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Dec 1999

Volume 25, Issue 12, pp. 929-1002

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Transport critical current in granular high-temperature superconductors

N. A. Bogolyubov

Low Temp. Phys. 25, 929 (1999); http://dx.doi.org/10.1063/1.593843 (7 pages) | Cited 2 times

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The temperature and size dependences of critical current in three Bi-based ceramic HTSC samples with a circular cross section and in a sample with a right triangular cross section in zero magnetic field are studied by a contactless technique. It is shown that the critical current of ceramic HTSC can be presented as the product of the temperature- and size-dependent factors. The temperature-dependent factor describes individual properties of the Josephson net of each sample, while the size-dependent factor is a homogeneous function whose exponent does not depend on the shape of the sample cross section. An analysis of experimental data is used to find the radial distribution of critical current density in round samples and to determine its dependence on the magnetic induction in granular HTSC. © 1999 American Institute of Physics.
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74.72.-h Cuprate superconductors
74.81.Bd Granular, melt-textured, amorphous, and composite superconductors
74.25.Sv Critical currents
74.50.+r Tunneling phenomena; Josephson effects

A crossover in the temperature behavior of the perpendicular upper critical magnetic field of layered superconductors and thin films

V. M. Gvozdikov

Low Temp. Phys. 25, 936 (1999); http://dx.doi.org/10.1063/1.593844 (7 pages) | Cited 4 times

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A mechanism which relates the upturn of the perpendicular upper critical magnetic field Hc2⊥(T) in layered superconductors and thin films with the structural inhomogeneity in the bulk of the sample, provided that the local critical temperature Tc∗ inside the inhomogeneity is higher than in the rest of the sample (Tc) is proposed. Within the Ginzburg–Landau approach an equation which describes two types of experimentally observed nonlinearities in Hc2⊥(T) near Tc for ISN (insulator-superconductor-normal metal) and NSN layer configurations, is found. In the NSN case a crossover from the linear branch Hc2⊥(T)∝(Tc−T), for fields H ⩽ Hm, to the nonlinear branch with the upturn, if H>Hm, takes place. The crossover field Hm is inversely proportional to the local enhancement of the critical temperature (Tc∗−Tc) and the distance R to the surface (the nearest surface, in case of a thin film). In the ISN case the upturn holds for H<Hm, whereas for higher fields Hc2⊥(T) crosses over to the linear branch. In the ISI case the Hc2⊥(T) is a linear function. © 1999 American Institute of Physics.
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74.25.Op Mixed states, critical fields, and surface sheaths
74.78.-w Superconducting films and low-dimensional structures
74.45.+c Proximity effects; Andreev reflection; SN and SNS junctions
74.10.+v Occurrence, potential candidates
74.20.De Phenomenological theories (two-fluid, Ginzburg-Landau, etc.)

Relaxation of the electric resistance of YBaCuO single crystals due to hydrostatic pressure and jumpwise temperature variation

M. A. Obolenskiĭ, D. D. Balla, A. V. Bondarenko, R. V. Vovk, A. A. Prodan, and T. F. Ivanova

Low Temp. Phys. 25, 943 (1999); http://dx.doi.org/10.1063/1.593845 (5 pages) | Cited 1 time

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The effect of hydrostatic pressure on pressure, temperature, and time dependences of the conductivity in the ab-plane of YBa2Cu3O7−x single crystals with different oxygen concentrations (0.1<x<0.5) is studied. It is shown that in oxygen-deficient samples with x>0.2, temperature and pressure variations may induce a nonequilibrium state in which effects associated with the variation of unit cell volume and redistribution of oxygen in Cu–O planes must be distinguished. Characteristic conductivity relaxation times under the effect of pressure and during annealing at room temperature are determined. It is concluded that these processes have the same origin. © 1999 American Institute of Physics.
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74.72.-h Cuprate superconductors
74.25.F- Transport properties
81.40.Gh Other heat and thermomechanical treatments
61.66.Bi Elemental solids
61.66.Dk Alloys
81.40.Rs Electrical and magnetic properties related to treatment conditions

Upper critical fields in superconductor–normal metal type superlattices in the Ginzburg–Landau approximation

V. N. Kushnir, A. Yu. Petrov, and S. L. Prishchepa

Low Temp. Phys. 25, 948 (1999); http://dx.doi.org/10.1063/1.593846 (5 pages) | Cited 5 times

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The application of the Ginzburg–Landau theory to the superconductor–normal metal–superconductor (SNS) superlattices is considered in the case when normal and superconducting layers have the same thickness. The temperature dependences of the transverse and longitudinal upper critical fields are considered. The theoretical curves are compared with the available experimental results on SNS superlattices [C. Coccorese et al., Phys. Rev. B57, 7922 (1998)]. It is shown that the theoretical model can provide a correct interpretation of the experimental results with the minimum number of fitting parameters. The peculiarities of the order parameter behavior at the dimensional crossover in a parallel magnetic field as a function of the sample symmetry axis are discussed, and practical recommendations are given for an experimental verification of the role of symmetry of SNS-type structures. © 1999 American Institute of Physics.
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74.78.Fk Multilayers, superlattices, heterostructures
74.20.De Phenomenological theories (two-fluid, Ginzburg-Landau, etc.)
74.25.Op Mixed states, critical fields, and surface sheaths
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Description of critical behavior of Ising ferromagnet in the ρ6 model approximation taking into account the confluent correction. II. Region below the phase transition point

I. V. Pylyuk

Low Temp. Phys. 25, 953 (1999); http://dx.doi.org/10.1063/1.593847 (9 pages)

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A scheme for calculating the thermodynamic characteristics of a 3D one-component spin system in the temperature range below the critical temperature is described on the basis of the sextic measure density (ρ6 model) taking into account the first confluent correction. A microscopic analog of the Landau free energy is calculated. The coefficients of average spin moment, leading critical amplitudes, and the amplitudes of the confluent correction to specific heat and susceptibility of the system are calculated for different values of effective radius of the exponentially decreasing interaction potential. Plots of temperature dependence of entropy and other thermodynamic characteristics in the vicinity of Tc are given for various values of effective radius. The variation of the free energy of the system at the phase transition point, average spin moment, and specific heat with increasing ratio of the potential effective radius to the constant of a simple cubic lattice is traced. The results of calculations and their comparison with the data obtained by other authors show that the ρ6 model provides a better quantitative description of the critical behavior of a 3D Ising ferromagnet than the ρ4 model. © 1999 American Institute of Physics.
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75.10.Hk Classical spin models
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
65.20.-w Thermal properties of liquids
65.40.gd Entropy
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)

Double-peaked character of the temperature dependence of resistance of perovskite manganites for a broadened ferromagnetic transition

A. I. Tovstolytkin, A. N. Pogorilyi, and S. M. Kovtun

Low Temp. Phys. 25, 962 (1999); http://dx.doi.org/10.1063/1.593848 (4 pages) | Cited 10 times

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The behavior of a system consisting of a mixture of paramagnetic semiconducting and ferromagnetic metallic phases is examined in the framework of the percolation model. As the temperature decreases below the Curie point, the paramagnetic phase fraction is assumed to decreases according to an exponential law. The evolution of the temperature variation of the resulting resistance is considered as a function of the magnetic transition broadening. An explanation is offered for the low-temperature resistance anomaly observed experimentally in a number of perovskite manganites. © 1999 American Institute of Physics.
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72.60.+g Mixed conductivity and conductivity transitions
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.-s Critical-point effects, specific heats, short-range order
75.50.Dd Nonmetallic ferromagnetic materials

Exact instanton solution for quantum tunneling in an uncompensated antiferromagnet

B. A. Ivanov and V. E. Kireev

Low Temp. Phys. 25, 966 (1999); http://dx.doi.org/10.1063/1.593849 (6 pages) | Cited 2 times

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An exact instanton solution describing macroscopic quantum tunneling for a small antiferromagnetic particle with uncompensated spin and biaxial quadratic anisotropy is constructed. The solution is valid for any relation between anisotropy parameters and relative value of uncompensated spin. The obtained solution is used for calculating the tunneling amplitude taking into account the pre-exponential factor. The amplitude is characterized by a nonanalytic dependence on the ratio of small parameters of the problem, viz., anisotropy in the basal plane and the value of uncompensated spin. © 1999 American Institute of Physics.
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75.45.+j Macroscopic quantum phenomena in magnetic systems
75.50.Tt Fine-particle systems; nanocrystalline materials
73.40.Gk Tunneling
75.30.Gw Magnetic anisotropy

Dynamics of domain walls and solitons in easy-plane magnets with weak exchange interaction

M. V. Gvozdikova and A. S. Kovalev

Low Temp. Phys. 25, 972 (1999); http://dx.doi.org/10.1063/1.593850 (7 pages) | Cited 4 times

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Internal spin dynamics of collinear domain walls and collinear soliton-like localized states in easy plane magnets is investigated. The dependence of internal mode frequencies of these excitations on the exchange interaction is determined using the essentially discrete Takeno–Homma model for low values of this interaction. © 1999 American Institute of Physics.
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75.60.Ch Domain walls and domain structure
75.30.Et Exchange and superexchange interactions
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
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Phenomenological model for Casimir attraction of a metal film

V. N. Dubrava and V. A. Yampol’skiĭ

Low Temp. Phys. 25, 979 (1999); http://dx.doi.org/10.1063/1.593851 (7 pages) | Cited 5 times

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The force of surface interaction (Casimir effect) between a bulk conductor and a metal film deposited on a dielectric substrate is studied by the method of quantum field theory. The film thickness is assumed to be much smaller than the skin depth at characteristic frequencies of fluctuation fields. The equations for one-particle Green’s function of the electromagnetic field in the metal film are solved on the basis of a simple phenomenological model. Namely, complex macroscopic electrodynamic properties of the film are described by introducing the surface permittivity determined by the conductivity 〈σ〉 averaged over sample thickness. The dependence of the Casimir attractive force on the specularity parameter ρ characterizing the interaction of conduction electrons with the film surface is predicted. The results of investigations demonstrate that the electronic and surface properties of metal films can in principle be studied experimentally by measuring the force of their Casimir attraction. © 1999 American Institute of Physics.
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73.61.At Metal and metallic alloys
03.70.+k Theory of quantized fields
77.22.Ch Permittivity (dielectric function)
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Characteristic energy losses of electrons in a two-dimensional electron gas in a magnetic field

A. M. Ermolaev and Babak Haghighi

Low Temp. Phys. 25, 986 (1999); http://dx.doi.org/10.1063/1.593852 (6 pages)

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The electron energy loss function is calculated in the random phase approximation for a two-dimensional electron gas in a quantizing magnetic field. Local states of electrons at impurity atoms are taken into consideration. The energy losses due to one-particle and collective excitations of two-dimensional electrons are determined. The activation of electrons localized at impurities leads to the emergence of steps on the dependence of loss function on the energy of an incident electron. Cerenkov losses associated with emission of magnetoplasmons appear starting from a threshold velocity of the electron. When the velocity exceeds the threshold value significantly, the losses are due only to spontaneous emission of magnetoplasmons. The corresponding loss function decreases in inverse proportion to the electron velocity. © 1999 American Institute of Physics.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
71.15.-m Methods of electronic structure calculations
79.20.Kz Other electron-impact emission phenomena
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.20.Hb Impurity and defect levels; energy states of adsorbed species
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Peculiarities of low-frequency excitation spectrum of CsDy1−xBix(MoO4)2

V. I. Kut’ko, S. S. Gerashchenko, and N. Yu. Nedbailo

Low Temp. Phys. 25, 992 (1999); http://dx.doi.org/10.1063/1.593853 (4 pages) | Cited 1 time

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Low-frequency IR transmission spectra for a series of isostructural single crystals of CsDy1−xBix(MoO4)2 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.5, and 0.7) are measured at low temperatures (T ≃ 6 K). It is found experimentally that absorption band broadening takes place in the Bi concentration range 0.2 ⩽ x ⩽ 0.6. The mechanism which may result in absorption spectrum blurring are considered. © 1999 American Institute of Physics.
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78.30.Hv Other nonmetallic inorganics
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On the theory of phase separation in systems metallized as a result of doping

M. A. Ivanov and V. M. Loktev

Low Temp. Phys. 25, 996 (1999); http://dx.doi.org/10.1063/1.593854 (3 pages) | Cited 3 times

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The mechanism of formation of an inhomogeneous ground state of weakly doped systems, consisting of the domains of metallic and insulating phases, is proposed. According to this mechanism, the formation of the charge-neutral metallic regions with the highest possible concentration of free charge carriers (and hence of the dopants generating them) is shown to be thermodynamically advantageous. © 1999 American Institute of Physics.
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71.30.+h Metal-insulator transitions and other electronic transitions

Sound attenuation in the superconducting amorphous alloy ZrTiCuNiBe

E. V. Bezuglyĭ, A. L. Gaiduk, V. D. Fil, W. L. Johnson, G. Bruls, B. Lüthi, B. Wolf, and S. V. Zherlitsyn

Low Temp. Phys. 25, 999 (1999); http://dx.doi.org/10.1063/1.593855 (4 pages)

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The superconducting energy gap and the parameter η determining the intensity of electron scattering at two-level systems in amorphous ZrTiCuNiBe are determined from the results of measurements of sound attenuation. The mechanism of adiabatic renormalization of the amplitude of coherent tunneling is used for a quantitative description of the peculiarities of sound absorption in the vicinity of Tc. © 1999 American Institute of Physics.
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74.81.Bd Granular, melt-textured, amorphous, and composite superconductors
74.25.Ld Mechanical and acoustical properties, elasticity, and ultrasonic attenuation
74.50.+r Tunneling phenomena; Josephson effects
62.65.+k Acoustical properties of solids
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