Mauro M. Doria

Torque properties of the orthogonal flux-line configuration

Abstract We consider extremely type-II uniaxial superconductors in the context of anisotropic London theory and study the high vortex density limit when the magnetic induction is tilted with respect to the axis of symmetry. We obtain some further properties of the vortex-flux arrangement recently considered by Preosti and Muzikar — two sets of orthogonal lines, along and perpendicular to the c -axis. We obtain the critical line in the diagram of anisotropy, γ, versus the Ginzburg-Landau constant, κ, below which such decomposition may occur. In the reversible region of the H - T diagram we propose a method, based on torque and magnetization experiments, to check the existence of this orthogonal state.

Coexistence of orthogonal flux lines in uniaxial superconductors

Abstract We consider extremely type II uniaxial superconductors in the context of anistropic London theory and study the Helmholtz free energy when the magnetic induction makes an angle of 45° with the symmetry axis. For straight vortex lines we find that the free energy is lower when the straight lines are in multiple directions, namely at θ = 0° and at 90°, instead of all oriented at 45°.

Vortex-line-lattices in anisotropic superconductors in an applied field

Abstract We consider a uniaxial anisotropic superconductor placed in an external magnetic field of magnitude H and arbitrary orientation and determine the vortex-line-lattices (VLL) that minimize the Gibbs free energy in the London approximation. To do this, we develop a numerical method based on simulated annealing that assumes only straight vortex-lines and VLL with only one vortex-line per unit cell. We pally the method to low fields to estimate the lower critical field Hc1 for each direction of H and the VLL shape as a function of H.

COLLECTIVE OSCILLATIONS IN SUPERCONDUCTING THIN FILMS IN THE PRESENCE OF VORTICES

A plasma wave propagates inside an anisotropic superconducting film sandwiched between two semi-infinite non-conducting bounding dieletric media. Along the c-axis, perpendicular to the film surfaces, an external magnetic field is applied. We show how vortices, known to cause dissipation and change the penetration depth, affect the propagative mode. We obtain the complex wave number of this mode and, using YBCO at 4 K as an example, determine a region where vortex contribution is dominant and dissipation is small.

Plasma waves in anisotropic superconducting films below and above the plasma frequency

Abstract We consider wave propagation inside an anisotropic superconducting film sandwiched between two semi-infinite non-conducting bounding dieletric media such that along c-axis, perpendicular to the surfaces, there is a plasma frequency ω p below the superconducting gap. Propagation is assumed to be parallel to the surfaces in the dielectric medium, where amplitudes decay exponentially. Below ω p , the amplitude also evanesces inside the film, and we retrieve the experimentally measured lower dispersion relation branch, ω α √ β , and the recently proposed higher frequency branch, ω , α 1/ √ β . Above ω p , propagation is of the guided wave type, i.e., a dispersive plane wave confined inside the film that reflects into the dielectric interfaces, and the modes are approximately described by w ≈ ω p √1 + ( β / β 0 ) 2 , where β 0 is discussed here.

The irreversibility line and a free energy expansion

Abstract The mixed state of extreme type II superconductors display two distinguishable regions, associated with a resistive and a true superconducting state. In the vicinity of Tc, I take that the description of these two regions, for the low temperature isotropic superconductors, is possible in terms of a free energy expansion. For large κ materials 1 find that de Almeida-Thouless transition and the coexistence of normal and superconducting phases are natural consequences of the free energy expansion proposed here.

Layered structure effects on three-dimensional superconductors

Abstract The London limit of the Lawrence-Doniach theory is expressed in terms of the local magnetic field and from this formulation a continuum model is derived, valid when the layers act coherently. The effects of the layered structure are seen at local variations of the magnetic field of the order of the inter-layer separation and at high values of the Josephson supercurrent, near its critical limit.