Anisotropic pinning in macroscopic electrodynamics of superconductors

Anisotropy of critical currents and electric fields in superconductors with strong pinning has been ascribed in the macroscopic model to features of the material equation system relating the electric field to the current density in a superconductor. The anisotropy of the pinning proper is described by an operator relating the pinning force density to the vectors of magnetic induction and Lorentz force. In the approximation of an extended critical state model, a feasible expression of this operator is given in the form of an algorithm based on the concept of a collective anisotropic potential well containing fluxoids. The current-carrying capacity of a strongly anisotropic niobium-titanium foil as a function of the orientation of the current density and applied field with respect to the principal axes of the material has been investigated in detail. Given measurements of the transverse electric fields in the foil under magnetic fields normal to the foil plane, we can plot cross sections of surfaces describing the pinning force density in the space of magnetic induction and Lorentz force. Zh. éksp. Teor. Fiz. 112, 1055–1081 (September 1997)     

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)     

Angular dependences of the peak effect in Yba2Cu3Ox single crystals

Angular dependences of the magnetization hysteresis loops have been studied at T=77 K on YBCO single crystals exhibiting the peak effect. The peak effect is shown to be related to the pinning of longitudinal vortices along the c axis at twin boundary-type ordered defects. The behavior of the peak effect at intermediate angles is explained by anisotropic magnetic field penetration into quasi-two-dimensional superconductors. In thin crystals with a dilute ordered-defect structure this can result in an enhanced peak effect due to formation of a vortex kink structure and to “internal” pinning of transverse vortex segments at Cu-O sheets. Fiz. Tverd. Tela (St. Petersburg) 39, 425–431 (March 1997)     

Field-asymmetric transverse magnetoresistance in a nonmagnetic quantum-size structure

A new phenomenon is observed experimentally in a heavily doped asymmetric quantum-size structure in a magnetic field parallel to the quantum-well layers — a transverse magnetoresistance which is asymmetric in the field (there can even be a channge in sign) and is observed in the case that the structure has a built-in lateral electric field. A model of the effect is proposed. The observed asymmetry of the magnetoresistance is attributed to an additional current contribution that arises under nonequilibrium conditions and that is linear in the gradient of the electrochemical potential and proportional to the parameter characterizing the asymmetry of the spectrum with respect to the quasimomentum. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 380–385 (10 September 1998)     

Simple formulas for the oscillating component of the electrical conductivity of layered chargeordered crystals

In the present paper, oscillations of the longitudinal component of the electrical conductivity of layered crystals are examined in electric and quantizing magnetic fields perpendicular to the layers. It is demonstrated that frequencies and amplitudes of longitudinal conductivity oscillations can be determined with sufficient accuracy through the chemical potential of the electron gas and effective width of the miniband caused by the charge ordering. In addition, based on an analysis of formulas for the transverse conductivity, it is established that the applicability limits for the transverse conductivity in the semiclassical approximation (for the magnetic field induction) in the field perpendicular to the layers are much wider than for the longitudinal conductivity. An immediate reason for this is the zero longitudinal velocity of current carriers in the extreme cross sections, which leads to the field dependence of the amplitudes of longitudinal conductivity oscillations stronger than of transverse ones. Calculated results are used to interpret experimental data obtained for the β-(ET)2IBr2 synthetic metals. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 34–43, May, 2006.     

A slow wave with the structure of an electromagnetic wave in piezoelectric and magnetostrictive media

The propagation of elastic waves in piezoelectric and magnetostrictive materials is considered theoretically. It is shown that an elastic wave in a piezoelectric can create not only a longitudinal electric field parallel to the wave normal (longitudinal piezoactivity) but also a transverse field of electric induction (transverse piezoactivity). The presence of a transverse induction field leads to the appearance of a magnetic field perpendicular to the direction of the wave normal and to the induction vector; therefore, the transverse-piezoactive wave is accompanied by a transverse wave having the structure of an electromagnetic wave and propagating with the speed of sound. Transverse-magnetostrictive elastic waves in magnetostrictive dielectrics are accompanied by a similar wave.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 32–36, October, 1976.     

Electrodynamics of hard superconductors in crossed magnetic fields

A model systematically accounting for the cutting of Abrikosov flux lines has been developed for the critical state of a hard superconductor in crossed dc and ac magnetic fields. The electrodynamic equations have been derived by minimizing the Gibbs free energy calculated using the proposed two-velocity hydrodynamic model. One velocity describes the motion of the vortex lattice as a whole, and the other describes the relative motion of the two intersecting sublattices. The resulting equations yield as special cases the previously known electrodynamic equations for hard superconductors. The model provides a natural explanation for the suppression of dc magnetization by a transverse ac magnetic field observed in our experiments. Zh. éksp. Teor. Fiz. 111, 1071–1084 (March 1997)     

Influence of frustration on magnetization processes in the ferrite CoFeCrO4

The behavior of isotherms of the magnetization σ(H) and the longitudinal λ_‖ and transverse λ_⊥ magnetostriction of the polycrystalline ferrite CoFeCrO₄ with frustrated magnetic structure has been investigated for the first time in magnetic fields up to H=50 kOe at T=4.2 K. It is found that the magnetization grows with increasing field due to two different paraprocesses. Fiz. Tverd. Tela (St. Petersburg) 41, 2042–2043 (November 1999)     

Anomalous absorption of ultrasound in gadolinium in a magnetic field

The absorption coefficient α _k for longitudinal ultrasonic (15 MHz) waves propagating transverse to the direction of a magnetic field H is measured in single crystal gadolinium. It is found that in fields H⩽600 Oe, the peak in α _k is shifted toward lower temperatures, while the absolute magnitude of the absorption rises with increasing H. It is shown on the basis of dynamic scaling that the anomalous behavior of α _k in fields H⩽600 Oe can be explained by introducing a magnetic field analog of the Landau-Khalatnikov relaxation mechanism. Fiz. Tverd. Tela (St. Petersburg) 39, 339–340 (February 1997)     

Reversible transitions in high - Tc cuprates based point contacts

The influence of electric fields and currents has been investigated in the high- superconductors YBaCuO and BiSrCaCuO using a point-contact geometry with Ag as the counterelectrode, which reveal switching transitions between states of a different resistance. The origin of this effect in point contacts is associated with electromigration of the oxygen, driven by the electric field as well as by the current-induced “electron wind”. The switching effect preserves its basic features at elevated temperatures up to room temperature and in high magnetic fields up to 10 T. Received 23 November 1998     

Experimental investigation of ultrasonic velocity anisotropy in magnetic fluids: Influence of grain–grain interaction

Magnetic field-induced dispersion of ultrasonic velocity in a Mn_0.7Zn_0.3Fe₂O₄ fluid (applied magnetic field is perpendicular to the ultrasonic propagation vector) is determined by employing continuous wave method. The magnitude of dispersion initially decreases with increasing field, then increases and reaches a plateau at higher fields. Results indicate that the velocity anisotropy is dominated by grain–grain interactions rather than grain–field interaction. At the critical temperature, the grain–grain interaction becomes weak as the transverse component of the particle/cluster moment is larger than the longitudinal one and the system reaches saturation even at low field. These observed variations in the field-induced anisotropy are analysed by incorporating the moment distribution of particles in Tarapov’s theory (J. Magn. Magn. Mater. 39, 51 (1983)).     

NMR line shape in anisotropic superconductors in a tilted magnetic field

NMR line shape has been constructed for anisotropic type-II superconductors in tilted magnetic fields, with inclusion of vortex-lattice magnetic-field nonuniformities and of the skin effect near the superconductor surface. The NMR line shape parameters are shown to change considerably when the external magnetic field changes direction. This makes it possible to obtain more detailed information about the characteristics of a superconductor, in particular, its anisotropy parameter. Fiz. Tverd. Tela (St. Petersburg) 40, 993–997 (June 1998)     

Energy exchange between a test charge and an unstable electron beam

An investigation is made of the interaction between a test charge and an electron beam which is convectively unstable against plasma oscillations. An analysis is made of the negative-mass instability of the longitudinal motion. Expressions are derived for the reaction force exerted by the electron plasma polarized by the moving charge and for the diffusion tensor arising from electric field fluctuations. The approximation of uniform rectilinear motion of a test particle is used to estimate the systematic and diffusion components of the rates of change in its energy. It is shown that in this case, interaction with an unstable electron beam leads to acceleration of the particle in respect to the longitudinal degree of freedom (antifriction) and under certain conditions enhances the friction effect (cooling) in respect to the transverse degrees of freedom. An assessment is made of the possibility of obtaining an overall positive friction effect by using a special “sweeping” procedure whereby the longitudinal velocity of the electron beam is changed in a specific manner consistent with the cooling process. Zh. Tekh. Fiz. 67, 58–61 (November 1997)     

Nonlinear surface magnetostatic waves in a ferrite semiconductor structure

A theoretical analysis is made of the propagation of a nonlinear surface magnetostatic wave in a planar ferrite semiconductor structure as a function of the carrier concentration in the semiconductor layer. It is shown that for certain concentrations the surface magnetostatic wave is unstable with respect to longitudinal perturbations and may propagate perpendicular to the magnetic field in the form of solitons. Zh. Tekh. Fiz. 69, 119–121 (June 1999)     

Spontaneous transverse response and amplified switching in superconductors with honeycomb pinning arrays

Using numerical simulations, we show that a novel spontaneous transverse response can appear when a longitudinal drive is applied to type-II superconductors with honeycomb pinning arrays in a magnetic field near certain filling fractions. This response is generated by dynamical symmetry breaking that occurs at fields away from commensurability. We find a coherent strongly amplified transverse switching effect when an additional transverse ac drive is applied. The transverse ac drive can also be used to control switching in the longitudinal velocity response. We discuss how these effects could be used to create new types of devices such as current effect transistors.     

Magnetoelectric response of the spin-Peierls compound CuGeO3

The effect of an electric field on the magnetic susceptibility in a pure CuGeO₃ single crystal at microwave frequencies is investigated. A quadratic effect of the electric field on the magnetic susceptibility, which increases with decreasing temperature, is observed in the spin-Peierls state at temperatures below 5 K. The observed effect is tentatively attributed to residual magnetism, due to distortions of the regular dimerized structure at domain walls with different values of the dimerization phase, and to the effect of the electric field on the interchain exchange interaction. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 646–651 (10 November 1996)     

Optical phenomena in bismuth-substituted ferrite-garnet films in external electric field

Magneto-electric properties of bismuth-substituted ferrite-garnet epitaxial films and yttrium ferrite-garnet films have been investigated by using optical polarimetry method, electromagneto-optical (EMO) effect. The EMO effect in the transverse electric (E) field was measured. In the yttrium ferrite-garnet films, the EMO effect in the specified geometry is not registered, though in a longitudinal E-field, it was observed by us. The EMO effect in separate multidomain areas of bismuth-substituted ferrite-garnet films was registered in the transverse electric field. A weak hysteresis appears in the magnetic-field dependences of the measured effect. A value of the EMO signal considerably depends on the external magnetic field, and in the homogeneously magnetized films, the EMO effect is practically absent. The irradiation of bismuth-substituted ferrite-garnet films by powerful laser impulse leads to increase of the EMO signal that can be a result of removal, in them, of nonuniform pressure.     

Anisotropy of the upper critical field and specific heat in V3Si — a superconductor with cubic symmetry

The specific heat of a V₃Si single crystal (T _c=17 K, H _c2=20 T) in magnetic fields up to 8 T isinvestigated experimentally for three orientations of the field relative to the crystallographic directions — H∥〈001〉, H∥〈110〉, and H∥〈111〉. Both the upper critical magnetic field and the specific heat of the mixed state are observed to depend on the orientation of the magnetic field relative to the crystallographic directions (anisotropy): The critical field reaches its maximum value and the specific heat its minimum value in a field along the 〈001〉 direction. The anisotropy scale in both phenomena increases as the magnetic field and reaches 3% in a 6 T field. The interrelationship of the upper critical field anisotropy and the specific-heat anisotropy in type-II superconductors is studied. It is shown that the anisotropy of the specific heat in the mixed state in weak fields can serve as a criterion for nontrivial pairing. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 26–29 (10 January 1999)     

Magnetic moment of a quasi-one-dimensional nanostructure in an inclined magnetic field

Shubnikov-de Haas oscillations in quasi-one-dimensional nanostructures (carbon nanotubes and quantum channels) are investigated. It is shown that two types of aperiodic oscillations arise in such systems: oscillations involving a change in the strength of the magnetic field, and oscillations involving a change in the angle of inclination of the field with respect to the symmetry axis of the system. It is found that the monotonic part of the magnetic moment lies in the plane of size confinement of the system and that the oscillating part has both longitudinal and transverse components. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 549–552 (10 April 1996)     

Nonlinear “brightening” of a film of nonabsorbing chiral nematic under selective reflection conditions

A strong decrease in the reflection coefficient of a film of a nonabsorbing chiral nematic (cholesteric) is observed experimentally in the region of selective reflection under the action of a powerful beam of circularly polarized light. The independence of the effect from the average power density (and its dependence only on the peak power density) allow it to be attributed to an increase in the pitch of the cholesteric helix to such a degree that it is completely unwound, an effect previously observed only in static and low-frequency electric and magnetic fields, in the strong field of the light wave. These are the first experiments in which, on account of the specially chosen irradiation conditions, the changes produced in the pitch of the helix by the field of the light wave can accumulate over time, so that a nonthermal mechanism can be invoked to explain the nonlinear brightening of a mirror made of a chiral nematic. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 403–407 (25 March 1996)