink dynamics in an easy-plane classical antiferromagnetic chain

The authors present an ansatz for kink excitations in an easy-plane classical antiferromagnetic chain with a magnetic field in the easy plane. The utility of the ansatz is that it presents the in-plane (XY) and out-of-plane (YZ) kinks as belonging to one continuously connected energy dispersion curve. Linear stability analysis applied to YZ kinks shows that there is a velocity-dependent critical field necessary for stability. The authors also present a set of results of a numerical integration of the equations of motion which verifies the YZ kink stability regimes, as well as showing that XY kinks are stable over a wide range of fields and velocities.

Vortex-vacancy interactions in two-dimensional easy-plane magnets

For a model with isotropic nearest-neighbor exchange combined with easy-plane exchange or single-ion anisotropies, the static effects of a magnetic vacancy site on a nearby magnetic vortex are analyzed on square, hexagonal, and triangular lattices. Numerical energy minimization and linear stability analysis using the vortex instability mode are employed. When the vortex is centered on a vacancy, the critical anisotropies where the stable vortex structure switches from out-of-plane to planar form are determined, and the vortex energies and magnetizations are found as functions of anisotropy. Consistent with square lattice calculations by Zaspel et al., the strength of anisotropy needed to stabilize a vortex in the planar form is reduced when the vortex is centered on a vacancy, for all three lattices studied. The vortex-on-vacancy energy is found to be smaller than the typical energy of a vortex centered between lattice sites in a system without vacancies. For a vortex separated from a vacancy, the energy found as a function of separation demonstrates an attractive potential between the two.

Soliton dynamics on a ferromagnetic chain

Presents the results of a numerical simulation of spin dynamics on a classical ferromagnetic chain subject to an easy-plane anisotropy and a magnetic field. The results show substantial deviation from the conventional sine-Gordon description and seriously modify interpretations of experimental and molecular dynamics simulation data for materials such as CsNiF3. The observed soliton-like solutions show a variety of effects including large off-easy-plane deviation, temporal oscillations and shock wave formation.