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Using the near-field microwave microscopy method, the temperature dependence of third harmonics power P3ω(T) of the MoN/Al superconductor–normal metal structures with the proximity effect is studied. The presence of a secondary (low-temperature) maximum and zeroings is detected in the P3ω(T) dependence of the MoN/Al structure, which disappear upon applying a weak magnetic field perpendicular to the plane of the structure.

—Temperature dependences of the third-harmonic power for the (Na0.3K0.7)xFe2 – ySe2 sodium–potassium ferroselenide single crystals have been investigated by nonlinear near-field microwave microscopy. The temperature dependence of magnetic susceptibility at a frequency of 100 kHz for this compound has been measured. The experimental results suggest a double-gap structure of the (Na0.3K0.7)xFe2 – ySe2 superconductor.

AbstractNarrow band emissions at 2.6–2.8 THz are observed out of liquid helium cooled 1 mm disk chips prepared of a wafer with the very low n type doped weak barrier GaAs–GaAlAs superlattice of 1000 periods. The emissions are at about 8.0–18.0 V pulsed voltage applied to the chips in region of the chips positive DC differential conductivity that guaranties absence of inhomogeneous electric field domains in the chips. The emission frequency bands are estimated with a cyclotron resonance filter; the measurements show that the band width is of about that of the THz quantum cascade laser. By using long voltage pulses the chip heating above 100 K is achieved without substantial change in emission power. We speculate that the emission is super luminescence (amplification) of whispering gallery modes in the chips as a result of inverted Wannier-Stark level transitions under bias. The results are the first world demonstration of THz stimulated emission in a simple superlattice within region of positive DC differential conductivity; they give strong impetus for development of THz and higher frequency sources based on such simple superlattices; the sources should well compete with the THz quantum cascade lasers in particular at elevated temperatures.

Annotation Within the framework of a simple two-band model, in the approximation of the method of effective parameters, the behavior of nonequilibrium carriers in solid solutions of the CdxHg1-xTe (MCT) type in an external electric field is considered. The consideration is carried out for the range of values of the parameter x, in which the specified compound demonstrates the behavior of a narrow-gap semiconductor. Estimates show that for the real parameters of MCT samples in a wide range of variation of the value of x (and hence the width of the band gap) at an electric field exceeding some characteristic value of the interband breakdown field (depending on the parameter x and other physical parameters) in such systems, it is possible to obtain an inverse population distributions of carriers by energy. Moreover, the magnitude of the amplification factor at direct interband transitions can reach values up to 101–102 cm−1. The response of samples prepared from thick (not quantum-dimensional in terms of their thickness) MCT films to an external pulsed electric field is studied experimentally. The field was applied to the samples using ohmic contacts. The current-voltage characteristics of such samples and the dependences of the radiation intensity from the samples on the amplitude of the applied voltage pulses were obtained experimentally. The characteristic features of the responses obtained experimentally in these systems are discussed, as well as the possible prospects and difficulties for obtaining the laser effect using interband breakdown in an electric field.

We present the results of creating a cooled microbolometer based on the cermet films of the silicon and chromium mixture. This material is used for manufacturing the freely hanging high-resistive microbolometers for the first time. The details of fabricating such microbolometers and the prospects for using cermet films to construct microbolometers are discussed. The first estimates of sensitivity of the fabricated microbolometers are given.

Features of a phase transition between 0 and π states in superconductor/ferromagnet/superconductor (SFS) Josephson structures with thin superconducting layers and a ferromagnetic barrier are studied experimentally and theoretically. The dependence of the critical temperature T c of a transition of the hybrid structure to a superconducting state on the thickness of superconducting layers d s is analyzed by a local method involving measurements of the nonlinear microwave response of the system by a near-field probe. An anomalous increase in the measured temperature T c at the reduction of the thickness d s is detected and is attributed to the 0-π transition.

We report the effect of disorder on superconducting phase transition of YBa 2 Cu 3 O 7 − x epitaxial thin films in external magnetic fields. The disorder was produced by several successive acts of oxygen ion implantation. Controlling a total accumulated dose of implanted ions nD, we carried out transport measurements in ab-plane for temperatures T below 91 K and external magnetic fields H up to 11 T. Temperature-field dependencies of in-plane resistivity allow us to analyze H − T phase diagrams for primary compound as well as for disordered structure. Considering the upper critical field Hc2 as the magnetic field which corresponds to a local resistivity drop at the onset of superconducting transition, we found out the following results. By gradual increasing of nD, the phase-transition line Hc2(T) suffers an unconventional critical-field slope reduction, while larger defect concentrations usually enhance the upper critical field in the vicinity of Tc0. Besides, for rather large nD, the curvature of Hc2(T) becomes upward for temperatures close to Tc0. Theoretical interpretation of the experimental data is developed in the framework of linearized Ginzburg-Landau theory with an inhomogeneous superconducting coherence length. A simple expression for the critical temperature Tc is obtained: T c = T c 0 ( 1 − h + α h 3 / 2 ) , where h is the dimensionless magnetic field and α is a constant which describes the defects in a specimen. The formula nicely fits our experimental results.

New intraband semiconductor lasers—Wannier–Stark lasers—based on simple GaAs (150 A, quantum well)/GaAlAs (19 A with an aluminum fraction of 12%, barrier) superlattices have been demonstrated. The amplification mechanism in these lasers is based on population inversion between the ground Wannier–Stark level in the superlattice quantum wells and the weakly populated upper Wannier–Stark level in the wells two, three, or four periods down in the applied potential. Multiple regions of intense stimulated microwave emission near voltages of 8, 13, and 20 V (i.e., in the vicinity of resonances between these Wannier–Stark levels of the superlattice) have been discovered in the laser chips. The stimulated emission emerges in the circuit formed by the chip and its wiring. The emission from one of the chips at a temperature of up to 150 K (near 20 V applied to the chip) occurs at a frequency of about 7.3 GHz and has an estimated power of up to 1 W. It has been shown that the negative conductivity responsible for the emission still persists at 300 K but the emission is unseen owing to high losses in the circuit at this temperature. The superlattice wafer has been grown by metalorganic chemical vapor deposition. It consists of 1000 periods and a stop layer, to produce a metal–superlattice–metal terahertz resonator. Terahertz radiation has not been observed owing to a low amplification, as compared to losses in the resonator. According to the performed experiments, calculations, and discussions, such superlattices as radiation sources in gigahertz, terahertz, and higher frequency ranges could compete with quantum cascade lasers under appropriate optimization of their parameters.

The results of complex analysis of the parameters of the GaAs/AlGaAs semiconductor superlattice with a complex unit cell using mathematical modeling and experimental methods of transmission electron microscopy, energy-dispersive analysis, and spectroscopy of photoluminescence and photocurrent are presented. Suppression of the intraband static negative differential conductivity under strong interaction conditions between minibands and the inharmonic electron-dispersion law is shown.

Using a double-layer Cd x Hg 1− x Te structure, we demonstrate stimulated emission at two wavelengths simultaneously, around 2 μm and around 3 μm under optical excitation by a pulsed Nd:YAG laser at temperatures T =77–150 K. Emission spectra of the structure for various temperatures are presented.

Emissions at 2.6 – 2.8 THz are observed from liquid helium cooled disk chips with metal - superlattice - metal cavities made of two low n type doped wafers with weak barrier GaAs-GaAlAs superlattice of 1000 periods. The emissions are at 8.6 – 18.8 Volts within region of rising chip current (at positive chip differential conductivity) that guarantees absence of inhomogeneous electric field domains. At 2.6 – 2.7 THz the emission tunable by 8 – 10 Volts voltage change is higher than the Bloch oscillations frequency; while at 2.8 THz at 12 – 18 Volts it is lower than ω B , The THz frequency bands are measured with a cyclotron resonance filter; the band width is of about that of the THz quantum cascade laser. By using long voltage pulses the chip heating above 100 K is achieved without substantial change in chip THz emission power. We speculate that the emission is super luminescence (amplification) of whispering gallery modes in the chips as a result of inverted Wannier-Stark level transitions under bias. The results advocate development of THz and higher frequency sources based on such simple superlattices; the sources should well compete with the THz quantum cascade lasers in particular at elevated temperatures.

Dual-wavelength stimulated emission from a double-layer CdxHg1 − xTe heterostructure optically pumped by a pulsed Nd:YAG laser at temperatures T = 77–150 K is reported. The emission spectral lines have been observed at wavelengths λ1 ∼ 2 μm and λ2 ∼ 3 μm. Emission spectra recorded at different temperatures are presented.

The results of studies of MOCVD growth regularities of GaAs/AlGaAs superlattices with narrow forbidden minibands are presented. The spectra of photoluminescence and X-ray diffraction are measured, the concentration distribution profiles of components are determined by secondary-ion mass spectrometry, and the concentration distribution of charge carriers are determined by the method of capacitance profiling. The relation of the growth modes of heterostructures to their crystalline characteristics and luminescent and electrical properties are studied. The photoluminescence measurements indicate the high quality of the superlattices. X-ray diffraction and the data on secondary ions confirm the high periodicity of the superlattices grown in the optimized modes. The nonlinear current-voltage characteristics with a region of negative differential conductivity at moderate voltages and subsequent current increase at higher voltages because of tunneling between the minibands is found for the superlattices grown in the optimal growth modes. Current oscillations at frequencies of ∼60 MHz were observed in the region of negative differential conductivity. The negative differential conductivity and oscillations confirm the presence of the electron localization effect in moderate electric fields in the first conductivity miniband, which emerges due to the Bragg reflection of carriers in the superlattice.

The time evolution of the magnetization normal component change in a garnet film with a labyrinthine domain structure under the action of circularly and linearly polarized laser pump pulses (the pulse duration is 5 ns; the wavelength is 527 nm) has been studied. The dynamic state of the magnetic film was registered using an induction method with a time resolution of 1 ns. It was found that for the initial state of the magnetic film with an equilibrium domain structure, the form of the photomagnetization pulse reflects the time evolution of a domain-wall motion. The domain-wall motion initiated by the circularly polarized laser pump pulse continues in the same direction for a time more than an order of magnitude exceeding the laser pulse duration. In general, the time evolution of the domain-wall movement occurs in three stages. The separation of the contributions to the photomagnetization from the polarization-dependent and polarization-independent effects was carried out. The photomagnetization pulses that reflect the contributions by the aforementioned effects differ by form, and more than two orders of magnitude by duration. Their form doesn't change under a magnetic bias field change, only the photomagnetization pulse amplitude does: for the polarization-dependent contribution, it's an even function of the field, and for the polarization-independent contribution, it's an odd function. The interconnection between the polarization-dependent and polarization-independent effects, on the one hand, and the domain-wall displacement and the change of the film's saturation magnetization, on the other hand, was identified and described.

Main requirements for the optimization of CdxHg1−xTe (MCT) structures with a view to increasing the wavelength of stimulated emission under optical pumping are discussed. A 2–2.5 μm stimulated emission from optimized MCT structures is observed experimentally at room temperature. The measured values of the gain in the active medium amount to 50 cm−1 at a 2 μm emission wavelength.

Experimental and theoretical studies of ferromagnet-superconductor hybrid systems consisting of a Josephson junction placed in the nonuniform field of submicron ferromagnetic particles are reviewed.

The experimental and numerical studies of the transport properties and wave functions of electrons in GaAs/AlxGa1-xAs superlattices with weak barriers in a strong electric field demonstrate the possibility of terahertz lasing on transitions between Wannier — Stark ladders, which can be tuned by applying voltage. The calculated gain can be as high as 500 cm-1, which exceeds several times the values characteristic of modern THz quantum cascade lasers. Thus, one would expect lasing to occur in a simple n+ — superlattice — n+ stripe structure. Such lasers, due to the possibility of wide tuning and simplicity of the superlattices they are based on, could compete with cascade lasers.

The main requirements for optimization of the MCT-based (CdxHg1 − xTe-based) structures for an increase in the wavelength of the stimulated radiation from them under optical pumping are discussed. The emergence of stimulated radiation in MCT range of 2–2.5 μm at room temperature for optimized MCT structures grown on GaAs substrates using the method of molecular beam epitaxy is shown experimentally. The obtained experimental data are the first results of observation of stimulated radiation from the MCT structures at these wavelengths at room temperature. The gain factor in the active medium measured for this case is very large and attains values of 50 cm−1, which allows one to hope that a further advance to a longer wavelengths is possible.

We report stimulated near-IR emission in optically pumped InGaAs/AlGaInAs heterostructures and stimulated- emission wavelength switching from 1.9 to 1.5 and then to 1.2 μm with increasing optical pump intensity. The wavelength switching behaviour of the heterostructures depends on their geometry (band-gap profile) and the competition between stimulated emissions at different frequencies in different parts of the system.

The results of experimental investigations for the I–V characteristics are presented for superlattices based on GaAs/AlxGa1−xAs with thin barriers in the electric-field region with an intense interminiband tunneling. The regular features for the I–V characteristics are found in the voltage range adequate to the static positive differential conductivity in the superlattice. On the basis of calculations for the Wannier-Stark levels, it is established that the observed features are associated with the resonant tunneling between these levels belonging to quantum wells located at a distance from 6–13 superlattice periods from each other. It is noted that the similar resonant delocalization of Wannier-Stark wave functions can lead to the existence of dynamic negative differential conductivity for the laser type of an appreciable value in such superlattices.

Current-voltage characteristics have been measured and low-frequency current instabilities have been studied for GaAs/Al x Ga1−x As superlattices with narrow forbidden minibands. At relatively low electric fields, a saw-like structure for current-voltage characteristics with alternating portions of positive and negative differential conductivity and spontaneous generation of low-frequency current oscillations with a complex frequency spectrum (varying from discrete to continuous) are observed. It is shown that the observed specific features of electron transport are caused by the spatial-temporal dynamics of electric-field domains (dipoles and monopoles). The effects of the bifurcation, hysteresis, and multistability of current-voltage characteristics are also observed. At high fields, regular features are observed and identified in the current-voltage characteristics; these features are caused by resonance tunneling of electrons between the levels of the Wannier-Stark ladders belonging to quantum wells separated by several periods.

A comprehensive investigation and comparison of the superconducting properties of bilayer and multilayer epitaxial heterostructures of IV–VI semiconductors exhibiting superconductivity at critical temperatures Tc⩽6.5K is carried out. The superconductivity of these systems is due to inversion of the bands in the narrow-gap semiconductors on account of the nonuniform stresses created by the grids of misfit dislocations arising at the interfaces during the epitaxial growth. It is found that Tc and the character of the superconducting transition of bilayer PbTe∕PbS heterostructures depend on the thickness d of the semiconductor layers and are directly related to the quality of the grids of misfit dislocations at the interfaces (the number and type of structural defects in the grids). Substantial differences in the behavior of bilayer sandwiches and superlattices are found. The minimum thickness d at which superconductivity appears is several times larger for bilayer than for multilayer systems. The upper crit...

The experimental observation of stimulated radiation of optically pumped CdxHg1 − x Te-based heterostructures in the wavelength range of 1.4–4.5 μm is reported. In the experiments, graded-gap Cd xHg1 − x Te samples grown on GaAs and Si substrates by molecular beam epitaxy were used. Superluminescence of such structures was observed at 77–300 K under the pulsed pumping of the samples by a Nd:YAG laser at a wavelength of 1.064 μm. At room temperature, stimulated radiation was observed at wavelengths of 1.4–1.7 μm. The obtained experimental data are the first results on the observation of stimulated radiation from graded-gap Cd x Hg1 − x Te structures on Si and GaAs substrates at these wavelengths at room temperature.

Low temperature electron transport under interminiband Zener tunneling and LO-phonon scattering in the second and higher minibands in GaAs/Alx Ga1ix As superlattices with weak barriers has been studied. At low electric fields multiple negative differential low-frequency conductivity regions and effects of complicated electron transport have been found. In stronger fields peculiarities of current - voltage curves attributed to resonant tunneling between Wannier-Stark levels of different minibands have been separated basing on calculated wavefunctions and energies of the levels. Calculated value of optical matrix element for inter-Wannier-Stark transitions is promising to achieve negative conductivity at these transitions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The local nonlinear microwave response of YBa2Cu3O7 − x films was measured by near-zone field microscopy with a spatial resolution of 50 μm, and YBa2Cu3O7 − x film microbridges were locally studied by low-temperature scanning microscopy with a spatial resolution of 4 μm. The microstructure of epitaxial YBa2Cu3O7 − x films was examined using x-ray diffraction and electron microscopy. A correlation is detected between the average crystallite size and the half-widths of the temperature dependences of the third-harmonic power (W TH) and the electron-beam-induced voltage (W EBIV). The experimental results are described in terms of a model of a two-phase medium taking into account the nonlinear I-V characteristic of the superconductor. For large crystallites, the nonlinear microwave response is shown to be caused by intracrystallite vortex pinning. As the average crystallite size decreases, an additional contribution to the nonlinear response appears due to the pinning of a magnetic flux by the Josephson network of crystallite boundaries. Calculations show that a three-fold increase in the crystallite size decreases the nonlinearity coefficient of YBa2Cu3O7 − x films by two orders of magnitude.

The experimental data on observation of spontaneous and stimulated emission from thin epitaxial CdxHg1−x Te films optically pumped by Nd: YAG laser radiation are reported. A simple theoretical model is suggested to describe the initiation of population inversion under these conditions. The parameters realized under the experimental conditions are theoretically estimated.

The properties of Mn-doped GaAs layers grown by laser deposition were investigated with measurements of Hall effect and magneto-optical Kerr effect (MOKE). The electrical and magnetic parameters of the layers were defined by growth temperature and quantity of sputtered Mn. It was shown that room-temperature ferromagnetism is revealed by MOKE and, after ruby laser 25 ns pulse annealing, by Hall effect measurements.

The effect of an array of ferromagnetic nanoparticles on the field-dependent critical current of the short overlap Josephson junction is experimentally studied. Large reversible variations of the maximum critical current are observed depending on the magnetic state of the particles. The pronounced commensurability effects are detected which are proved by the additional peaks of magnetic field induced diffraction pattern.

A photomagnetization effect is observed for a (Tm,Bi)3(Fe,Ga)5O12 garnet film at T = 300 K due to the restructurization of its multidomain structure. Photomagnetization (ΔM) emerges under the action of circularly polarized laser radiation in the spectral range of 450–600 nm at lasing power P < 600 kW/cm2 and pulse duration τi ∼ 7ns. The dependences of ΔM are measured on the polarization of light and magnetic field, the latter being perpendicular to the surface of the film. The maximum value of ΔM is observed for the circular polarization of light without a magnetic field. Moreover, ΔM changes sign with as the circular polarization changes sign and is zero for linear polarization.

A method for diagnostics of the interaction of electron states in multilayer heterostructures in a dc electric field is suggested. The method is based on measurements of the resonance response of the harmonic amplitude of the current in a heterostructure to modulated laser radiation. The method is used for studying tunneling between the Wannier-Stark levels in the superlattice with strongly coupled quantum wells.

Stimulated emission of radiation from CdxHg1−xTe samples pumped by a Nd:YAG laser at temperature T≈77 K is obtained. Both spontaneous and stimulated emission is observed for the wavelengths λ in the range 3300–3600 nm. Experimental emission spectra are presented. The special features of the spectra and the possible applications of the structures are discussed.

The effect of a system of ferromagnetic particles on the field-dependent critical current of a Josephson junction is experimentally studied for junctions of different geometries. For edge junctions, the effect of commensurability between the periodic magnetic field of the particles and the Josephson vortex lattice is observed. The effect manifests itself in additional maxima of the field-dependent critical current. For overlap junctions, giant (greater than sixfold) variations of the maximum critical current are observed depending on the magnetic state of the particles. The changes in the “Fraunhofer” pattern of the overlaped Josephson junctions are attributed to the formation of Abrikosov vortices due to the effect of uniformly magnetized particles. The effects revealed in the experiments can be used to analyze the inhomogeneous magnetic field of a system of submicron particles and to control the transport properties of Josephson junctions.

The surface morphology and superconducting properties of YBaCuO epitaxial films prepared through magnetron sputtering from targets of different cation composition were systematically studied. It was shown that small changes in the growth conditions and relatively small variations in the cation composition of the condensate noticeably affect the surface morphology of the films and their structural and superconducting properties, thus offering an efficient way of controlling the YBCO film parameters. It was found that the 90° off-axis configuration of the magnetron sputtering system permits realization of growth conditions in which the grown films do not contain CuO precipitates and exhibit good superconducting properties (Tc≥88 K, jc(77 K)≥4×106 A/cm2).

Magnetic field measurements above type-II superconductor films in a remanent magnetization state have been used to determine two dimensional surface current distributions. Using the Biot-Savart law for the surface current density, integral equations (of 2-D convolution type) for two components of current have been obtained. These equations have been solved by employing Tikhonov's method of generalized discrepancy, and the surface current pattern in superconductors as well as the magnetic field distribution on the film surface have been obtained. Current peculiarities related to various kinds of inhomogeneities, specific character of the film form, and a laser pulse effect have been observed. In particular, it was discovered that a laser pulse can signficantly redistribute the film currents without changing the total number of current vortices in the film.

Using a local technique, we study the microwave radiation power P3ω(T,Pω,Hdc) at the triple frequency of the main signal as a function of the temperature T, the input power Pω, and the external magnetic field Hdc perpendicular to the superconductor surface for YBa2Cu3O7 (YBCO) films, monocrystals, and polycrystals and for Nb films. The most distinctive feature of the temperature dependence P3ω(T) of nonlinear response of superconductors is a maximum of the nonlinearity near the critical temperature Tc. Spatial distributions of the third-harmonic power are obtained for YBCO films at various temperatures. These distributions are indicative of a nonuniform distribution of Tc over the superconductor surface. Additional nonlinearity maxima are discovered in the dependence P3ω(T) for YBCO films and monocrystals at temperatures about 2Tc/3. These maxima are probably related to the existence of several superconducting phases with different critical temperatures. For Nb films, the second nonlinearity maximum in the dependence P3ω(T) appears only in the presence of an external magnetic field. The experimental data are interpreted within the framework of a two-fluid model of a superconductor, which takes into account the phenomenological nonlinear relationship between the vector potential A and the supercurrent js(A). The origin of nonlinearity in the studied superconductors is discussed.

HTS thin films continue to be of interest for passive device applications at microwave frequencies, but nonlinear effects may limit their performance. We report a set of experimental data on local third-harmonic generation at microwave frequencies (0.5 GHz) in YBa/sub 2/Cu/sub 3/O/sub 7/ and Nb thin films from 4.2 K to T/sub c/. For local investigations of the nonlinear response a probe with inductive coupling was developed. The map of the nonlinear microwave response of YBa/sub 2/Cu/sub 3/O/sub 7/ thin film is plotted below T/sub c/ with high resolution. The third-harmonic power is measured as a function of temperature, input power and dc-magnetic field on some places of the film. The temperature dependence demonstrates a peak of nonlinearity below T/sub c/ for YBa/sub 2/Cu/sub 3/O/sub 7/ and Nb films and a minimum of nonlinearity at low temperatures for HTS films. The behavior of the third-harmonic power vs the input power can be described by the power-law with the exponent n/spl sim/1.5-3. The correlation between the depinning current density J/sub p/ and the nonlinear microwave response is also demonstrated.

This work is concerned with the behaviour of a regular 2D rectangular lattice of Ni3Fe nanoparticles, determined by the dipole interaction between them. The samples under study, prepared by electron-beam lithography, consisted of about 105 particles approximately 50 nm in size. The magnetization curves were studied by Hall magnetometry for different external magnetic field orientations at 4.2 K and 77 K. The results indicate a collective behaviour of the system. The magnetization curves depend on the external magnetic field direction and temperature; the system exhibits multistability. A model system of interacting 3D magnetic dipoles forming a rectangular lattice was numerically simulated by solving a system of stochastic Landau-Lifshitz equations. The multistability of the system and steps in the magnetization curves were obtained. It is shown analytically that the shape of the magnetization curves depends on the character of the interaction in the system.

The temperature dependence of the local energy barrier to formation of the mixed state in YBaCuO thin-film superconducting samples has been determined. The measurement technique is based on use of a small ferromagnetic particle as the magnetic field source. It is found that the energy barrier to creation of vortices (for the field oriented parallel to the CuO planes) is anomalously small while the dependence of the corresponding threshold current jc(T) differs substantially from the temperature dependence of the pair-breaking current. The experimental results are interpreted in terms of the model of a Josephson medium. The observed temperature dependence of jc points to a strong suppression of the superconducting order parameter at the intergrain boundaries, which for the most probable type of boundaries: superconductor-insulator-superconductor, is evidence of anisotropic pairing.

Direct local measurements of the characteristics of a mixed state induced by a magnetic field of a small ferromagnetic particle have been carried out in thin film YBaCuO samples, using a scanning Hall probe. The results provide evidence for an extremely low energy barrier for entry of vortex half-loops into high quality films and further breaking of these half-loops into vortex-antivortex pairs. The observed phenomena are consistent with the Josephson medium model. We obtained the temperature dependence of the intergrain critical current density, which points to strong pair breaking effects at the grain boundaries.

Direct local measurements of the characteristics of a mixed state induced by a magnetic field of a small ferromagnetic particle have been carried out in thin film Y-Ba-Cu-O samples, using a scanning Hall probe. The results provide evidence for an extremely low energy barrier for entry of vortex half-loops into high quality films and further breaking of these half-loops into vortex-antivortex pairs. The observed phenomena are consistent with the Josephson medium model. We obtained the temperature dependence of the intergrain critical current density, which points to strong pair breaking effects at the grain boundaries.

Collective effects arising in a two-dimensional lattice of nanosize magnetic particles as a result of the dipole interparticle interaction are investigated by Hall magnetometry. The experimental system consists of 105 permalloy particles having a diameter of ∼40 nm and a height of ∼40 nm and forming a lattice with a rectangular unit cell (90 nm×180 nm). We attribute the characteristic features observed in the magnetization curves to quasi-one-dimensionality of the experimental lattice of particles and to the formation of solitons in chains of dipoles.

The X-ray data, DC and microwave properties as functions of the YBaCuO epitaxial films thickness in the range from 5 to 300 nm are investigated. The films were deposited by laser ablation on LaAlO/sub 3/ substrates. Films over 9 nm thickness have T/sub c0/=89 K, J/sub c/(77 K)>3 MA/cm/sup 2/ and the thickness dependence of their effective microwave surface resistance R/sub eff/ at 77 K, 10 GHz is in good agreement with the local electrodynamic theory for the thickness-independent intrinsic surface resistance R/sub s/=0.25 m/spl Omega/ and penetration depth /spl lambda/=250 nm. Decreasing a film thickness to less than a few tens nanometers provides a decrease of orthorhombicity parameter of YBCO cell, and to less than 9 nm - a reduction of transition temperature. The ultrathin films showed no degradation during thermocycling without a protection layer.

The crystal structure and transport properties of epitaxial c-oriented YBa2Cu3O7−x films are investigated for high-T c layer thicknesses from 5 to 300 nm. The films were prepared by laser deposition. Films less than 30 nm thick become predominantly single-domain in the direction of the c axis. As the thickness decreases, the orthorhombicity parameter of the YBaCuO lattice decreases, which correlates with the critical temperature degradation observed in films less than 9 nm thick. The obtained thickness dependence of the effective microwave surface resistance of a YBaCuO film agrees well with the computational result obtained in the framework of local electrodynamics for samples with a constant microwave conductance.

The results are reported on the observation of the induced emission from CdxHg1−x Te samples optically pumped by a pulsed Nd:YAG laser at temperatures T

Edge-type Josephson junctions with a silicon nitride spacer possessing small capacitance have been fabricated at the butt-joint of two superconducting niobium films. The main results of experimental investigation of the electrical properties of these structures are presented.

Transport properties of GaAs–GaAlAs weak barrier superlattices are studied by computer simulation and observation of current–voltage curves and terahertz emission. Existence of terahertz dynamic negative differential conductivity is demonstrated in the simulation. The observed terahertz emission is considered as due to incoherent Bloch oscillation emission. Prospects for the terahertz Bloch oscillator based on such superlattices are discussed.

The experimental results of the film parameter evaluation based on recording detective coil signal produced by the film in external or remanent magnetic field subject to a laser pulse at any specific spot in the film are presented. The mechanisms of the film response to the laser irradiation based on Bean-like model for the inhomogeneous superconductive film are discussed. >

YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCo) thin films on R-plane sapphire up to 5-cm-diam with ZrO/sub 2/:Y (YSZ) buffer layers were fabricated by in-situ off-axis DC magnetron sputtering-and laser ablation. The film DC properties, critical current density (J/sub c/), effective microwave surface resistance (R/sub s/), X-ray data and surface morphology as functions of deposition temperature (T/sub d/) from 600 to 750/spl deg/C were studied. We have found out that the two processes simultaneously and in different ways affect the YBCO layers superconducting properties versus T/sub d/. The results show that films deposited at both ends of T/sub d/ range have equal critical temperatures but their J/sub c/ and R/sub s/ differ up to an order of magnitude. High-homogeneity 5-cm-diam films with J/sub c/>4 MA/cm/sup 2/ and R/sub s/ >