Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K30: Magnetic Field Effects in Superconductors |
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Sponsoring Units: DCMP DMP Chair: Carmen Almasan, Kent State Room: LACC 406B |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K30.00001: Meissner - London state in superconductors of different shapes Ruslan Prozorov , Makariy Tanatar , Kyuil Cho , Naufer Nusran , Kamal Joshi , Erik Timmons The distributions of vector magnetic fields inside and outside the superconducting samples of different shapes was calculated numerically with high precision by solving Maxwell-London equations by using a full three - dimensional adaptive finite element method. While the ellipsoids reproduce exact analytical solutions, the results for cuboids and finite thickness discs (cylinders) are novel. The solutions are explored for different aspects of sample geometry (i.e., shape, aspect ratio, corner sharpness, orientation of magnetic field etc. and London penetration depth. By analyzing volume magnetic susceptibility we provide new phenomenological formulas for demagnetization correction (quite different from the previous attempts to describe non-ellipsoidal shapes). Indeed, the same formulas can be used for uniformly magnetized magnetic samples. The results of this study are of great practical importance for the analysis of actual magnetic measurements. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K30.00002: Dynamics of the Meissner effect Jorge Hirsch When a metal in a magnetic field becomes superconducting, the magnetic field is expelled from its interior. A Faraday electric field exists during this process that opposes the flux expulsion (Lenz's law). Yet, both electrons and ions acquire angular momentum in direction opposite to that dictated by the Faraday field. The process is reversible under ideal conditions. The dynamics of this process has not been explained within the conventional theory of superconductivity. I argue that this is because the conventional theory cannot explain this process. I explain the dynamics of the Meissner effect using physics that is not contained in the conventional theory, namely: (i) superconductors expel negative charge [1], and (ii) the normal state charge carriers are holes [2],[3]. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K30.00003: London penetration depth in d-wave superconductors in the presence
of a soft internal phase mode Hulikal Krishnamurthy , Aabhaas Mallik , Vijay Shenoy We have recently shown that a d-wave superconductor, in presence of strong correlations, |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K30.00004: Magnetic field dependent thermodynamics of spontaneous translational symmetry breaking at edges of d-wave superconductors Patric Holmvall , Mikael Fogelström , Tomas Lofwander We present results for the magnetic field dependent thermodynamics of a low-temperature phase with spontaneous loop currents at edges of layered d-wave superconductors. As we showed in Ref. [1] for zero external magnetic field, time-reversal symmetry and translational symmetry along the edge are broken spontaneously in a second order phase transition at a temperature T^{*}≈0.18 T_{c} with a jump in the specific heat, where T_{c} is the superconducting transition temperature. The phase with broken symmetry is characterized by a gauge invariant superfluid momentum p_{S} that forms a non-trivial vector field with a chain of sources and sinks along the edges with a period of approximately 12ξ_{0}, where ξ_{0} is the superconducting coherence length. At finite external magnetic field, explicitly breaking time-reversal symmetry, the phase transition is still of second order and is associated with spontaneous breaking of translational symmetry along the edge and formation of the non-trivial p_{S} vector field. Due to a competing paramagnetic response at the edges, the phase transition temperature T^{*} is slowly suppressed with increasing magnetic field strength, but survives into the mixed state. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K30.00005: Onset of surface superconductivity beyond the Saint-James–de Gennes limit Hongyi Xie , Vladimir Kogan , Maxim Khodas , Alex Levchenko We revisit the problem of the surface superconductivity nucleation focusing on the detailed study of the critical field $H_{c3}$ as a function of temperature and disorder. Using the semiclassical Eilenberger formalism, we find that away from the Ginzburg-Landau region the ratio between the nucleation critical field $H_{c3}$ and the upper critical field $H_{c2}$ deviates strongly from the Saint-James–de Gennes limit. In particular, the $H_{c3}/H_{c2}$ is found to be a nonmonotonic function of temperature, which reaches the maximum for a set of parameters corresponding to a crossover region from ballistic to diffusive scattering. We also analyze the robustness of the nucleated phases with respect to diffusive scattering off the sample boundary by solving exactly corresponding eigenvalue problem of an integral equation for the critical field. The implications of these results for the transport in superconductors of various geometries near $H_{c3}$ are briefly discussed. In particular, we present results for the mechanism of magnetoconductivity oscillations due to surface superconductivity effects. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K30.00006: On electromagnetic screening in superconductors Da Wang It's a common belief that the magnetostatic field and longitudinal electrostatic field screenings in superconductors are governed by London and Thomas-Fermi lengths, respectively. However, more and more well-designed experiments showed that such a belief may be incorrect or at least incomplete. In this regard, we revisit this old problem on electromagnetic field screening in superconductors from different points of view. Based on a classical diffusive Drude model combined with the BCS picture, we find the dynamical transverse field penetration depth strongly depends on ωτ where τ is the collision time of the unpaired electrons. Surprisingly, the static transverse field penetration depth is determined only by paired electrons at finite temperature. On the other hand, in order to go beyond the classical picture and incorporate the wave property of electrons, we solve the Bogoliubov-de Gennes equation combined with Maxwell equations self-consistently. We find the longitudinal electrostatic field screening length can be much longer than Thomas-Fermi theory. Our results explain the experiments very well and broaden our understanding of the old problem. |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K30.00007: Estimating the Superconducting Superheating Field in Time-Dependent Ginzburg-Landau Theory using Bifurcation Analysis Alden Pack , Mark Transtrum The expulsion of magnetic fields is a hallmark of superconductivity known as the Meissner effect. In the presence of an applied magnetic field, the Meissner state is thermodynamically stable up to a critical magnetic field (Hc for type I superconductors and Hc1 for type II superconductors). However, the Meissner state may persist as a metastable state up to the so-called "superheating field", Hsh. Understanding the dependence of Hsh on material and geometry is an important question for improving performance of particle accelerators. We numerically study the superheating transition in time-dependent Ginzburg-Landau theory using finite-element methods. At the superheating field, the equations exhibit a saddle-node bifurcation. We use techniques from numerical analysis of dynamical systems to estimate Hsh. We estimate the time for the system to equilibrate at small values of the applied field and extrapolate to where the equilibration time diverges. We explore the dependence on Hsh on material and geometric properties of interest in accelerator physics. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K30.00008: Interplay of bulk and edge-assisted pinning in superconducting stripes Gregory Kimmel , Ivan Sadovskyy , Andreas Glatz The complex dynamics of superconducting vortices in the strong pinning regime is still a major challenge in the field. This dynamics depends qualitatively not only on the shape of individual inclusions and their concentration but also on the spatial arrangement of these inclusions. Utilizing the time-dependent Ginzburg-Landau model and state-of-art large-scale GP GPU simulations, we present a systematic study of the combined effect of a pinning potential generated by the edge of the superconductor and non-superconducting inclusions embedded in the superconducting matrix. In particular, we found that pinning centers in the vicinity of the superconductor’s boundary reduce the effective pinning potential of the boundary and quantified the effect. Our results give important insights into the pinning mechanisms near superconducting boundaries, which can be used, in particular, for increasing the quality factor of superconducting radio frequency cavities. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K30.00009: Study of RF flux penetration on Nb for SRF Applications Bakhrom Oripov , Steven Anlage Superconducting Radio Frequency (SRF) cavities are being widely used in new generations of particle accelerators. A novel near-field magnetic field microwave microscope was successfully built to identify defects on Nb surfaces that limit the accelerating gradient. Our microscope can measure local nonlinear response (3^{rd} harmonic voltage V_{3f}) from Nb bulk and thin film superconductor surfaces under conditions approaching B_{surface} ~ 600mT and frequencies in the multi-GHz range. Using our probe, we performed local microwave measurements of V_{3f}(T,H_{rf}) and its dependence on temperature and rf input power. We observed that below a temperature dependent onset rf field amplitude H_{0}(T), V_{3f}(T,H) is below our noise floor. For fields H_{rf} > H_{ 0}(T) there exists relatively high nonlinear response signal with periodic dips at H_{rf} = H_{1}(T), H_{2}(T), H_{3}(T)… Similar behavior is observed in both bulk Nb and thin film Nb samples. The origins of this response is being investigated with the most likely explanation being nonlinearity generated by Josephson Junctions on the surface where such periodicity would be expected. As temperature approaches T_{c}, H_{0}(T) goes to zero, thus providing a tool for local measurement of T_{c}. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K30.00010: Enhancement of Fulde-Ferrell-Larkin-Ovchinnikov instability in ultraclean layered superconductors by orbital quantization Alexei Koshelev , Kok Wee Song We investigate the superconducting instability for a layered superconductor in the magnetic field oriented perpendicular to the layers and study conditions for the formation of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state with the modulation of the order parameter between the layers, i. e., along the magnetic field. It is well known that within the quasiclassical approximation such modulation only appears at anomalously high Maki parameter corresponding to very large Zeeman splitting energy and/or very shallow band. We found, however, that the Landau-quantization effects strongly promote formation of the interlayer FFLO state with large modulation period. The conditions for such state can be satisfied in many layered materials provided they are sufficiently clean. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K30.00011: Superconducting Vortex Motion on Magnetic Potentials Made with Arrays of Spin-Ice Nanomagnets. Jose Vicent , Victor Rollano , Fernando Valdes-Bango , Alvaro Munoz-Noval , Alicia Gomez , Jose Martin , Maria Velez , Elvira Gonzalez Electron Beam Lithography has been used to pattern arrays with honeycomb shapes on Si substrates. Sputtering and lift-off techniques are used to obtain a honeycomb array of Co connected nanolines. On top of this array a Nb superconducting film is deposited and a cross-shaped bridge is defined for transport measurements. We have explored the vortex dynamics of these hybrid samples. Different scenarios are studied depending on the magnetic states in the Co honeycomb network: i) Demagnetized state, what allows studying vortex motion and cage potentials; ii) Remanent magnetic states, which allow studying vortex motion in magnetic potentials with magnetic frustration showing different spin-ice configurations: 1) Remanent magnetic state achieved by saturating magnetic fields perpendicular to two opposite sides of the hexagon and 2) Remanent magnetic state obtained with saturating magnetic fields parallel to two opposite sides of the hexagon. In both cases, the different magnetic polarity of the hexagon vertices attracts (pinning centers) or repels (antipining centers) vortices and, in comparison with the demagnetized case, a distinct behavior of the vortex motion is achieved. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K30.00012: Vortex avalanches in superconducting/ferromagnetic hybrids Vitalii Vlasko-Vlasov , Fabiano Colauto , Timothy Benseman , Daniel Rosenmann , Wai-Kwong Kwok We study the potential for manipulating ultrafast vortex dynamics in thermo-magnetic avalanches (TMA) using magnetic stripes patterned on top of superconducting films. Tuning the stripe magnetization induces lines of strong magnetic potential for Abrikosov vortices and promotes guided slow vortex motion and directional TMA jumps. The cross-polarized stripes cause the emission of flux branches away from the TMA stamp reducing the TMA power and limiting possible harmful effect of the TMA. We find that TMAs are repeatedly launched from specific spots at the sample perimeter with enhanced barrier for vortex entry. The flux accumulation in these spots followed by accelerated discharge of vortices significantly reduces the threshold of the applied field ramping speed for the TMA creation, which is orders of magnitude smaller than predicted by the existing TMA theory. We observe a regular preferred propagation of the TMA dendrites into maximum trapped flux regions and enhanced TMA activity at the receding branches of magnetization, pointing to the effect of the vortex-antivortex annihilation on the TMA front motion. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K30.00013: Vortex Confinement in S/F heterostructures Cinzia Di Giorgio , Fabrizio Bobba , Anna Maria Cucolo , Steven Moore , Valentyn Novosad , Volodymyr Yefremenko , Goran Karapetrov , Maria Iavarone Low temperature scanning tunneling microscopy and magnetic force microscopy have been used to directly visualize the superconducting vortex behavior in hybrid structures consisting of striped ferromagnetic materials (such as Py or Co/Pd) and conventional superconductors (such as Nb or Pb). We observe that strong confinement potential profoundly affects the vortex spatial distribution eventually leading to vortex cluster formation. High resolution scanning tunneling microscopy has been used to obtain detailed information of the local electronic density of states outside and inside the vortex clusters. |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K30.00014: Enhancement of Superconductivity due to Magnetic Fields in Composite D-wave Superconductors Mauro Schiulaz , Christopher Baldwin , Christopher Laumann , Boris Spivak We study the response of composite D-wave superconductors to weak magnetic fields, at temperatures much lower than the bulk critical temperature. Such systems are formed by superconducting droplets, embedded into a normal metal host in a random fashion. Such system can be modeled as a network of XY spins, with strongly disordered Josephson couplings. We show that the application of a weak magnetic field enhances the superconducting fraction of the system in a non analytic way, as well as the critical temperature. This has to be compared with homogeneous superconductors, whose superconducting properties are always reduced by magnetic fields. |
Wednesday, March 7, 2018 10:48AM - 11:00AM |
K30.00015: Enhanced flux expulsion in superconductor-ferromagnet proximity coupled systems. Rhea Stewart , Machiel Flokstra , Stephen Lee , Nathan Satchell , Gavin Burnell , Sean langridge , Thomas Prokscha , Hubertus Luetkens , Andreas Suter , Elvezio Morenzoni In mesoscopic systems the Meissner response of a superconducting film can be very different from its bulk behaviour. In normal (N) superconductor (S) bilayer systems screening can be greatly enhanced depending on the relative material properties and interface conditions^{1}. Furthermore, with the addition of ferromagnetic (F) layers comes the possibility of generating a paramagnetic screening response due to spin triplet pairs^{2}. Such modifications to the screening manifest in the flux profile across the sample which we can directly probe using Low energy muon spin spectroscopy (LE-μSR)^{3}. We present our recent LE-μSR results on bi and trilayer systems of N, S and F layers. We find the expected NS bilayer enhancement which can be modelled successfully in the Quasiclassical framework. A large discrepancy between experiment and theory occurs, however, with the addition of an F layer. In this case we find a further unexpected enhancement to the flux profile. The F layer enhancement can be tuned by modifying the interface conditions. This work is valuable for understanding more complex S, F, N systems in which there is much interest. |
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