Dynamic magnetic susceptibility of a superconductor with a relaxing magnetic moment

Within the framework of the diffusion approximation for the dynamic magnetic flux, a model is constructed for the dynamic magnetic susceptibility of a superconductor with a nonuniform and time-dependent magnetic field distribution in the sample. The possibility of appearance of peaks in the temperature (or frequency) dependence of χ″ in a structurally homogeneous superconducting sample in the presence of a magnetic-field-induced nonuniformity of the diffusion parameter is demonstrated. The character of the temporal evolution of the magnetic susceptibility during relaxation of the magnetic moment of the sample is predicted and its properties are investigated. Fiz. Tverd. Tela (St. Petersburg) 39, 811–815 (May 1997)     

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.     

Topological structure of the fiber bundle of fermion eigenvectors on a lattice in a magnetic field

It is shown that for an arbitrary lattice Hamiltonian of fermions in a uniform magnetic field the Chern number of the complete fiber bundle of the magnetic Bloch states of this Hamiltonian is equal to zero. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 367–368 (10 March 1996)     

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)     

Magnetic response of a two-dimensional degenerate electron gas in nanostructures with cylindrical symmetry

An investigation is made of the magnetic response of nanostructures with cylindrical symmetry located in a longitudinal magnetic field. Analytic expressions are obtained for the magnetic moment of the nanostructures, cylinders and bracelets. It is shown that the magnetic moment describes Aharonov-Bohm oscillations. The profile of the oscillations and the position of the oscillation maxima are studied. In the limit T→0 the curves of the magnetic response as a function of the magnetic field flux contain “beak”-shaped kinks, and the positions of the points at which these kinks occur are determined. The temperature dependence of the magnetic response is studied and the influence of the spin-magnetic interaction on the magnetic response of the nanostructures is examined. It is shown that this interaction destroys the periodicity of the magnetic response with respect to flux and gives rise to a monotonic term in the response. Zh. éksp. Teor. Fiz. 115, 1450–1462 (April 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)     

Effect of a magnetic field on the electroluminescence intensity of single-crystal ZnS

After exposure of single-crystal ZnS with microscopic twinning to magnetic fields of 1–10 T, the integral electroluminescence brightness is found to increase by several times. It is proposed that the magnetic field facilitates relaxation of a metastable state of the structural defects. Fiz. Tverd. Tela (St. Petersburg) 41, 1944–1947 (November 1999)     

Magnetic lines of force in a Tornado trap

Numerical methods are used to analyze the structure of the magnetic field of an ideal Tornado trap; i.e., a trap consisting of linear currents whose magnitude and location in space are such as to produce a spherical separatrix with a radius η _c between the helices of the trap and partition the magnetic field into two regions so that the lines of force from the working volume η<η _c do not pass beyond its boundary. It is found that the magnetic lines of force of the working volume form a series of regions, each with its own properties, and do not penetrate from one region to another. In particular, one of the regions is a layer of magnetic toroidal surfaces adjacent to the inner surface of the separatrix. Zh. Tekh. Fiz. 67, 30–34 (January 1997)     

Shape of the magnetic resonance line in a thin film on the surface of an anisotropic superconductor

The shape of the EPR line in a thin (=λ/2, where λ is the London penetration depth of the magnetic field in the superconductor) paramagnetic film deposited on the surface of an anisotropic superconductor is calculated in an oblique magnetic field with allowance for the inhomogeneity of the local magnetic field of the Abrikosov vortex lattice. It is shown that, as the tilt angle of the external magnetic field is varied, the shape of the EPR line changes noticeably. This fact can give additional information about the superconductor parameters (the symmetry type of the vortex lattice and the anisotropy parameter of the superconductor). Fiz. Tverd. Tela (St. Petersburg) 41, 386–388 (March 1999)     

Donors in a strong magnetic field and elastic magnetic-impurity resonance in diamondlike semiconductors

The possibility of resonance during elastic intravalley scattering in n-type semiconductors is investigated in connection with the crossing (due to anisotropy of the effective mass) of the energy levels of excited states of a shallow donor as functions of the magnetic field. The hybridization of states of different frequencies in the vicinity of a crossing is attributed to the emergence of a nonzero dipole moment of the excited impurity atom and, accordingly, a long-range potential, which creates carrier-transport anomalies. The lower part of the donor spectrum is calculated as a function of the magnetic field in Si with B∥〈001〉 and in Ge with B∥〈111〉 or B∥〈110〉. A crossing occurs in Ge in the field range 9.9 T<B<16.7 T and in Si in the field range 10.5 T<B<37.7 T. The characteristic longitudinal relaxation time and the transverse conductivity, which are determined by scattering at excited donors in the presence of the hybridization of states, are calculated. Zh. éksp. Teor. Fiz. 112, 975–1010 (September 1997)     

Effect of a magnetic field on the plasticity and photo-and electroluminescence of ZnS single crystals

It is found that a ∼10 T magnetic field influences the microhardness and the photo-and electroluminescence of ZnS single crystals containing microtwins. It is established that a magnetic field irreversibly changes the properties of the crystal, leading to relaxation of the metastable states of structural defects. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 2, 114–118 (25 January 1999)     

Properties of a magnetic impurity in a metal

The effect of a polarized conduction-electron cloud back on a magnetic impurity dissolved in a metal is studied. It is shown that at a temperature T _c much higher than the Kondo temperature the system becomes unstable against symmetry breaking and that a state with 〈S _z 〉 ≠ 0 is established. The behavior of 〈 S _z 〉 is derived for all temperatures and magnetic fields except for a very narrow region around T _c and for very low temperatures. The minute role of Kondo-type processes in establishing the symmetry-broken state is pointed out. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 184–189 (10 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Exchange mechanism for localization of phonons near the surface of a magnetically ordered crystal

It is shown that first-principles inclusion of nonuniform exchange interaction leads to the creation of a new type of generalized shear acoustic wave propagating near the mechanically free surface of a magnet. Criteria are formulated that can be used to specify conditions under which an “exchange” type of surface acoustic wave can exist at the boundary between a magnetic and a nonmagnetic medium once the spectrum of magnetoelastic oscillations of the unbounded magnetic crystal is known. Fiz. Tverd. Tela (St. Petersburg) 40, 299–304 (February 1998)     

Local correlations of different eigenfunctions in a disordered wire

The correlation of the local density of states 〈ρ_ɛ(r ₁)ρ_{ɛ + ω}(r ₂)〉 in quasi-one-dimensional disordered wires in a magnetic field is calculated under the assumption that |r ₁ − r ₂| is much smaller than the localization length. This amounts to finding the zero mode of the transfer-matrix Hamiltonian for the supersymmetric σ model, which is done exactly by mapping to the three-dimensional Coulomb problem. Both the regimes of level repulsion and level attraction are obtained, depending on |r ₁ − r ₂|. We demonstrate that the correlations of different eigenfunctions in the quasi-one-dimensional and strictly one-dimensional cases are dissimilar. The text was submitted by the authors in English.     

Effect of a magnetic field on thermally activated tunneling ionization of impurity centers in semiconductors

An expression for the probability of thermally activated tunneling ionization in an electric field in the presence of a magnetic field is obtained. It is shown that the logarithm of the ionization probability is proportional to the squared electric field, and the coefficient of proportionality decreases with increasing magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 763–767 (25 November 1998)     

Anisotropy of a hole magnetic polaron in a semimagnetic semiconductor

It is shown that the shape of a hole magnetic polaron in a semimagnetic semiconductor with the sphalerite structure is anisotropic: The polaron is strongly oblate in the direction of its magnetic moment. When the anisotropy of the hole spectrum is taken into account, the properties of the polaron depend on the orientation of its spin with respect to the crystallographic axes. Specifically, the binding energy of the polaron is maximum when the spin is oriented along the [111] axis. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 209–213 (10 February 1996)     

Macroscopic coherent tunneling in a small particle of an uncompensated antiferromagnet in a strong magnetic field

A new macroscopic quantum tunneling effect is predicted for a particle of an uncompensated ferritin-type antiferromagnet in a noncollinear phase induced by a strong magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 369–374 (10 March 1999)     

The excitonic spectrum of germanium in a high magnetic field

In recently reported experiments with uniaxially deformed germanium in a magnetic field [V. B. Timofeev and A. V. Chernenko, JETP Lett. 61, 617 (1995)], it was found that applying a magnetic field of sufficiently high intensity results in the appearance of a new line in the optical spectrum of the excitons. In the present paper a mechanism is proposed which can provide an explanation for this experimentally observed spectral feature. The new spectral line may be attributed to the formation of strongly bound biexcitonic molecules in the quantum state ³Π_u. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 405–409 (25 March 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Anisotropic momentum transfer in low-dimensional electron systems in a magnetic field

In low-dimensional systems with an asymmetric quantizing potential, an asymmetric electron energy spectrum ε(p)≠ε(−p), where p is the electron momentum, arises in the presence of a magnetic field. A consequence of such an energy spectrum is that momentum transfer to the electron system in mutually opposite directions in the presence of an external perturbation is different. Therefore, in the presence of a standing electromagnetic wave momentum is transferred from the wave to the electrons, which gives rise to a new type of electromotive force. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 551–555 (25 October 1997)     

Photoinduced changes in the space-modulated magnetic order of a FeBO<Subscript>3</Subscript>:Mg single crystal

An effect of nonpolarized white light on the modulated magnetic structure of a FeBO₃:Mg single crystal, which arises in this light-plane weak ferromagnet in the low temperature range during technical magnetization, has been revealed. It has been found that the degree of the light action on the magnetic state of FeBO₃:Mg depends both on its duration and on the orientation of the spontaneous magnetization vector M of the crystal during illumination. Interpretation of the results obtained has been performed in the context of the “magnetic ripple” theory on the assumption that the absorbed light induces additional uniaxial magnetic anisotropy in the easy plane of the crystal and that the anisotropy axis is collinear to the vector M during illumination.