Energetics of a strongly correlated Fermi gas

The energy of the two-component Fermi gas with the s-wave contact interaction is a simple linear functional of its momentum distribution:

     

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.     

Current-voltage behaviour of Schottky diodes fabricated on p-type silicon for radiation hard detectors

Current-voltage (I-V) measurements were carried out on Schottky diodes fabricated on undoped and on metal-doped p-type silicon. The metals used are gold, platinum, erbium and niobium. The I-V data were used to extract the saturation current, the ideality factor and the Schottky barrier height for each of the five diodes. These parameters were correlated to the defect levels generated by the metals in silicon. The results show that in all cases the silicon has become relaxation-like after doping since the device current is Ohmic. This is in agreement with the existence of the midgap defect in all the doped devices as compiled from the literature. Such metal doped (or relaxation) devices have been found to perform better as radiation-hard particle detectors.     

Terahertz pulse emission from semiconductor surfaces illuminated by femtosecond Yb:KGW laser pulses

The amplitudes of terahertz pulses emitted from the surfaces of InAs, InSb, InGaAs, GaAs and Ge after their excitation by femtosecond 1 μm laser pulses was compared. It has been found that this effect is most efficient in p-type InAs. The mechanisms leading to the terahertz emission are investigated and discussed. It has been concluded that in the majority of the investigated semiconductors the main contribution to THz pulse emission comes from the electrical-field-induced optical rectification effect.     

Nanointegration of ZnO with Si and SiC

The study is dedicated to some aspects of the controlled heteroepitaxial growth of nanoscaled ZnO structures and an investigation of their general and dimension mediated properties. ZnO nanostructures were synthesized by optimized MOCVD process via two growth approaches: (i) catalyst free self-organized growth of ZnO on Si substrates and (ii) ZnO heteroepitaxy on p-type hexagonal 4H-SiC substrates. The SiC substrate was prepared by sublimation epitaxy and served as a template for the ZnO epitaxial growth. The epitaxial growth of n-ZnO on p-SiC resulted in a regular matrix of well-faceted hexagonally shaped ZnO single crystals. The achievement of ZnO integration with Si encompasses controlled growth of vertically oriented nanosized ZnO pillars. The grown structures were characterized by transmission electron microscopy and microphotoluminescence. Low concentration of native defects due to a stoichiometry balance, advanced optical emission, (excitonic type near-band-edge emission and negligible defect related luminescence) and continuous interfaces (epitaxial relationship ZnO_[0 0 0 1]/SiC_[0 0 0 1]) are evidenced. The ZnO nanopillars were further probed as field emitters: the grown structures exhibits advanced field emission properties, which are explained in term of dimensionality and spatial uniformity of the nanopillars. The present results contribute to understanding and resolving growth and device related issues of ZnO as a functional nanostructured material.     

Electrical performance of Ti/Pt thin films on glass-ceramic substrates after high temperature loading

In this study, the influence of post-deposition annealings (PDA) up to temperatures of T _PDA=700°C on the room-temperature resistivity of e-beam evaporated titanium/platinum (Ti/Pt) bi-layers on low temperature co-fired (LTCC) substrates covered with a glass encapsulate is investigated. The thickness of the platinum top layer is varied between 24 and 95 nm (titanium film thickness: 5 nm) and between 23 and 90 nm (titanium film thickness: 15 nm), respectively. In the “as-deposited” state and up to post-deposition annealing temperatures of T _PDA=450°C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. When applying, however, solely the Fuchs-Sondheimer model for evaluation, the effective mean free path for electrons is substantially above the value reported for crystalline platinum at room temperature. Compared to similar investigations on smooth Si/SiO₂ substrates yielding interpretable results within this theoretical approach, this is due to the increase of the thickness-dependent fraction in film resistivity which is strongly affected by the enhanced LTCC/glass surface roughness. At T _PDA>600°C, diffusion of titanium into the platinum top layer and the roughening of the LTCC/glass substrate dominate the electrical behavior, both causing an increase in film resistivity above average. In contrast to Si/SiO₂ substrates, thermal induced grooving effects in the Pt top layer play a minor role as the temperature coefficients of expansion of metallization and glass-ceramic substrate match better and the effective temperature difference for stress generation is lower due a glass softening temperature of about 450°C.