The influence of interlayer exchange coupling on magnetic ordering in Fe/V superlattices

The relation between the interlayer exchange coupling and magnetic order is addressed, using Fe/V(0 0 1) superlattices as a model system. Large decrease in the ordering temperature (T_c) is observed with decreasing interlayer exchange coupling. The effective exponents of the magnetization were determined to be larger than 0.5 for all the samples, which is strongly deviating from the classical values of both two- and three-dimensional systems. This effect can partially be ascribed to the presence of boundaries, invoked by the finite number of magnetic layers.     

Current-temperature measurements of a SBD evaporated onto inductively coupled plasma cleaned germanium

Inductively coupled plasma (ICP) etching has been used primarily on compound semiconductors. There are however compelling reasons to study the effects of ICP etching on Ge. Pd Schottky barrier diodes (SBDs) were resistively evaporated onto Ge (1 1 1) that was ICP etched at a rate of 60 Å per minute for three or ten minute intervals. Although plasma cleaning is known to introduce defects that were observed with DLTS, the diodes exhibited excellent current-voltage characteristics when cooled down to 80 K. Current-temperature (IT) scans that were recorded from 20 K up to 300 K after cooling under reverse bias showed no effect of recombination/generation (RG). On the other hand, IT scans that were recorded after cooling under zero or forward bias clearly exhibited RG effects in the 100-240 K temperature range. This effect was found to be completely reversible. In addition, ICP etching leads to superior devices when compared to devices manufactured by RF sputter deposition.     

Shaping nanoparticles and their optical spectra with photons

An experimental method to tailor the shape and the optical absorption spectra of metallic nanoparticles is presented. It exploits the influence of laser irradiation on particle growth by self-assembly of atoms deposited on a substrate surface. By applying nanosecond light pulses of appropriate fluence and three different wavelengths, oblate silver particles with three fixed axial ratios have been fabricated. Their optical extinction spectra were measured with s- and p-polarized light and are dominated by plasmon resonances at fixed photon energies determined by the axial ratio. Possible applications of such tailormade nanoclusters include catalytic converters and optical components with narrow-band extinction, the magnitude and center frequency of which can be specified in advance. Received: 9 September 1999 / Published online: 20 October 1999     

Plasmon-Assisted Phase-Matched Second- and Third-Harmonic Generation in Single-Negative Heterostructures

Dispersion relations of surface plasmon polaritons （SPPs） in sandwiched optical systems are studied. The system is actually a kind of SPP waveguides, with two kinds of single negative material （SNG） as core and cladding layers, respectively. Since both TM and TE polarized SPPs can be excited in the structure, the dispersion of SPPs becomes more abundant and leads to colorful nonlinear opticM properties. The authors demonstrate the effective phase-matched second and third-harmonic generation （SHG, THG） assisted by the coupled SPPs. A cascaded second-order nonlinear process can Mso be achieved in the structure when the thickness of the core layer is properly selected, leading to the simultaneous SHG and THG. Further investigations show that much easier phase-matching can be fulfilled in the SNG waveguide array. Our results would be of potential use for surface-enhanced frequency conversion device such as light emitters or lasers.     

Frequency dependence of the ferromagnetic resonance width in magneto-impedance measurements

Giant magneto-impedance has shown large sensitivities that are of great interest in practical applications. Above certain frequencies the ferromagnetic resonance controls the magneto-impedance behavior, and the resonance width limits the maximum achievable magneto-impedance ratio. In this work we present the evolution of the resonance width as a function of the frequency, determined through magneto-impedance and microwave absorption measurements on a NiFe–Au multilayer thin film. The width of the resonance can be fitted to a curve in the form of Δf∼f⁻¹. This is explained by means of a simple model taking into account all inhomogeneities in the sample through a Gaussian distribution of anisotropy fields, as suggested by the shape of the hysteresis loop.     

Kohn-Sham theory of an atomic Fermi gas

We point out that, when repulsive interactions between two fermions are not integrable, as the case may be for atomic fermions, the original Kohn-Sham density functional must be revised.     

Bulk and surface localized modes on a magnetic nonlinear impurity

We study localized modes on a single magnetic impurity positioned in the bulk or at the surface of a one-dimensional chain, in the presence of a magnetic field B acting at the impurity site. The strong on-site nonlinear interaction U between two electrons of opposite spin at the impurity site, modelled here as a nonlinear local term, and the presence of the external field induce a strong correlation between parallel and antiparallel spin bound states. We find that, for an impurity in the bulk, a localized vector mode (with up and down spin components) is always possible for any given value of U and B, while for a surface impurity, a minimum value of both, U and B is needed to create a vector mode. In this case, up to two localized modes are possible, but only one of them is stable. The presence of the surface seems to destabilize the bulk mode in the parameter region U∼B, creating a “forbidden strip” region in parameter space, bounded by U=B+V and U=B−V, approximately.     

Power spectrum analysis of the average-fluctuation density separation in interacting particle systems

The power spectrum analysis using the Lomb-Scargle false alarm probability statistic shows a clear separation between the average and fluctuating parts of the state density in embedded two-body random matrix ensembles with a mean-field for both fermion and boson systems as well as in the nuclear shell model.     

Fermion gases in magnetic fields: a semiclassical treatment

The study of quantum degenerate gases has received much interest in these last years essentially thanks to the extremely important experimental results of the achievement of Bose-Einstein condensation of atoms and, very recently, of almost complete degeneracy of atomic fermion gases. Here we want to present the results of a semi-analytical method for the study of an interacting degenerate fermion gas based on semiclassical kinetic theory; special care has been devoted to the study of a rotating electron gas, in a cylindrically symmetrical configuration, radially confined by a uniform magnetic field. The model will lead to a particular Thomas-Fermi equation which is generalized to take into account finite temperature and average velocity of the gas, and which is further developed to consider the effects of external fields. Received 10 March 2000     

Atomic-level studies of superconducting and nonsuperconducting YBa2Cu3O7−x

Atomic-resolution images from superconducting and nonsuperconducting samples of YBa₂Cu₃O_7–x have been obtained by field ion microscopy. Both phases of the material exhibit a similar layered structure in the field ion images. Transmission electron micrographs of superconducting field ion tips show vividly the twinned structure of the orthorhombic phase. Time-of-flight (atom-probe) mass spectra from the two materials differ primarily in an enhanced molecular oxygen signal from the superconducting samples indicating that oxygen in the O(4) sites (CuO chains) is field desorbed preferentially as molecular oxygen ions.