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Register a new userThe Nature of the Superconducting phase Transitions in Strongly type-II Superconductors in the Pauli Paramagnetic limit
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Superconducting phase transitions in strongly type-II superconductors in the Pauli paramagnetic limit are considered within the framework of the Gorkov-Ginzburg-Landau approach in the lowest Landau level approximation for both s and d-wave electron pairing. Simple analytical expressions for the quadratic and quartic coefficients in the order parameter expansion of the superconducting free energy are derived without relying on gradient or wavenumber expansions. The existence of a changeover from continuos to discontinuos superconducting phase transitions predicted to occur in the clean limit is shown to depend only on the dimensionality of the underlying electronic band structure. Such a changeover can take place in the quasi 2D regime below a critical value of a 3D-2D crossover parameter.
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Using large scale Monte Carlo simulations on a uniformly frustrated 3DXY model, we report a first order vortex lattice melting transition in clean, isotropic extreme type-II $kappa to infty$ superconductors. This work clarifies an important issue: the unpinned vortex liquid is always incoherent with no phase coherence in any directions for all anisotropies. Previous claims of a disentangled vortex liquid for isotropic superconductors based on simulations, are due to finite size effects. We explicitly show that the effective vortex-line tension vanishes precisely at the superconducting phase transition in zero magnetic field. This loss of line tension is accompanied by an abrupt change in the connectivity of the vortex tangle across the superconductor. We also obtain results indicating that the connectivity of the vortex tangle changes in a similar way even in finite magnetic field, and suggest that this could also be associated with a genuine phase-transition.
Dynamics of vortices in strongly type-II superconductors with strong disorder is investigated within the frustrated three-dimensional XY model. For two typical models in [Phys. Rev. Lett. {bf 91}, 077002 (2003)] and [Phys. Rev. B {bf 68}, 220502(R) (2003)], a strong evidence for the finite temperature vortex glass transition in the unscreened limit is provided by performing large-scale dynamical simulations. The obtained correlation length exponents and the dynamic exponents in both models are different from each other and from those in the three-dimensional gauge glass model. In addition, a genuine continuous depinning transition is observed at zero temperature for both models. A scaling analysis for the thermal rounding of the depinning transition shows a non-Arrhenius type creep motion in the vortex glass phase, contrarily to the recent studies..
The sharp suppression of the de-Haas van-Alphen oscillations observed in the mixed superconducting (SC) state of the heavy fermion compound URu$_{2}$Si$% _{2}$ is shown to confirm a theoretical prediction of a narrow double-stage SC phase transition, smeared by fluctuations, in a 3D paramagnetically-limitted superconductor. The predicted scenario of a second order transition to a nonuniform (FFLO) state followed by a first order transition to a uniform SC state, obtained by using a non-perturbative approach, is also found to be consistent with recent thermal conductivity measurements performed on this material.
The magnetic field distribution around the vortices in TmNi2B2C in the paramagnetic phase was studied experimentally as well as theoretically. The vortex form factor, measured by small-angle neutron scattering, is found to be field independent up to 0.6 Hc2 followed by a sharp decrease at higher fields. The data are fitted well by solutions to the Eilenberger equations when paramagnetic effects due to the exchange interaction with the localized 4f Tm moments are included. The induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor.
The superconductivity in the Bi-II phase of elemental Bismuth (transition temperature $T_{rm c}simeq3.92$ K at pressure $psimeq 2.80$ GPa) was studied experimentally by means of the muon-spin rotation as well as theoretically by using the Eliashberg theory in combination with Density Functional Theory calculations. Experiments reveal that Bi-II is a type-I superconductor with a zero temperature value of the thermodynamic critical field $B_{rm c}(0)simeq31.97$~mT. The Eliashberg theory approach provides a good agreement with the experimental $T_{rm c}$ and the temperature evolution of $B_{rm c}$. The estimated value for the retardation (coupling) parameter $k_{rm B}T_{rm c}/omega_{rm ln} approx 0.07$ ($omega_{rm ln}$ is the logarithmically averaged phonon frequency) suggests that Bi-II is an intermediately-coupled superconductor.
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