13 research outputs found

    Power-law spin correlations in a perturbed honeycomb spin model

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    We consider spin-12\frac{1}{2} model on the honeycomb lattice~\cite{Kitaev06} in presence of a weak magnetic field hα1h_{\alpha }\ll 1. Such a perturbation destroys exact integrability of the model in terms of gapless fermions and \textit{static} Z2Z_{2} fluxes. We show that it results in appearance of a long-range tail in the irreducible dynamic spin correlation function: sz(t,r)sz(0,0)hz2f(t,r)% \left\langle \left\langle s^{z}(t,r)s^{z}(0,0)\right\rangle \right\rangle \propto h_{z}^{2}f(t,r), where f(t,r)[max(t,r)]4f(t,r)\propto \lbrack \max (t,r)]^{-4} is proportional to the density polarization function of fermions

    Gapful electrons in a vortex core in granular superconductors

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    We calculate the quasiparticle density of states (DoS) inside the vortex core in a granular superconductor, generalizing the classical solution applicable for dirty superconductors. A discrete version of the Usadel equation for a vortex is derived and solved numerically for a broad range of parameters. Electron DoS is found to be gapful when the vortex size ξ\xi becomes comparable to the distance between neighboring grains ll. Minigap magnitude EgE_g grows from zero at ξ1.4l\xi \approx 1.4 l to third of superconducting gap Δ0\Delta_0 at ξ0.5l\xi \approx 0.5 l. The absence of low-energy excitations is the main ingredient needed to understand strong suppression of microwave dissipation recently observed in a mixed state of granular Al

    Electrostatics of Vortices in Type II Superconductors

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    In a type II superconductor the gap variation in the core of a vortex line induces a local charge modulation. Accounting for metallic screening, we determine the line charge of individual vortices and calculate the electric field distribution in the half space above a field penetrated superconductor. The resulting field is that of an atomic size dipole deaBz^{\bf d} \sim e a_{{\rm B}} {\bf {\hat z}}, aB=2/me2a_{{\rm B}} = \hbar^2/m e^2 is the Bohr radius, acting on a force microscope in the pico to femto Newton range.Comment: 9 pages, late

    Quiet SDS Josephson Junctions for Quantum Computing

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    Unconventional superconductors exhibit an order parameter symmetry lower than the symmetry of the underlying crystal lattice. Recent phase sensitive experiments on YBCO single crystals have established the d-wave nature of the cuprate materials, thus identifying unambiguously the first unconventional superconductor. The sign change in the order parameter can be exploited to construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting a degenerate ground state and a double-periodic current-phase characteristic. Here we discuss how to make use of these special junction characteristics in the construction of a quantum computer. Combining such junctions together with a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit --- a solid state implementation of a quantum bit which remains optimally isolated from its environment.Comment: 4 pages, 2 ps-figure

    Vortex Dynamics and the Hall-Anomaly: a Microscopic Analysis

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    We present a microscopic derivation of the equation of motion for a vortex in a superconductor. A coherent view on vortex dynamics is obtained, in which {\it both} hydrodynamics {\it and} the vortex core contribute to the forces acting on a vortex. The competition between these two provides an interpretation of the observed sign change in the Hall angle in superconductors with mean free path ll of the order of the coherence length ξ\xi in terms of broken particle-hole symmetry, which is related to details of the microscopic mechanism of superconductivity.Comment: 12 pages, late
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