Vortex State Of Superconductors Research Articles

Induced Anisotropy by the Inclusion of Defects of Variable $$T_\mathrm{c}$$ T c in a Superconducting Disk

Static and non-static properties of the superconducting vortex state in a splice of weak and strong superconducting regions were studied. We considered a superconducting disk with triangular and circular defects, which lead to the appearance of anisotropy in the sample. This anisotropy is included in the time-dependent Ginzburg–Landau equations through a varied \(T_\mathrm{c}\), which leads to distinct differences in the magnetization, supercurrent, vorticity, and free energy curves. Several unconventional vortex states were found, some comprising vortex clusters or exhibiting asymmetry, which are formed owing to the competing interactions of vortices with different Meissner currents that can be found in the sample due to anisotropy.

Paramagnetic excited vortex states in superconductors

We consider excited vortex states, which are vortex states left inside a superconductor once the external applied magnetic field is switched off and whose energy is lower than of the normal state. We show that this state is paramagnetic and develop here a general method to obtain its Gibbs free energy through conformal mapping. The solution for any number of vortices in any cross section geometry can be read off from the Schwarz - Christoffel mapping. The method is based on the first order equations used by A. Abrikosov to discover vortices.

Dependence of vortex states in superconductors on a chiral helimagnet and an applied magnetic field

We study effects of a chiral helimagnet (CHM) on vortex states in a superconductor, solving the Ginzburg–Landau equations in a chiral helimagnet/superconductor bilayer system. We found that vortices form a periodically modulated triangular lattice, because the magnetic field from the chiral helimagnet HCHM oscillates spatially. An increase of a critical current is expected, because vortices are pinned by the HCHM.

Coupling between temporal and spatial chaos of vortex state in superconductors with periodical pinning arrays

Vortex motion in the background of many vortices is investigated by a mean field approach. Effects of the vortex–vortex coupling, the driving frequency, and the vortex viscosity on the vortex motion have been studied to reveal the interaction between the spatial and temporal chaos. It is found that the mean-field approach is a good approximation to describe the vortex motion in one dimensional vortex system. The vortex motion under the damping mode is a kind of self-organized motion. The spatial chaos can dominate the chaotic behavior of the system.

25pRK-4 パウリ常磁性の強い超伝導渦糸格子における反強磁性揺らぎの効果(磁性理論,領域8,強相関系:高温超伝導,強相関f電子系など)

25pRK-4 パウリ常磁性の強い超伝導渦糸格子における反強磁性揺らぎの効果(磁性理論,領域8,強相関系:高温超伝導,強相関f電子系など)

Upper critical field in the organic superconductorβ″−(ET)2SF5CH2CF2SO3: Possibility of Fulde-Ferrell-Larkin-Ovchinnikov state

We report upper critical-field measurements in the metal-free-all-organic superconductor ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\text{ET})}_{2}{\text{SF}}_{5}{\text{CH}}_{2}{\text{CF}}_{2}{\text{SO}}_{3}$ obtained from measuring the in-plane penetration depth using the tunnel diode oscillator technique. For magnetic field applied parallel to the conducting planes the low-temperature upper critical fields are found to exceed the Pauli limiting field calculated by using a semiempirical method. Furthermore, we found a signature that could be the phase transition between the superconducting vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov state in the form of a kink just below the upper critical field and only at temperatures below 1.23 K.

Transport anisotropy as a probe of the interstitial vortex state in superconductors with artificial pinning arrays

We show using simulations that when interstitial vortices are present in superconductors with periodic pinning arrays, the transport in two perpendicular directions can be anisotropic. The degree of the anisotropy varies as a function of field due to the fact that the interstitial vortex lattice has distinct orderings at different matching fields. The anisotropy is most pronounced at the matching fields but persists at incommensurate fields, and it is most prominent for triangular, honeycomb, and kagome pinning arrays. Square pinning arrays can also show anisotropic transport at certain fields in spite of the fact that the perpendicular directions of the square pinning array are identical. We show that the anisotropy results from distinct vortex dynamical states and that although the critical depinning force may be lower in one direction, the vortex velocity above depinning may also be lower in the same direction for ranges of external drives where both directions are depinned. For honeycomb and kagome pinning arrays, the anisotropy can show multiple reversals as a function of field. We argue that when the pinning sites can be multiply occupied such that no interstitial vortices are present, the anisotropy is strongly reduced or absent.

Novel Magnetic Photonic Crystal Structures for Magnetic Field Sensors and Visualizers

Optimized one-dimensional (1-D) magnetophotonic crystals greatly increase the sensitivity of magnetooptical sensors, which are widely used in magnetooptical imaging to observe the magnetic domain patterns in magnetic materials, to observe the vortex states in superconductors, to detect small bits in magnetooptical recording media, to visualize defects in ferromagnetic objects, and to measure the value and spatial distribution of stray magnetic fields. This paper examines the properties of such devices operating in the optimized reflection (doubled Faraday rotation) mode and discusses the use of 1-D magnetophotonic crystals as sensors.

Vortex states in superconductors with strong Pauli-paramagnetic effect

Using the quasiclassical theory, we analyze the vortex structure of strong-paramagnetic superconductors.There, induced paramagnetic moments are accumulated exclusively around the vortex core. We quantitatively evaluate the significant paramagnetic effect in the H-dependence of various quantities, such as low temperature specific heat, Knight shift, magnetization and the flux line lattice (FLL) form factor. The anomalous H-dependence of the FLL form factor observed by the small angle neutron scattering in CeCoIn_5 is attributable to the large paramagnetic contribution.

Tuning the Quantum Stability and Superconductivity of Ultrathin Metal Alloys

Quantum confinement of itinerant electrons in atomically smooth ultrathin lead films produces strong oscillations in the thickness-dependent film energy. By adding extra electrons via bismuth alloying, we showed that both the structural stability and the superconducting properties of such films can be tuned. The phase boundary (upper critical field) between the superconducting vortex state and the normal state indicates an anomalous suppression of superconducting order just below the critical temperature, Tc. This suppression varies systematically with the film thickness and the bismuth content and can be parametrized in terms of a characteristic temperature, Tc* (less than Tc), that is inversely proportional to the scattering mean free path. The results indicate that the isotropic nature of the superconductive pairing in bulk lead-bismuth alloys is altered in the quantum regime.

Penetrations and dynamics of vortices in mesoscopic superconducting plates

Nano-technologies enable us to control quantum states. Vortex states in superconductor whose dimension is order of the coherence length would be applied in electronic devices in near future. In this presentation, we report the penetration process and dynamics of vortices in mesoscopic superconducting plate with anti-dot obtained by solving the time dependent Ginzburg–Landau (TDGL) equations.As to a regular square superconducting plate with an anti-dot located at the center, for suddenly applying the field perpendicular to the plate, vortices tend to penetrate into the plate from four central points of the each square side keeping the fourfold rotational symmetry. After the penetration, we found the vortex exclusion process that residual vortices for stability of the system are expelled. For introducing external current that breaks symmetry of the system.

Electronic thermal conductivity in a superconducting vortex state

The longitudinal component of the electronic thermal conductivity κxx in a superconducting vortex state is calculated as a function of magnetic field B. Calculations are performed by taking account of the spatial dependence of normal Green’s function g, which was neglected in the previous studies using the Brandt–Pesch–Tewordt method. We discuss the possibility of using κxx(B) as a probe of the pair potential symmetry.

Recent progress in vortex studies by tunneling spectroscopy

Among the methods used to study the vortex state in superconductors, scanning tunneling spectroscopy (STS), is unique in its ability to measure in real space the variations in the local quasiparticle density of states. Thus, as opposed to magnetic imaging, STS gives direct access to the coherence length rather than to the penetration length. Here we discuss two novel methods which enhance the capabilities of STS as a tool for the study of the vortex state.In the first one, called Lazy Fisherman [A. Kohen et al., Appl. Phys. Lett. 86 (2005) 212503], the scanning tunneling microscope’s tip is kept fixed at a selected location while the vortices are being moved by varying the applied magnetic field. By continuously acquiring the local tunneling conductance spectra, dI/dV(V), we detect the changes in the local density of states under the tip due to the vortex motion. With no need for scanning, the method permits one to extend the study of vortices to samples in which scanning is difficult or even impossible due to surface non-uniformity and allows one to detect faster vortex dynamics. To illustrate the approach we study single crystal samples of MgB2.In the second STS method, we replace the commonly used normal metal STM tip by a superconducting (SC) tip which we produce either by mechanically breaking a Nb wire under vacuum in the STM chamber [A. Kohen et al., Physica C 49 (2005) 18] or by gluing a piece of a crystal of MgB2 [F. Giubileo et al., Phys. Rev. Lett. 87 (2001) 177008]. The use of a SC tip enhances the energy resolution of STS in comparison to that obtained with a normal metal tip. The method is illustrated by using Nb and MgB2 tips to perform a simultaneous topographic and spectroscopic imaging on 2H–NbSe2.

Effect of impurities and random pinning on the superconducting vortex state

The presence of impurities affects the quasiparticles via potential scattering and pinning of the vortices. We calculate self-consistently the local density of states of an inhomogeneous superconductor in a magnetic field with a random distribution of impurities. The quasiparticles are only significantly affected by the impurities in their vicinity. In contrast, the random distribution of vortices increases the density of states at low energies by filling the gap in the s-wave case and creating a finite value in the d-wave case. The dominant effect on the quasiparticles is due to the vortex scattering.

Interplay of disorder and magnetic field in the superconducting vortex state

We calculate the density of states of an inhomogeneous superconductor in a magnetic field where the positions of vortices are distributed completely at random. We consider both the cases of s-wave and d-wave pairing. For both pairing symmetries either the presence of disorder or increasing the density of vortices enhances the low energy density of states. In the s-wave case the gap is filled and the density of states is a power law at low energies. In the d-wave case the density of states is finite at zero energy and it rises linearly at very low energies in the Dirac isotropic case (\alpha_D=t/\Delta_0=1, where t is the hopping integral and \Delta_0 is the amplitude of the order parameter). For slightly higher energies the density of states crosses over to a quadratic behavior. As the Dirac anisotropy increases (as \Delta_0 decreases with respect to the hopping term) the linear region decreases in width. Neglecting this small region the density of states interpolates between quadratic and back to linear as \alpha_D increases. The low energy states are strongly peaked near the vortex cores.

Delocalization in two-dimensional disordered Bose systems and depinning transition in the vortex state in superconductors.

We investigate a two-dimensional (2D) Bose system with the long range interactions in the presence of disorder. Formation of the bound states at strong impurity sites gives rise to a depletion of the superfluid density. We predict the intermediate superfluid state where the condensate and localized bosons are present simultaneously. We find that interactions suppress localization and that with the increase of the boson density the system experiences a sharp delocalization crossover into a state where all bosons are delocalized. We map our results onto a 3D system of vortices in type II superconductors in the presence of columnar defects; the intermediate superfluid state maps to an intermediate vortex liquid where vortex liquid neighbors pinned vortices. We predict the depinning crossover within the vortex liquid and depinning induced vortex lattice-Bose glass melting.

On sine-Gordon vortex states in high-temperature superconductors, quantum antiferromagnets and sonie other two-dimensional systems

Abstract A review of vortex configurations in two-dimensional sine-Gordon systems, including Josephson junctions, will be given starting from a simple vortex configuration (as described by Hudak in 1982) to recent results on vortex lattices in high-temperature superconductors and to some other systems. Josephson effects and vortex states in superconductors and intrinsic Josephson effects on high-temperature superconductor stacked junctions in multilayers are discussed.

Non-quantized penetration of magnetic field in the vortex state of superconductors

As first pointed out by Bardeen and Ginzburg in the early sixties, the amount of magnetic flux carried by vortices in superconducting materials depends on their distance from the sample edge, and can be smaller than one flux quantum, phi0 = h/2e (where h is Planck's constant and e is the electronic charge). In bulk superconductors, this reduction of flux becomes negligible at submicrometre distances from the edge, but in thin films the effect may survive much farther into the material. But the effect has not been observed experimentally, and it is often assumed that magnetic field enters type II superconductors in units of phi0. Here we measure the amount of flux introduced by individual vortices in a superconducting film, finding that the flux always differs substantially from phi0. We have observed vortices that carry as little as 0.001phi0, as well as 'negative vortices', whose penetration leads to the expulsion of magnetic field. We distinguish two phenomena responsible for non-quantized flux penetration: the finite-size effect and a nonlinear screening of the magnetic field due to the presence of a surface barrier. The latter effect has not been considered previously, but is likely to cause non-quantized penetration in most cases.

Site-Selective Nuclear Magnetic Relaxation Time in a Superconducting Vortex State

The temperature and field dependences of the site-selective nuclear spin relaxation time T_1 around vortices are studied comparatively both for s-wave and d-wave superconductors, based on the microscopic Bogoliubov-de Gennes theory. Reflecting low energy electronic excitations associated with the vortex core, the site selective temperature dependences deviate from those of the zero-field case, and T_1 becomes faster with approaching the vortex core. In the core region, T_1^{-1} has a new peak below the superconducting transition temperature T_c. The field dependence of the overall T_1(T) behaviors for s-wave and d-wave superconductors is investigated and analyzed in terms of the local density of states. The NMR study by the resonance field dependence may be a new method to probe the spatial resolved vortex core structure in various conventional and unconventional superconductors.

History dependence of peak effect in CeRu [formula omitted] and V [formula omitted]Si: an analogy with the random field Ising systems

We present results of transport measurements showing distinct path dependence of the electrical resistance in the superconducting vortex state of single crystal samples of CeRu 2 and V 3 Si. Resistance measured in the vortex state of both the systems prepared by field cooling (FC), indicates a relatively higher degree of disorder than when it is prepared by isothermal variation of field. Small oscillations of magnetic field modify the resistance in the FC state, highlighting the metastable nature of that state. An analogy is drawn with the FC state of the random-field Ising systems.