Cycloidal vortex motion in easy-plane ferromagnets due to interaction with spin waves

The dynamics of a non-planar vortex in a two-dimensional easy-plane ferromagnet of finite size is studied. Spin dynamics simulations show small cycloidal oscillations of the vortex around its mean path. In contrast to an earlier phenomenological theory we give a physical explanation: The oscillations occur due to the interaction of the vortex with coherent spin waves which are excited by this vortex at the moment when it starts to move, in order to conserve the total energy and angular momentum. The calculation of these quantities yields the frequencies and amplitudes of the cycloidal oscillations in good agreement with the simulation data. Received 9 December 2002 Published online 4 June 2003 RID="a" ID="a"e-mail: franz.mertens@uni-bayreuth.de     

Internal mode mechanism for collective energy transport in extended systems

We study directed energy transport in homogeneous nonlinear extended systems in the presence of homogeneous ac forces and dissipation. We show that the mechanism responsible for unidirectional motion of topological excitations is the coupling of their internal and translation degrees of freedom. Our results lead to a selection rule for the existence of such motion based on resonances that explain earlier symmetry analysis of this phenomenon. The direction of motion is found to depend both on the initial and the relative phases of the two harmonic drivings, even in the presence of noise.     

Survival of He+ ions during grazing scattering from a Ag(111) surface

He+ ions as well as neutral He atoms with keV energies are scattered under a grazing angle of incidence from a clean and atomically flat Ag(111) surface. From a comparison of ion fractions observed after scattering of He+ ions and He atoms we find for energies below some keV small but defined fractions of ions that have survived the complete scattering event with the surface. This feature allows us to clear up the microscopic interaction scenario for Auger neutralization of He+ ions at a Ag(111) surface. The Auger neutralization rates are 2 to 3 orders of magnitude smaller than conventional rates derived from experiments for He+-metal systems and agree with recent calculations.     

Energy transfer from fast atomic projectiles to a crystal lattice under channeling conditions

Neutral Ne atoms with keV energies are scattered under channeling conditions, i.e., at a glancing angle of incidence, from a LiF(001) surface. By means of a time-of-flight method with a pulsed neutral beam, energy distributions for scattered projectiles are obtained. We find for this specific system that the small energy transferred to the crystal lattice during channeling via binary collisions with large impact parameters dominates the dissipation of projectile energy, whereas all other excitations of the solid can be brought to a negligible level.     

Relaxation and deorientation of110Ag(T 1/2=24.4s) nuclei produced by capture of polarized neutrons in the silver halides

Polarized¹¹⁰Ag nuclei are produced in the silver halides by capture of polarized neutrons at temperatures below 30 K and magnetic field strengths up to 6 kOe. The depolarization process is studied by observation of the β decay asymmetry as a function of magnetic field, temperature and of the radio frequency field strength in NMR signals. The depolarization is caused by a field dependent deorientation process and by temperature dependent spin-lattice relaxation. The deorientation is due to a succession of coupling steps of the nuclear spin with electromagnetic fields of defects generated as a consequence of the capture process, and the field dependence of the polarization can be understood as a decoupling curve. The temperature dependence of the spin-lattice relaxation is in accordance with the theory of quadrupolar relaxation above 18 K if an empirical phonon spectrum is used for the calculation. At lower temperatures the experimental relaxation rate is anomalously high, which may be due to resonance modes connected with recoil lattice defects.     

A note on the characteristic function of supergames

Theα- andβ-characteristic functions of a supergame coincide.     

Diffraction of light by two spatially separated parallel ultrasonic waves of different frequency

A theory is developed for the diffraction of light by two spatially separated parallel ultrasonic progressive waves of different frequency. The preliminary theories ofRaman andNath [C. V.Raman and N. S.Nath, Proc. Indian Acad. Sci. A2, 406–412; 413–420 (1935)] for normal and oblique incidence are taken to be valid. The resulting equations are extensions of earlier results of R.Mertens, Z. Physik160, 291–296 (1960). The predicted periodicity of the diffraction spectrum with increasing sound beam separation agrees with the well known periodicity of the light intensity distributions in the Fresnel zone of the phase grating formed by the first ultrasonic wave. Results of numerical calculations are presented to illustrate features of the theoretical results, as reflected in the first order of diffraction for 3.0 and 6.0 Mc ultrasonic waves in water.     

Spin-flip probability in the inelastic scattering of 16.9 MeV neutrons from 12C

The neutron spin-flip probability S(θ) for inelastic scattering of unpolarized 16.9 MeV neutrons to the 4.44 MeV state of ¹²C has been determined by measuring the absolute directional correlation between the scattered neutrons and the subsequent E2 deexcitation γ- radiation emitted perpendicular to the scattering plane. Time-of-flight techniques with carbon recoil detection in a plastic scintillator were used to separate elastically and inelastically scattered neutrons. The neutron spin-flip data were found to be in close agreement with 20.0 MeV proton spin-flip results. Comparison of the measurement is made with the predictions of a microscopic antisymmetrized distorted wave calculation in which the direct reaction mechanism is supplemented by a two step resonance contribution. This theoretical analysis reveals the presence of a strong quadrupole resonance at 20.5 MeV excitation in ¹²C. The results are also influenced to a lesser extent by the E1 giant dipole resonance.     

A duality theorem on a pair of simultaneous functional equations

Given P and Q convex compact sets in R^kandR^s, respectively, and u a continuous real valued function on P × Q, we consider the following pair of dual problems: Problem I—Minimize ? so that ?: $\text{P × Q → R}\text{and}\text{? ?}\text{Cav}{}_{\mathrm{p}}\text{Vex}{}_{\mathrm{q}}\text{×}\text{max}\text{(u, ?)}$. Problem II—Maximize g so that g: P × Q → R and g ? Vex_q × Cav_pmin(u, g). Here Cav_p is the operation of concavification of a function with respect to the variable p?P (for each fixed q?Q). Similarly, Vex_q is the operation of convexification with respect to q?Q. Maximum and minimum are taken here in the partial ordering of pointwise comparison: $\text{? ? g}\text{means}\text{?(p, q) ? g(p, q) ?(p, q) ? P × Q}$. It is proved here that both problems have the same solution which is also the unique simultaneous solution of the following pair of functional equations: (i) $\text{? =}\text{Vex}{}_{\mathrm{q}}\text{max}\text{(u, ?)}$. (ii) $\text{? =}\text{Cav}{}_{\mathrm{p}}\text{min}\text{(u, ?)}$. The problem arises in game theory, but the proof here is purely analytical and makes no use of game-theoretical concepts.