On the penetration depth of a strong field into superconductors

The field dependence of the magnetic penetration depth over the entire range of stability and metastability of the Meissner state was determined within the framework of the Ginzburg-Landau theory. A simple interpolation formula is suggested.     

Polarization effects induced by a magnetic field in intrinsically granular superconductors

Based on the previously suggested model of nanoscale dislocation-induced Josephson junctions and their arrays, we study the magnetic-field-induced electric polarization effects in intrinsically granular superconductors. In addition to the new phenomenon of chemomagnetoelectricity, the model also predicts a few other interesting effects, including charge analogs of Meissner paramagnetism (at low fields) and a “fishtail” anomaly (at high fields). The conditions under which these effects can be experimentally measured in nonstoichiometric high-T_c superconductors are discussed.     

On magnetic field penetration into a granulated superconductor

The frequency dependence of the magnetic response of YBa₂Cu₃O_7−x high-temperature supercon-ducting ceramics is studied. The experimental results are obtained for different values of the constant ((0–99)×10⁻⁴ T) and variable ((0.18–7.24)×10⁻⁴ T) components of the external magnetic field in the frequency range from 60 Hz to 1 MHz. A simple qualitative model of the distributed Josephson medium is offered to account for the results obtained.     

The penetration of a local magnetic field through large sheets of type-I superconductors

The penetration of a local magnetic field, applied perpendicularly to a large sheet of type-I superconducting material, is studied. It is found that perfect shielding occurs up to an applied field somewhat stronger that the thermodynamic critical field B_c. Using the method of images it is shown that at the threshold the maximum value of the flux density along the surface of the sheet just reaches the value B_c.     

Magnetic field penetration into half-space for type-II superconductors of the second kind

Equations that simulate the magnetic induction and current density distributions in half-space in view of the power I-V characteristic are derived. The magnetization front velocity is determined for a given mean rate of external magnetic field variation at the boundary of the sample. An integral condition for the electrical resistance (nonlinearly depending on the magnetic field) under which the magnetic flux penetrates into the sample with a finite rate is found. An analytical solution that simulates the power variation of the magnetic field at the boundary is given. The Bean generalized model describing the current density distribution near the critical current is considered. It is shown that solutions like shock waves may arise beyond the applicability domain of the Bean model.     

On the penetration of the magnetic field into a superconductor

It is known that in superconductors the exponential decay of the magnetic field is an approximation, which breaks down if the dimension of a Cooper pair ξ _f is of the order or smaller than the London penetration depth δ. The appearance of a nonlocal relation between current and field yields deviations from the exponential decay especially a sign reversal of the field at a certain distance. This sign reversal is connected with a change: of the surface energy in superconductors and of the structure of fluxoids together with their interaction. In this paper we present results on the decay of magnetic field which is calculated from the exact BCS-integral-kernel for weak fields. As a result, the nonlocal effects in the framework of BCS-theory can be described in good approximation by the ratio of the London penetration depth δ(T, l) and the dimension of Cooper pairs ξ _f (T, l). The evaluations show, that one has still sign reversal, i.e. large nonlocal effects, in Type II superconductors with a κ(T _c )?,1.6. It should be mentioned that the limit κ?1.6 coincides roughly with the experimentally observed region of attraction of fluxoids. In addition results on the penetration depths are summarized.     

Critical magnetic field ratio of anisotropic magnetic superconductors

The upper critical field, the lower critical field and the critical magnetic field ratio of anisotropic magnetic superconductors are calculated by Ginzburg–Landau theory analytically. The effect of the Ginzburg–Landau parameter (κ₀), magnetic susceptibility (χ) and magnetic-to-anisotropic parameter ratio (θ) on the critical field ratio are considered. We find that the value of critical field ratio increases with increasing κ₀ and θ, and decreases with increasing χ. The highest and the lowest value of critical field ratio is found in the diamagnetic superconductors and the ferromagnetic superconductors, respectively.     

超导体磁场穿透深度测量中的数据分析问题

讨论了目前在超导体磁场穿透深度λ测量中常用的几种数据分析方法.着重指出对于那些只能测量λ随温度的变化值Δλ(T),而无法测量其绝对值的实验而言,一种合适的数据分析方法对于我们从实验中得出正确的结论尤其重要.而微分归一法将有效地克服目前此类实验中常用数据处理方法的不足,从而实现理论与实验间的无参量比较. 关键词： 超导体 磁场穿透深度     

Sensitivity estimate of magnetic field sensors based on high-temperature superconducting yttrium cuprate

Experimental studies of the interaction of variable and constant magnetic fields with supercon-ducting YBa₂Cu₃O_{7 ? x} ceramics are presented. A technique for estimating the minimal noise level of magnetic field sensors based on superconducting yttrium cuprate is suggested. Ways to reach the ultimate sensitivity of such devices are considered.     

Ion separation due to magnetic field penetration into a multispecies plasma

The magnetic field, the electron density, and the ion velocities in a multispecies plasma conducting a high fast-rising current are determined using simultaneous spectroscopic measurements. It is found that ion separation occurs in which a light-ion plasma is pushed ahead while a heavy-ion plasma lags behind the magnetic piston. We show that most of the momentum imparted by the magnetic field pressure is taken by the reflected light ions, and most of the dissipated magnetic field energy is converted into kinetic energy of these ions, even though their mass is only a small part of the total plasma mass. Such species separation with implications to the momenta and energy partitioning is shown to be of a general nature.     

Specific features of helium penetration into single-crystal and nanocrystalline copper under deformation in liquid helium

This paper reports on experimental data on the penetration of helium atoms into single-crystal and nanocrystalline copper samples subjected to tensile and compressive strains at T=4.2 K, respectively. The dependences of the helium concentration N in the samples on the strain ? and the curves of helium extraction in the temperature range 300–1000 K at different strains ? are determined. It is found that the dependences N(?) and σ(?) correlate qualitatively with each other for single-crystal copper and do not correlate for nanocrystalline copper. This is associated with the different mechanisms of deformation in these samples. The deformation proceeds through the dislocation mechanism in single-crystal copper and through the jumpwise (twinning, rotational) mechanisms in nanocrystalline copper during local heating in regions of plastic shears. These factors are also responsible for the considerable difference between the curves of helium extraction from samples of both types. The curves of helium extraction exhibit two maxima for single-crystal copper and five maxima for nanocrystalline copper samples. The results obtained are discussed in terms of both the dynamic dislocation pipe diffusion and grain-boundary mechanisms of particle penetration from the surrounding medium into copper through different-type moving defects under applied stresses and due to the gradient of the chemical potential at the metal-surrounding medium interface.     

Two regimes of vortex penetration into platelet-shaped type-II superconductors

Vortex penetration into a thin superconducting strip of a rectangular cross section is considered at an increasing applied magnetic field H _a , taking an interplay between the Bean-Livingston and the geometric barriers in the sample into account. We calculate the magnetic field H _p at which the penetration begins and show that two regimes of vortex penetration are possible. In the first regime, vortices appearing at the corners of the strip at H _a = H _p immediately move to its center, where a vortex dome starts to develop. In the second regime, the penetration occurs in two stages. In the first stage, at H _a < H _p , tilted vortices penetrate into the edge regions of the strip, where novel domes are shown to be formed at the top, bottom, and lateral surfaces. In the second stage, at H _a = H _p , the vortex propagation to the center becomes possible. The difference between the regimes manifests itself in slightly different dependences of the magnetic moment of the strip on H _a .     

The critical magnetic field of anisotropic two-band magnetic superconductors

The upper and lower critical fields, and the critical field ratio of an anisotropic two-band magnetic superconductor in the Ginzburg–Landau (GL) scenario is derived analytically. The temperature-dependent upper critical field is investigated and applied to Fe-based superconductors. We find that a very high value of zero-temperature upper critical field in Fe-based superconductors can be found in the negative differential susceptibility region. The temperature-dependent upper critical field is presented in two formulas, in the empirical view and in the GL two-band view, which agrees with the experimental results.     

Specific features of chemisorption on a size-quantized film in an external quantizing magnetic field

The energy of an electron of an adatom chemisorbed on a size-quantized film is investigated as a function of the external quantizing magnetic field. Consideration is given to the cases when the external magnetic field is directed parallel and transverse to the film surface. It is demonstrated that, as the magnetic field increases, the energy of chemisorption jumpwise decreases.     

Specific features of magnetic properties of three-component magnetic superlattices based on Fe/Co/Mo

Synthesis of three-component magnetic superlattices based on the Fe/Co/Mo system by cathode sputtering in an electron-oscillating discharge and their complex investigations have been performed for the first time. The fields have been investigated by X-ray diffraction, vibrational magnetometry, and Mössbauer spectroscopy with the use of completely computerized systems. Oscillations of the main magnetic parameters with a change in the Mo thickness and giant spontaneous magnetization have been found. The properties of three-component superlattices are compared with those of the analogous system based on Fe/Mo.     

Infinite penetration of a projectile into a granular medium

An object falling in a fluid reaches a terminal velocity when the drag force and its weight are balanced. Contrastingly, an object impacting into a granular medium rapidly dissipates all its energy and comes to rest always at a shallow depth. Here we study, experimentally and theoretically, the penetration dynamics of a projectile in a very long silo filled with expanded polystyrene particles. We discovered that, above a critical mass, the projectile reaches a terminal velocity and, therefore, an endless penetration.