Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T25: Superconductivity: Theory, Mainly Vortices |
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Sponsoring Units: DCMP Chair: Maxim Korshunov, University of Florida Room: D166 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T25.00001: Microscopic investigation of vortex-vortex interaction in conventional and unconventional superconductors Masaru Kato, Yuhei Niwa Recently, we have investigated the vortex structures in nano-sized superconductors. We found the interference of vortex bound states around multiple vortices. And their interaction is affected by such quasi-particle interference. Vortex structures becomes different from which phenomenological theory predicts [1]. Therefore we clarify how such quasi-particle structure changes the vortex-vortex interaction. In order for this, we investigate the quasi-particle structures around a pair of vortices, using the Bogoliubov-de Gennes equation in the elliptical coordinates, where two vortices sit at two foci We expand quasi-particle wave functions by the (modified) Mathieu function. From the numerical results, we discuss the distance dependence of interference of the quasi-particle bound states and free energies. We will extend our method to unconventional superconductors. \\[4pt] [1] H. Suematsu, T. Ishida, T. Koyama, M. Machida, M. Kato, J. Phys. Soc. Jpn. 79, no.12 (2010) in press. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T25.00002: Type-1.5 superconductivity in multiband systems: the effects of interband couplings Egor Babaev, Johan Carlstrom, Martin Speight Two-component superconductors can posses a ``type-1.5" state which falls outside the usual type-I/type-II dichotomy. In this regime two vortices attract one another at long range but repel at shorter ranges. Multiple vortices thus should form clusters in cases where their interaction could be approximately described by a superposition of such nonmonotonic two-body forces and one can define a negative interface energy inside a cluster and at the same there one can define a positive interface energy associated with the cluster's boundary. We describe the appearance of type-1.5 regimes in the case of two bands with various kinds of substantial interband couplings such as Josephson coupling, mixed gradient coupling and density-density interactions. We show that in these cases the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by mixed combinations of the density fields. Talk based on arXiv:1009.2196 and Phys. Rev. Lett. 105, 067003 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T25.00003: Proposed Aharonov-Casher interferometry of non-Abelian vortices in chiral p-wave superconductors Eytan Grosfeld, Babak Seradjeh, Smitha Vishveshwara We propose a two-path vortex interferometry experiment based on the Aharonov- Casher effect for detecting the non-Abelian nature of vortices in a chiral p-wave superconductor. The effect is based on observing vortex interference patterns upon enclosing a finite charge of externally controllable magnitude within the interference path. We predict that when the interfering vortices enclose an odd number of identical vortices in their path, the interference pattern disappears only for non-Abelian vortices. When pairing involves two distinct spin species, we derive the mutual statistics between half quantum and full quantum vortices and show that, remarkably, our predictions still hold for the situation of a full quantum vortex enclosing a half quantum vortex in its path. We discuss the experimentally relevant conditions under which these effects can be observed. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T25.00004: Microscopic theory of vortex interaction in two-band superconductors and type-1.5 superconductivity Mihail Silaev, Egor Babaev In the framework of self-consistent microscopic theory we study the structure and interaction of vortices in two-gap superconductor taking into account the interband Josephson coupling. The asymptotical behavior of order parameter densities and magnetic field is studied analytically within the microscopic theory at low temperature. At higher temperatures, results consistent with Ginzburg-Landau theory are obtained. It is shown that under quite general conditions and in a wide temperature ranges (in particular outside the validity of the Ginzburg-Landau theory) there can exist an additional characteristic length scale of the order parameter density variation which exceeds the London penetration length of magnetic field due to the multi-component nature of superconducting state. Such behavior of order parameter density variation leads to the attractive long-range and repulsive short-range interaction between vortices. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T25.00005: Hairy balls and flux lines in superconductors Mark Laver, Ted Forgan Many physical phenomena originate from geometrical effects rather than from local physics. For example, the hairy ball theorem --- a hairy sphere cannot be combed --- is fulfilled by the atmospheric circulation with the existence of stratospheric polar vortices, and the fact that there is always at least one place on Earth where the horizontal wind is still. We examine the consequences of the hairy ball theorem for the flux line lattice (FLL). We find that discontinuities must exist in lattice shape as a function of field direction relative to the crystal. The remarkable ways in which the hairy ball theorem is fulfilled are demonstrated for FLL's in superconducting niobium. We show that extraordinary, unconventional flux line lattice shapes that spontaneously break the underlying crystal symmetry are surprisingly likely across all Type-II superconductors, both conventional and unconventional. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T25.00006: Boundary Wess-Zumino-Novikov-Witten model, BCS superconductivity, and Maxwell-Bloch theory Tigran Sedrakyan, Victor Galitski We establish an exact correspondence between the discrete-state pairing Hamiltonian (Richardson model) and the Wess-Zumino-Novikov-Witten (WZNW) model modified by an additional boundary operator. We solve this boundary WZNW model exactly and from this solution re-derive the Richardson equations of the pairing Hamiltonian. As an example of practical applications of the boundary WZNW model we use the obtained results to derive solution to the Maxwell Bloch theory of a two-level laser with damping and pumping. We use the results to calculate various observable characteristics of a laser: (i) the complex electrical field amplitude, (ii) the polarization of the laser medium, (iii) the population inversion. We discuss the relation of our results to recent experimental data. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T25.00007: Geometric phases of d-wave vortices in a model of lattice fermions Zhenyu Zhou, Alexander Seidel, Oskar Vafek We study the local and topological features of Berry phases associated with the adiabatic transport of vortices in a d-wave superconductor of lattice fermions. At half filling, where the local Berry curvature must vanish due to symmetries, the phase associated with the exchange of two vortices is found to vanish as well, implying that vortices behave as bosons. Away from half filling, and in the limit where the magnetic length is large compared to the lattice constant, the local Berry curvature gives rise to an intricate flux pattern within the large magnetic unit cell. This renders the Berry phase associated with an exchange of two vortices highly path dependent. However, it is shown that ``statistical'' fluxes attached to the vortex positions are still absent. Despite the complicated profile of the Berry curvature away from half filling, we show that the average flux density associated with this curvature is tied to the average particle density. This is familiar from dual theories of bosonic systems, even though in the present case, the underlying particles are fermions. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T25.00008: Quasiparticle scattering from vortices in d-wave superconductors: Superflow and Berry phase contributions Adam C. Durst, Sriram Ganeshan, Manas Kulkarni In the vortex state of a d-wave superconductor, massless Dirac quasiparticles are scattered from magnetic vortices via a combination of two basic mechanisms: effective potential scattering due to the superflow swirling about the vortices and Aharonov-Bohm scattering due to the Berry phase acquired by a quasiparticle upon circling a vortex. First, we consider the superflow contribution by calculating the differential cross section for a quasiparticle scattering from the effective non-central potential of a single vortex. Next, we consider the Berry phase contribution, which results in branch cuts between neighboring vortices across which the quasiparticle wave function changes sign. Here, the simplest problem that captures the physics is that of scattering from a single finite branch cut that stretches between two vortices. Elliptical coordinates are natural for this two-center problem and we proceed by separating the massless Dirac equation in elliptical coordinates. The separated equations take the form of the Whittaker-Hill equations, which we solve to obtain radial and angular eigenfunctions. With these eigenfunctions in hand, we construct the scattering cross section via partial wave analysis. We discuss the scattering effect of each mechanism, superflow and Berry phase, leaving the important issue of interference between the two mechanisms to future work. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T25.00009: Ground states of multi-band type-I and type-1.5 superconductors and interlaced type-I/type-II layered superconducting structures in external magnetic field Julien Garaud, Johan Carlstrom, Egor Babaev We report a numerical study of magnetic field-induced structures in multiband/multi-component superconductors and type-I/type-II multilayers. The magnetic ground state in these different regimes shows very rich structure formation. In particular we report vortex cluster formation in the cases of strong interband Josephson coupling. The results in particular can be applied to layered structures manufactured from interlaced layers of type-I and type-II superconductors yielding effectively the type-1.5 superconducting behavior with tunable intercomponent couplings. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T25.00010: Properties of vortex clusters and intercluster interaction in type-II and type-1.5 two-band superconductors and type-I/type-II superconducting bilayers Johan Carlstrom, Julien Garaud, Egor Babaev We discuss magnetic flux-carrying vortex states in multiband type-II and type-1.5 superconductors and interlaced type-I/type-II superconducting multilayers. Especially we focus on the case where there is a substantial disparity in characteristic variations of superfluid densities in bands or superconducting layers. We discuss the properties of vortex clusters in the type-1.5 regime both in the cases of strong and weak interband Josephson coupling, including interaction between vortex clusters with different numbers of vortices. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T25.00011: Novel time-dependent Ginzburg-Landau simulation of extreme type-II superconductors with a focus on the Nernst signal and its fluctuations Sangwoo Chung, Paata Kakashvili, Carlos Bolech Recently, different transport coefficients have been measured in High-$T_{c}$ superconductors to pinpoint the nature of the pseudogap phase. In particular, the thermoelectric coefficients received a considerable attention both theoretically and experimentally. We numerically simulate the Nernst effect in extreme type-II superconductors using the time-dependent Ginzburg-Landau equations. We report the sign reversal of the thermoelectric coefficient, $\alpha_{xy}$, at temperatures close to the mean-field transition temperature $T^{MF}_{c}(H)$, which qualitatively agrees with recent experiments on high-$T_{c}$ materials. We also discuss the noise power spectrum of $\alpha_{xy}$, which shows $1/f^{\beta}$ behavior. Based on this observation, we propose an experiment to determine different regimes of vortex dynamics by measuring the noise correlations of the Nernst signal. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T25.00012: Temperature dependence of the superheating field in type-II superconductors Mark Transtrum, Gianluigi Catelani, James Sethna The expulsion of an applied magnetic field is a hallmark characteristic of superconductivity. For a sufficiently large external field, the superconducting state transitions to a normal metal (type-I) or a flux-lattice state (type-II) at a field $H_{c1}$. The superconducting state is metastable and persists up to a field above $H_{c1}$, the so-called superheating field. We numerically solve the semi-classical equations of Eilenberger for the anomalous Green's functions, order parameter, and vector potential for a clean superconductor in an external magnetic field. We use a linear stability analysis to explore the local stability of the free energy to two-dimensional fluctuations, mapping the stability onto an eigenvalue problem of a linear operator. We systematically calculate the dependence of the superheating field on both temperature and the Ginzburg-Landau parameter $\kappa$. We compare our results with the analogous calculation for Ginzburg-Landau theory, which is valid only near the critical temperature, and to experimental measurements. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T25.00013: Molecular dynamic simulation on the rule of the defect size on critical current at low temperature Abdalla Obeidat, Hadeel Abulahim Molecular dynamics have been used to study the effect of the pinning center sizes on the critical current density of driven vortex lattices interacting with periodic arrays of pinning sites in two dimensions. In our study, we assumed that the radii of the pinning centers are much larger than the coherence length of the vortices. The critical current density has been studied at different temperatures for several values of pinning strengths. The overdamped equation of vortex motion has been solved taking into account the vortex-vortex repulsion, the thermal force, the attractive vortex-pinning interaction, and the driving Lorentz force. We found that the critical current density is independent of pinning size at low temperatures. [Preview Abstract] |
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