2,359 research outputs found

    Vortex glass transitions in disordered three-dimensional XY models: Simulations for several different sets of parameters

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    The anisotropic frustrated 3D XY model with strong disorder in the coupling constants is studied as a model of a disordered superconductor in an applied magnetic field. Simulations with the exchange Monte Carlo method are performed for frustrations f=1/5 and f=1/4, corresponding to two different values of the magnetic field along the z direction. The anisotropy is also varied. The determination of the helicity modulus from twist histograms is discussed in some detail and the helicity modulus is used in finite size scaling analyses of the vortex glass transition. The general picture is that the behavior in [Phys. Rev. Lett. 91, 077002 (2003)] is confirmed. For strong (e.g. isotropic) coupling in the z direction the helicity modulus fails to scale and it is argued that this is due to a too small effective randomness of such systems for the accessible system sizes

    Counterflow measurements in strongly correlated GaAs hole bilayers: evidence for electron-hole pairing

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    We study interacting GaAs bilayer hole systems, with very small interlayer tunneling, in a counterflow geometry where equal currents are passed in opposite directions in the two, independently contacted layers. At low temperatures, both the longitudinal and Hall counterflow resistances tend to vanish in the quantum Hall state at total bilayer filling ν=1\nu=1, demonstrating the pairing of oppositely charged carriers in opposite layers. The temperature dependence of the counterflow Hall resistance is anomalous compared to the other transport coefficients: even at relatively high temperatures (\sim600mK), it develops a very deep minimum, with a value that is about an order of magnitude smaller than the longitudinal counterflow resistivity.Comment: 4+ pages, 4 figure

    Dynamics of vortex glass phase in strongly type II superconductors

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    Dynamics of vortices in strongly type-II superconductors with strong disorder is investigated within the frustrated three-dimensional XY model. For two typical models in [Phys. Rev. Lett. {\bf 91}, 077002 (2003)] and [Phys. Rev. B {\bf 68}, 220502(R) (2003)], a strong evidence for the finite temperature vortex glass transition in the unscreened limit is provided by performing large-scale dynamical simulations. The obtained correlation length exponents and the dynamic exponents in both models are different from each other and from those in the three-dimensional gauge glass model. In addition, a genuine continuous depinning transition is observed at zero temperature for both models. A scaling analysis for the thermal rounding of the depinning transition shows a non-Arrhenius type creep motion in the vortex glass phase, contrarily to the recent studies..Comment: 6 pages, 5 figure

    Localized systems coupled to small baths: from Anderson_{nderson} to Zeno_{eno}

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    We investigate what happens if an Anderson localized system is coupled to a small bath, with a discrete spectrum, when the coupling between system and bath is specially chosen so as to never localize the bath. We find that the effect of the bath on localization in the system is a non-monotonic function of the coupling between system and bath. At weak couplings, the bath facilitates transport by allowing the system to 'borrow' energy from the bath. But above a certain coupling the bath produces localization, because of an orthogonality catastrophe, whereby the bath 'dresses' the system and hence suppresses the hopping matrix element. We call this last regime the regime of "Zeno-localization", since the physics of this regime is akin to the quantum Zeno effect, where frequent measurements of the position of a particle impede its motion. We confirm our results by numerical exact diagonalization

    Permutation-Symmetric Multicritical Points in Random Antiferromagnetic Spin Chains

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    The low-energy properties of a system at a critical point may have additional symmetries not present in the microscopic Hamiltonian. This letter presents the theory of a class of multicritical points that provide an interesting example of this in the phase diagrams of random antiferromagnetic spin chains. One case provides an analytic theory of the quantum critical point in the random spin-3/2 chain, studied in recent work by Refael, Kehrein and Fisher (cond-mat/0111295).Comment: Revtex, 4 pages (2 column format), 2 eps figure

    Universality and Crossover of Directed Polymers and Growing Surfaces

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    We study KPZ surfaces on Euclidean lattices and directed polymers on hierarchical lattices subject to different distributions of disorder, showing that universality holds, at odds with recent results on Euclidean lattices. Moreover, we find the presence of a slow (power-law) crossover toward the universal values of the exponents and verify that the exponent governing such crossover is universal too. In the limit of a 1+epsilon dimensional system we obtain both numerically and analytically that the crossover exponent is 1/2.Comment: LateX file + 5 .eps figures; to appear on Phys. Rev. Let

    Zero Temperature Dynamics of the Weakly Disordered Ising Model

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    The Glauber dynamics of the pure and weakly disordered random-bond 2d Ising model is studied at zero-temperature. A single characteristic length scale, L(t)L(t), is extracted from the equal time correlation function. In the pure case, the persistence probability decreases algebraically with the coarsening length scale. In the disordered case, three distinct regimes are identified: a short time regime where the behaviour is pure-like; an intermediate regime where the persistence probability decays non-algebraically with time; and a long time regime where the domains freeze and there is a cessation of growth. In the intermediate regime, we find that P(t)L(t)θP(t)\sim L(t)^{-\theta'}, where θ=0.420±0.009\theta' = 0.420\pm 0.009. The value of θ\theta' is consistent with that found for the pure 2d Ising model at zero-temperature. Our results in the intermediate regime are consistent with a logarithmic decay of the persistence probability with time, P(t)(lnt)θdP(t)\sim (\ln t)^{-\theta_d}, where θd=0.63±0.01\theta_d = 0.63\pm 0.01.Comment: references updated, very minor amendment to abstract and the labelling of figures. To be published in Phys Rev E (Rapid Communications), 1 March 199

    Numerical Results for the Ground-State Interface in a Random Medium

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    The problem of determining the ground state of a dd-dimensional interface embedded in a (d+1)(d+1)-dimensional random medium is treated numerically. Using a minimum-cut algorithm, the exact ground states can be found for a number of problems for which other numerical methods are inexact and slow. In particular, results are presented for the roughness exponents and ground-state energy fluctuations in a random bond Ising model. It is found that the roughness exponent ζ=0.41±0.01,0.22±0.01\zeta = 0.41 \pm 0.01, 0.22 \pm 0.01, with the related energy exponent being θ=0.84±0.03,1.45±0.04\theta = 0.84 \pm 0.03, 1.45 \pm 0.04, in d=2,3d = 2, 3, respectively. These results are compared with previous analytical and numerical estimates.Comment: 10 pages, REVTEX3.0; 3 ps files (separate:tar/gzip/uuencoded) for figure