J. B. Sokoloff

Negative Refraction and Left-Handed Electromagnetism in Microwave Photonic Crystals

We demonstrate the negative refraction of microwaves in a metallic photonic crystal prism. The spectral response of the photonic crystal prism, which manifests both positive and negative refraction, is in complete agreement with band-structure calculations and numerical simulations. The validity of Snells law with a negative refractive index is confirmed experimentally and theoretically. The negative refraction observed corresponds to left-handed electromagnetism that arises due to the dispersion characteristics of waves in a periodic medium. This mechanism for negative refraction is different from that in metamaterials.

Theory of atomic level sliding friction between ideal crystal interfaces

Recent theoretical work on atomic level sliding friction is summarized. Some previous analytic results are verified by numerical simulations, and finite‐size scaling arguments based on several time scales appropriate to finite crystals undergoing shear motion are used to interpret the results of the simulations and to give insight into the methods by which energy is dissipated in such a shearing process. One conclusion is that the existence of a lifetime for the lattice vibrations plays an important role in determining the velocity dependence of the force of friction, for any finite‐size crystal. Finally the force of friction found in recent microbalance experiments between a solid rare‐gas‐element film on a metallic substrate is calculated by the present methods and compared to experiment. The main conclusion is that the observed friction is probably due to atomic level defects such as substitutional impurities. Larger‐scale defects on the surface contribute a much smaller value.

Refraction of electromagnetic energy for wave packets incident on a negative-index medium is always negative.

We analyze refraction of electromagnetic wave packets on passing from an isotropic positive to an isotropic negative-refractive-index medium. We definitively show that in all cases the energy is always refracted negatively. For localized wave packets, the group refraction is also always negative.

Static Friction between Elastic Solids due to Random Asperities

Several workers have established that the Larkin domains for two three-dimensional nonmetallic elastic solids in contact with each other at a disordered but atomically flat interface are enormously large, implying that there should be negligible static friction per unit area in the macroscopic solid limit. In contrast, the present Letter argues that when the Larkin domains are calculated for disorder on the multiasperity scale, they are much smaller than the interface size. This can account for the virtual universal occurrence of static friction.

Spin‐wave spectrum for barium ferrite

We describe the calculation of the spin‐wave spectrum for barium ferrite, a complex exchange‐coupled hexagonal ferrimagnetic compound, using a method first used in a similar calculation for yttrium iron garnet (YIG). The exchange integrals are calculated by fitting the Weiss molecular field approximation to the sublattice magnetizations while including single‐ion anisotropy. The value for the space‐averaged stiffness constant as calculated from the acoustic mode of the spin‐wave spectrum is 2.5×10−9 Oe cm2, which compares favorably with values obtained by domain wall resonance, considering the accuracy of such measurements.

Intrinsic ferrimagnetic resonance linewidth of barium ferrite due to spin‐wave scattering by trigonal site single‐particle excitations

Time‐dependent two‐magnon scattering was previously proposed as a mechanism to explain the large magnitude of the ferrimagnetic resonance (FMR) linewidth of barium ferrite as a function of frequency. In the present work, it is shown that a quantum mechanical mechanism like the Kasuya–Le Craw process (KL)1 but with the phonon excitation replaced by a single‐particle excitation of a trigonal site iron ion, which moves in an anharmonic potential well, gives a linewidth contribution of less than a tenth of an Oersted and proportional to the frequency, as in the KL mechanism. We conclude, based on this work and our previous work on the KL mechanism, that neither of these mechanisms can explain the observed FMR linewidths in barium ferrite at any frequency.

Frequency-dependence of the ferromagnetic-resonance linewidth of barium ferrite

Ferromagnetic resonance was measured in both a swept frequency mode of operation, in which the magnetic field was fixed, and a field swept mode, using field modulation techniques. Single‐crystal spheres of 0.381, 0.305, and 0.483 mm in diameter were inserted in the waveguide and transmission was observed in the measurement. The g values for all the spheres averaged to 2.052±0.011 and the uniaxial anisotropy field was 16.4 kOe.Our measurements show that the linewidth of barium ferrite is nearly independent of resonant frequency from 48 to 105 GHz. This is in disagreement with the Kasuya–LeCraw two‐magnon–one‐phonon mechanism, which would predict a linewidth linear with the resonant frequency. Previous measurements [J. Magn. Magn. Mater. 54‐57, 1141 (1986); IEEE Trans. Magn. MAG‐22, 984 (1986)], however, show a strong temperature dependence, which rules out magnon scattering from static defects as the primary contributor to the intrinsic linewidth. These results are consistent with a single scattering proce...

Measurements and simulations of micron size coplanar waveguides

Measurements and simulations have been done on a coplanar waveguide, which consists of a pair of slots which taper down to a separation and width of micron size. The purpose of this device is to permit one to concentrate microwaves or millimeter waves on magnetic samples of the order of a micron in order to do ferrimagnetic resonance (FMR) studies on such small samples. The transmission coefficient as a function of frequency found in the simulations agrees quite well with the measurements. The simulations show that the magnetic field at the pinch is about a factor of several thousand larger than the field of the incident wave. Results for the circuit parameters found from the simulations will be compared to the values for these parameters measured for this device, and the prospects for using the device for FMR studies on micron and submicron magnetic particles will be discussed.

Strongly Temperature Dependent Sliding Friction for a Superconducting Interface

A sudden drop in mechanical friction between an adsorbed nitrogen monolayer and a lead substrate occurs when the lead passes through the superconducting transition temperature. We attribute this effect to a sudden drop at the superconducting transition temperature of the substrate Ohmic heating. The Ohmic heating is due to the electronic screening current that results from the sliding adsorbed film.

Ferrimagnetic resonance lineshape asymmetry due to Suhl instabilities

We present calculations of the ferrimagnetic resonance lineshapes resulting from second order Suhl instabilities for thin films and spheres. We find that whereas a spherical sample has a lineshape which is symmetrical around the resonant frequency, a thin film has an asymmetrical lineshape. The calculations are in agreement with measurements that we have performed of the lineshape as a function of input power for thin film samples of both barium ferrite and yttrium iron garnet. When the magnetic field direction is changed from perpendicular to parallel to the film plane, the asymmetry of the lineshape at magnetic resonance changes in opposite sense relative to the resonant field. Theoretical estimates of the critical microwave field necessary for second order Suhl instabilities to occur are in agreement with measured critical fields.