Cyclic hydrocarbons: nanoscopic (π)-SQUIDs?

A nonperturbative method for calculating persistent currents in molecules and nanoscopic quantum rings is presented. Starting from the extended Hubbard model on a ring threaded by an Aharonov-Bohm flux, a feedback term through which the current can generate magnetic flux is added. Another extension of the Hamiltonian describes the energy stored in the internally generated field. This model is evaluated using exact diagonalization and an iterative scheme to find the minima of the free energy with respect to the current. The magnetic properties due to electron delocalization of conjugated hydrocarbons like benzene [magnetic anisotropy, magnetic susceptibility exaltation, nucleus-independent chemical shift (NICS)] — that have become important criteria for aromaticity — can be examined using this model. A possible novel mechanism for a permanent orbital magnetic moment in quantum rings analogous to the one in π-SQUIDs is found in the framework of the proposed model. The quantum rings must satisfy two conditions to exhibit this kind of permanent orbital magnetic moment: a negative Drude weight and an inductivity above the critical level.     

Noncollinear orientation of external magnetic field and flux lines penetrating an isotropic hard superconductor

Penetration by Abrikosov flux lines of an isotropic hard superconductor in the critical state induced by changes in the orientation of external magnetic field has been theoretically investigated. The analysis has been based on the microscopic nonlocal model taking into account forces of bulk and surface pinning, alongside magnetic forces of interaction of the row of penetrating vortices with existing flux lines, Meissner currents, and vortex images. New vortices penetrate a superconductor only when the angle through which the field is rotated is larger than a certain critical value. It has been determined that the alignment of entering vortices is essentially different from that of the applied magnetic field. The feasibility of detecting noncollinearity effects is discussed. Zh. éksp. Teor. Fiz. 114, 1804–1816 (November 1998)     

On the geometry of relativistic magnetofluid flows

This paper deals with the construction of “magnetic vorticity” vector using Greenberg's theory of spacelike congruences for the trajectories of magnetic fields. A set of propagation equations is derived for the geometrical invariants associated with the congruences of magnetic field lines and fluid flow lines. Some applications of these propagation equations are made. A generalization of Ferraro's law of isorotation is obtained employing the propagation equation forω ² along the magnetic field lines.     

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)     

Modeling the wave parameters of a narrow slotted transmission line based on a superconducting film

An analytical expression for calculating the wave parameters of a narrow-slot transmission line is found in the quasistatic approximation. A two-fluid model is used for analyzing processes in a superconducting film of thickness comparable to the London penetration depth. The wave parameters of the slotted line are calculated under assumptions about the current distribution near the edges of the slot which are analogous to those used previously for analyzing microstrip and coplanar lines. Zh. Tekh. Fiz. 67, 94–97 (September 1997)     

On interrelation between inward turbulent flux and lower order rational magnetic surfaces at plasma edge

The interrelation between experimentally measured inward turbulent flux and lower order rational magnetic surfaces is demonstrated by the example of system with externally imposed magnetic surfaces—L-2M stellarator [Plasma Phys. Control. Fusion 50, 045001 (2008)]. In this note we show that average turbulent flux change sign from outward to inward in the vicinity of lower order rational magnetic surface located at plasma edge. There exists an upper threshold in plasma density for inward flux observation.     

Laser spectroscopy on molecular beam with a time-of-flight mass spectrometer operating in a strong magnetic field

Experimental set-up for studying effects of a strong magnetic field on a structure and a decay dynamics of molecules, was designed and constructed. A vacuum chamber, in which a molecular beam propagated, was mounted in a bore of a superconducting magnet. Laser light crossed the molecular beam in the magnetic field and excited the molecules. Fragment or parent ions produced through sequential decay processes, were extracted by an electric field parallel to the magnetic field and detected by a microchannel plate. By measuring the time-of-flight from the photo-excitation to the ion-detection, a species of ions —mass and charge state— was identified. A performance of the set-up was demonstrated using the resonance enhanced multiphoton ionization process through the X²Π-A²Σ⁺ transition of nitric oxide (NO) molecules. A mass resolution m/Δm ≥180±6 was obtained in the field up to 10 T. This was the first successful result demonstrating the sufficient mass resolution obtained by the time-of-flight technique in the strong magnetic field up to 10 T. Parent NO⁺ ions were selectively detected by the mass spectrometer and the ion current was measured as a function of the frequency of the laser light. Rotational transition lines were measured with a sufficient S/N ratio in the field up to 10 T.     

Magnetoplasmon replica in the recombination radiation spectra of a quasi-two-dimensional electron gas in GaAs/AlGaAs quantum wells

The radiative recombination spectra of two-dimensional electrons with free photoexcited holes are investigated for a wide variety of GaAs/ AlGaAs quantum wells, with different thicknesses and electron densities. It is found that for certain, close to integral, filling factors an intense line corresponding to an Auger process — radiative recombination with the emission of an additional magnetoplasmon — appears in the luminescence spectrum. The new line is shifted to lower energies with respect to the zero Landau level, and the magnetic field dependence of the energy splitting between these lines agrees with the theoretical concepts of the dispersion of magnetoplasmon excitations. This makes it possible to estimate the magnetoplasmon energy at the roton minimum. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 539–544 (25 October 1997)     

Magnetoplastic effect and spin-lattice relaxation in a dislocation-paramagnetic-center system

A magnetic induction threshold B _c above which the magnetoplastic effect — depinning of dislocations from paramagnetic pinning centers — can be observed in samples placed in a magnetic field is predicted and observed in Al, NaCl, and LiF crystals. The existence of a threshold is associated with the fact that for B<B _c the spin-lattice relaxation time τ_sl in a dislocation-paramagnetic-center system is less than the time required for spin evolution in a magnetic field resulting in the removal of the spin forbiddenness of an electronic transition that “switches off” the dislocation-pinning-center interaction. It is shown that the threshold field B _c is sensitive to temperature and x-ray irradiation of the samples. A new method for measuring the spin-lattice relaxation time in paramagnetic centers on dislocations is proposed on the basis of the data obtained. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 628–633 (25 April 1996)     

Stochastic resonance between limit cycles. Spring pendulum in a thermostat

The effect of white noise on phase synchronization is studied numerically for a classical model of a spring pendulum with a multiple ratio of the frequencies of small oscillations (Vitt-Gorelik model). It is shown that in the model investigated a Fermi resonance regime occurs for a system in a thermostat. A new type of nonlinear dynamics is found — stochastic resonance between limit cycles. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 585–589 (25 April 1999)     

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)     

Rigidly connected magnetic lines: twisting and winding of magnetic lines

The dynamical process of magnetic flux variation in a fluid’s stream tube is described by constructing \(1+1+ (2)\) decomposition of the gradient of fluid’s 4-velocity. The necessary and sufficient conditions are obtained for a spacelike congruence to be a congruence of rigidly connected spacelike curves. The evolution of magnetic flux in a magnetic tube is explored under the assumptions that magnetic lines are rigidly connected and the chemical potential of the fluid is constant along a magnetic tube. The interplay between magnetic and stream tubes is demonstrated. It is shown that the growth of magnetic energy in a magnetic tube cannot exceed to that of a stream tube. It is found that the proper time variation of twist of magnetic lines is caused by gravitation inside a neutron star if magnetic lines are rigidly connected and charge neutrality condition holds. Helmholtz-like magnetic vorticity flux conservation in a magnetic tube constituted by rigidly connected geodetic magnetic lines is derived under the assumption that the charge neutrality condition holds. It is shown that the winding of frozen-in poloidal magnetic field due to differential rotation requires meridional circulation in an axisymmetric stationary hydromagnetic configuration.     

Polarization of light: Fourth-order effects and polarization-squeezed states

The polarization properties of a monochromatic beam of light are ordinarily determined by three numbers, for example, the Stokes parameters. However, three numbers are no longer sufficient when intensity fluctuations in the polarized modes (or the correlation between them) are recorded. It is shown that in this case nine parameters, which can be arranged into 3×3 matrices, must be prescribed. The transformation properties of these matrices under polarization converters and the invariants of the matrices are analyzed. Specifically, the fourth-order polarization P ₄ is introduced. Several examples are examined of light with “hidden” polarization—light which is not polarized in the ordinary sense (P ₂=0) but is polarized in fourth order (P ₄≠0)—as well as “polarization-squeezed” light in which the quantum fluctuations of the Stokes parameters are suppressed. Zh. éksp. Teor. Fiz. 111, 1955–1983 (June 1997)     

Two-loop Majorana neutrino mass and magnetic moment in a gauge model

Two-loop contributions to Majorana mass and transition magnetic moment in a gauge model not in conflict with decaying neutrino dark matter (DDM) hypothesis have been studied. Another variant of an earlier model [J Dhar and S Dev,Pramana — J. Phys. 39 541 (1992)] consistent with the DDM hypothesis is proposed and is shown to lead to large enough neutrino magnetic moment and consistent with the phenomenological constraints on neutrino mass.     

Nonlinear dynamics of the critical state in hard superconductors and composites based on them

The nonlinear dynamics of the magnetic flux within a superconducting plate in response to the continuous rise in temperature over the course of the entire process of applying the magnetic field is investigated within the critical-state model. The results of numerical simulations based on a method developed to solve the system of Fourier and Maxwell equations with an unknown internal magnetic flux penetration boundary are compared with the corresponding analytical expressions of the isothermal theory. It is shown that the difference between the isothermal and nonisothermal models increases as the heat transfer coefficient decreases and as the rate of increase in the magnetic field strength and the transverse dimensions of the superconductor increase. The errors appearing in the isothermal approximation are very significant in the case of a thermally insulated, massive conductor. Consequently, the calculated values of the thermal losses occurring during the time period preceding the flux jump in the isothermal approximation can be significantly lower than the corresponding nonisothermal values. Zh. Tekh. Fiz. 67, 29–33 (September 1997)     

Possibility of the appearance of spin autowaves in a model of a ferromagnet with nonuniform dissipation

A model of a ferromagnet with nonuniform dissipation is introduced for the Landau-Lifshitz equations. It is shown that in this model a ferromagnet can be regarded as an oscillating active medium where the formation of autowave structures — spin autowaves, pacemakers, and spiral waves — is possible. Their wave characteristics, expressed in terms of the parameters of the medium, are found for a special case. Fiz. Tverd. Tela (St. Petersburg) 39, 513–515 (March 1997)     

High-pressure microwave discharge in an above-breakdown field. Streamer branching

A new effect — the splitting of the tips (branching) of a microwave streamer arising from a pre-existing dense plasma cloud in an above-threshold electric field — is obtained numerically on the basis of a planar two-dimensional model. The causes of this phenomenon and the factors suppressing it are found. An expression is obtained for the value of the cloud radius above which branching occurs. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 15–19 (10 January 1999)     

Stability limits, structure, and relaxation of a mixed state in superconducting films with an edge barrier

The interval of external magnetic fields where a mixed state can exist in a superconducting film with a fixed value of the flux is determined. The equilibrium and reversible magnetization curves of the sample are calculated, and the dynamics of the magnetic relaxation in the film is described. Nonuniform deformation of the Abrikosov lattice is predicted. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 405–410 (10 March 1997)