1,036,906 research outputs found

    Superconductor-insulator transition in Coulomb disorder

    Full text link
    Superconductor-insulator transition driven by the decreasing concentration of electrons nn is studied in the case of the disorder potential created by randomly positioned charged impurities. Electrons and Cooper pairs (formed by an non-Coulomb attraction) nonlinearly screen the random potential of impurities. Both electrons and Cooper pairs can be delocalized or localized in the resulting self-consistent potential. The border separating the superconductor and insulator phases in the plane of the concentration of electrons and the length of the Cooper pair is found. For a strong disorder the central segment of this border follows the BEC-BCS crossover line defined for a clean sample.Comment: 4.5 pages, introduction rewritten, a dozen of references added, 2D case adde

    Hopping conductivity of a suspension of nanowires in an insulator

    Full text link
    We study the hopping conduction in a composite made of straight metallic nanowires randomly and isotropically suspended in an insulator. Uncontrolled donors and acceptors in the insulator lead to random charging of wires and hence finite bare density of states at the Fermi level. Then the Coulomb interactions between electrons of distant wires result in the soft Coulomb gap. At low temperatures the conductivity is due to variable range hopping of electrons between wires and obeys the Efros-Shklovskii (ES) law lnσ(TES/T)1/2\ln\sigma \propto -(T_{ES}/T)^{1/2}. We show that TES1/(nL3)2T_{ES} \propto 1/(nL^3)^2, where nn is the concentration of wires and LL is the wire length. Due to enhanced screening of Coulomb potentials, at large enough nL3nL^3, the ES law is replaced by the Mott law.Comment: 5 pages, 5 figure

    On Low-Energy Effective Action of Noncommutative Hypermultiplet Model

    Get PDF
    We consider the noncommutative hypermultiplet model within harmonic superspace approach. The 1-loop four-point contributions to the effective action of selfinteracting q-hypermultiplet are computed. This model has two coupling constants instead of a single one in commutative case. It is shown that both these coupling constants are generated by 1-loop quantum corrections in the model of q-hypermultiplet interacting with vector multiplet. The holomorphic effective action of q-hypermultiplet in external gauge superfield is calculated. For the fundamental representation there is no UV/IR-mixing and the holomorphic potential is a star-product generalization of a standard commutative one. For the adjoint representation of U(N) gauge group the leading contributions to the holomorphic effective action are given by the terms respecting for the UV/IR-mixing which are related to U(1) phase of U(N) group.Comment: 13 pages, 4 figures. v2: minor changes, refs adde

    A simple model of Coulomb disorder and screening in graphene

    Full text link
    We suggest a simple model of disorder in graphene assuming that there are randomly distributed positive and negative centers with equal concentration N/2N/2 in the bulk of silicon oxide substrate. We show that at zero gate voltage such disorder creates two-dimensional concentration n0N2/3n_0 \sim N^{2/3} of electrons and holes in graphene. Electrons and holes reside in alternating in space puddles of the size R0N1/3R_0 \sim N^{-1/3}. A typical puddle has only one or two carriers in agreement with recent scanning single electron transistor experiment.Comment: 2.5 pages, twice longer than previous versio

    Spin-fluctuation theory beyond Gaussian approximation

    Full text link
    A characteristic feature of the Gaussian approximation in the functional-integral approach to the spin-fluctuation theory is the jump phase transition to the paramagnetic state. We eliminate the jump and obtain a continuous second-order phase transition by taking into account high-order terms in the expansion of the free energy in powers of the fluctuating exchange field. The third-order term of the free energy renormalizes the mean field, and fourth-order term, responsible for the interaction of the fluctuations, renormalizes the spin susceptibility. The extended theory is applied to the calculation of magnetic properties of Fe-Ni Invar.Comment: 20 pages, 4 figure

    Two-loop low-energy effective action in Abelian supersymmetric Chern-Simons matter models

    Get PDF
    We compute two-loop low-energy effective actions in Abelian Chern-Simons matter models with N=2 and N=3 supersymmetry up to four-derivative order. Calculations are performed with a slowly-varying gauge superfield background. Though the gauge superfield propagator depends on the gauge fixing parameter, it is shown that the obtained results are independent of this parameter. In the massless case the considered models are superconformal. We demonstrate that the superconformal symmetry strongly restricts the form of two-loop quantum corrections to the effective actions such that the obtained terms have simpler structure than the analogous ones in the effective action of three-dimensional supersymmetric electrodynamics (SQED) with vanishing topological mass.Comment: 1+30 pages; v2: references added, misprints corrected; v3: minor changes, published versio

    Conductance noise in interacting Anderson insulators driven far from equilibrium

    Full text link
    The combination of strong disorder and many-body interactions in Anderson insulators lead to a variety of intriguing non-equilibrium transport phenomena. These include slow relaxation and a variety of memory effects characteristic of glasses. Here we show that when such systems are driven with sufficiently high current, and in liquid helium bath, a peculiar type of conductance noise can be observed. This noise appears in the conductance versus time traces as downward-going spikes. The characteristic features of the spikes (such as typical width) and the threshold current at which they appear are controlled by the sample parameters. We show that this phenomenon is peculiar to hopping transport and does not exist in the diffusive regime. Observation of conductance spikes hinges also on the sample being in direct contact with the normal phase of liquid helium; when this is not the case, the noise exhibits the usual 1/f characteristics independent of the current drive. A model based on the percolative nature of hopping conductance explains why the onset of the effect is controlled by current density. It also predicts the dependence on disorder as confirmed by our experiments. To account for the role of the bath, the hopping transport model is augmented by a heuristic assumption involving nucleation of cavities in the liquid helium in which the sample is immersed. The suggested scenario is analogous to the way high-energy particles are detected in a Glaser's bubble chamber.Comment: 15 pages 22 figure

    Non-Linear Compton Scattering of Ultrashort and Ultraintense Laser Pulses

    Full text link
    The scattering of temporally shaped intense laser pulses off electrons is discussed by means of manifestly covariant quantum electrodynamics. We employ a framework based on Volkov states with a time dependent laser envelope in light-cone coordinates within the Furry picture. An expression for the cross section is constructed, which is independent of the considered pulse shape and pulse length. A broad distribution of scatted photons with a rich pattern of subpeaks like that obtained in Thomson scattering is found. These broad peaks may overlap at sufficiently high laser intensity, rendering inappropriate the notion of individual harmonics. The limit of monochromatic plane waves as well as the classical limit of Thomson scattering are discussed. As a main result, a scaling law is presented connecting the Thomson limit with the general result for arbitrary kinematics. In the overlapping regions of the spectral density, the classical and quantum calculations give different results, even in the Thomson limit. Thus, a phase space region is identified where the differential photon distribution is strongly modified by quantum effects.Comment: 31 pages, 10 figure

    Fractional Chemotaxis Diffusion Equations

    Get PDF
    We introduce mesoscopic and macroscopic model equations of chemotaxis with anomalous subdiffusion for modelling chemically directed transport of biological organisms in changing chemical environments with diffusion hindered by traps or macro-molecular crowding. The mesoscopic models are formulated using Continuous Time Random Walk master equations and the macroscopic models are formulated with fractional order differential equations. Different models are proposed depending on the timing of the chemotactic forcing. Generalizations of the models to include linear reaction dynamics are also derived. Finally a Monte Carlo method for simulating anomalous subdiffusion with chemotaxis is introduced and simulation results are compared with numerical solutions of the model equations. The model equations developed here could be used to replace Keller-Segel type equations in biological systems with transport hindered by traps, macro-molecular crowding or other obstacles.Comment: 25page
    corecore