Optical properties of Si/Si0.87Ge0.13 multiple quantum well wires

Nanometer-scale wires cut into a Si/Si0.87Ge0.13 multiple quantum well structure were fabricated and characterized by using photoluminescence and photoreflectance at temperatures between 4 and 20 K. It was found that, in addition to a low-energy broadband emission at around 0.8 eV and other features normally observable in photoluminescence measurements, fabrication process induced strain relaxation and enhanced electron-hole droplets emission together with a new feature at 1.131 eV at 4 K were observed. The latter was further identified as a transition related to impurities located at the Si/Si0.87Ge0.13 heterointerfaces

A new security architecture for SIP based P2P computer networks

Many applications are transferred from C/S (Client/Server) mode to P2P (Peer-to-Peer) mode such as VoIP (Voice over IP). This paper presents a new security architecture, i.e. a trustworthy authentication algorithm of peers, for Session Initialize Protocol (SIP) based P2P computer networks. A mechanism for node authentication using a cryptographic primitive called one-way accumulator is proposed to secure the P2P SIP computer networks. It leverages the distributed nature of P2P to allow for distributed resource discovery and rendezvous in a SIP network, thus eliminating (or at least reducing) the need for centralized servers. The distributed node authentication algorithm is established for the P2P SIP computer networks. The corresponding protocol has been implemented in our P2P SIP experiment platform successfully. The performance study has verified the proposed distributed node authentication algorithm for SIP based P2P computer networks

Prospect of Making Ceramic Shell Mold by Ceramic Laser Fusion

Manufacturing prototypical castings by conventional investment casting not only takes several weeks, but also is prohibitively expensive. Z Corporation in USA, EOS GmbH and IPT in Germany employ the techniques of 3DP and SLS respectively to make directly ceramic shell molds for metal castings. Although those techniques dramatically reduce time expenditure and production cost, each layer cannot be thinner than 50 µm because of using powder to pave layers. The dimensional accuracy and roughness of the castings still cannot meet the specification of precision casting. Therefore, in this paper the ceramic laser fusion (CLF) was used to pave layers. Each layer can be thinner than 25 µm, so that the step effect can be diminished and the workpiece surface can be smoother; drying time will be shortened dramatically. Moreover, the inherent solid-state support formed by green portion has the capability of preventing upward and downward deformation of the scanned cross sections. In order to make shell mold which meets the roughness requirement (Rq=3.048µm) of the precision casting, following issues have to be further studied: (1) design a proper ceramic shell mold structure, (2) design a paving chamber for paving a complete green layer which can be easily collapsed, (3) cut down drying time, (4) optimize laser scanning process parameters with the smallest distortion, (5) eliminate sunken area, (6) reduce layer thickness to less than13µm, (7) control power to guarantee the energy uniformly absorbed by workpiece, and (8) develop a method which can directly clean green portion in cavity from gate.Mechanical Engineerin

Energy levels of a parabolically confined quantum dot in the presence of spin-orbit interaction

We present a theoretical study of the energy levels in a parabolically confined quantum dot in the presence of the Rashba spin-orbit interaction (SOI). The features of some low-lying states in various strengths of the SOI are examined at finite magnetic fields. The presence of a magnetic field enhances the possibility of the spin polarization and the SOI leads to different energy dependence on magnetic fields applied. Furthermore, in high magnetic fields, the spectra of low-lying states show basic features of Fock-Darwin levels as well as Landau levels.Comment: 6 pages, 4 figures, accepted by J. Appl. Phy

Theory for the nonequilibrium dynamics of flexible chain molecules: relaxation to equilibrium of pentadecane from an all-trans conformation

We extend to nonequilibrium processes our recent theory for the long time dynamics of flexible chain molecules. While the previous theory describes the equilibrium motions for any bond or interatomic separation in (bio)polymers by time correlation functions, the present extension of the theory enables the prediction of the nonequilibrium relaxation that occurs in processes, such as T-jump experiments, where there are sudden transitions between, for example, different equilibrium states. As a test of the theory, we consider the ``unfolding'' of pentadecane when it is transported from a constrained all-trans conformation to a random-coil state at thermal equilibrium. The time evolution of the mean-square end-to-end distance after release of the constraint is computed both from the theory and from Brownian dynamics (BD) simulations. The predictions of the theory agree very well with the BD simulations. Furthermore, the theory produces enormous savings in computer time. This work is a starting point for the application of the new method to nonequilibrium processes with biological importance such as the helix-coil transition and protein folding.Comment: 11 pages total, including 2 Postscript figures; submitted to Journal of Chemical Physic

Optical band edge shift of anatase cobalt-doped titanium dioxide

We report on the optical properties of magnetic cobalt-doped anatase phase titanium dioxide Ti_{1-x}Co_{x}O_{2-d} films for low doping concentrations, 0 <= x <= 0.02, in the spectral range 0.2 to 5 eV. For well oxygenated films (d << 1) the optical conductivity is characterized by an absence of optical absorption below an onset of interband transitions at 3.6 eV and a blue shift of the optical band edge with increasing Co concentration. The absence of below band gap absorption is inconsistent with theoretical models which contain midgap magnetic impurity bands and suggests that strong on-site Coulomb interactions shift the O-band to Co-level optical transitions to energies above the gap.Comment: 5 pages, 4 figures, 1 table; Version 2 - major content revisio

Phase glass and zero-temperature phase transition in a randomly frustrated two-dimensional quantum rotor model

The ground state of the quantum rotor model in two dimensions with random phase frustration is investigated. Extensive Monte Carlo simulations are performed on the corresponding (2+1)-dimensional classical model under the entropic sampling scheme. For weak quantum fluctuation, the system is found to be in a phase glass phase characterized by a finite compressibility and a finite value for the Edwards-Anderson order parameter, signifying long-ranged phase rigidity in both spatial and imaginary time directions. Scaling properties of the model near the transition to the gapped, Mott insulator state with vanishing compressibility are analyzed. At the quantum critical point, the dynamic exponent $z_{\rm dyn}\simeq 1.17$ is greater than one. Correlation length exponents in the spatial and imaginary time directions are given by $\nu\simeq 0.73$ and $\nu_z\simeq 0.85$, respectively, both assume values greater than 0.6723 of the pure case. We speculate that the phase glass phase is superconducting rather than metallic in the zero current limit.Comment: 14 pages, 4 figures, to appear in JSTA

Three-Dimensional Modelling and Simulation of the Ice Accretion Process on Aircraft Wings

© 2018 Chang S, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.In this article, a new computational method for the three-dimensional (3D) ice accretion analysis on an aircraft wing is formulated and validated. The two-phase flow field is calculated based on Eulerian-Eulerian approach using standard dispersed turbulence model and second order upwind differencing with the aid of commercial software Fluent, and the corresponding local droplet collection efficiency, convective heat transfer coefficient, freezing fraction and surface temperature are obtained. The classical Messinger model is modified to be capable of describing 3D thermodynamic characteristics of ice accretion. Considering effects of runback water, which is along chordwise and spanwise direction, an extended Messinger method is employed for the prediction of the 3D ice accretion rates. Validation of the newly developed model is carried out through comparisons with available experimental ice shape and LEWICE codes over a GLC-305 wing under both rime and glaze icing conditions. Results show that good agreement is achieved between the current computational ice shapes and the compared results. Further calculations based on the proposed method over a M6 wing under different test conditions are numerically demonstrated.Peer reviewedFinal Published versio

Avalanches and Correlations in Driven Interface Depinning

We study the critical behavior of a driven interface in a medium with random pinning forces by analyzing spatial and temporal correlations in a lattice model recently proposed by Sneppen [Phys. Rev. Lett. {\bf 69}, 3539 (1992)]. The static and dynamic behavior of the model is related to the properties of directed percolation. We show that, due to the interplay of local and global growth rules, the usual method of dynamical scaling has to be modified. We separate the local from the global part of the dynamics by defining a train of causal growth events, or "avalanche", which can be ascribed a well-defined dynamical exponent $z_{loc} = 1 + \zeta_c \simeq 1.63$ where $\zeta_c$ is the roughness exponent of the interface. We observe that the avalanche size distribution obeys a power-law decay with an exponent $\kappa \simeq 1.25$.Comment: 7 pages, (5 figures available upon request), REVTeX, RUB-TP3-93-0