Hermann Rauh

Finite-element simulations of hysteretic ac losses in a bilayer superconductor/ferromagnet heterostructure subject to an oscillating transverse magnetic field

Numerical simulations of hysteretic ac losses in a bilayer superconductor/ferromagnet heterostructure subject to an oscillating transverse magnetic field are performed within the quasistatic approach, calling upon the ANSYSfinite-element software program and exploiting magnetostatic-electrostatic analogs. It is shown that one-sided magnetic shielding of a thin, type-II superconductor strip can lead to an enhancement or, respectively, a reduction of hysteretic ac losses as compared to those for a nonmagnetic support, depending on the amplitude of the applied magnetic field.

The effect of a superconducting surface layer on the optical properties of a dielectric photonic composite

The effect of a strongly anisotropic superconducting surface layer on the transmittance, reflectance and absorptance of a one-dimensional, layered dielectric composite with periodically alternating, isotropic constituents for linearly polarized, normally incident electromagnetic radiation is studied both analytically and numerically. The underlying model of the electric permittivity of the superconducting constituent permits photonic excitation at frequencies both below and above the superconductor pair breaking frequency as well as thermal and normal scattering right up to the superconductor critical temperature. The optical properties addressed reveal traits such as band-like patterns of the transmittance and reflectance, but also step-like or smeared-out patterns of the reflectance and absorptance, displaying a marked reference to the particular type of polarization by virtue of the anisotropy of the superconducting layer covering the dielectric composite. Thus, in switching from transverse electric to transverse magnetic polarization, the maximum optical selectivity can become gigantic, given an appropriate thickness of the superconducting layer, with a moderate dependence on temperature. This fact offers unique possibilities regarding practical applications of such a novel photonic composite as an efficient polarization filter for electromagnetic radiation tunable via the thickness of the covering layer and temperature.

Superconducting wire subject to synchronous oscillating excitations: Power dissipation, magnetic response, and low-pass filtering

Numerical simulations of a type-II superconducting wire subject to an ac transport current and oscillating transverse magnetic field are performed using the theory of the critical state. Time-dependent distributions of the current and the density of magnetic flux, the local power dissipation, and cycles of the magnetic moment are displayed. Noticeable inhomogeneous dissipation and field distortions are exposed. Results for hysteretic ac losses are reported too, and significant differences to predictions of available approximate formulae identified. Finally, a distinct low-pass filtering effect intrinsic to the wire’s magnetic response is revealed.

Hysteretic ac losses in a superconductor strip between flat magnetic shields

Hysteretic ac losses in a thin, current-carrying superconductor strip located between two flat magnetic shields of infinite permeability are calculated using Beans model of the critical state. Exact numerical calculations and approximate analytical forms delineating the penetration of magnetic flux and the energy dissipated during a cycle of the ac transport current, per unit length of the strip, are derived. For the shields oriented parallel to the plane of the strip, the penetration of the self-induced magnetic field is found to be enhanced, with the analytical current dependence of the ac loss conditionally resembling that of an isolated superconductor slab; for the shields oriented perpendicular to the plane of the strip, the penetration of the self-induced magnetic field is found to be impaired, with the analytical current dependence of the ac loss conditionally duplicating that of a regular set of curved superconducting tapes extending longitudinally around a surface of cylindrical shape. Thus, hysteretic ac losses can strongly augment or, respectively, wane when the shields approach the strip.

Magnetic dipole–vortex interaction in a bilayered superconductor/soft-magnet heterostructure

The penetration of the magnetic field of an infinitesimal magnetic dipole into a bilayered type-II superconductor/soft-magnet heterostructure is studied on the basis of the classical London approach. The energies associated with the presence of a straight magnetic vortex in the superconductor layer are derived and the critical dipole moment for the first vortex appearance is obtained when the magnetic dipole faces the superconductor or the soft-magnet constituent. This shows that the presence of the latter constituent can lead to an improvement of the superconducting properties of the heterostructure by either inhibiting vortex penetration into the superconductor constituent or enhancing vortex pinning in the superconductor constituent.

Enhancement of the current in a superconductor strip by means of curved superconducting shields

The distribution of the sheet current in a superconductor strip located between superconducting shields of various geometries is studied on the basis of exact analytic forms. Whereas the current becomes increasingly uniform when flat shields approach the strip, strong inhomogeneities are found to develop for curved shields, with concomitant enhancements by several times of the maximum total current predicted in the marginal Meissner state; a trait which could be favorably exploited regarding superconductor electronics applications.

Novel design of a smart magnet/superconductor heterostructure

Current distributions in a thin, flat superconductor ring located between two coaxial cylindrical soft magnets of high permeability are studied. Such a heterostructure protects the flux-free state of the ring even in the presence of strong total supercurrents, preventing entry of magnetic flux from the edges of the ring. A Fredholm integral equation of the first kind governing the current distribution in the Meissner state of the ring is derived and solved numerically for different relative permeabilities of the magnets and various distances between the magnets and the ring. This reveals that the current distribution tends to become homogeneous when the relative permeabilities are increased, the effect already saturating at values of these quantites of a few hundred. When the distances between the magnets and the ring are reduced, the current peaks near the circumferences of the ring decrease, thereby contributing to the large total supercurrent and, concomitantly, to the strong longitudinal component of the magnetic field.

Optical transmittance of photonic structures with logarithmically similar dielectric constituents

The optical transmittance of one-dimensional, logarithmically similar, layered stacks of pairs of lossless dielectric plates for linearly polarized, normally incident electromagnetic radiation is studied both analytically and numerically. Resorting to an analysis of the respective photonic modes, the dependence on frequency of the optical property addressed is established for a singly logarithmic stack as well as for two of its symmetric derivatives, an outbound doubly logarithmic stack and an inbound doubly logarithmic stack. Given specified constraints on the structural and dielectric parameters that delineate these stacks, the following key results come to the fore: the transmittance of the singly logarithmic stack shows single oscillatory maxima of almost full height at low frequencies and total quench at larger frequencies; the transmittance of the outbound doubly logarithmic stack shows twinned oscillatory maxima of full height at low frequencies, supplemented with a sharp, full resonance at a higher frequency against a background of suppressed transparency; the transmittance of the inbound doubly logarithmic stack shows twinned oscillatory maxima of full height at low frequencies, supplemented with a series of pairs of sharp spikes of full transparency for higher frequencies against a background of suppressed transparency. Thus, one-dimensional photonic structures generated by logarithmically similar dielectric constituents are capable of unfolding desirable and useful optical properties: transparency at low frequencies, but opacity, or selectivity, at higher frequencies, in one single photonic device.

Distribution of the sheet current in a magnetically shielded superconducting filament

Abstract The distribution of the transport current in a superconducting filament aligned parallel to the flat surface of a semi-infinite bulk magnet is studied theoretically. An integral equation governing the current distribution in the Meissner state of the filament is derived and solved numerically for various filament–magnet distances and different relative permeabilities. This reveals that the current is depressed on the side of the filament adjacent to the surface of the magnet and enhanced on the averted side. Substantial current redistributions in the filament can already occur for low values of the relative permeability of the magnet, when the distance between the filament and the magnet is short, with evidence of saturation at moderately high values of this quantity, similar to the findings for magnetically shielded strips.

Finite-element simulations of hysteretic alternating current losses in a magnetically coated superconducting tubular wire subject to an oscillating transverse magnetic field

Numerical simulations of hysteretic ac losses in a tubular superconductor/paramagnet heterostructure subject to an oscillating transverse magnetic field are performed within the quasistatic approach, calling upon the COMSOL finite-element software package and exploiting magnetostatic-electrostatic analogues. It is shown that one-sided magnetic shielding of a thin, type-II superconducting tube by a coaxial paramagnetic support results in a slight increase of hysteretic ac losses as compared to those for a vacuum environment, when the support is placed inside; a spectacular shielding effect with a possible reduction of hysteretic ac losses by orders of magnitude, however, ensues, depending on the magnetic permeability and the amplitude of the applied magnetic field, when the support is placed outside.