An experimental study for a miniature Stirling refrigerator

Experimental results of a miniature two-stage Stirling cryocooler are introduced. The influence of filling gas pressure and refrigeration temperature on the refrigerating capacity along with the relationship between parameters was measured. The valley pressure corresponding to the lowest refrigeration temperature and the cooldown time versus operating pressure are discussed. The coefficient of performance and thermodynamic efficiency of the cryocooler are calculated based on experimental data

Spin Dynamics in the Second Subband of a Quasi Two Dimensional System Studied in a Single Barrier Heterostructure by Time Resolved Kerr Rotation

By biasing a single barrier heterostructure with a 500nm-thick GaAs layer as the absorption layer, the spin dynamics for both of the first and second subband near the AlAs barrier are examined. We find that when simultaneously scanning the photon energy of both the probe and pump beams, a sign reversal of the Kerr rotation (KR) takes place as long as the probe photons break away the first subband and probe the second subband. This novel feature, while stemming from the exchange interaction, has been used to unambiguously distinguish the different spin dynamics ($T_2^{1*}$ and $T_2^{2*}$) for the first and second subbands under the different conditions by their KR signs (negative for $1^{st}$ and positive for $2^{nd}$). In the zero magnetic field, by scanning the wavelength towards the short wavelength, $T_2^{1*}$ decreases in accordance with the D'yakonov-Perel' (DP) spin decoherence mechanism. At 803nm, $T_2^{2*}$(450ps) becomes ten times longer than $T_2^{1*}$(50ps). However, the value of $T_2^{2*}$ at 803nm is roughly the same as the value of $T_2^{1*}$ at 815nm. A new feature has been disclosed at the wavelength of 811nm under the bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times ($T_2^{1*}$ and $T_2^{2*}$) and the effective $g^*$ factors ($|g^*(E1)|$ and $|g^*(E2)|$) all display a sudden change, due to the "resonant" spin exchange coupling between two spin opposite bands.Comment: 9pages, 3 figure

Electrical Control of Dynamic Spin Splitting Induced by Exchange Interaction as Revealed by Time Resolved Kerr Rotation in a Degenerate Spin-Polarized Electron Gas

The manipulation of spin degree of freedom have been demonstrated in spin polarized electron plasma in a heterostructure by using exchange-interaction induced dynamic spin splitting rather than the Rashba and Dresselhaus types, as revealed by time resolved Kerr rotation. The measured spin splitting increases from 0.256meV to 0.559meV as the bias varies from -0.3V to -0.6V. Both the sign switch of Kerr signal and the phase reversal of Larmor precessions have been observed with biases, which all fit into the framework of exchange-interaction-induced spin splitting. The electrical control of it may provide a new effective scheme for manipulating spin-selected transport in spin FET-like devices.Comment: 8 pages, 3 figures ; added some discussion

Lattice Boltzmann study on Kelvin-Helmholtz instability: the roles of velocity and density gradients

A two-dimensional lattice Boltzmann model with 19 discrete velocities for compressible Euler equations is proposed (D2V19-LBM). The fifth-order Weighted Essentially Non-Oscillatory (5th-WENO) finite difference scheme is employed to calculate the convection term of the lattice Boltzmann equation. The validity of the model is verified by comparing simulation results of the Sod shock tube with its corresponding analytical solutions. The velocity and density gradient effects on the Kelvin-Helmholtz instability (KHI) are investigated using the proposed model. Sharp density contours are obtained in our simulations. It is found that, the linear growth rate $\gamma$ for the KHI decreases with increasing the width of velocity transition layer ${D_{v}}$ but increases with increasing the width of density transition layer ${D_{\rho}}$. After the initial transient period and before the vortex has been well formed, the linear growth rates, $\gamma_v$ and $\gamma_{\rho}$, vary with ${D_{v}}$ and ${D_{\rho}}$ approximately in the following way, $\ln\gamma_{v}=a-bD_{v}$ and $\gamma_{\rho}=c+e\ln D_{\rho} ({D_{\rho}}<{D_{\rho}^{E}})$, where $a$, $b$, $c$ and $e$ are fitting parameters and ${D_{\rho}^{E}}$ is the effective interaction width of density transition layer. When ${D_{\rho}}>{D_{\rho}^{E}}$ the linear growth rate $\gamma_{\rho}$ does not vary significantly any more. One can use the hybrid effects of velocity and density transition layers to stabilize the KHI. Our numerical simulation results are in general agreement with the analytical results [L. F. Wang, \emph{et al.}, Phys. Plasma \textbf{17}, 042103 (2010)].Comment: Accepted for publication in PR

Improved approximation algorithm for k-level UFL with penalties, a simplistic view on randomizing the scaling parameter

The state of the art in approximation algorithms for facility location problems are complicated combinations of various techniques. In particular, the currently best 1.488-approximation algorithm for the uncapacitated facility location (UFL) problem by Shi Li is presented as a result of a non-trivial randomization of a certain scaling parameter in the LP-rounding algorithm by Chudak and Shmoys combined with a primal-dual algorithm of Jain et al. In this paper we first give a simple interpretation of this randomization process in terms of solving an aux- iliary (factor revealing) LP. Then, armed with this simple view point, Abstract. we exercise the randomization on a more complicated algorithm for the k-level version of the problem with penalties in which the planner has the option to pay a penalty instead of connecting chosen clients, which results in an improved approximation algorithm