6,542 research outputs found

    Comment on the anisotropic impurity scattering in superconductors

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    We discuss a case of a strong anisotropic impurity scattering within the model introduced in our previous paper [Phys. Rev. B54, 15463 (1996)] and clarify our former statement about a possible enhancement of the critical temperature in this scattering regime. We show that for the anisotropy of the impurity potential determined by the functions from the non-identity irreducible representations of the crystal point group, the critical temperature decreases with the impurity scattering rate which is consistent with a generally understood role of disorder in superconductors.Comment: 6 pages, RevTe

    Maximum screening fields of superconducting multilayer structures

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    It is shown that a multilayer comprised of alternating thin superconducting and insulating layers on a thick substrate can fully screen the applied magnetic field exceeding the superheating fields HsH_s of both the superconducting layers and the substrate, the maximum Meissner field is achieved at an optimum multilayer thickness. For instance, a dirty layer of thickness 0.1  μ\sim 0.1\; \mum at the Nb surface could increase Hs240H_s\simeq 240 mT of a clean Nb up to Hs290H_s\simeq 290 mT. Optimized multilayers of Nb3_3Sn, NbN, some of the iron pnictides, or alloyed Nb deposited onto the surface of the Nb resonator cavities could potentially double the rf breakdown field, pushing the peak accelerating electric fields above 100 MV/m while protecting the cavity from dendritic thermomagnetic avalanches caused by local penetration of vortices

    Pair-breaking in iron-pnictides

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    The puzzling features of the slopes of the upper critical field at the critical temperature TcT_c, Hc2(Tc)TcH^\prime_{c2}(T_c)\propto T_c, and of the specific heat jump ΔCTc3\Delta C\propto T_c^3 of iron-pnictides are interpreted as caused by a strong pair-breaking

    Nonadiabatic effects of rattling phonons and 4f excitations in Pr(Os{1-x}Ru{x})4Sb12

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    In the skutterudite compounds the anharmonic 'rattling' oscillations of 4f-host ions in the surrounding Sb12 cages are found to have significant influence on the low temperature properties. Recently specific heat analysis of Pr(Os{1-x}Ru{x})4Sb12 has shown that the energy of crystalline electric field (CEF) singlet-triplet excitations increases strongly with Ru-concentration x and crosses the almost constant rattling mode frequency ω0\omega_0 at about x ~ 0.65. Due to magnetoelastic interactions this may entail prominent nonadiabatic effects in inelastic neutron scattering (INS) intensity and quadrupolar susceptibility. Furthermore the Ru- concentration dependence of the superconducting Tc, notably the minimum at intermediate x is explained as a crossover effect from pairforming aspherical Coulomb scattering to pairbreaking exchange scattering.Comment: 12 pages, 5 figures; to appear in Phys. Rev.

    Unusual field and temperature dependence of Hall effect in graphene

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    We calculate the classic Hall conductivity and mobility of the undoped and doped (or in the gate voltage) graphene as a function of temperature, magnetic field, and carrier concentration. Carrier collisions with defects and acoustic phonons are taken into account. The Hall resistivity varies almost linearly with temperature. The magnetic field dependence of resistivity and mobility is anomalous in weak magnetic fields. There is the square root contribution from the field in the resistivity. The Hall mobility diverges logarithmically with the field for low doping.Comment: 4 pages, 5 figures, typos correcte

    Weak anisotropic impurity scattering in unconventional superconductors

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    The effect of weak anisotropic (momentum-dependent) impurity scattering in unconventional superconductors has been investigated. It is shown that the anisotropic scattering can lead either to a small reduction or a small enhancement of the isotropic pair-breaking effect. The influence of the anisotropy of the scattering potential becomes significant for the order parameters with large Fermi surface average values. In that case an unexpected enhancement (up to 10%) of the critical temperature over the critical temperature in the absence of impurities is predicted for a small impurity concentration.Comment: 12 pages, RevTeX, 3 PostScript figure

    Influence of high magnetic fields on superconducting transition of one-dimensional Nb and MoGe nanowires

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    The effects of strong magnetic field on superconducting Nb and MoGe nanowires with diameter 10\sim10 nm have been studied. We have found that the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase slips is applicable in a wide range of magnetic fields and describes well the temperature dependence of the wire resistance, over eleven orders of magnitude. The field dependence of the critical temperature, TcT_{c}, extracted from the LAMH fits is in good quantitative agreement with the theory of pair-breaking perturbations that takes into account both spin and orbital contributions. The extracted spin-orbit scattering time agrees with an estimate τsoτ(c/Ze2)4\tau_{so}\simeq \tau(\hbar c/ Ze^{2})^{4}, where τ\tau is the elastic scattering time and ZZ is the atomic number.Comment: accepted for publication in Physical Review Letter

    Differential conductance of point contacts between an iron-based superconductor and a normal metal

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    We present a theoretical description of the differential conductance of point contacts between a normal metal and a multi-band superconductor with extended s\pm pairing symmetry. We demonstrate that the interband impurity scattering broadens the coherent peak near the superconducting gap and significantly reduces its height even at relatively low scattering rates. This broadening is consistent with a number of recent experiments performed for both tunnel junctions and larger diffusive contacts. Our theory helps to better evaluate the energy gap of iron-based superconductors from point contact Andreev spectroscopy measurements.Comment: 5 pages, 4 figure

    Ultra-cold fermions in real or fictitious magnetic fields: The BCS-BEC evolution and the type-I--type-II transition

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    We study ultra-cold neutral fermion superfluids in the presence of fictitious magnetic fields, as well as charged fermion superfluids in the presence of real magnetic fields. Charged fermion superfluids undergo a phase transition from type-I to type-II superfluidity, where the magnetic properties of the superfluid change from being a perfect diamagnet without vortices to a partial diamagnet with the emergence of the Abrikosov vortex lattice. The transition from type-I to type-II superfluidity is tunned by changing the scattering parameter (interaction) for fixed density. We also find that neutral fermion superfluids such as 6^6Li and 40^{40}K are extreme type-II superfluids, and that they are more robust to the penetration of a fictitious magnetic field in the BCS-BEC crossover region near unitarity, where the critical fictitious magnetic field reaches a maximum as a function of the scattering parameter (interaction).Comment: 4+ pages with 2 figure

    Fermion States on the Sphere S2S^2

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    We solve for the spectrum and eigenfunctions of Dirac operator on the sphere. The eigenvalues are nonzero whole numbers. The eigenfunctions are two-component spinors which may be classified by representations of the SU(2) group with half-integer angular momenta. They are not the conventional spherical spinors but special linear combinations of those.Comment: Talk given at the Fifth Workshop on Quantum Field Theory under the Influence of External Conditions. 4p
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