We study the effect of critical fluctuations on the (B, T ) phase diagram in extreme type-II superconductors in zero and finite magnetic field. In zero magnetic field the critical fluctuations are transverse phase-fluctuations of the complex scalar Ginzurg-Landau order parameter, which when excited thermally will induce line-defects in the form of closed vortex loops into the system. The distribution function D(p) of vortex loops of perimeter p changes from an exponential function D(p) ∼ p −α exp(−ε(T )p/kBT ) to a power law distribution D(p) ∼ p −α at the zero-field critical temperature T = Tc. We find that the long-wavelength vortex-line tension vanishes as ε(T ) ∼ |T − Tc| γ ; γ ≈ 1.45, as T → Tc. At T = Tc, an extreme type-II superconductor suffers an unbinding of large vortex loops of order the system size. When this happens, the connectivity of the thermally excited vortex-tangle of the system changes abruptly. When amplitude fluctuations are included, it is shown that they are far from being critical at the superconducting transition temperature Tc. The vortex-loop unbinding can therefore not be reparametrized in terms of critical amplitude fluctuations of the original local Ginzburg-Landau order parameter. The loss of phase-stiffness in the Ginzburg-Landau order parameter, the anomaly in specific heat, the loss of vortex-line tension, and the change in the connectivity of the vortex-tangle are all found at the same temperature, the critical temperature of the superconductor. At zero magnetic field, unbinding of vortex-loops of order the system size can be phrased in terms of a global U (1)-symmetry breaking involving a local complex disorder field which is dual to the order parameter of the usual Ginzburg-Landau theory. There is one parameter in the theory that controls the width of the critical region, and for the parameters we have used, we show that a vortex-loop unbinding gives a correct picture of the zero-field transition even in the presence of amplitude fluctuations. A key result is the extraction of the anomalous dimension of the dual field directly from the statistics of the vortex-loop excitations of the Ginzburg-Landau theory in the phase-only approximation. In finite magnetic fields, the first order vortex-line lattice (VLL) melting transition is accompanied by a loss of longitudinal superfluid stiffness; this is true also for the isotropic case. A scaling analysis of the vortex lattice melting line is carried out, yielding two different scaling regimes for the vortex lattice melting line, namely a high-field scaling regime and a distinct low-field 3DXY scaling regime. We also find indications of an abrupt change in the connectivity of the vortex-tangle in the vortex liquid along a line TL(B), which at low enough fields appears to coincide with the VLL melting transition line within the resolution of our numerical calculations. We study the temperature at which this phenomenon takes place as a function of system size and shape. Our results show that this temperature decreases and appears to s...

The statistical mechanics of the flux-line lattice in extreme type-II superconductors is studied within the framework of the uniformly frustrated anisotropic three-dimensional XY -model. It is assumed that the externally applied magnetic field is low enough to invalidate the lowest Landaulevel approach to the problem. A finite-field counterpart of an Onsager vortex-loop transition in extreme type-II superconductors renders the vortex liquid phase-incoherent when the Abrikosov vortex lattice undergoes a first order melting transition. For the magnetic fields considered in this paper, corresponding to filling fractions f given by 1/f = 12, 14, 16, 20, 25, 32, 48, 64, 72, 84, 96, 112, and 128, the vortex liquid phase is not describable as a liquid of well-defined field-induced vortex lines. This is due to the proliferation of thermally induced closed vortex-loops with diameters of order the magnetic length in the problem, resulting in a "percolation transition" driven by non-field induced vortices also transverse to the direction of the applied magnetic field. This immediately triggers flux-line lattice melting and loss of phase-coherence along the direction of the magnetic field. Due to this mechanism, the field induced flux lines loose their line tension in the liquid phase, and cannot be considered to be directed or well defined. In a non-relativistic 2D boson-analogy picture, this latter feature would correspond to a vanishing mass of the bosons. Scaling functions for the specific heat are calculated in zero and finite magnetic field. From this we conclude that the critical region is of order 10% of Tc for a mass-anisotropy Mz/M = 3, and increases with increasing mass-anisotropy. The entropy jump at the melting transition is calculated in two ways as a function of magnetic field for a mass-ansitropy slightly lower than that in Y BCO, namely with and without a T -dependent prefactor in the Hamiltonian originating at the microscopic level and surfacing in coarse grained theories such as the one considered in this paper. In the first case, it is found to be ∆S = 0.1kB per pancake-vortex, roughly independent of the magnetic field for the filling fractions considered here. In the second case, we find an enhancement of ∆S by a factor which is less than 2, increasing slightly with decreasing magnetic field. This is still lower than experimental values of ∆S ≈ 0.4kB found experimentally for Y BCO using calorimetric methods. We attribute this to the slightly lower mass-anisotropy used in our simulations. 74.25.Dw, 74.25.Ha,74.60.Ec

We study the interplay between a novel vortex-loop unbinding in finite magnetic field at T=T_V and flux-line lattice (FLL) melting at T=T_M in type-II superconductors. The FLL melts due to nucleation of vortex loops parallel to c-axis, connected to flux lines. For moderate anisotropy, phase coherence along c-axis is lost at T_V > T_M due to an ab-plane vortex-loop unbinding with loops located close to thermal FLL fluctuations. For large anisotropy, phase coherence along c-axis is lost at T_V < T_M due to nucleation of ab-plane vortex-loops uncorrelated to flux lines.Comment: 4 pages, 3 postscript figure

Monte-Carlo simulations in conjunction with finite-size scaling analysis are used to investigate the (H, T )-phase diagram in uniaxial anisotropic high-Tc superconductors, both in zero magnetic field (B = 0) and in intermediate magnetic fields (0 ≪ B ≪ Bc2) for various mass-anisotropies. The model we consider is the uniformly frustrated anisotropic Villain Model, which is dual to the Lattice London Model with an infinite London penetration length. The quantities we consider are various helicity moduli, the structure function, the specific heat, and the distribution of closed non-field induced vortex loops as a function of the loop-size. In zero magnetic field, and for all anisotropies considered, we find one single second order phase transition, mediated by an Onsager vortex-loop unbinding transition, or blowout. This is the superconductor-normal metal transition. A comparison with numerical simulations and a critical scaling analysis of the zero-field loop-transition yields the same exponent of the loop-distribution function at the critical point. In the intermediate magnetic field regime, we find two anomalies in the specific heat. The first anomaly at a temperature Tm is associated with the melting transition of the flux-line lattice. The Lindemann-ratio at the melting is given by cL ≈ 0.24. The second anomaly at a temperature Tz is one where phase coherence in the BCS order parameter across the sample along the field direction is destroyed. We argue that Tm = Tz in the thermodynamic and continuum limit. Hence, there is no regime where the flux-line lattice melts into a disentangled flux-line liquid. The loss of phase coherence parallel to the magnetic field in the sample is argued to be due to the proliferation of closed non-field induced vortex loops on the scale of the magnetic length in the problem, resulting in flux-line cutting and recombination. In the flux-line liquid phase, therefore, flux-lines appear no longer to be well defined entities. Above the melting temperature, the system always exhibits an incoherent vortex-liquid phase characterized by lack of phase coherence in the BCS order parameter parallel to the magnetic field. For increasing anisotropy, we resolve a delta-function peak in the specific heat. A finite-size scaling analysis of the delta-function peak specific heat anomaly at the melting transition is used to extract the discontinuity of the entropy at the melting transition. This entropy discontinuity is found to increase rapidly with mass-anisotropy, at least for not too layered compounds.

We consider current-induced domain wall motion and, the reciprocal process, moving domain wall-induced current. The associated Onsager coefficients are expressed in terms of scattering matrices. Uncommonly, in (Ga,Mn)As, the effective Gilbert damping coefficient alphaw and the effective out-of-plane spin-transfer torque parameter betaw are dominated by spin-orbit interaction in combination with scattering off the domain wall, and not scattering off extrinsic impurities. Numerical calculations give alphaw approximately 0.01 and betaw approximately 1 in dirty (Ga,Mn)As. The extraordinarily large betaw parameter allows experimental detection of current or voltage induced by domain wall motion in (Ga,Mn)As.