Partitioning technique for a discrete quantum system

We develop the partitioning technique for quantum discrete systems. The graph consists of several subgraphs: a central graph and several branch graphs, with each branch graph being rooted by an individual node on the central one. We show that the effective Hamiltonian on the central graph can be constructed by adding additional potentials on the branch-root nodes, which generates the same result as does the the original Hamiltonian on the entire graph. Exactly solvable models are presented to demonstrate the main points of this paper.Comment: 7 pages, 2 figure

Using Agent Solutions and Visualization Techniques to Manage Cloud-based Education System

Over the past few years, there are many requests from academic institutions, eLearning developers, education businesses owners, and global enterprises concerning cloud-based education systems. Nowadays, a range of software and applications have been created for managing teaching and learning resources via internet. Many of them have been even trying to integrate all the educational resources into a single cloud system. This paper proposes using agent technologies and visualization solutions to manage cloud-based education systems to match streamline of day to day business and operations. It focuses on adopting agents for University of Westminster’s Cloud computing education system and mobile learning project. It shows how intelligent agents can be used as a good tool for cloud-based education service and associated applications provision and management within Software as Service (SaaS) level

Finite Temperature Phase Diagram of a Two-Component Fermi Gas with Density Imbalance

We investigated possible superfluid phases at finite temperature in a two-component Fermi gas with density imbalance. In the frame of a general four-fermion interaction theory, we solved in the BCS region the gap equations for the pairing gap and pairing momentum under the restriction of fixed number densities, and analyzed the stability of different phases by calculating the superfluid density and number susceptibilities. The homogeneous superfluid is stable only at high temperature and low number asymmetry, the inhomogeneous LOFF survives at low temperature and high number asymmetry, and in between them there exists another possible inhomogeneous phase, that of phase separation. The critical temperatures and the orders of the phase transitions among the superfluid phases and normal phase are calculated analytically and numerically. The phase diagram we obtained in the temperature and number asymmetry plane is quite different from the one in temperature and chemical potential difference plane for a system with fixed chemical potentials.Comment: Final published versio

Evidence for Antiferromagnetic Order in La2−x_{2-x}Cex_{x}CuO4_{4} from Angular Magnetoresistance Measurements

We investigated the in-plane angular magnetoresistivity (AMR) of $T^{^{\prime}}$-phase La$_{2-x}$Ce$_{x}$CuO$_{4}$ (LCCO) thin films ($x=0.06-0.15$) fabricated by a pulsed laser deposition technique. The in-plane AMR with $\mathbf{H}\parallel ab$ shows a twofold symmetry instead of the fourfold behavior found in other electron-doped cuprates such as Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ and Nd$_{2-x}$Ce$_{x}$CuO$_{4}$. The twofold AMR disappears above a certain temperature, $T_{D}$. The $T_{D}(x)$ is well above $T_{c}(x)$ for $x=0.06$ ($\sim 110$ K), and decreases with increasing doping, until it is no longer observed above $T_{c}(x)$ at $x=0.15$. This twofold AMR below $T_{D}(x)$ is suggested to originate from an antiferromagnetic or spin density wave order.Comment: to be published in Phys. Rev. B, Vol. 80 (2009

LOFF Pairing vs. Breached Pairing in Asymmetric Fermion Superfluids

A general analysis for the competition between breached pairing (BP) and LOFF pairing mechanisms in asymmetric fermion superfluids is presented in the frame of a four fermion interaction model. Two physical conditions which can induce mismatched Fermi surfaces are considered: (1) fixed chemical potential asymmetry $\delta\mu$ and (2) fixed fermion number asymmetry $\alpha$. In case (1), the BP state is ruled out because of Sarma instability and LOFF state is thermodynamically stable in a narrow window of $\delta\mu$. In case (2), while the Sarma instability can be avoided and both the BP and LOFF states can survive provided $\alpha$ is less than the corresponding critical value, the BP state suffers magnetic instability and the LOFF state is always thermodynamically stable. While the LOFF window in case (2) is much larger than the one in the conventional case (1), for small $\alpha$ the longitudinal superfluid density of the LOFF state is negative and it suffers also magnetic instability.Comment: 12 pages, 13 figures, published in Physical Review B. Notice: an algebra error in Equation (39) correcte

Fermion Cooper Pairing with Unequal Masses: Standard Field Theory Approach

The fermion Cooper pairing with unequal masses is investigated in a standard field theory approach. We derived the superfluid density and Meissner mass squared of the U(1) gauge field in a general two species model and found that the often used proportional relation between the two quantities is broken down when the fermion masses are unequal. In weak coupling region, the superfluid density is always negative but the Meissner mass squared becomes mostly positive when the mass ratio between the pairing fermions is large enough. We established a proper momentum configuration of the LOFF pairing with unequal masses and showed that the LOFF state is energetically favored due to the negative superfluid density. The single plane wave LOFF state is physically equivalent to an anisotropic state with a spontaneously generated superflow. The extension to finite range interaction is briefly discussed.Comment: 13 pages, 2 figures, published version, erratum will appear soo

Paramagnetic Meissner Effect and Finite Spin Susceptibility in an Asymmetric Superconductor

A general analysis of Meissner effect and spin susceptibility of a uniform superconductor in an asymmetric two-component fermion system is presented in nonrelativistic field theory approach. We found that, the pairing mechanism dominates the magnetization property of superconductivity, and the asymmetry enhances the paramagnetism of the system. At the turning point from BCS to breached pairing superconductivity, the Meissner mass squared and spin susceptibility are divergent at zero temperature. In the breached pairing state induced by chemical potential difference and mass difference between the two kinds of fermions, the system goes from paramagnetism to diamagnetism, when the mass ratio of the two species increases.Comment: 17pages, 2 figures, published in Physical Review

Phase field theory of polycrystalline solidification in three dimensions

A phase field theory of polycrystalline solidification is presented that is able to describe the nucleation and growth of anisotropic particles with different crystallographic orientation in three dimensions. As opposed with the two-dimensional case, where a single orientation field suffices, in three dimensions, minimum three fields are needed. The free energy of grain boundaries is assumed to be proportional to the angular difference between the adjacent crystals expressed here in terms of the differences of the four symmetric Euler parameters. The equations of motion for these fields are obtained from variational principles. Illustrative calculations are performed for polycrystalline solidification with dendritic, needle and spherulitic growth morphologies.Comment: 7 pages, 4 figures, submitted to Europhysics Letters on 14th February, 200

Goldstone-Mode Phonon Dynamics in the Pyrochlore Cd2Re2O7

We have measured the polarized Raman scattering spectra of Cd2Re2O7, the first superconducting pyrochlore, as a function of temperature. For temperatures below the cubic-to-tetragonal structural phase transition (SPT) at 200K, a peak with B1 symmetry develops at zero frequency with divergent intensity. We identify this peak as the first observation of the Goldstone phonon in a crystalline solid. The Goldstone phonon is a collective excitation that exists due to the breaking of the continuous symmetry with the SPT. Its emergence coincides with that of a Raman-active soft mode. The order parameter for both features derives from an unstable doubly-degenerate vibration (with Eu symmetry) of the O1 atoms which drives the SPT.Comment: 4+ pages, 4 figures. Updated figures and text. Accepted to PR