A granular high-temperature superconductive film in the field of a magnetostatic surface wave

The reflection of a magnetostatic surface wave from a strip of a granular high-temperature superconductor is investigated. An expression obtained by the method of perturbations describes the field of the scattered wave. The coefficient of reflection of the MSSW from the HTSC strip is calculated as a function of frequency and temperature. It is shown that the reflectance is very low at temperatures above critical and that the HTSC ceramic behaves as an ordinary superconductor. The reflectance rises sharply when the temperature drops below critical. As the critical current decreases, the reflectance drops off and the film loses the high-temperature properties. An expression is derived for the loss resistance of the MSSW in a granular HTSC film. The loss resistance amounts to a few kiloohms and falls off sharply as the film transitions to the superconductive state. The results can also be developed for periodic HTSC structures and used in the design of frequency-selective devices, as well as high-speed bolometric photodetectors. Tomsk State University of Control Systems and Electronics. Translated Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 97–102, July, 1998.     

Influence of the pinning of Abrikosov vortices on the propagation of surface magnetostatic waves in a ferromagnet-superconductor structure

The influence of surface-layer vortex pinning in a type-II superconductor on the propagation of surface magnetostatic waves in a ferromagnet-superconductor structure is analyzed. The pinning is assumed to be strong enough to prevent vortex displacement under the influence of the Lorentz force generated by the surface magnetostatic waves, so that the ground state of the superconductor is determined by the elastic properties of the vortex lattice and by pinning. In the given model the problem reduces to the analysis of the wave spectrum in the scattered field created by the disordered vortex surface layer. A calculation shows that the influence of this field on the surface magnetostatic-wave spectrum is slight and, hence, degradation of the shielding properties of the superconductor does not take place in the presence of strong vortex pinning (as opposed to the ferromagnet-ideal superconductor structure). Fiz. Tverd. Tela (St. Petersburg) 40, 32–35 (January 1998)     

Surface acoustic waves in a piezo-electric with a resonant high temperature superconductor planar structure

We have analyzed the propagation of SAW in a periodic planar structure made from a granular high-temperature superconductor formed on the surface of a piezoelectric material. A number of essential features in the properties of the SAW were revealed. In the region of the N-S transition in a band where the Josephson current at the intergranular junctions of the superconductor is dominant the frequency and temperature dependences of the SAW parameters can have a resonant character. An estimate showed that the resonant damping peak can reach 100 dB/cm while the shift ΔV/V of the SAW velocities in the resonance region exceeds 10⁻². The sharp temperature dependences of these parameters allow one to use the effect to make bolometric acoustoelectronic photodetectors and other transducers. Tomsk State Academy for Control Systems and Radio Electronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 32–41, August, 1996.     

Bragg Interaction of Surface Magnetostatic Waves with a Periodic Granular High-Temperature Superconducting Structure

Bragg interaction of surface magnetostatic waves with a periodic granular high-temperature superconducting structure is studied. A dispersion equation for coupled waves describing the characteristics of surface magnetostatic waves is derived. Resonance wave absorption near the critical temperature is revealed. The magnetostatic wave reflection coefficients of semi-infinite and finite periodic high-temperature superconducting structures are calculated. The results obtained can be used in designing tunable frequency selectors and transient bolometric photodetectors.     

On the mechanisms of transport losses in high-temperature superconductors

Monte Carlo simulations of a high-temperature superconductor carrying an alternating transport current have been performed. The influence of defect concentration and external magnetic field on the transport energy losses is investigated. We show that there are two components of the losses in a superconductor carrying an alternating transport current: a hysteresis component related to the superconductor remagnetization by the self-current field and a dynamic component related to the steady flow of vortices-antivortices and their annihilation in the superconductor.     

Magnetic flux penetration into high-temperature superconductors in the vortex liquid state under the blow-up conditions

The problem of magnetic field penetration into a type-II high-temperature superconductor that is in the weakly pinned vortex-liquid phase is considered. A magnetic field on the superconductor boundary rises with time in the blow-up regime. A model hydrodynamic equation describing the magnetic induction distribution in the vortex-liquid phase for thermomagnetic motion of the flux is derived. Analytical expressions for the depth and rate of magnetic field penetration into the superconductor are found. It is demonstrated that these quantities depend on parameters of the problem: index of power n in the boundary regime characterizing the penetration rate of vortices into the superconducting half-space and a parameter describing the effect of random pinning forces and thermal fluctuations on the magnetic flux distribution.     

The propagation of magnetostatic surface waves in ferrite/superconductor structures

An electrodynamic model that describes the dispersion of magnetostatic surface waves in ferrite/superconductor structures is suggested. On its basis, a new approach to determining the microwave sheet resistance R _S of superconducting films in a magnetic field is elaborated. The values calculated (R _S =0.20–0.96 mΩ) agree with results obtained by the Tauber method. For YIG/YBCO structures, the controllable phase shift is about 1.5π when the depth of magnetostatic wave penetration into the YBCO film varies from 2.0 to 0.8 μm.     

高T_c超导体YBCO磁通钉扎的计算机模拟研究

临界电流密度Jc是影响高温超导体在强电领域应用的一个重要参数,在实际应用中,特别在外加磁场下,临界电流密度与超导材料的磁通钉扎性质密切相关.因此,磁通钉扎一直是高温超导体研究中的一个重要领域.由于高温超导体磁通钉扎力密度Fp的标度律存在,本文根据D.Dew-Hughes总结的钉扎力函数,主要存在两种主要作用类型(正常相和△K).我们将D.Dew-HugBes给出的钉扎力密度Fp标度函数改进为一个简化的具有物理意义的函数表达式.结合文献中已有的实验数据,我们对YBcO进行了计算机模拟,确定了它的磁通钉扎类型,模拟的研究结果与实际情况比较吻合.     

Magnetostriction of thin flat superconductor in a transverse magnetic field

Magnetostriction loops of a hard superconductor are calculated analytically. The geometry under consideration is a thin flat superconductor in a magnetic field perpendicular to the flat face of the sample. The analytical representation of the results is given in a relationship with the magnetic flux distribution inside a sample in the mixed state of superconductivity. The simulations are consistent with the measurements on the high-temperature superconductor LaSrCuO.     

Nonlinear properties of multiphase high-temperature superconductors of the Bi–Sr–Ca–Cu–O system in the temperature range of the superconducting transition

The nonlinear characteristics of high-temperature superconductors of the Bi–Sr–Ca–Cu–O system have been experimentally investigated in the temperature range of the superconducting transition under the influence of a harmonic alternating magnetic field. The effect of the generation of odd harmonics in the signal of response to a harmonic alternating magnetic field for multiphase high-temperature superconductors containing regions with different values of the critical temperature in their bulk has been observed for the first time. The mechanism of harmonic generation in a superconductor in the resistive state, which is associated with the switch effect, i.e., with the redistribution of eddy current density between the local regions of the superconductor, has been considered.     

Dispersion of magnetostatic surface waves in ferrite-htsc structures near the phase transition

We present a detailed study of the dispersion properties of surface magnetostatic waves in layered structures of ferrite and high temperature superconductor (HTSC) layers. We solve the propagation problem for the surface magnetostatic waves under a quasistatic approximation, and obtain the dispersion relation. We present an analysis of this dispersion near the phase transition temperature in the HTSC layer. The calculations show that when the HTSC material transforms to the superconducting state the dispersion properties of the magnetostatic waves changes suddenly: the damping decreases by 4–10 dB and the phase velocity increases by a factor of 2–4. These results support experiments which have been performed, in which surface magnetostatic waves have been studied in a YIG film with gallium impurities and a YBaCuO film on a lithium niobate substrate.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 50–55, March, 1995.     

Effect of the rotation of an external magnetic field on the internal flux distribution in superconducting ceramics: Experiments with polarized neutrons

The behavior of a magnetic flux in a ceramic high-temperature superconductor with rotation of a magnetic field through a specified angle has been investigated using polarized neutrons. Two models of penetration of a weak magnetic field into a granular superconductor have been discussed. Based on the obtained distributions of the rotation angle and the magnitude of the magnetic field inside the sample, the choice is made in favor of one of these models.     

Influence of magnesium oxide nanoparticles embedded in Bi-2212/Ag tapes on their superconducting properties

A MgO-doped Bi₂Sr₂CaCu₂O _y (Bi-2212) high-temperature superconductor is investigated experimentally. The influence of the MgO particle concentration and size on the high-temperature superconductivity parameters is studied. It is shown that, when incorporated into the superconductor, MgO particles do not change their structure. The optimal parameters of the particles exerting the strongest effect on the material are found. The optimal size of the particle is in the range 40–50 nm for a cubic structure. Measures should be taken against clustering. Ways of particle preparation and incorporation into the superconductor are considered. The superconductivity of the material with embedded MgO particles is investigated. At temperatures close to 20 K, the magnetic field of doped tapes exceeds that of undoped ones by a factor of 2.5–10.0 depending on the MgO concentration. The temperature dependences of the superconducting properties of the doped tapes at different magnetic fields are presented.     

Longitudinal critical current in a ferrite-type-II superconductor structure

The effect of the interaction of Abrikosov vortices with the magnetization on the longitudinal vortical instability in a layered ferromagnet-type-II superconductor structure is analyzed. It is shown that in the vicinity of the orientational phase transition in the magnet, where the transverse magnetic susceptibility is large, the magnitude of the longitudinal critical current in the structure can be almost 1.5 times smaller than in the isolated superconductor. The reason for this is compensation of stray field sources outside the superconductor by “magnetic charges” arising from a jump in the transverse magnetization on the surface of the magnet. A structure is considered in which the thickness of the superconductor significantly exceeds the London penetration depth of the magnetic field and the wavelength of the critical mode. For this reason (in light of the absence of high-quality bulk high-temperature superconductors), to experimentally study the described phenomenon it is necessary to use conventional low-temperature superconductors. Fiz. Tverd. Tela (St. Petersburg) 39, 231–235 (February 1997)     

Electrodynamic stabilization conditions for high-temperature superconducting composites with different types of current-voltage characteristic nonlinearity

The current instability is studied in high-temperature superconducting current-carrying elements with I–V characteristics described by power or exponential equations. Stability analysis of the macroscopic states is carried out in terms of a stationary zero-dimensional model. In linear temperature approximation criteria are derived that allow one to find the maximum allowable values of the induced current, induced electric field intensity, and overheating of the superconductor. A condition is formulated for the complete thermal stabilization of the superconducting composite with regard to the nonlinearity of its I–V characteristic. It is shown that both subcritical and supercritical stable states may arise. In the latter case, the current and electric field intensity are higher than the preset critical parameters of the superconductor. Conditions for these states depending on the properties of the matrix, superconductor’s critical current, fill factor, and nonlinearity of the I–V characteristic are discussed. The obtained results considerably augment the class of allowable states for high-temperature superconductors: it is demonstrated that there exist stable resistive conditions from which superconductors cannot pass to the normal state even if the parameters of these conditions are supercritical.     

Thermal effects and diamagnetic response of a current-carrying YBCO film

The temperature dependence of the dissipative part of the response to an ac magnetic field has been studied for an YBCO high-temperature superconductor film carrying a transport current. The response is found to exhibit a thermal hysteresis, whose magnitude depends on the transport current. The hysteresis is assigned to the current-carrying superconductor having two stable equilibrium states within a certain thermostat temperature interval. The existence of these states governed by specific features of the release and removal of heat.     

基于LabVIEW的高温超导材料特性测试实验

高温超导体临界特性的测量是近代物理实验中最为经典的实验之一,本文通过使用GM制冷机,利用Lab-VIEW虚拟仪器软件和高性能采集卡,改进了传统的测量高温超导材料临界特性的实验装置.本测试系统不仅可以测量高温超导体在不同温度下的临界电流,还可以测量其失超传播特性.该实验对于学生了解超导体特性和培养严谨的科学作风十分有益.     

Propagation of magnetostatic waves in a ferrite plus high-T c superconductor structure in the presence of transport current in the superconductor

A thin-film structure consisting of a ferrite and a high-T _c superconductor was used to investigate the effct of the transport current in the superconductor on the amplitude-frequency characteristic and dispersion of surface magnetostatic waves (MSWs) in a ferrite film. It was found that the nature of energy transfer between the MSWs and the superconducting film undergoes a significant change as the transport current is varied. In particular, in one of the current ranges, energy can be transferred both from the MSWs to the superconductor and back again, whereas in another range it can only be efficently transferred to the superconductor. Fiz. Tverd. Tela (St. Petersburg) 39, 2195–2202 (December 1997)     

<Emphasis Type="Italic">I–V</Emphasis> characteristics and current instability in a high-temperature superconductor with a nonuniform temperature distribution over the cross section

The effect of a nonuniform temperature distribution in the cross section of a high-temperature superconductor on the formation of its I–V characteristic and the conditions for conservation of a stable distribution of the input current are analyzed. The equations permitting us to determine the boundary values of the electric field and current above which current instability can be developed with allowance for the dimensional effect are derived. It is shown that the input current quench determining the maximal permissible value of the input current decreases if thermal inhomogeneity of electrodynamic states is taken into account in theoretical analysis of its stability conditions. It is found that the limiting admissible values of the electric field and current are nonlinear functions of the thermal resistance of the superconductor, its critical parameters, and external heat-exchange conditions. Therefore, intense cooling of the superconductor does not result in an increase in the current corresponding to the emergence of instability in proportion to the increase in its cross-sectional area or critical current density.     

Magnetization reversal processes in layered high-temperature superconductors with ferromagnetic impurities

The self-consistent interaction of a vortex system of a high-temperature superconductor and ferromagnetic impurities, including single impurities and their clusters, has been considered in the model of a layered high-temperature superconductor. For different temperatures and concentrations of ferromagnetic impurities, the magnetization reversal loops have been calculated by the Monte Carlo method taking into account an ensemble of ferromagnetic particles with different orientations of their easy magnetization axes with respect to the direction of an external magnetic field and for different magnetic anisotropy energies. It has been demonstrated that there is a nonlinear interaction of the high-temperature superconductor with ferromagnetic impurities, in which the initially thermodynamically reversible character of the magnetization reversal of the ferromagnetic ensemble can become irreversible. For a periodic lattice of clusters of ferromagnetic impurities, the magnetization curves of the high-temperature superconductor have been calculated for different sizes and configurations of the clusters. It has been revealed that, when extended defects are oriented parallel to the direction of the entrance of vortices in the sample, the length of the defects does not affect the remanent magnetization. It has been shown that the inclusion of the interaction between the magnetic moments inside the impurity cluster leads to a decrease in the magnetization reversal loop, the coercivity, and, accordingly, the energy loss due to magnetization reversal.