Thermodynamically stable noncomposite vortices in mesoscopic two-gap superconductors

In mesoscopic two-gap superconductors with sizes of the order of the coherence length noncomposite vortices are found to be thermodynamically stable in a large domain of the $T - H$ phase diagram. In these phases the vortex cores of one condensate are spatially separated from the other condensate ones, and their respective distributions can adopt distinct symmetries. The appearance of these vortex phases is caused by a non-negligible effect of the boundary of the sample on the superconducting order parameter and represents therefore a genuine mesoscopic effect. For low values of interband Josephson coupling vortex patterns with $L_1 \neq L_2$ can arise in addition to the phases with $L_1 =L_2$, where $L_1$ and $L_2$ are total vorticities in the two condensates. The calculations show that noncomposite vortices could be observed in thin mesoscopic samples of MgB$_{2}$.Comment: 5 pages, 3 figures, to be published in Europhysics Letter

Numerical bifurcation study of superconducting patterns on a square

This paper considers the extreme type-II Ginzburg-Landau equations that model vortex patterns in superconductors. The nonlinear PDEs are solved using Newton's method, and properties of the Jacobian operator are highlighted. Specifically, it is illustrated how the operator can be regularized using an appropriate phase condition. For a two-dimensional square sample, the numerical results are based on a finite-difference discretization with link variables that preserves the gauge invariance. For two exemplary sample sizes, a thorough bifurcation analysis is performed using the strength of the applied magnetic field as a bifurcation parameter and focusing on the symmetries of this system. The analysis gives new insight in the transitions between stable and unstable states, as well as the connections between stable solution branches.Comment: 31 page

Spin reversal in Fe8 under fast pulsed magnetic fields

We report measurements on magnetization reversal in the Fe8 molecular magnet using fast pulsed magnetic fields of 1.5 kT s−1 and in the temperature range of 0.6–4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of ~4 K. We also observe shifts in the maxima of the relaxation within each resonance field with temperature that suggest the emergence of a thermal instability—a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability

Spin reversal in Fe8 under fast pulsed magnetic fields

We report measurements on magnetization reversal in the Fe8 molecular magnet using fast pulsed magnetic fields of 1.5 kT s−1 and in the temperature range of 0.6–4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of ~4 K. We also observe shifts in the maxima of the relaxation within each resonance field with temperature that suggest the emergence of a thermal instability—a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability

Type-1.5 Superconductors

We demonstrate the existence of a novel superconducting state in high quality two-component MgB2 single crystalline superconductors where a unique combination of both type-1 (kappa_1 0.707) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamer-like vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.Comment: accepted in Phys. Rev. Let

Giant vortices, vortex rings and reentrant behavior in type-1.5 superconductors

We predict that in a bulk type-1.5 superconductor the competing magnetic responses of the two components of the order parameter can result in a vortex interaction that generates group-stabilized giant vortices and unusual vortex rings in the absence of any extrinsic pinning or confinement mechanism. We also find within the Ginzburg-Landau theory a rich phase diagram with successions of behaviors like type-1 -> type-1.5 -> type-2 -> type-1.5 as temperature decreases.Comment: 5 pages, 4 figure

Single-electron transport through the vortex core levels in clean superconductors

We develop a microscopic theory of single-electron transport in N-S-N hybrid structures in the presence of applied magnetic field introducing vortex lines in a superconductor layer. We show that vortex cores in a thick and clean superconducting layer are similar to mesoscopic conducting channels where the bound core states play the role of transverse modes. The transport through not very thick layers is governed by another mechanism, namely by resonance tunneling via vortex core levels. We apply our method to calculation of the thermal conductance along the magnetic field.Comment: 4 pages, 1 figur

Preface

Preface - The second International Conference on Mathematical Modeling in Physical Sciences (IC-MSQUARE) took place at Prague, Czech Republic, from Sunday 1 September to Thursday 5 September 2013

Antivortices due to competing orbital and paramagnetic pair-breaking effects

Thermodynamically stable vortex-antivortex structures in a quasi-two-dimensional superconductor in a tilted magnetic field are predicted. For this geometry, both orbital and spin pair-breaking effects exist, with their relative strength depending on the tilt angle \Theta. The spectrum of possible states contains as limits the ordinary vortex state (for large \Theta) and the Fulde-Ferrell-Larkin-Ovchinnikov state (for \Theta=0). The quasiclassical equations are solved near H_{c2} for arbitrary \Theta and it is shown that stable states with coexisting vortices and antivortices exist in a small interval close to \Theta=0. The results are compared with recent predictions of antivortices in mesoscopic samples.Comment: 11 pages, 3 figure