56 research outputs found
Magnetic Field of an In-Plane Vortex Inside and Outside a Layered Superconducting Film
In the present work we study an anisotropic layered superconducting film of
finite thickness. The film surfaces are considered parallel to the face of
the crystal. The vortex lines are oriented perpendicular to the film surfaces
and parallel to the superconducting planes. We calculate the local field and
the London free energy for this geometry. Our calculation is a generalization
of previous works where the sample is taken as a semi-infinite superconductor.
As an application of this theory we investigate the flux spreading at the
superconducting surface.Comment: One REVTeX file containing a figure in it and two PS figures. To
appear in Physical Review
Vortices in a mesoscopic superconducting circular sector
In the present paper we develop an algorithm to solve the time dependent
Ginzburg-Landau (TDGL) equations, by using the link variables technique, for
circular geometries. In addition, we evaluate the Helmholtz and Gibbs free
energy, the magnetization, and the number of vortices. This algorithm is
applied to a circular sector. We evaluate the superconduting-normal magnetic
field transition, the magnetization, and the superconducting density.
Furthermore, we study the nucleation of giant and multi-vortex states for that
geometry.Comment: One REVTeX file and 8 figure
Difusão de calor e o pico central em transições de fase estruturais
Orientador: Armando Fernandes da Silva MoreiraDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: O acoplamento entre polarização e flutuações em temperatura em cristais ferro e antiferroelétricos que apresentam transições de fase estruturais é estudado através do formalismo de Mori. Partindo de considerações termodinâmicas, escolhemos como variáveis dinâmicas as componentes de Fourier da polarização local, da primeira derivada temporal desta polarização local e da temperatura local (esta última definida em termos da componente de Fourier da densidade de energia local). Discutimos leis de conservação para estas variáveis e a relação entre elas e as quantidades microscópicas que descrevem a dinâmica de redes, a saber, os operadores campo de fonon e momento de fonon. Encontramos uma expressão para o fator de estrutura dinâmico que tem a mesma forma que aquela deduzida por técnicas de função de Green. No limite de longos comprimentos de onda e longe da região critica o fator de estrutura dinâmico apresenta um par de Brillouin e um pico central enquanto que próximo à região critica o par de Brillouin desaparece e o pico central divergeAbstract: The coupling between polarization and temperature fluctuations in ferro and antiferroelectric crystals which undergo structural phase transitions is studied through Mori's formalism. Using thermodynamic considerations, we choose as dynamical variables the Fourier components of the local polarization, of its first time derivative and of the local temperature (the latter defined in terms of the Fourier components of the local energy density). We discuss conservation laws for these variables and their relationship to microscopic quantities describing the lattice dynamics, namely, the phonon field and phonon momentum operators. We obtain an expression for dynamical structure factor which has the same shape as the one derived using Green¿s functions. In the long-wavelength limit and far from the critical region the structure factor exhibits a Brillouin pair and a central peak, whereas close to the critical region the Brillouin pair vanishes and the central peak divergesMestradoFísicaMestre em Físic
Ultra-fast Kinematic Vortices in Mesoscopic Superconductors: The Effect of the Self-Field
Within the framework of the generalized time-dependent Ginzburg-Landau
equations, we studied the influence of the magnetic self-field induced by the
currents inside a superconducting sample driven by an applied transport
current. The numerical simulations of the resistive state of the system show
that neither material inhomogeneity nor a normal contact smaller than the
sample width are required to produce an inhomogeneous current distribution
inside the sample, which leads to the emergence of a kinematic
vortex-antivortex pair (vortex street) solution. Further, we discuss the
behaviors of the kinematic vortex velocity, the annihilation rates of the
supercurrent, and the superconducting order parameters alongside the vortex
street solution. We prove that these two latter points explain the
characteristics of the resistive state of the system. They are the fundamental
basis to describe the peak of the current-resistance characteristic curve and
the location where the vortex-antivortex pair is formed.Comment: 9 pages, 6 figures. Accepted for publication in Scientific Report
Vortex-Antivortex annihilation dynamics in a square mesoscopic superconducting cylinder
The dynamics of the annihilation of a vortex-antivortex pair is investigated.
The pair is activated magnetically during the run of a simulated hysteresis
loop on a square mesoscopic superconducting cylinder with an antidot inserted
at its center. We study the nucleation of vortices and antivortices by first
increasing the magnetic field, applied parallel to the axis of the sample, from
zero until the first vortex is created. A further increase of the field pulls
the vortex in, until it reaches the antidot. As the polarity of the field is
reversed, an antivortex enters the scene, travels toward the center of the
sample and eventually the pair is annihilated. Depending on the sample size,
its temperature, and Ginzburg-Landau parameter, the vortex-antivortex encounter
takes place at the antidot or at the superconducting sea around it. The
position and velocity of the vortex and antivortex singularities were evaluated
as a function of time. The current density, magnetization and order parameter
topology were also calculated.Comment: One REVTeX file and 5 EPS figure
Vortices in superconductors with a columnar defect: finite size effects
In the present work we investigate the behavior of a vortex in a long
superconducting cylinder near to a columnar defect at the center. The
derivations of the local magnetic field distribution and the Gibbs free energy
will be carried out for a cylinder and a cavity of arbitrary sizes. From the
general expressions, it considered two particular limits: one in which the
radius of the cavity is very small but the radius of the superconducting
cylinder is kept finite; and one in which the radius of the superconducting
cylinder is taken very large (infinite) but the radius of the cavity is kept
finite. In both cases the maximum number of vortices which are allowed in the
cavity is determined. In addition, the surface barrier field for flux entrance
into the cavity is calculated.Comment: 13 pages, 1 figur
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