Anomalies of the photo-response and thermal boundary resistance of a YBaCuO/YSZ structure

The photoresponse of a YBaCuO/ZrO₂ bolometric structure was measured under modulated (λ = 630 nm) and pulsed (τ ≈ 7 – 8 ns; λ = 337 nm) laser excitation. The shape of the measured photoresponse was interpreted by a thermal model; nevertheless, the pulse amplitude for vanishing YBaCuO film resistance was 5–6 times greater than predicted; the thermal boundary resistance R_Bd between YBaCuO and YSZ was evaluated ? 10^?2 K × cm²/Watt, which is considerably larger than estimated theoretically for the similar situation of YBaCuO/MgO [5].     

Burning rate of homogeneous energetic materials with thermal expansion and varying thermal properties in the condensed phase

We present a study of the one-dimensional flame structure of combusting solid propellants that focuses on the effects of thermal expansion and variable thermal properties in the condensed phase. A nonlinear heat equation is derived for a burning thermo-elastic solid with temperature-dependent specific heat, thermal expansion, and thermal conductivity coefficients. It is solved for different modelling approximations both analytically and numerically. Explicit expressions are derived for the regression rate of the propellant surface as functions of surface temperature and thermal expansion parameters. A simple one-step reaction model of the gas phase is used to study the full structure of propellent flame and illuminate the influence of temperature-dependent material properties on the regression rate, surface temperature, and flame stand-off distance. Results are displayed for HMX and compared with experimental data and numerical simulation with fair success.     

On the use of high-Tc superconductors for Perot-Fabry mirrors coatings

In a very rough experiment using two MgO plates, coated with a 200 Å thick YBaCuO film to increase their reflectivity, and facing each other their YBaCuO films to make a Perot-Fabry (PF) interferometer, we have observed a 43% maximum transmission and a finesse around 33 at v=29 cm^–1, in good accordance with a new phenomenological model for YBaCuO. Much higher performances can be expected by optimizing the YBaCuO film thickness.     

Structure and transport properties of ultrathin YBa2Cu3O7−x films

The crystal structure and transport properties of epitaxial c-oriented YBa₂Cu₃O_7−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. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 608–613 (25 April 1996)     

Macrostructure of high-temperature superconducting YBaCuO thin films grown by laser ablation

The superconducting parameters and macrostructure of YBaCuO thin films grown by laser ablation on SrTiO₃ substrates are influenced by the substrate temperature during growth. In a study of this influence it is found that the macrostructure has significant bearing on the superconducting parameters of the films and on the critical current, in particular. For J _c⩽10⁵ A/cm² the films have a distinctly pronounced granular structure, while for J _c>10⁵ A/cm² the films do not have any kind of block structure. Zh. Tekh. Fiz. 68, 48–51 (February 1998)     

Critical thickness and stress relaxation in YBaCuO (123) strained epitaxial layers and YBaCuO based strained superlattices

An energy model has been used to calculate the critical thickness h _c of YBaCuO thin films and YBaCuO based superlattices within an isotropic or anisotropic approximation. The critical thickness of single layers calculated from the anisotropic model (16 nm) is in good agreement with the previously published experimental values which are spread out from 4 to 20 nm. In the case of superlattices, relaxation appears to be governed by the critical thickness of the elementary sub-layers and is then better evaluated through the calculation performed for YBaCuO single layers. XRD measurements on YBa₂Cu₃O₇/PrBa₂Cu_3?xGa_xO₇ superlattices grown on {100{ SrTiO₃ have evidenced a tetragonal stress in the YBaCuO ab plane which remains expanded when the YBaCuO elementary layer thickness is lower than 4.8 nm (4 YBaCuO cells). However the critical temperature of the shortest period superlattices is only slightly affected by this expanded stress in contrast to the effect of an elastic stress externally applied along the ab plane of YBaCuO thin films.     

Study of the critical current in YBaCuO films under reactor-neutron irradiation in the low-temperature helium circuit in the 25–300 K range

Changes in the temperature and dose dependences of I-V characteristics and of the critical current j _c of YBaCuO films on MgO and SrTiO₃ substrates under neutron irradiation has been studied at 25–80 K. The transport properties of YBaCuO films on MgO (M1) and SrTiO₃ (S1) substrates were found to behave differently. It is shown that the M1 films have granular structure, and their transport properties were considered within the concept of percolation over weak intergranular links. The S1 are single-crystal films, and their properties are analyzed within a resistive-state theory associated with thermal activation of Abrikosov vortices. It is shown that the degradation rate of the critical temperature T _c of S1 films is 3.5 times smaller than that of the M1 films. The dose dependence of j _c has an exponential character, j _c=j _c(0)exp(−kΦ), where k is related to the number of displaced atoms per neutron and is the same for the M1 and S1 films, irrespective of the irradiation temperature. The pinning energy has been derived from the I-V characteristics and it has been found that the U/kT ratio lies within 20–25 and is independent of neutron flux. It is shown that the radiation-induced disorder changes the pinning mechanism, from pinning at the boundaries of misoriented crystals to that at spatial inhomogeneities, apparently radiation induced. Fiz. Tverd. Tela (St. Petersburg) 40, 1961–1967 (November 1998)     

High pressure cell for neutron diffraction investigations

Abstract

The design of a titanium-zirconium clamped cylinder-piston type pressure cell for neutron diffraction investigations under hydrostatic pressure up to 10 kbars without supports is described. It is the first time that Freon-11 has been used as a hydrostatic pressure transmitting medium. The following results carried out at the room temperature are presented: the discovery of the transition from I to 111 phase in a LiKSO₄ single crystal and the results of the investigation on the influence of pressure upon the structure of the YBaCuO HTSC obtained by means of the powder diffraction method.     

RAA Electron-Phonon Interaction Cannot Support High TC in YBaCuO

The e-ph interaction in YBaCuO is calculated and the nonlinear Eliashberg equation is solved by the newly introduced bifurcation theory and parameter imbedding method. The transition temperature is low from the traditional (RA.4) e-ph model. The cause of some authors' improper high T_C result is discussed.     

Thermal stabilization of anomalies in inhomogeneous conducting structures

We study the thermal mechanism of suppression of the anomalies in the nonlinear characteristics of inhomogeneous media. A generalized expression for the effective nonlinear conductivity is derived that allows for heat transfer from hot regions. We study the nature of the divergences in two-and three-dimensional inhomogeneous structures as depending on the local parameters and the microgeometry of the system. Finally, we show that in the critical region the effective nonlinear conductivity may be much higher than the conductivity of the components. Zh. éksp. Teor. Fiz. 115, 1818–1832 (May 1999)     

Specific heat of magnetic clusters with S=2 YBa2Cu3O7−δ

An analysis is made of published data from measurements of the temperature-and-field dependence of the specific heat in YBaCuO high-temperature superconductors at temperatures below 10 K. It is shown that the “excess” contribution to the specific heat can be explained by spin splittings in copper-oxygen clusters with S=2. The magnitudes of the necessary splittings agree with the available magnetic resonance data. Fiz. Tverd. Tela (St. Petersburg) 39, 1320–1322 (August 1997)     

Fordy-Kulish model and spinor Bose-Einstein condensate

Abstract

A three-component nonlinear Schrodinger-type model which describes spinor Bose-Einstein condensate ( BEC) is considered. This model is integrable by the inverse scattering method and using Zakharov-Shabat dressing method we obtain three types of soliton solutions. The multi-component nonlinear Schrödinger type models related to symmetric spaces C.I ? Sp(4)/U(2) is studied.     

Ultraviolet detectors based on ZnO films by thermal oxidation of Zn metallic films

Metallic Zn films were deposited on glass substrates by electron-beam evaporation. ZnO films were synthesized by thermal oxidation of Zn metallic films in air. At the annealing temperature of 550 °C, ZnO nanowires appeared on the surface, which mainly result from the decrease of oxidation rate. A ZnO ultraviolet photodetector was fabricated based on a metal-semiconductor-metal planar structure. The detector showed a large UV photoresponse with an increase of two orders of magnitude. It is concluded that promising UV detectors can be obtained on ZnO films by thermal oxidation of Zn metallic films. The ways of performing spectral response measurements for polycrystalline ZnO films are also discussed.      

The photovoltaic effect in poly(alkylthiophene) films on a silicon substrate

A study of photovoltage was made for a series of sandwich structures on the basis of poly(3-dode-cylthiophene) films having characteristic thicknesses 100 and 500 nm and being deposited on n-Si and p-Si substrates from a solution. Semitransparent Al and Au electrodes were obtained on the surfaces of these films by thermal evaporation. A clear photoresponse was obtained in films on an n-Si substrate. Two distinct spectral components of the photovoltage were observed in the 1.3-to 3.6-eV (900–300 nm) energy range for incident quanta. The first component corresponds to the absorption edge of the Si substrate (1.4–1.6 eV). The other corresponds to the π-π* absorption of the polythiophene films (1.7–2.1 eV). The dependences of the photovoltage upon radiation intensity are different for these two spectral components. The relaxation time of the photoresponse for the second component, corresponding to the absorption in the film, is 10–20 min. This is 3–4 orders of magnitude higher than the relaxation time for the first component. A model of the potential barrier at the polythiophene/n-Si interface, allowing one to explain the main experimental results, is proposed. This barrier is formed as a result of the chemical interaction of the polythiophene molecules with the substrate.     

Distribution of the local magnetic field of the vortex lattice near the surface of an anisotropic superconductor in oblique external magnetic fields

The distribution of the magnetic field in the unit cell of the Abrikosov vortex lattice near the surface of a uniaxial, anisotropic, type-II superconductor in an oblique external magnetic field is determined on the basis of the London model for the cases in which the symmetry axis is perpendicular and parallel to the boundary of the superconductor. The distribution of the local magnetic field is obtained as a function of the distance from the surface of the superconductor and the inclination angle of the external field. It is shown for an YBaCuO high-T _c superconductor that the investigation of the distribution function of the local magnetic field as a function of the angle of the external magnetic field relative to the symmetry axis and to the surface of the superconductor can yield important information about the anisotropic properties of the superconductor. Fiz. Tverd. Tela (St. Petersburg) 39, 1935–1939 (November 1997)     

Low-temperature conduction of a system of tunnel-coupled quantum dots in YBaCuO and LaSrMnO dielectric films

This paper reports on new results of experimental investigations into the nature of the electrical resistivity ρ(T) ? const at T<T _crit for YBaCuO and LaSrMnO dielectric films. The films are prepared by pulsed laser deposition and contain nanocrystalline clusters with metallic conductivity. Dependences of the electrical resistivity ρ(T) ? const are observed for the epitaxial films YBaCuO (T _crit=10 K) with a tetragonal structure after exposure to KrF excimer laser radiation and for the as-prepared amorphous films LaSrMnO (T _crit?160 K). The effect of interest (ρ(T) ? const) manifests itself in the case when the optical spectra of the studied samples contain portions attributed to absorption by free charge carriers. The inference is made that this effect can be associated with tunneling conduction in a system of quantum dots.     

Relaxation into tunnel induced nonequilibrium states in metal-oxide semiconductor structures

The relaxation of a metal-oxide semiconductor structure from deep depletion towards a tunnel-induced non-equilibrium steady state is addressed in this work. A simple model was constructed, taking into account thermal generation, tunneling of both types of carriers and impact ionization. Experimental results obtained on p-and n-type Si substrates and oxides thinner than 6.5 nm are shown to be well fitted by the proposed model. A map describing the possible behavior patterns for a structure with given oxide thickness and effective generation velocity is presented. Fiz. Tverd. Tela (St. Petersburg) 41, 862–864 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Structure of the mixed state induced in thin YBaCuO superconducting films by the field of a small ferromagnetic particle

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 j _c(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 j _c 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. Zh. éksp. Teor. Fiz. 116, 1735–1749 (November 1999)     

Nature of the microwave response of a YBaCuO Josephson junction on a bicrystalline substrate

The response of YBaCuO Josephson junctions on a bicrystalline substrate to the action of microwave radiation is found to contain, besides the Josephson response, a contribution associated with the tunneling of quasiparticles through channels formed by localized states in the region of the Josephson junction. It is shown that this contribution is associated with an increase in the conduction through these channels under the action of the radiation on the junction. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 416–420 (25 September 1996)     

The possibility of new high-transmission, high finesse, Perot-Fabry interferometers for far ir spectroscopy

Finesse as high as 260 and peak transmission close to 98% at frequencies around 30 cm^–1 are predicted at 10 K by coating the Perot-Fabry (PF) mirrors with a suitable thickness of High Tc (HTC) superconductor YBaCuO. In fact, Finesse will be limited by the lack of parallelism and the lack of perfect flatness of the mirrors to values around 100. These predictions are based upon phenomenological models for both the YBaCuO coating and the MgO support, i.e. from direct transmission measurements of MgO and YBaCuO in the Far IR, which had to be fitted by the models.