Advanced Data Reduction and Control of High-Precision Low-Temperature Experiments

No Heading Advances in low-temperature techniques allow us to measure temperature with a precision approaching one part in 10¹¹. Using an rf-biased Josephson-array voltage source, we can control the power dissipated in electric heaters to the same level. At this level of precision, the algorithm used for thermal control and its sensitivity to the non-white character of the input noise become important factors limiting the temperature stability of fundamental physics experiments. We consider the problem of measuring slowly changing data in the presence of both while noise and sudden, short noise spikes. Such data is obtained in high-resolution experiments in earth-orbit, where the thermometers are struck and heated by charged particles. We devise control schemes that improve upon traditional methods. Specifically, a proportional-integral/proportional controller has a better step response, and applying a Kalman filter to the input signal significantly decreases noise injected by the feedback loop. We also describe a new algorithm for spike removal that is robust and has a fixed worst-case execution time. It is significantly faster than the algorithm used in experiments previously flown in space.PACS numbers: 84.40.Xb, 84.40.Ua, 89.20.Ff, 89.20.Kk     

Lateral epitaxial overgrowth of GaN films on patterned sapphire substrates fabricated by wet chemical etching

A promising technique of lateral epitaxial overgrowth, namely CantiBridge epitaxy, is developed and demonstrated in order to reduce the threading dislocation density in GaN films. Using metalorganic chemical vapor deposition, the GaN films are grown on patterned sapphire fabricated by wet chemical etching, instead of traditional dry etching. The image of atomic force microscopy shows that the threading dislocations in CantiBridge-epitaxy GaN are reduced sharply, which makes a promising to realize the high-performance GaN-based optoelectronic devices.     

Cathodoluminescence and Raman research of V-shape inverted pyramid in HVPE grown GaN film

Thick GaN films with high quality have been grown on (0001) sapphire substrate in a home-made vertical HVPE reactor. Micron-size hexagonal pits with inverted pyramid shape appear on the film surface, which have six triangular {10-11} facets. These {10-11} facets show strong luminescence emission and are characteristic of doped n-type materials. Broad red emission is suppressed in {10-11} facets and is only found at the flat region out of the pit, which is related with the decreasing defects on {10-11} facets. Low CL emission intensity is observed at the apex of V-shape pits due to the enhanced nonradiative recombination. Raman spectra show that there are higher carrier concentration and low strain in the pit in comparison to the flat region out of the pit. The strain relaxation may be the main mechanism of the V-shape pits formation on the GaN film surface.     

Investigation of the interface properties of MOVPE grown AlGaN/GaN high electron mobility transistor (HEMT) structures on sapphire

We have developed a virtual GaN substrate on sapphire based on a two-step growth method. By optimizing the growth scheme for the virtual substrate we have improved crystal quality and reduced interface roughness. Our Al_0.22Ga_0.78N/GaN HEMT structure grown on the optimized semi-insulating GaN virtual substrate, exhibits Hall mobilities as high as 1720 and 7350 cm²/Vs and sheet carrier concentrations of 8.4 × 1012 and 10.0 × 1012 cm^− 2 at 300 K and 20 K, respectively. The presence of good AlGaN/GaN interface quality and surface morphology is also substantiated by X-Ray reflectivity and Atomic Force Microscopy measurements. A simplified transport model is used to fit the experimental Hall mobility.     

Effect of sintering process on the microstructures of Bi2O3-doped yttria stabilized zirconia

Variations of microstructures in Bi₂O₃-doped yttria stabilized zirconia (YSZ) with conventional furnace and microwave sintering were investigated in this work. The results demonstrated that a small amount of addition of Bi₂O₃ was effective in reducing the sintering temperature of YSZ from 1500 °C to 1200 °C and promoting the densification rate of the ceramics. It is interesting that microwave sintering is found to suppress the evaporation rate of Bi₂O₃ and formation of the monoclinic-ZrO₂ or other amorphous phases. Compared to conventional furnace sintering, significant improvement in density of Bi₂O₃-doped YSZ at lower sintering temperatures with microwave sintering was observed. Rapid heating rate and short sintering time for restricting serious segregation at grain boundary were observed as well. Employing microwave sintering at the same sintered condition, the density of a specimen was evidently increased by 4.59% in comparison to the specimen sintered with a conventional furnace sintering.     

Noise Immunity of Pulse Wave Contour Analysis

Measurement Techniques - This article is devoted to analyzing the noise immunity of pulse waveform contour analysis. Different approaches to the implementation of the pulse waveform contour...     

Vortex Lattice Structures of a Bose-Einstein Condensate in a Rotating Lattice Potential

We study vortex lattice structures of a trapped Bose-Einstein condensate in a rotating lattice potential by numerically solving the time-dependent Gross-Pitaevskii equation. By rotating the lattice potential, we observe the transition from the Abrikosov vortex lattice to the pinned vortex lattice. We investigate the transition of the vortex lattice structure by changing conditions such as angular velocity, strength, and lattice constant of the rotating lattice potential.     

Hydrodynamics of Superfluid Bose Gases in an Optical Lattice at Finite Temperatures

Starting from an effective action for the order parameter field, we derive a coupled set of generalized hydrodynamic equations for a Bose condensate in an optical lattice at finite temperatures. Using the linearized hydrodynamic equations, we study the microscopic mechanism of the Landau instability due to the collisional damping process between the condensate and noncondensate atoms. It is shown that the Landau criterion for the stability of a superfluid in a uniform system is modified due to the presence of the periodic optical lattice potential.     

Influence of oxygen partial pressure on the structure and photoluminescence of direct current reactive magnetron sputtering ZnO thin films

The effects of oxygen partial pressure on the structure and photoluminescence (PL) of ZnO films were studied. The films were prepared by direct current (DC) reactive magnetron sputtering with various oxygen concentrations at room temperature. With increasing oxygen ratio, the structure of films changes from zinc and zinc oxide phases, single-phase ZnO, to the (002) orientation, and the mechanical stresses exhibit from tensile stress to compressive stress. Films deposited at higher oxygen pressure show weaker emission intensities, which may result from the decrease of the oxygen vacancies and zinc interstitials in the film. This indicates that the emission in ZnO film originates from the oxygen vacancy and zinc interstitial-related defects. From optical transmittance spectra of ZnO films, the plasma edge shifts towards the shorter wavelength with the improvement of film stoichiometry.     

Landau and Dynamical Instabilities of Bose-Einstein Condensates in a Kronig-Penney Potential

We study elementary excitations of Bose-Einstein condensates in a one-dimensional periodic potential and discuss the stability of superfluid flow based on the Kronig-Penney model. We analytically solve the Bogoliubov equations and calculate the excitation spectrum. The Landau and dynamical instabilities occur in the first condensate band when the superfluid velocity exceeds certain critical values as in a sinusoidal potential. It is found that the onset of the Landau instability coincides with the point where the perfect transmission of low-energy excitations is forbidden, while the dynamical instability occurs when the effective mass is negative. The condensate band has a swallow-tail structure when the periodic potential is shallow compared to the mean-field energy. We find that the upper side of a swallow-tail is dynamically unstable although the excitation spectrum has a linear dispersion reflecting the positive effective mass.     

Dynamical Properties of Vortices in a Bose-Einstein Condensate in a Rotating Lattice

We present simulation results of the vortex dynamics in a trapped Bose-Einstein condensate in the presence of a rotating optical lattice. Changing the potential amplitude and the relative rotation frequency between the condensate and the optical lattice, we find a rich variety of dynamical phases of vortices. In particular, when the optical lattice rotates faster than the condensate, the competition between the pinning force and the interactions by nucleated interstitial vortices leads to the melting of vortex lattice, yielding a vortex liquid phase.     

The Noise Immunity of Teraohmmeters Based on Current Measuring Instruments

The results of an investigation of existing teraohmmeter circuits using the criterion of noise immunity are described. Based on this criterion, the most optimum circuit, which has a measurement time of not more than 30 sec, is chosen. The noise level at a frequency of 50 Hz is estimated. The range of measurements in the experiments is 1 M – 1000 G.     

Many-body Physics of a Quantum Fluid of Exciton-Polaritons in a Semiconductor Microcavity

Some recent results concerning nonlinear optics in semiconductor microcavities are reviewed from the point of view of the many-body physics of an interacting photon gas. Analogies with systems of cold atoms at thermal equilibrium are drawn, and the peculiar behaviours due to the non-equilibrium regime pointed out. The richness of the predicted behaviours shows the potentialities of optical systems for the study of the physics of quantum fluids.     

Synthesis and field emission properties of needle-shaped SnO2 nanostructures with rectangular cross-section

Large-scale novel needle-shaped SnO₂ nanostructures were produced via a thermal evaporation method and their possible growth mechanism is proposed. The products, which gradually become thinner to form a sharp tip, had preferred <110> growth directions. Efficient and stable field emission is obtained from these SnO₂ nanostructures. The current density is up to 3 mA/cm² at 5.93 V/μm and the fluctuation of FE (field emission) currents is as small as 8% over 2 h. Our results imply that needle-shaped SnO₂ nanostructures are promising candidate for FE displays.     

Evolution of composition distribution of Si-capped Ge islands on Si(001)

The evolution of the composition distribution and microstructures of Ge islands on Si(001) during the Si overgrowth was investigated by atomic force microscopy combined with selective wet etching procedures. With increasing Si coverage to 5.4 nm, the uncapped Ge islands were found to change their shapes dramatically from domes to truncated pyramids, nanorings and eventually to the fully buried islands. Different atomic composition profiles in SiGe islands were observed at different Si coverages. Especially, the nanorings were found to have a Ge-rich core with a Si-rich periphery. Based on the experimental results, the Ge redistribution in islands during Si capping is not only correlated with the intermixing between Si capping layer and Ge islands, but also a strain-driven process.     

3d transition metal doping of semiconducting boron carbides

The introduction metallocenes, in particular ferrocene (Fe(η⁵-C₅H₅)₂), cobaltocene (Co(η⁵-C₅H₅)₂), and nickelocene (Ni(η⁵-C₅H₅)₂), together with the carborane source molecule closo-1,2-dicarbadodecaborane, during plasma enhanced chemical vapor deposition, will result in the transition metal doping of semiconducting boron carbides. Here we report using ferrocene to introduce Fe dopants, and a semiconducting boron-carbide homojunction has been fabricated. The diode characteristics are very similar to those fabricated with Co and Ni doping.     

Microstructure and electrical properties of Sm2O3 doped Bi2O3-based ZnO varistor ceramics

The dependence of the bulk density, microstructure and dc electrical properties of bismuth oxide (Bi₂O₃)-based zinc oxide (ZnO) varistor ceramics for various samarium oxide (Sm₂O₃) contents was investigated. The value of bulk density was found to 5.43-5.50 g cm⁻³ with Sm₂O₃ (mol%) content. The maximum value of bulk density is observed to be 5.50 for 0.30 mol% Sm₂O₃ containing varistor ceramics. The grain sizes for all the samples calculated from the scanning electron micrographs were found to decrease as Sm₂O₃ increases. The presence of ZnO phases, Bi-rich phases, spinel phases and Sm₂O₃ phases were observed in the samples by the energy dispersive X-ray analysis and X-ray diffraction analysis. As the Sm₂O₃ amount increased in the Bi₂O₃-based ZnO varistor ceramics, the nonlinear coefficient, α increased up to 0.10 mol%, reaching a maximum value of 58 and then decreased. The breakdown electric field, E_b, increased with the increase of Sm₂O₃ content with a maximum value of 3220 V cm⁻¹ for the 0.75 mol% Sm₂O₃ doped ZnO varistor ceramics. The leakage current, I_L, showed a minimum value of 1.10 μA for the composition of 0.30 mol% Sm₂O₃ doped Bi₂O₃-based ZnO varistor ceramics. The 0.30 mol% Sm₂O₃-doped Bi₂O₃-based ZnO varistor ceramics sintered at 1200 °C exhibited a good stability for dc accelerated aging stress of 0.90 V_1 mA/90 °C/12 h.     

ZnTe/Si heterostructures grown by isothermal closed space sublimation

ZnTe films with different thickness were grown onto (001) Si substrates using isothermal closed space sublimation by alternated sublimation of Zn and Te sources. As a result ZnTe polycrystalline films were obtained with a strong [111] orientation as revealed by X-ray diffraction patterns. The reason for this polycrystalline nature of the samples comes from incomplete removal of SiO₂ from the Si substrate surface. A preferential adsorption of Te in the first stages of the growth was concluded from Rutherford backscattering spectra analysis.     

水下耦合机械噪声源的量化识别及试验

目前水下机械噪声源及其传递路径识别效果较难。为此,将盲源分离算法和传递路径分析方法融合和集成。视多振源信号为卷积混叠,结合LU分解,提出一种新的非正交联合块对角化方法进行耦合振动源的分离。将分离振源作为工况传递路径分析方法的输入振源,建立水下机械振动噪声源识别算法,并对潜艇舱段模型的水下振动-声辐射试验对算法进行验证。结果表明,与现存方法相比,该盲源分离算法具有易实现、收敛速度快、精度高等优点;所集成的源识别算法在水下声场预报和振源贡献量排序中的性能均优于振源耦合时的结果,与实际情况吻合好,达到了高效、准确地识别机械噪声源的目的。