Superfluid-Mott-Insulator Transition in a One-Dimensional Optical Lattice with Double-Well Potentials

We study the superfluid-Mott-insulator transition of ultracold bosonic atoms in a one-dimensional optical lattice with a double-well confining trap using the density-matrix renormalization group. At low density, the system behaves similarly as two separated ones inside harmonic traps. At high density, however, interesting features appear as the consequence of the quantum tunneling between the two wells and the competition between the "superfluid" and Mott regions. They are characterized by a rich step-plateau structure in the visibility and the satellite peaks in the momentum distribution function as a function of the on-site repulsion. These novel properties shed light on the understanding of the phase coherence between two coupled condensates and the off-diagonal correlations between the two wells.Comment: 5 pages, 7 figure

Accurate determination of tensor network state of quantum lattice models in two dimensions

We have proposed a novel numerical method to calculate accurately the physical quantities of the ground state with the tensor-network wave function in two dimensions. We determine the tensor network wavefunction by a projection approach which applies iteratively the Trotter-Suzuki decomposition of the projection operator and the singular value decomposition of matrix. The norm of the wavefunction and the expectation value of a physical observable are evaluated by a coarse grain renormalization group approach. Our method allows a tensor-network wavefunction with a high bond degree of freedom (such as D=8) to be handled accurately and efficiently in the thermodynamic limit. For the Heisenberg model on a honeycomb lattice, our results for the ground state energy and the staggered magnetization agree well with those obtained by the quantum Monte Carlo and other approaches.Comment: 4 pages 5 figures 2 table

Electron spin relaxation in cubic GaN quantum dots

The spin relaxation time $T_{1}$ in zinc blende GaN quantum dot is investigated for different magnetic field, well width and quantum dot diameter. The spin relaxation caused by the two most important spin relaxation mechanisms in zinc blende semiconductor quantum dots, {i.e.} the electron-phonon scattering in conjunction with the Dresselhaus spin-orbit coupling and the second-order process of the hyperfine interaction combined with the electron-phonon scattering, are systematically studied. The relative importance of the two mechanisms are compared in detail under different conditions. It is found that due to the small spin orbit coupling in GaN, the spin relaxation caused by the second-order process of the hyperfine interaction combined with the electron-phonon scattering plays much more important role than it does in the quantum dot with narrower band gap and larger spin-orbit coupling, such as GaAs and InAs.Comment: 8 pages, 5 figures, PRB 79, 2009, in pres

Temporal eye-voice span as a dynamic indicator for cognitive effort during speech processing: A comparative study of reading aloud and sight translation

This chapter examines the dynamic latency between human translators’ reading input and speaking output during reading aloud and sight translation. It aims to determine whether the temporal eye-voice span (EVS) at sentence level could work as a dynamitic indicator of cognitive effort during speech processing. Thirty participants performed both the reading aloud and sight translation tasks with either English or Chinese texts. Their eye movements and speech outputs were recorded by an eye-tracker and an audio recorder, respectively. EVS at sentence initial and sentence terminal positions in the reading aloud and sight translation tasks were analyzed. The results show that the lengths of both sentence-initial and sentence-terminal EVS in sight translation tasks are significantly longer than those in reading aloud tasks. This is in line with results of total gaze fixation duration and fixation count, which are closely related to cognitive effort. Further correlation tests show that both initial and terminal EVS yield a positive although weak correlation with the fixation indexes in the sight translation tasks, while discrepant results emerge in the reading aloud tasks. Hence, we suggest that temporal EVS can be used to discriminate different types of reading-speaking tasks and has the potential to serve as a dynamic indicator of cognitive effort during sight translation

Phase diagram of the frustrated, spatially anisotropic S=1 antiferromagnet on a square lattice

We study the S=1 square lattice Heisenberg antiferromagnet with spatially anisotropic nearest neighbor couplings $J_{1x}$, $J_{1y}$ frustrated by a next-nearest neighbor coupling $J_{2}$ numerically using the density-matrix renormalization group (DMRG) method and analytically employing the Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of the anisotropy, within both methods we find quantum fluctuations to stabilize the N\'{e}el ordered state above the classically stable region. Whereas SBMFT suggests a fluctuation-induced first order transition between the N\'{e}el state and a stripe antiferromagnet for $1/3\leq J_{1x}/J_{1y}\leq 1$ and an intermediate paramagnetic region opening only for very strong anisotropy, the DMRG results clearly demonstrate that the two magnetically ordered phases are separated by a quantum disordered region for all values of the anisotropy with the remarkable implication that the quantum paramagnetic phase of the spatially isotropic $J_{1}$-$J_{2}$ model is continuously connected to the limit of decoupled Haldane spin chains. Our findings indicate that for S=1 quantum fluctuations in strongly frustrated antiferromagnets are crucial and not correctly treated on the semiclassical level.Comment: 10 pages, 10 figure