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Register a new userMagnetic-field-induced 1st order transition to FFLO state at paramagnetic limit in 2D superconductors
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We have recently reported the first direct calorimetric observation of a magnetic-field-induced first-order phase transition into a high-field FFLO superconducting state at the Clogston-Chandrasekar `Pauli paramagnetic limit $H_p$ in a 2D superconductor $kappa-{textrm{(BEDT-TTF)}}_2{textrm{Cu}}{textrm{(NCS)}}_2$. The high-field state is both higher entropy and strongly paramagnetic, as thermodynamically required for the FFLO state. Here we compare our results with theoretical predictions for the field dependence of the high-field FFLO state in the 2D limit, revealing tentative evidence for transitions between FFLO states of differing order parameter. We also present calorimetric evidence for a 1st order phase transition into the FFLO state for a second 2D organic superconductor: ${beta}^{primeprime}-{textrm{(BEDT-TTF)}}_2{textrm{SF}}_5{textrm{(CH)}}_2{textrm{(CF)}}_2{textrm{(SO)}}_3$.
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There are strong experimental evidences of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state formation in layered organic superconductors in parallel magnetic field. We study theoretically the interplay between the orbital effect and the FFLO modulation in this case and demonstrate that the in-plane critical field anisotropy drastically changes at the transition to the FFLO state. The very peculiar angular dependence of the superconducting onset temperature which is predicted may serve for unambiguous identification of the FFLO modulation. The obtained results permit us to suggest the modulated phase stabilization as the origin of the magnetic-field angle dependence of the onset of superconductivity experimentally observed in (TMTSF)$_{2}$ClO$_{4}$ organic conductors.
DC and finite frequency transport measurements of thin films of amorphous indium oxide that were driven through the critical point of superconductor-insulator transition by the application of perpendicular magnetic field are presented. The observation of non-monotonic dependence of resistance on magnetic field in the insulating phase, novel transport characteristics near the resistance peak and finite superfluid stiffness in the insulating phase are all discussed from the point of view that suggests a possible relation between the conduction mechanisms in the superconducting and insulating phases. The results are summarized in the form of an experimental phase diagram for disordered superconductors in the disorder-magnetic field plane.
The Landau-Ginzburg-Wilson hamiltonian is studied for different values of the parameter $lambda$ which multiplies the quartic term (it turns out that this is equivalent to consider different values of the coherence length $xi$ in units of the lattice spacing $a$). It is observed that amplitude fluctuations can change dramatically the nature of the phase transition: for small values of $lambda$ ($xi/a > 0.7$), instead of the smooth Kosterlitz-Thouless transition there is a {em first order} transition with a discontinuous jump in the vortex density $v$ and a larger non-universal drop in the helicity modulus. In particular, for $lambda$ sufficiently small ($xi/a cong 1$), the density of bound pairs of vortex-antivortex below $T_c$ is so low that, $v$ drops to zero almost for all temperature $T<Tc$.
We investigate the antiferromagnetic (AF) order in the d-wave superconducting (SC) state at high magnetic fields. A two-dimensional model with on-site repulsion U, inter-site attractive interaction V and antiferromagnetic exchange interaction J is solved using the mean field theory. For finite values of U and J, a first order transition occurs from the normal state to the FFLO state, while the FFLO-BCS phase transition is second order, consistent with the experimental results in CeCoIn_5. Although the BCS-FFLO transition is continuous, the Neel temperature of AF order is discontinuous at the phase boundary because the AF order in the FFLO state is induced by the Andreev bound state localized in the zeros of FFLO order parameter, while the AF order hardly occurs in the uniform BCS state. The spatial structure of the magnetic moment is investigated for the commensurate AF state as well as for the incommensurate AF state. The influence of the spin fluctuations is discussed for both states. Since the fluctuations are enhanced in the normal state for incommensurate AF order, this AF order can be confined in the FFLO state. The experimental results in CeCoIn_5 are discussed.
We review and analyze magnetization and specific heat investigations on type-II superconductors which uncover remarkable evidence for the magnetic field induced fnite size effect and the associated 3D to 1D crossover which enhances thermal fluctuations.
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