Magneto-optic study of spatial magnetic-field distribution relaxation in an HTSC film strip after transport current turn-on

The paper provides the first demonstration of the efficiency of applying the magneto-optic method to studies of the spatial and temporal magnetic-field relaxation in an YBa₂Cu₃O₇ film strip after the transport current is switched on. It is shown that the evolution of magnetic flux distribution is adequately described in terms of a modified Bean model with time-dependent critical current. At a time 50 ms after the current is switched on, the critical current of the samples studied decreases by ≈15%. This proves the significance of thermally activated magnetic flux motion (creep) in the regime investigated. The magnetic vortex pinning energy has been estimated as U ₀≈20 kT. Fiz. Tverd. Tela (St. Petersburg) 41, 965–968 (June 1999)     

Breakdown of the linear current regime in periodic structures

We study the effect of a weak nonlinearity in media on the linear regime of current flow in two-dimensional periodic structures with two equal component concentrations. We find that the asymptotic behavior of the electric field and current as functions of the distance between the angles in heterogeneous media is determined by the parameter h=σ ₂/σ ₁ (here σ ₁ and σ ₂ are the linear conductivities of the cells) and the external magnetic field B. This dependence leads to divergence of the higher-order moments of field and current at certain critical values h _c and B _c and to divergence of the response functions related to the higher-order moments. For square cells the effective nonlinear conductivity diverges at h⩽h _c, with . For structures of general shape we find the dependence of h _c on the angles and the external magnetic field. We show that for a given structure the linear regime of current flow in the system can be reversibly transformed into a nonlinear one by varying the magnetic field strength. The critical field B _c is approximately determined from the condition ω _c τ∼1, where ω _c and τ ⁻¹ are, respectively, the cyclotron frequency and the collision rate. Finally, we discuss the feasibility of detecting these effects experimentally. Zh. éksp. Teor. Fiz. 112, 643–660 (August 1997)     

Critical current of a nonuniform Josephson junction at an intergrain boundary with a random distribution of dislocations

The critical current of a nonuniform intergrain Josephson junction is calculated on the assumption that the superconductivity is suppressed by local stresses produced by randomly distributed boundary dislocations. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 32–37 (10 January 1997)     

Anomalous behavior of textures in a magnetic field

It is shown that a qualitatively new type of transition to a nonlinear phase is possible in two-dimensional two-phase textures in a magnetic field. The anomalous growth of the effective response (higher-order correlation functions of the current) near the critical magnetic field is studied. The dependence of the critical magnetic field on the angles of microinclusions is calculated. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 115–119 (25 July 1997)     

Low-field electrodynamics of polycrystalline ternary molybdenum chalcogenides

The penetration of very weak magnetic fields into the polycrystalline low-temperature superconductor SnMo₆S₈ is studied experimentally. The experimental results agree satisfactorily with the theory of the critical state in the framework of low-field electrodynamics. It is found that the critical current density in this case does not depend on the dc magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 184–188 (10 February 1999)     

Critical current of an inhomogeneous Josephson junction at an intergrain boundary with a random distribution of dislocations

The critical current J _c(θ) of an intergrain boundary is calculated as a function of the contact misorientation angle θ of the granules. It is assumed that the ordering parameter is suppressed in regions near boundaries with an enhanced mechanical stress induced by randomly distributed surface dislocations. The stress distribution function is determined using a probabilistic approach. Assuming that the weak coupling at the boundary is Josephson coupling, an analytic expression is found for the angular dependence J _c(θ) (for tilt and twist boundaries). The magnitude of the residual critical current of a boundary in a strong magnetic field is estimated. Fiz. Tverd. Tela (St. Petersburg) 40, 393–402 (March 1998)     

Sound velocity dispersion in a solution with two critical points

The dispersion of the sound velocity in a solution with a region of stratification has been investigated experimentally in the frequency range from 2.6 MHz to 12 GHz. It is found that the dispersion of the sound velocity below the lower critical temperature of stratification reaches 22% and depends linearly to the temperature above the upper critical temperature of separation, the dispersion of the sound velocity is much smaller and has a different linear dependence on the temperature. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 166–168 (10 August 1996)     

Wavevector-Dependent Susceptibility in <Emphasis Type="Italic">Z</Emphasis>-Invariant Pentagrid Ising Model

We study the q-dependent susceptibility χ(q) of a Z-invariant ferromagnetic Ising model on a Penrose tiling, as first introduced by Korepin using de Bruijn's pentagrid for the rapidity lines. The pair-correlation function for this model can be calculated exactly using the quadratic difference equations from our previous papers. Its Fourier transform χ(q) is studied using a novel way to calculate the joint probability for the pentagrid neighborhoods of the two spins, reducing this calculation to linear programming. Since the lattice is quasiperiodic, we find that χ(q) is aperiodic and has everywhere dense peaks, which are not all visible at very low or high temperatures. More and more peaks become visible as the correlation length increases—that is, as the temperature approaches the critical temperature. Supported in part by NSF Grant No. PHY 01-00041.     

Transformation of beams of dia-, para-, and ferromagnetic particles by the magnetic field of a linear current

Some possibilities of transformation and trapping of beams of small (10⁻¹–10⁻⁴ cm) dia-, para-, and ferromagnetic particles by the magnetic field of a linear, constant current are investigated. Zh. Tekh. Fiz. 67, 138–141 (August 1997)     

Current filamentation and macroturbulence in superconductors in rotating magnetic fields

The onset of specific magnetic structures associated with the formation of extended current filaments is observed in YBCO single crystals in a rotating magnetic field. Like current filaments in a plasma, they are unstable and decay into current macrovortices. The appearance of filaments is explained by the formation of closed flux rings along magnetization-reversal fronts and collapse of these rings with formation of Meissner cylinders. The total current along the surface and in the vortex shell of the cylinder (the latter current dominates in high-T _c superconductors) exceeds the critical current in the same volume located far from a filament. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 253–257 (10 February 1997)     

Radio-frequency absorption study of the parameters of high-T c materials irradiated with fast reactor neutrons

The magnetic properties of YBa₂Cu₃O_7−x ceramics of various densities and Bi₂Sr₂CaCu₂O₈ quasicrystals irradiated with neutrons at fluences of 10¹⁶–10¹⁹ cm⁻² are investigated by a radio-frequency method (in the frequency range f=100–150 MHz). The electromagnetic absorption is used to estimate the parameters of a Josephson medium: the lower critical field, the critical current density, the grain size and intergranular distances, and the penetration depth of the magnetic field in the sample. The hysteresis of electromagnetic absorption in the indicated superconductors is investigated; the hysteresis effect can be used to obtain data on flux pinning processes and flux creep dynamics. It is shown that an increase in the critical current density is observed as a result of the injection of radiation-induced pinning centers. Fiz. Tverd. Tela (St. Petersburg) 39, 28–34 (January 1997)     

The statistical distribution of magnetic critical currents determined by HTSC film morphology

An analysis is made of the geometric and morphological properties of YBCO superconducting films and of their effect on the magnetic and transport phenomena. A study is presented of the statistical characteristics of critical currents derived from the variation of trapped magnetic flux induced by passing a pulsed transport current. It is shown that the critical currents in the materials under study have a specific statistical distribution, whose main properties are due to the morphology of their structure and can be determined by geometric-probability analysis. The superconducting film is considered as a percolation system. An empirical normal-phase cluster-area distribution function was used to derive the distribution function of magnetic critical currents, which describes adequately the experimental data on how transport current affects trapped magnetic flux. The critical current for transition of a film to the resistive state has been calculated. Fiz. Tverd. Tela (St. Petersburg) 41, 1739–1742 (October 1999)     

Fluctuation of the delay time of nonhysteretic Josephson junctions during a linear current rise

The influence of thermal fluctuations on the delay time of nonhysteretic Josephson junctions during a linear rise in the current through them is investigated in two cases: a) during a slow rise, and b) at a high rate of increase. Zh. Tekh. Fiz. 68, 129–130 (September 1998)     

Effect of asymmetry of CuO2 layers in copper oxides on the anomalous neutron scattering near T c

Reasons for critical magnetic scattering of neutrons near T _c in copper oxides with CuO₂ layers whose nearest environment has no “up-down” symmetry are discussed. The intracrystalline electric field, which threads the CuO₂ planes on account of the asymmetry, induces coupling between the spin and momentum of the current carriers. This coupling is shown to result in a manifestation of virtual Cooper pairs in the imaginary part of the spin susceptibility. Thus spin density fluctuations as well as current fluctuations should participate in the scattering. A way of experimentally distinguishing between the two mechanisms is pointed out. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 363–368 (10 March 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Self-organization of the critical state in a two-dimensional multijunction SQUID under closed boundary conditions

A model of a closed system with self-organization is presented. This is a simplified model of a multijunction SQUID in an ac magnetic field. In our closed system, a self-organized critical state is realized on account of the fact that current dumping, which gives rise to self-organization in open systems, is replaced here by a fundamentally different mechanism — annihilation of the currents. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 2, 119–125 (25 January 1999)     

Ultranarrow beams of electromagnetic radiation in media with a Kerr nonlinearity

The vector structure of a self-channeling electromagnetic field is determined by solving the complete system of Maxwell’s equations in a transparent medium with a Kerr nonlinearity. Self-channelling with an asymmetric angular distribution of the field occurs at powers several times the critical self-focusing power. As the power is increased, a universal (self-similar) field structure develops in which only the scales change as the power is varied. Self-channelling with a channel width much smaller than the (linear) wavelength of the light, i.e., a “needle of light” with an extreme concentration of radiant power, is found to occur. Zh. éksp. Teor. Fiz. 116, 458–468 (August 1999)     

Heat release accompanying current injection into a composite superconductor in a stable superconducting state

An investigation is made of the Joule dissipation of energy in the superconducting phase of a composite superconductor as the current in it increases. It is shown that at high rates of current injection the dissipative processes are characterized by an appreciable release of heat, which substantially influences the critical energies for the loss of superconductivity and the velocity of the normal zone. An analytical expression is given for calculating the power of the thermal losses during current injection. Zh. Tekh. Fiz. 69, 125–128 (April 1999)     

Characteristics of the statistical distribution of the current transmitted by a cathode spot of a vacuum arc in variously oriented magnetic fields

The influence of variously oriented uniform magnetic fields on the cathodic attachment of a low-current vacuum arc with electrodes made of oxygen-free copper and CuCr30 composition is studied. It is found that, if the current is fixed, cathode spots in the arc attachment are distributed by the normal law in the entire range of variation of the amplitude of magnetic induction vector B and angle α between this vector and the normal to the cathode surface. The parameters of the distribution depend on the magnetic field and cathode material. The magnetic field dependence is appreciable only when angle α exceeds some critical value α* (α* ≈ 30° and ≈45° for cathodes made of copper and CuCr30, respectively). At α > α*, the parameters of the distribution become strongly dependent on α, while the B dependence remains weak. Only when α → π/2 does the field amplitude have a pronounced effect on the parameters of the distribution. From the obtained results, we determine the statistical characteristics of the distribution of the mean current transmitted by a cathode spot in variously oriented magnetic fields. The found relationships make it possible to explain the peculiarities of the structure of the cathodic attachment of the high-current vacuum arc stabilized by an external axial magnetic field.     

Electron spectrum and stability of the saturated ferromagnetic state in a Hubbard model with strong correlations

A Hubbard model with infinite Coulomb repulsion is studied in a many-electron operator representation. A picture of the density of states is constructed using expressions for the one-particle Green’s functions of first order in 1/z. Its behavior is studied near the Fermi level, especially Condon effects. The stability of saturated ferromagnetism is examined. The corresponding critical current carrier densities are found for semielliptical and rectangular seed densities of state, and for square and cubic lattices. These results are compared with earlier work. Fiz. Tverd. Tela (St. Petersburg) 41, 1057–1063 (June 1999)     

Effect of normal electrons in superconductors on the current-voltage characteristic of a Josephson junction

It is established that the conductivity of normal electrons in superconductors plays a large role in the current-voltage characteristic of a Josephson junction having a high critical current density, described on the basis of a nonlocal electrodynamics. It is shown in the resistive limit that the normal-electron contribution increases the current through the Josephson junction, while in the capacitive limit it decreases the contribution of Cherenkov resonances. Fiz. Tverd. Tela (St. Petersburg) 41, 582–587 (April 1999)