Gauge invariance in the theoretical description of time-resolved angle-resolved pump/probe photoemission spectroscopy

Nonequilibrium calculations in the presence of an electric field are usually performed in a gauge, and need to be transformed to reveal the gauge-invariant observables. In this work, we discuss the issue of gauge invariance in the context of time-resolved angle-resolved pump/probe photoemission. If the probe is applied while the pump is still on, one must ensure that the calculations of the observed photocurrent are gauge invariant. We also discuss the requirement of the photoemission signal to be positive and the relationship of this constraint to gauge invariance. We end by discussing some technical details related to the perturbative derivation of the photoemission spectra, which involve processes where the pump pulse photoemits electrons due to nonequilibrium effects.

Statistics of an adiabatic charge pump

We investigate the effect of time-dependent cyclic-adiabatic driving on the charge transport in a quantum junction. We propose a nonequilibrium Green's function formalism to study the statistics of the charge pumped (at zero bias) through the junction. The formulation is used to demonstrate charge pumping in a single electronic level coupled to two (electronic) reservoirs with time-dependent couplings. An analytical expression for the average pumped current for a general cyclic driving is derived. It is found that for zero bias, for a certain class of driving, the Berry phase contributes only to the odd cumulants. In contrast, a quantum master equation formulation does not show a Berry-phase effect at all.

Microstructure and wear resistance of in situ TiCp composite coating by laser cladding

A new in situ method was realized by one step laser cladding to produce Ni-base alloy composite coating reinforced by in situ reacted and gradiently distributed TiCp particles. The submicron TiCp particles were formed and uniformly distributed because of the in situ reaction and trapping effect under the rapid solidification condition. And, TiCp particles were of gradient distribution on a macro scale and their volume fraction increased from 1.86% at the layer/substrate interface to a maximum 38.4% at the surface of the layer. Furthermore, the in situ generated TiCp/gamma-Ni interfaces were free from deleterious surface reactions. Additionally, the clad coating also revealed a high microhardness of gradient variation with the layer depth and the superior abrasive wear resistance.

Application of laser cladding technology to improve the interface of thermal barrier coatings

The present study is focused on improvement of the adhesion properties of the interface between plasma-sprayed coatings and substrates by laser cladding technology (LCT), Within the laser-clad layer there is a gradient distribution in chemical composition and mechanical properties that has been confirmed by SEM observation and microhardness measurement. The residual stress due to mismatches in thermal and mechanical properties between coatings and substrates can be markedly reduced and smoothed out. To examine the changes of microstructure and crack propagation in the coating and interface during loading, the three-point bending test has been carried out in SEM with a loading device. Analysis of the distribution of shear stress near the interface under loading has been made using the FEM code ANSYS, The experimental results show clearly that the interface adhesion can be improved with LCT pretreatment, and the capability of the interface to withstand the shear stress as well as to resist microcracking has been enhanced.

Investigation on Laser Cladding MOSi2 Powder on Steel

The feasibility of the fabrication of coatings for elevated-temperature structural applications by laser cladding MoSi2 powder on steel was investigated. A dense and crack-free fine coating, well-bonded with the substrate has been obtained by this technique. This coating consists of FeMoSi, Fe2Si and a small amount of Mo5Si3 due to dilution of the substrate in the coating. The microstruelure of the coating is characterized of typical fine dendrites. The dendrites are composed of FeMoSi primary phase, and the interdendritic areas are two eutectic phases of FeMoSi and Fe2Si. The hardness of the coating reaches 845 Hv(0.5), 3.7 times larger than that of the steel substrate (180Hv(0.5)).

Investigation of Laminar Plasma Remelting/Cladding Processing

Investigation of remelting and cladding processing with laminar plasma jets on several metals has been conducted looking for possible development of a new surface modification technique. The remelting tests illustrated that the new method could evidently improve the material microstructure and properties of cast iron. The cladding was done with Al2O3 ceramic powder on stainless steel. The energy dispersive spectra (EDS) analysis was used to determine the distribution of the major cladding element in the plasma-processed layers, for which the microstructure observations and hardness measurements were also performed.

Characteristics of Gas Flow Within a Micro Diffuser/Nozzle Pump

英文摘要: The gas flow characteristics for various shapes of micro diffuser/nozzles have been experimentally investigated. The micro diffuser/nozzles with the lengths of 70 mu m, 90 mu m, 125 mu m and the taper angles of 7 degrees, 10 degrees, 14 degrees are designed and fabricated based on silicon micromachining technology for optimizing and comparing. The flat-wall diffuser/nozzle is 40 mu m x 5 mu m in depth and width. An experimental setup is designed to measure the gas flow rates under controlled temperature and pressure condition. Optimized values for the taper angle and the length of the diffuser/nozzle are experimentally obtained.

ZnO film based surface acoustic wave micro-pump

In this study, a micro-pump unit based on surface acoustic wave (SAW) on piezoelectric ZnO film is designed and fabricated as a micro-fluidic device. It employs a mechanical wave, which is generated electrically using an aluminum interdigital transducer (IDT), and propagates on the surface of the ZnO film. The ZnO film was used in this study because it has a high electromechanical coefficient and an excellent bonding with various substrate materials, in particular silicon. The sputtering parameters for ZnO film deposition have been optimized, and the ZnO films with different thickness from 1 micron to 5.5 microns were prepared. The film properties have been characterized using different methods, such as scanning electron microscopy, X-ray diffraction and atomic force microscopy. Aluminum IDT with a finger width and spacing of 8 microns was patterned on the ZnO film using a lift-off process. The frequency generated was measured using a network analyzer, and it varies from 130 MHz to 180 MHz as a function of film thickness. A signal generator was used to generate the frequency with a power amplifier to amplify the signal, which was then applied to aluminum IDT to generate the surface acoustic wave. If a liquid droplet exists on the surface carrying the acoustic wave, the energy and the momentum of the SAW will be coupled into the fluid, causing the liquid to vibrate and move on film surface. The strength of this movement is determined by the applied voltage and frequency. The volume of the liquid drop loaded on the SAW device in this study is of several hundreds of nanoliters. The movement of the liquid inside the droplet and also on the ZnO film surface can be demonstrated. The performance of ZnO SAW device was characterized as a function of film thickness. © 2007 IOP Publishing Ltd.

In situ formation by laser cladding of a TiC composite coating with a gradient distribution

An in situ method was developed to produce an Ni alloy composite coating reinforced by in situ reacted TiC particles with a gradient distribution, using one-step laser cladding with a pre-placed powder mixture on a 5CrMnMo steel substrate. Dispersed and ultra-fine TIC particles were formed in situ in the coating. Most. of the TiC particles, with a marked gradient distribution, were uniformly distributed within interdendritic regions because of the trapping effect of the advancing solid-liquid interface. In addition, the TiC-gamma-Ni interfaces generated in situ were found to be free from any deleterious surface reaction. Finally, the microhardness also showed a gradient variation, with the highest value of 1250 Hv0.2 and the wear properties of the coating were significantly enhanced.

Novel Fe70Zr10Ni6Al4Si6B4 thick metallic glass coating produced by laser cladding

A novel multicomponent thick metallic glass coating has been synthesised by laser cladding. The maximum coating thickness was I mm. The clad cooling rate restrained the epitaxial growth of dendrites in the metallic glass coating. The metallic glass had high glass forming ability with a wide supercooled liquid region ranging from 59 to 70 K. The metallic glass coating also revealed high hardness and good corrosion resistance.

Synthesis of thick Ni66Cr5Mo4Zr6P15B4 amorphous alloy coating and large glass-forming ability by laser cladding

A thick amorphous alloy (a-alloy) coating was synthesized by laser cladding. The a-alloy had a multicomponent chemistry, i.e., Ni66Cr5MO4Zr6P15B4 (in atom%). The maximum thickness of the coating is 0.8 mm. The a-alloy coating had large glass-forming ability (GFA) with wide supercooled liquid region (SLR) ranging from 52 to 61 K through the coating. The reason for high GFA in the a-alloy coating was discussed. (C) 2002 Published by Elsevier Science B.V.