911 research outputs found

    Finite-Size Scaling of the Level Compressibility at the Anderson Transition

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    We compute the number level variance Σ2\Sigma_{2} and the level compressibility χ\chi from high precision data for the Anderson model of localization and show that they can be used in order to estimate the critical properties at the metal-insulator transition by means of finite-size scaling. With NN, WW, and LL denoting, respectively, system size, disorder strength, and the average number of levels in units of the mean level spacing, we find that both χ(N,W)\chi(N,W) and the integrated Σ2\Sigma_{2} obey finite-size scaling. The high precision data was obtained for an anisotropic three-dimensional Anderson model with disorder given by a box distribution of width W/2W/2. We compute the critical exponent as ν1.45±0.12\nu \approx 1.45 \pm 0.12 and the critical disorder as Wc8.59±0.05W_{\rm c} \approx 8.59 \pm 0.05 in agreement with previous transfer-matrix studies in the anisotropic model. Furthermore, we find χ0.28±0.06\chi\approx 0.28 \pm 0.06 at the metal-insulator transition in very close agreement with previous results.Comment: Revised version of paper, to be published: Eur. Phys. J. B (2002

    An exact-diagonalization study of rare events in disordered conductors

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    We determine the statistical properties of wave functions in disordered quantum systems by exact diagonalization of one-, two- and quasi-one dimensional tight-binding Hamiltonians. In the quasi-one dimensional case we find that the tails of the distribution of wave-function amplitudes are described by the non-linear sigma-model. In two dimensions, the tails of the distribution function are consistent with a recent prediction based on a direct optimal fluctuation method.Comment: 13 pages, 5 figure

    Interacting particles at a metal-insulator transition

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    We study the influence of many-particle interaction in a system which, in the single particle case, exhibits a metal-insulator transition induced by a finite amount of onsite pontential fluctuations. Thereby, we consider the problem of interacting particles in the one-dimensional quasiperiodic Aubry-Andre chain. We employ the density-matrix renormalization scheme to investigate the finite particle density situation. In the case of incommensurate densities, the expected transition from the single-particle analysis is reproduced. Generally speaking, interaction does not alter the incommensurate transition. For commensurate densities, we map out the entire phase diagram and find that the transition into a metallic state occurs for attractive interactions and infinite small fluctuations -- in contrast to the case of incommensurate densities. Our results for commensurate densities also show agreement with a recent analytic renormalization group approach.Comment: 8 pages, 8 figures The original paper was splitted and rewritten. This is the published version of the DMRG part of the original pape

    Weak delocalization due to long-range interaction for two electrons in a random potential chain

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    We study two interacting particles in a random potential chain by a transfer matrix method which allows a correct handling of the symmetry of the two- particle wave function, but introduces an artificial ¨bag¨ interaction. The dependence of the two-particle localization length lambta 2on disorder, interaction strength and range is investigated. Our results demonstrate that the recently proposed enhancement of lambta 2 as compared to the results for single particles is vanishingly small for a Hubbard interaction. For longer-range interactions, we observe a small enhancement but with a different disorder dependence than proposed previously

    High-pressure phases and transitions of the layered alkaline earth nitridosilicates SrSiN2 and BaSiN2

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    We investigate the high-pressure phase diagram of SrSiN2 and BaSiN2 with density-functional calculation. Searching a manifold of possible candidate structures, we propose new structural modifications of SrSiN2 and BaSiN2 attainable in high-pressure experiments. The monoclinic ground state of SrSiN2 transforms at 3 GPa into an orthorhombic BaSiN2 type. At 14 GPa a CaSiN2-type structure becomes the most stable configuration of SrSiN2. A hitherto unknown Pbcm modification is adopted at 85 GPa and, finally, at 131 GPa a LiFeO2-type structure. The higher homologue BaSiN2 transforms to a CaSiN2 type at 41 GPa and further to a Pbcm modification at 105 GPa. Both systems follow the pressure-coordination rule: the coordination environment of Si increases from tetrahedral through trigonal bipyramidal to octahedral. Some high-pressure phases are related in structure through simple group–subgroup mechanisms, indicating displacive phase transformations with low activation barriers

    Thermoelectric phenomena in disordered open quantum systems

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    Using a stochastic quantum approach, we study thermoelectric transport phenomena at low temperatures in disordered electrical systems connected to external baths. We discuss three different models of one-dimensional disordered electrons, namely the Anderson model of random on-site energies, the random-dimer model and the random-hopping model - also relevant for random-spin models. We find that although the asymptotic behavior of transport in open systems is closely related to that in closed systems for these noninteracting models, the magnitude of thermoelectric transport strongly depends on the boundary conditions and the baths spectral properties. This shows the importance of employing theories of open quantum systems in the study of energy transport.Comment: 5 pages, 2 figures, revised versio

    Enhanced Charge and Spin Currents in the One-Dimensional Disordered Mesoscopic Hubbard Ring

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    We consider a one-dimensional mesoscopic Hubbard ring with and without disorder and compute charge and spin stiffness as a measure of the permanent currents. For finite disorder we identify critical disorder strength beyond which the charge currents in a system with repulsive interactions are {\em larger} than those for a free system. The spin currents in the disordered repulsive Hubbard model are enhanced only for small UU, where the magnetic state of the system corresponds to a charge density wave pinned to the impurities. For large UU, the state of the system corresponds to localized isolated spins and the spin currents are found to be suppressed. For the attractive Hubbard model we find that the charge currents are always suppressed compared to the free system at all length scales.Comment: 20 RevTeX 3.0 pages, 8 figures NOT include
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