Nonequilibrium phase transitions of vortex matter in three-dimensional layered superconductors

Large-scale simulations on the three-dimensional (3D) frustrated anisotropic XY model have been performed to study the nonequilibrium phase transitions of vortex matter in weak random pinning potential in layered superconductors. The first-order phase transition from the moving Bragg glass to the moving smectic is clarified, based on thermodynamic quantities. A washboard noise is observed in the moving Bragg glass in 3D simulations for the first time. It is found that the activation of the vortex loops plays the dominant role in the dynamical melting at high drive.     

Thermodynamics of superconductors with a disorder induced increased coulomb repulsion

We show that thermodynamic properties can be used to determine whether, the degradation in T_c with increasing disorder observed in some A-15 compounds, is due to a reduction in electronic density of states and/or electron-phonon interaction or is due instead to an increase in Coulomb repulsion $\text{μ}{}^1$ coming from increased localization as suggested by Anderson et al.     

Josephson pancakes in layered superconductors

We consider a pancake vortex in a layered superconductor with linear defects in the superconducting planes. We treat these defects as Josephson junctions. We show that the tunneling Josephson current through these junctions results in a pancake with a superconducting core. We find the mobility of a Josephson pancake.     

Fulde-Ferrell-Larkin-Ovchinnikov state in layered superconductors

The phase diagram of a layered superconductor in a large parallel magnetic field is calculated. This includes a calculation of the lower critical field beyond which the superconductor is in the FFLO-state and possesses a spatially modulated order parameter and spin polarization. The order parameter, spin polarization, free-energy density, and phase boundaries of this unconventional superconducting phase are evaluated numerically in the complete B–T plane. The analysis suggests that the transition at the lower critical field is of second order, and not of first order, as previously assumed. The order parameter of the FFLO-state merges continuously into the uniform superconducting state.     

Unconventional superconductivity pairing induced by antiferromagnetic spin fluctuations in layered organic superconductors

The unconventional character of the superconductivity in organic compounds κ-(ET)₂X is ascribed to an antiferromagnetic spin fluctuation induced pairing. Since the band structure involves two bands (±), we assume that the large amplitude spin fluctuations arise from the band with the best nesting properties (band +), while superconductivity pairing occurs in the other band (-).We show that the nesting properties may mimic either a chemical pressure (deuterization) or a hydrostatic one. Indeed, a change of the nesting ratio t₁/t₂, according to our model, induces a modification of the Fermi surface topology. The spin fluctuations strength in the system is affected and consequently the calculated effective coupling constant of superconductivity for the even-parity singlet pairing channel. Our theory appears to be qualitatively consistent with major experimental reports.     

Topological transitions in multi-band superconductors

The search for Majorana fermions has been concentrated in topological insulators or superconductors. In general, the existence of these modes requires the presence of spin–orbit interactions and of an external magnetic field. The former implies in having systems with broken inversion symmetry, while the latter breaks time reversal invariance. In a recent paper, we have shown that a two-band metal with an attractive inter-band interaction has non-trivial superconducting properties, if the k$k$-dependent hybridization is anti-symmetric in the wave-vector. This is the case, if the crystalline potential mixes states with different parities as for orbitals with angular momentum l$l$ and l+1$l+1$. In this paper we take into account the effect of an external magnetic field, not considered in the previous investigation, in a two-band metal and show how it modifies the topological properties of its superconducting state. We also discuss the conditions for the appearance of Majorana fermions in this system.     

Boundary induced phase transitions in driven lattice gases with metastable states

We study the effect of metastability onto boundary induced phase transitions in a driven lattice gas. The phase diagram for open systems, parameterized by the input and output rates, consists of two regions corresponding to the free flow and jammed phase. Both have been entirely characterized. The microscopic states in the high density phase are shown to have an interesting striped structure, which undergoes a coarsening process and survives in the thermodynamic limit.     

Isotope and multiband effects in layered superconductors

In this review we consider three classes of superconductors, namely cuprate superconductors, MgB(2) and the new Fe based superconductors. All of these three systems are layered materials and multiband compounds. Their pairing mechanisms are under discussion with the exception of MgB(2), which is widely accepted to be a 'conventional' electron-phonon interaction mediated superconductor, but extending the Bardeen-Cooper-Schrieffer (BCS) theory to account for multiband effects. Cuprates and Fe based superconductors have higher superconducting transition temperatures and more complex structures. Superconductivity is doping dependent in these material classes unlike in MgB(2) which, as a pure compound, has the highest values of T(c) and a rapid suppression of superconductivity with doping takes place. In all three material classes isotope effects have been observed, including exotic ones in the cuprates, and controversial ones in the Fe based materials. Before the area of high-temperature superconductivity, isotope effects on T(c) were the signature for phonon mediated superconductivity-even when deviations from the BCS value to smaller values were observed. Since the discovery of high T(c) materials this is no longer evident since competing mechanisms might exist and other mediating pairing interactions are discussed which are of purely electronic origin. In this work we will compare the three different material classes and especially discuss the experimentally observed isotope effects of all three systems and present a rather general analysis of them. Furthermore, we will concentrate on multiband signatures which are not generally accepted in cuprates even though they are manifest in various experiments, the evidence for those in MgB(2), and indications for them in the Fe based compounds. Mostly we will consider experimental data, but when possible also discuss theoretical models which are suited to explain the data.