Properties of Si δ-layers embedded in GaAs

An ultrathin, a few monolayers (MLs) thick Si δ-layer, has been embedded in GaAs. The Si δ-layer properties have been electrically and structurally characterized. A conductivity transition has been observed, when going from free carrier transport in thin (1 ML) ordered δ-layers to the disordered conditions for Si δ-layers thicker than 4 MLs. Two novel emission bands are observed in photoluminescence (PL) for Si δ-layers in the width range 1–4 MLs, but solely at below bandgap excitation. The pronounced 2D properties of these δ-layers have been clearly demonstrated by an observed blue shift of the PL characterization as the thickness of the Si δ-layer is reduced. The so-derived results on the transition energies and the electronic structure are in good agreement with theoretical predictions obtained by a self-consistent approach.     

Mott transition, antiferromagnetism, and d-wave superconductivity in two-dimensional organic conductors

We study the Mott transition, antiferromagnetism, and superconductivity in layered organic conductors using the cellular dynamical mean-field theory for the frustrated Hubbard model. A d-wave superconducting phase appears between an antiferromagnetic insulator and a metal for t'/t=0.3-0.7 or between a nonmagnetic Mott insulator (spin liquid) and a metal for t'/t>or=0.8, in agreement with experiments on layered organic conductors including kappa-(ET)2Cu2(CN)3. These phases are separated by a strong first-order transition. The phase diagram gives much insight into the mechanism for -wave superconductivity. Two predictions are made.     

Electronic structure of the Cu(111) surface

Self-consistent electronic structure calculations are reported on bulk Cu, and 3- and 5-layer Cu films. These yield a size insensitive work function, φ = 5.0±.1 eV, and a surface energy of 0.75 eV, in agreement with experiment. Good size convergence of the film potential permits the construction of a self-consistent potential for an 11-layer Cu(111) film, whose spectral properties we studied. A prominent p-like surface band was found within 0.1 eV of experiment, serving as a check on the surface potential.     

Transport and AC loss properties of the repaired multifilamentary REBCO superconducting tapes

For near-future applications of REBa₂Cu₃O₇ (REBCO) coated conductors to electric power cables, transformers and Superconducting Magnetic Energy Storage (SMES), the long taped wires with high performance in the transport properties have been designed and fabricated. Moreover, in order to drastically reduce AC losses in perpendicular field configuration, advanced multifilament YBCO coated conductors (MFYCCs) fabricated with technique of a laser scribing process have been also developed. In the present study, from engineering viewpoints to utilize such advanced conductors, we evaluated the transport and AC loss properties of short MFYCCs with a repaired part or a joint by a diffusion joint technique with the saddle-shaped pickup coil method.     

Reabsorption in a neodymium fibre laser

Resonance absorption, the reabsorption of laser light by the laser transition of the dopant material itself, is found to be the limiting factor for fibre lasers when extremely long fibres are used. We report on measurements of the temperature dependence of the losses in a 8.5 m long commercial Nd³⁺ doped fibre (York, ND 95020/E). To determine the reabsorption losses, the threshold power has been measured as a function of temperature between 243 K and 413 K. The results are compared with theory assuming a ⁴l level energy of 2110 cm^-1 and an absorption cross-section of δ = 1.15 x 10^-20cm². The comparison shows that reabsorption from the thermally populated ⁴l level is the dominant contribution to the measured losses.     

A compact UHV package for microfabricated ion-trap arrays with direct electronic air-side access

We have demonstrated a new apparatus for operating microfabricated ion-trap arrays in a compact ultra-high-vacuum setup with excellent optical and electrical access. The approach uses conventional components, materials and techniques in a unique fashion. The microtrap chip is mounted on a modified ceramic leadless chip carrier, the conductors of which serve as the vacuum feedthrough. The chip carrier is indium-sealed to stainless-steel components to form vacuum seals, resulting in short electrical path lengths of ≤20 mm from the trap electrodes under vacuum to air side. The feedthrough contains conductors for the radio-frequency trap drive, as well as 42 conductors for DC electrodes. Vacuum pressures of ～1 × 10^?11 mbar are achieved, and ions have been confined and laser cooled in a microtrap chip. The apparatus enables accurate measurements of radio-frequency voltage amplitudes on the trap electrodes, yielding an excellent agreement between measured and modelled trap efficiencies. This feature is of significant use in establishing initial operation of new devices. The principle of the connectivity scheme presented here is applicable to larger ceramic chip carriers containing many more conductors.     

Coulomb bare interactions in inhomogeneous 4-layer graphene structures

We investigate Coulomb bare interactions in 4-layer graphene systems, taking into account the inhomogeneity of dielectric environment. By solving Poisson equation in momentum space, 16 elements of Coulomb potential tensor have been formed with analytical expressions. The formulae illustrate that Coulomb bare interactions in inhomogeneous 4-layer graphene system differ noticeably from those in homogeneous one. Nevertheless, both intra- and interlayer Coulomb potentials become independent with dielectric constants of contacting media, but they are decided by those of dielectric layer surrounding the system in long wavelength approximation. Besides, numerical results demonstrate that Coulomb bare interactions decrease considerably with the increase in carrier density in graphene sheets as well as their separated distance. In all investigations, the inhomogeneity of dielectric background should not be neglected for improvement in the model.     

Diffraction of a Bose-Einstein condensate from a magnetic lattice on a microchip

We experimentally study the diffraction of a Bose-Einstein condensate from a magnetic lattice, realized by a set of 372 parallel gold conductors which are microfabricated on a silicon substrate. The conductors generate a periodic potential for the atoms with a lattice constant of 4 microm. After exposing the condensate to the lattice for several milliseconds we observe diffraction up to fifth order by standard time of flight imaging techniques. The experimental data can be quantitatively interpreted with a simple phase imprinting model. The demonstrated diffraction grating offers promising perspectives for the construction of an integrated atom interferometer.     

The similarities in loss creation in LTS and HTS cables

The coupling current losses represent an essential contribution to AC losses in most practical superconducting conductors. The origin of this loss type is well known, being caused by induced currents in different loops consisting of superconducting and non-superconducting parts. However, the ‘current pattern' in different conductor types (strands, flat or round cables, more complicated cable structures, CICC) varies appreciably. These differences are mainly due to geometrical effects (size and shape of filaments and/or strands, their spatial distribution, conductor aspect ratio, demagnetization effects). Although the general knowledge about AC losses in low temperature cable structures is by far not complete (mainly at higher frequencies, in inhomogeneous fields and for inhomogeneous cable structure), an attempt is made to summarize those results which can be adopted to high T_c conductors and some remarks are made about new features of AC losses in these conductor types.     

Computation of AC losses in high-temperature superconductors

A formulation for the computation of AC losses in technical HTS conductors by using commercial FEM packages developed for two-dimensional computation of electromagnetic problems is presented. The formulation takes into account the real current density–electric field characteristic of a conductor and the spatial dependence of the current density. Having presented the formulation, example runs comparing transport current loss behaviour between HTS and LTS conductors are given.     

Electrical characteristics of the waveguide grating system

Electromagnetic wave propagation is conside-red through a waveguide system consisting of arbitrary number of short-period gratings (i.e. whose period is much shorter than the wave-length). We assume that gratings are formed by ideal conductors having arbitrary cross-section. In order to "stitch" together the fields on gratings, equivalent boundary conditions are used.We have analyzed and studied experimentally the electrical characteristics of an adjustable frequency filter. The filter consists of three short-period gratings formed by conductors with circular cross-section.Experimental results are in good agreement with theory.     

Preparation of dielectric mirrors from high-refractive index contrast amorphous chalcogenide films

We report the preparation of planar 15-layer dielectric mirrors by a thermal evaporation of alternating high refractive index contrast amorphous chalcogenide Sb-Se and Ge-S layers, exhibiting a high-reflection band around 1.55 μm. The layer deposition quality and the thickness accuracy of such prepared chalcogenide multilayers were then checked using transmission electron microscopy. The layer thickness deviation of chalcogenide layers did not exceed ∼7 nm in comparison with the desired thicknesses. The width of Sb-Se/Ge-S layer boundary was approximately ∼3 nm, which is in good agreement with the surface roughness values of thermally evaporated Sb-Se and Ge-S single layers. The optical properties of the prepared 15-layer dielectric mirrors were consistent in temperature range of 20-120 °C; however, at higher temperatures there started apparent structural changes of Sb-Se films, which were followed by their crystallization. Excellent optical properties of chalcogenide materials in the infrared range make them interesting for applications, e.g., in optics and photonics.     

Small-bore fluorocarbon polymer-coated silver hollow glass waveguides for Er:YAG laser light

Several types of fluorocarbon polymer (FCP)-coated silver hollow glass waveguides have been fabricated for Er:YAG laser delivery by using the improved wet chemical technique and dynamic coating procedure. The straight losses of 2 m long 700 μmØ and 540 μmØ waveguides are 0.4 and 1.0 dB, respectively. The transmission losses of these waveguides are below 1.5 dB even when the waveguides are bent to 180° with the bending radius of either 20 or 15 cm. The waveguides with the small diameters of 320 and 200 μm have also been developed for clinical treatment, which exhibit low enough transmission losses for Er:YAG laser light.     

Impact of interface relaxation on the nanoscale corrugation in Pb/Si(1 1 1) islands

The nanoscale hexagonal pattern observed in scanning tunneling microscopy (STM) for 3-layer and 4-layer Pb islands on Si(1 1 1) is studied theoretically. We found that besides thickness the atomic rearrangement at the Pb/Si interface plays an important role in determining the STM patterns. Electronic structures of the Pb film on Si(1 1 1) obtained from fully relaxed and unrelaxed Pb films are qualitatively different. Simulated STM images for Pb films with different stacking also show that the corrugation patterns are sensitive to the buried Pb-Si interfacial structure.     

Effect of oxygen on the stability of Ag islands on Si(111)-7 × 7

We have used scanning tunneling microscopy to probe the effect of oxygen exposure on an ensemble of Ag islands separated by a Ag wetting layer on Si(111)-7 × 7. Starting from a distribution dominated by islands that are 1 layer high (measured with respect to the wetting layer), coarsening in ultrahigh vacuum at room temperature leads to growth of 2-layer islands at the expense of 1-layer islands, which is expected. If the sample is exposed to oxygen, 3-layer islands are favored, which is unexpected. There is no evidence for oxygen adsorption on top of Ag islands, but there is clear evidence for adsorption in the wetting layer. Several possible explanations are considered.     

Investigation of interface roughness cross-correlation properties of optical thin films from total scattering losses

The interface roughness and interface roughness cross-correlation properties affect the scattering losses of high-quality optical thin films. In this paper, the theoretical models of light scattering induced by surface and interface roughness of optical thin films are concisely presented. Furthermore, influence of interface roughness cross-correlation properties to light scattering is analyzed by total scattering losses. Moreover, single-layer TiO₂ thin film thickness, substrate roughness of K9 glass and ion beam assisted deposition (IBAD) technique effect on interface roughness cross-correlation properties are studied by experiments, respectively. A 17-layer dielectric quarter-wave high reflection multilayer is analyzed by total scattering losses. The results show that the interface roughness cross-correlation properties depend on TiO₂ thin film thickness, substrate roughness and deposition technique. The interface roughness cross-correlation properties decrease with the increase of film thickness or the decrease of substrates roughness. Furthermore, ion beam assisted deposition technique can increase the interface roughness cross-correlation properties of optical thin films. The measured total scattering losses of 17-layer dielectric quarter-wave high reflection multilayer deposited with IBAD indicate that completely correlated interface model can be observed, when substrate roughness is about 2.84 nm.     

基于电磁谐振分离的宽带低雷达截面超材料吸波体

基于超材料的电磁谐振特性, 设计、制作了一种极化无关的宽带低雷达散射截面 (radar cross section, RCS)超材料吸波体. 通过场分布和反演法分析了其吸波机理, 利用波导法和空间波法测试了其吸波率和RCS特性. 理论分析表明: 在平面波的作用下, 该吸波体对某一吸波频率在不同的位置分别提供电谐振和磁谐振, 对不同的吸波频率, 利用不同的介质层提供主要的能量损耗, 从而有效减弱了电磁耦合, 保证了宽频带的强吸收特性. 实验结果表明: 设计的三层结构吸波体吸波率达90%以上的带宽是单层结构的4.25倍, RCS减缩10 dB以上的带宽为5.1%, 其单元尺寸为0.17λ, 厚度仅为0.015λ. 该吸波体的低RCS特性还具有极化无关、宽入射角的特点, 且通过改变吸波体的夹层结构可以实现工作带宽的灵活调节. 关键词： 超材料吸波体 雷达散射截面 宽带 电磁谐振     

Simulation of prefactor anomalies in superionic conductors in the case of arbitrary energy losses

Based on a sticking-probability formulation, we have considered a breakdown of absolute rate theory in relation to prefactor anomalies in superionic conductors in the case of arbitrary energy losses of ions. We present simulation results concerning the prefactor dependence on the magnitude of the average energy loss in the energy-transfer range from 10⁻³ to 0.46 kT and the energy dependence of the sticking probability for ions.     

Low-Field MR Coils: Comparison between Strip and Wire Conductors

This work describes how the cross-sectional shape of radio-frequency coil conductors affects coils performance. This is of particular importance at low Larmor frequencies such as those of low-field magnetic resonance imaging systems where conductor and capacitor losses are the dominant power dissipation mechanisms. We demonstrate that conductors having a circular cross section allow the coil to achieve significantly better performance than the one built using flat strips. The change in coil quality factor due to conductor geometry was verified to be due only to changes in the conductors’ resistance and not their inductance. The results are not limited to low-field proton imaging but they are equally applicable to other situations where the Larmor frequency is in the megahertz range, including nuclear magnetic resonance of other nuclear species at intermediate fields.