Anomalous fluctuation regimes at the FFLO transition

Recently some experimental evidences have been obtained in favour of the existence of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state in heavy-fermion superconductor CeCoIn_{5} and organic superconductor -(BETS)_{2}FeCl_{4}. However the unambiguous identification of FFLO state remains very difficult. We present the theoretical studies of the Gaussian fluctuations near the tricritical point (where the FFLO modulation appears) and demonstrate that the behavior of the fluctuational specific heat, paraconductivity and diamagnetism is qualitatively different from the usual superconducting transition. Special values of the critical exponent and the crossovers between different fluctuational regimes may provide a unique test for the FFLO state appearance.Comment: 6 pages, 2 figures. Work supported by ANR Extreme Conditions Correlated Electrons (ANR-06-BLAN-0220

Band structure of magnetic excitations in the vortex phase of a ferromagnetic superconductor

International audienceMagnetic excitations in a ferromagnetic superconductor in the presence of an Abrikosov vortex lattice have been studied using the phenomenological London and Landau-Lifshitz equations. Due to the periodicity of the vortex field the magnon spectrum has a band structure, similar to the structure of the electon spectrum in a crystal lattice. The gaps between adjacent bands have been calculated using an analog of the weak-binding approximation. When the applied magnetic field is altered the band structure undergoes a qualitative transformation due to commensurability effects, connected with the nonmonotonicity of the magnon spectrum in the Meissner state. In dirty samples the energy gaps may be smeared out because of the dissipation connected with vortex motion. In sufficiently clean samples the gaps manifest themselves as maxima in the frequency dependence of the microwave reflectivity coefficient

Resonance in-plane magnetic field effect as a means to reveal the Fulde-Ferrell-Larkin-Ovchinnikov state in layered superconductors

International audienceThe vector potential of a parallel magnetic field produces a modulation of the interlayer coupling between adjacent superconducting layers. In some cases the period of this modulation can coincide with the period of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulation of the superconducting order parameter. Such a resonance condition results in cusps on the temperature and in-plane angular dependencies of the upper critical field Hc2. This effect can open up a possibility to unambiguously evidence a spatially modulated superconducting phase in layered conductors. Remarkably, the proposed signature of the FFLO state is directly based on the main feature of the FFLO state, the spatial modulations of the order parameter

Giant Mesoscopic Fluctuations and Long-Range Superconducting Correlations in Superconductor-Ferromagnet Structures

International audienceThe fluctuating superconducting correlations emerging in dirty hybrid structures under the conditions of the strong proximity effect are demonstrated to affect the validity range of the widely used formalism of Usadel equations at mesoscopic scales. In superconductor ferromagnet structures these giant mesoscopic fluctuations originating from the interference effects for the Cooper pair wave function in the presence of the exchange field can be responsible for an anomalously slow decay of superconducting correlations in a ferromagnet even when the noncollinear and spin orbit effects are negligible. The resulting sample to sample fluctuations of the Josephson current in superconductor ferromagnetic superconductor junctions and the local density of states in superconductor ferromagnetic hybrid structures can provide an explanation of the long range proximity phenomena observed in mesoscopic samples with collinear magnetization

Field-Direction Dependence of the Upper Critical Field in Organic Superconductors

International audienceIn this work, we investigate the anisotropy of the in-plane critical field in conventional and spatially modulated phases of layered superconductors within the quasi-classical approach, taking into account the interlayer Josephson coupling. We show that the anisotropy of the onset of superconductivity may change dramatically in the FFLO state as compared with the conventional superconducting phase

Nonsinusoidal current-phase relation in strongly ferromagnetic and moderately disordered SFS junctions

We study the Josephson current in a junction comprising two superconductors linked by a strong ferromagnet in presence of impurities. We focus on a regime where the electron (and hole) motion is ballistic over the exchange length and diffusive on the scale of the weak link length. The current-phase relation is obtained for both two- and three dimensional ferromagnetic weak links. In the clean limit, the possibility of temperature-induced 0- transitions is demonstrated while the corresponding critical current versus temperature dependences are also studied.Comment: 10 pages, 7 figure

Crossover from crossing to tilted vortex phase in Bi2Sr2CaCu2O8+δ single crystals near ab-plane

International audienceIn extremely anisotropic layered superconductors of Bi2Sr2CaCu2O8+δ the stacks of vortex pancakes (PV) and the Josephson vortex (JV) interpenetrate, and due to PV-JV mutual pinning energy, weakly interact and form various tilted and crossing lattice structures including vortex chains, stripes, mixed chain + lattice phases, etc. In order to study these phenomena, it is decisive to have excellent quality of samples and the ideal experimental techniques. The vortex phases in high-quality Bi2Sr2CaCu2O8+δ single crystals were studied by in-plane resistivity measurement and local ac magnetic permeability. The sharp crossover was shown by both techniques, deep in the vortex solid state separating the Abrikosov dominant 'strong pinning' phase from the Josephson dominant 'weak pinning' phase. Those two vortex states were recognized as the mixed chain + lattice vortex phase and chains (tilted) vortex phase, respectively

On the theory of the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase

We demonstrate that the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase may be very different depending on the field orientation relative to the crystalline axes. We calculate numerically the upper critical field near the tricritical point taking into account the modulation of the order parameter along the magnetic field as well as the higher Landau levels. For s-wave superconductors with the anisotropy described by an elliptical Fermi surface we propose a general scheme of the analysis of the angular dependence of upper critical field at all temperatures on the basis of the exact solution for the order parameter. Our results show that the transitions (with tilting magnetic field) between different types of mixed states may be a salient feature of the FFLO phase. Moreover we discuss the reasons for the first-order phase transition into the FFLO state in the case of CeCoIn5 compound.Comment: 7 figure

Oscillations of magnetization and conductivity in anisotropic Fulde-Ferrell-Larkin-Ovchinnikov superconductors

We derive the fluctuational magnetization and the paraconductivity of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductors in their normal state. The FFLO superconducting fluctuations induce oscillations of the magnetization between diamagnetism and unusual paramagnetism which originates from the competition between paramagnetic and orbital effects. We also predict a strong anisotropy of the paraconductivity when the FFLO transition is approached in contrast with the case of a uniform BCS state. Finally building a Ginzburg-Levanyuk argument, we demonstrate that these fluctuation effects can be safely treated within the Gaussian approximation since the critical fluctuations are proeminent only within an experimentally inaccessible temperature interval

Decoupling of superconducting layers in magnetic superconductor RuSr_{2}GdCu_{2}O_{8}

We propose the model for magnetic properties of the magnetic superconductor RuSr$_{2}$GdCu$_{2}$O$_{8}$, which incorporates the theory of the superconducting/ferromagnetic multilayers. The transition line $T_{d}(h)$, on which the Josephson coupled superconducting planes are decoupled, i.e. $j_{c}(T_{d})=0$, is calculated as a function of the exchange energy $h$. As the result of this decoupling a nonmonotonic behavior of magnetic properties, like the lower critical field $H_{c1}$, Josephson plasma frequency, etc. is realized near (or by crossing) the $T_{d}(h)$ line. The obtained results are used in analyzing the newly discovered antiferromagnetic ruthenocuprate RuSr$_{2}$GdCu$_{2}$O$_{8}$ with possible weak ferromagnetic order in the RuO planes.Comment: 12 pages, 3 figs embede