arXiv subjects

Ernst Helmut Brandt

Publications and source records attributed to Ernst Helmut Brandt.

At least 19 recordsLinked to original sources

arXiv2011

Lamellar mesophase nucleated by Josephson vortices at the melting of the vortex lattice in $Bi_2Sr_2CaCu_2O_{8+delta}$

The local effect of the Josephson vortices on the vortex lattice melting process in \BSCCO crystals in the presence of an in-plane field $H_{ab}$ is studied by differential magneto-optical imaging. The melting process is found to commence along the Josephson vortex stacks, forming a mesomorphic phase of periodic liquid and solid lamellas, the direction and spacing of which are controlled by $H_{ab}$. The reduction of the local melting field $H_m$ along the Josephson vortex stacks is more than an order of magnitude larger than the reduction of the average bulk $H_m$ by $H_{ab}$.

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arXiv2011

Suppression of geometrical barrier in $Bi_2Sr_2CaCu_2O_{8+δ}$ crystals by Josephson vortex stacks

Differential magneto-optics are used to study the effect of dc in-plane magnetic field on hysteretic behavior due to geometrical barriers in $Bi_2Sr_2CaCu_2O_{8+δ}$ crystals. In absence of in-plane field a vortex dome is visualized in the sample center surrounded by barrier-dominated flux-free regions. With in-plane field, stacks of Josephson vortices form vortex chains which are surprisingly found to protrude out of the dome into the vortex-free regions. The chains are imaged to extend up to the sample edges, thus providing easy channels for vortex entry and for drain of the dome through geometrical barrier, suppressing the magnetic hysteresis. Reduction of the vortex energy due to crossing with Josephson vortices is evaluated to be about two orders of magnitude too small to account for the formation of the protruding chains. We present a model and numerical calculations that qualitatively describe the observed phenomena by taking into account the demagnetization effects in which flux expulsion from the pristine regions results in vortex focusing and in the chain protrusion. Comparative measurements on a sample with narrow etched grooves provide further support to the proposed model.

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arXiv2010

Electrodynamics of Superconductors Exposed to High Frequency Fields

The electric losses in a bulk or film superconductor exposed to a parallel radio-frequency magnetic field may have three origins: In homogeneous vortex-free superconductors losses proportional to the frequency squared originate from the oscillating normal-conducting component of the charge carriers which is always present at temperatures $T>0$. With increasing field amplitude the induced supercurrents approach the depairing current at which superconductivity breaks down. And finally, if magnetic vortices can penetrate the superconductor they typically cause large losses since they move driven by the AC supercurrent.

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arXiv2010

Attractive vortex interaction and the intermediate-mixed state of superconductors

The magnetic vortices in superconductors usually repel each other. Several cases are discussed when the vortex interaction has an attractive tail and thus a minimum, leading to vortex clusters and chains. Decoration pictures then typically look like in the intermediate state of type-I superconductors, showing lamellae or islands of Meissner state or surrounded by Meissner state, but with the normal regions filled with Abrikosov vortices that are typical for type-II superconductors in the mixed state. Such intermediate-mixed state was observed and investigated in detail in pure Nb, TaN and other materials 40 years ago; last year it was possibly also observed in MgB$_2$, where it was called "a totally new state" and ascribed to the existence of two superconducting electron bands, one of type-I and one of type-II. The complicated electronic structure of MgB$_2$ and its consequences for superconductivity and vortices are discussed. It is shown that for the real superconductor MgB$_2$ which possesses a single transition temperature, the assumption of two independent order parameters with separate penetration depths and separate coherence lengths is unphysical.

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arXiv2009

Vortex-vortex interaction in thin superconducting films

The properties of vortices in superconducting thin films are revisited. The interaction between two Pearl vortices in an infinite film is approximated at all distances by a simple expression. The interaction of a vortex with a regular lattice of real or image vortices is given. The two spring constants are calculated that one vortex in the vortex lattice feels when the surrounding vortices are rigidly pinned or are free. The modification of these London results by the finite size of real films is obtained. In finite films, the interaction force between two vortices is not a central force but depends on both vortex positions, not only on their distance. At the film edges the interaction energy is zero and the interaction force is peaked. Even far from the edges the vortex interaction considerably deviates from the Pearl result and is always smaller than it.

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arXiv2009

Boundary condition for Ginzburg-Landau theory of superconducting layers

Electrostatic charging changes the critical temperature of superconducting thin layers. To understand the basic mechanism, it is possible to use the Ginzburg-Landau theory with the boundary condition derived by de Gennes from the BCS theory. Here we show that a similar boundary condition can be obtained from the principle of minimum free energy. We compare the two boundary conditions and use the Budd-Vannimenus theorem as a test of approximations.

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arXiv2008

Non-linear theory of deformable superconductors

Interaction of the superconducting condensate with deformations of the crystal lattice is formulated assuming the electrostatic potential of Bernoulli type and the effect of strain on material parameters. In the isotropic approximation it is shown that within the Ginzburg-Landau theory both contributions can be recast into the local but non-linear interaction term of the free energy.

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arXiv2008

The Vortex Lattice in Ginzburg-Landau Superconductors

Abrikosov's solution of the linearized Ginzburg-Landau theory describing a periodic lattice of vortex lines in type-II superconductors at large inductions, is generalized to non-periodic vortex arrangements, e.g., to lattices with a vacancy surrounded by relaxing vortices and to periodically distorted lattices that are needed in the nonlocal theory of elasticity of the vortex lattice. Generalizations to lower magnetic inductions and to three-dimensional arrangements of curved vortex lines are also given. Finally, it is shown how the periodic vortex lattice can be computed for bulk superconductors and for thick and thin films in a perpendicular field for all inductions B and Ginzburg-Landau parameters kappa.

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arXiv2008

Muon Spin Rotation and the Vortex Lattice in Superconductors

The magnetic field probability P(B) is calculated from the Ginzburg-Landau theory for various lattices of vortex lines in type-II superconductors: Ideal triangular lattices, lattices with various shear strains and with a super lattice of vacancies, and lattices of short vortices in films whose magnetic field ''mushrooms'' near the surface.

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arXiv2008

Longitudinal magnetic field increases critical current in superconducting strip

The critical state of the vortex lattice in a thin strip is considered for the case when first a perpendicular magnetic field is applied, then a longitudinal field, and then again the perpendicular field is increased. This longitudinal field can strongly enhance the critical currents in the strip since the vortices are inclined and the currents flow in the strip plane.

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arXiv2008

Surface deformation caused by the Abrikosov vortex lattice

In superconductors penetrated by Abrikosov vortices the magnetic pressure and the inhomogeneous condensate density induce a deformation of the ionic lattice. We calculate how this deformation corrugates the surface of a semi-infinite sample. The effect of the surface dipole is included.

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arXiv2007

Interaction between ionic lattices and superconducting condensates

The interaction of the ionic lattice with the superconducting condensate is treated in terms of the electrostatic force in superconductors. It is shown that this force is similar but not identical to the force suggested by the volume difference of the normal and superconducting states. The BCS theory shows larger deviations than the two-fluid model.

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arXiv2006

The concept of correlated density and its application

The correlated density appears in many physical systems ranging from dense interacting gases up to Fermi liquids which develop a coherent state at low temperatures, the superconductivity. One consequence of the correlated density is the Bernoulli potential in superconductors which compensates forces from dielectric currents. This Bernoulli potential allows to access material parameters. Though within the surface potential these contributions are largely canceled, the bulk measurements with NMR can access this potential. Recent experiments are explained and new ones suggested. The underlying quantum statistical theory in nonequilibrium is the nonlocal kinetic theory developed earlier.

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arXiv2006

Vortex induced deformation of the superconductor crystal lattice

Deformation of the superconductor crystal lattice caused by Abrikosov vortices is formulated as a response of the elastic crystal lattice to electrostatic forces. It is shown that the lattice compression is linearly proportional to the electrostatic potential known as the Bernoulli potential. Eventual consequences of the crystal lattice deformation on the effective vortex mass are discussed.

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arXiv2006

Thin Ohmic or superconducting strip with an applied ac electric current

The complex impedance, currents, and electric and magnetic fields are calculated as functions of resistivity and frequency or London depth for a long thin strip with applied ac current. Both Ohmic and superconducting strips are considered. While the inductance per unit length of the strip depends on the strip length logarithmically, the sheet current, magnetic field, resistance, and magnetic susceptibility are independent of this length. It is found that the enhancement of resistance by the skin effect in thin Ohmic strips is much weaker (logarithmic) than in thick wires.

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arXiv2005

Response of thin-film SQUIDs to applied fields and vortex fields: Linear SQUIDs

In this paper we analyze the properties of a dc SQUID when the London penetration depth λis larger than the superconducting film thickness d. We present equations that govern the static behavior for arbitrary values of Λ= λ^2/d relative to the linear dimensions of the SQUID. The SQUID's critical current I_c depends upon the effective flux Φ, the magnetic flux through a contour surrounding the central hole plus a term proportional to the line integral of the current density around this contour. While it is well known that the SQUID inductance depends upon Λ, we show here that the focusing of magnetic flux from applied fields and vortex-generated fields into the central hole of the SQUID also depends upon Λ. We apply this formalism to the simplest case of a linear SQUID of width 2w, consisting of a coplanar pair of long superconducting strips of separation 2a, connected by two small Josephson junctions to a superconducting current-input lead at one end and by a superconducting lead at the other end. The central region of this SQUID shares many properties with a superconducting coplanar stripline. We calculate magnetic-field and current-density profiles, the inductance (including both geometric and kinetic inductances), magnetic moments, and the effective area as a function of Λ/w and a/w.

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arXiv2005

Thin superconductors and SQUIDs in perpendicular magnetic field

It is shown how the static and dynamic electromagnetic properties can be calculated for thin flat superconducting films of any shape and size, also multiply connected as used for SQUIDs, and for any value of the effective magnetic London penetration depth Lambda. As examples, the distributions of sheet current and magnetic field are obtained for rectangular and circular films without and with slits and holes, in response to an applied perpendicular magnetic field and to magnetic vortices moving in the film. The self energy and interaction of vortices with each other and with an applied magnetic field and/or transport current are given. Due to the long ranging magnetic stray field, these energies depend on the size and shape of the film and on the vortex position even in large films, in contrast to the situation in large bulk superconductors. The focussing of magnetic flux into the central hole of square films without and with a radial slit is compared.

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arXiv2004

Critical state in type-II superconductors of arbitrary shape

The well-known Bean critical state equations in general are not sufficient to describe the critical state of type-II superconductors when the sample shape is not symmetric. We show how one can find the critical state in superconductors of arbitrary shape. Analyzing a simple example of nonsymmetry, we demonstrate that in the general case, a perturbation of the current distribution in the critical state propagates into the sample smoothly in a diffusive way. This is in contrast to the usual Bean critical state where the current distribution changes abruptly at a narrow front.

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