I–V characteristics and the electric-field effect in melt-grown YBa2Cu3Ox/Y2BaCuO5 HTSC crystals

The effect of an electric field E=120 MV/m in the electrode-insulator-superconductor system on I–V curves obtained at 77 K on two types of single-crystal samples cut from monolithic superconducting YBa₂Cu₃O_x/Y₂BaCuO₅ has been studied. The Y211 nonsuperconducting phase in the ingot was in the form of precipitates ≈1 μm in size. It has been found that an electric field applied to samples with a comparatively low Y211 content (volume fraction 8%) does not affect the critical current I _c while reducing the resistance R at currents slightly above I _c . In samples containing more than 35% Y211 phase, electric field results in an increase of I _c and a decrease of R for I>I _c . Data on the critical temperature T _c and the temperature dependence of I _c have also been obtained. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 7, 2000, pp. 1172–1175. Original Russian Text Copyright ? 2000 by Smirnov, Orlova, Sengupta, Goretta.     

Experimental evidence for Coulomb charging effects in submicron Bi-2212 stacks

Focused ion beam (FIB) and ion milling techniques are developed for the fabrication of Bi₂Sr₂CaCu₂O_8+1d (Bi-2212) stacked junctions with in-plane size L ab ranging from several microns down to the submicron scale without degradation of superconducting transition temperature T _c. It is found that the behavior of submicron junctions (L _ab<1 μm) is quite different from that of larger ones. The critical current density is considerably suppressed, the hysteresis and multibranched structure of the current-voltage (I-V) characteristics are eliminated, and a periodic structure of current peaks appears reproducibly on the I-V curves at low temperatures. The period ΔV of the structure is consistent with the Coulomb charging energy of a single pair, ΔV=e/C, where C is the effective capacitance of the stack. It is considered that this behavior originates from the Coulomb blockade of the intrinsic Josephson tunneling in submicron Bi-2212 stacks. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 75–80 (10 January 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Formation of high-T c phases of the system Bi-Pb-Sr-Ca-Cu-O in a fluxed KCl melt

The influence of the conditions of synthesis and annealing on the ratio of the volumes and the temperature T _c ⁿ at which transition to the superconducting state begins is investigated on the basis of diamagnetic susceptibility measurements and x-ray phase analysis for bismuth 2212 and 2223 phases obtained from solution in a KCl melt. It is found that the value of T _c ⁿ for the 2212 phase decreases as the temperature and the holding time are increased in synthesis. The 2212–2223 transition in the fluxed KCl melt takes place in the presence of an oxygen deficiency, and the width of the transition interval to the superconducting state of the 2223 phase depends on the cooling rate of the fluxed melt after isothermal holding and also on subsequent annealing in air. Temperatures T _c ⁿ =107 K, 90 K, and 20 K are observed for unannealed crystals of the 2223 phase with average dimensions 50×50 μm. Air annealing leads to oxygen saturation of their lattice, and T _c ⁿ =107 K throughout the entire structure of the crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1761–1763 (October 1997)     

Determination of the depth of formation of magnetooptical effects in CoNi films

The optical constants of CoNi films with magnetic properties that are nonuniform across their thickness are determined in reflected light by two methods, viz., optical and magnetooptical measurements. The values of the parameters L=λ/4πk and Z ₀=λ/8n, one of which (specifically, the one which has the smaller value at a given value of λ) determines the depth of formation of reflective magnetooptical effects (l _mo) according to the current theories, are calculated on the basis of the values obtained for the optical constants n and k of the films (λ is the wavelength of the light used, and n and k are the refractive index and the absorption coefficient of the magnet). It is established for the CoNi films investigated that l _mo is determined by L and varies from about 200 to 300 ? in the range 0.33 μm⩽λ⩽0.83 μm. In CoNi films, which are inhomogeneous across their thickness and are characterized by significant variation of the magnetic properties over distances ∼l _mo, variation of the form of the magnetization curves determined by measuring the equatorial Kerr effect is observed as λ increases. Zh. Tekh. Fiz. 68, 69–72 (February 1998)     

Effect of elastic stresses on the IR spectra of lattice vibrations in epitaxial ZnSe films on (001)GaAs substrates

The lattice IR reflection spectra of epitaxial ZnSe films are studied for different thicknesses on a (001)GaAs substrate. The frequency of the TO mode is found to increase for films with thicknesses of 0.8 and 1.2 μm that exceed the critical value d _cr ≈0.1 μm for ZnSe/GaAs pairs. The effect is explained by the existence of regions with residual stress in the film. Fiz. Tverd. Tela (St. Petersburg) 41, 1948–1952 (November 1999)     

Strain-induced photoreflectance spectra in the vicinity of the E 0 transition in GaAs/Si and InP/Si heterostructures

A study is reported of the structure of photoreflectance (PR) spectra in the vicinity of the E ₀ transition from thin (d=1–5 μm) n-GaAs and n-InP films (n=10¹⁶–10¹⁷ cm⁻³) grown epitaxially on Si(001) substrates. A quantitative analysis of the spectra involving multi-component fitting shows that the electronic optical transition from the {3/2;±1/2} subband provides a dominant contribution to the intermediate-field electromodulation component in both systems. The splitting observed in the GaAS/Si PR spectra near the main peak are accounted for not by the strain-induced valence-band splitting but rather by a spectral superposition of the intermediate-field component due to the {3/2;±1/2} subband with a low-energy excitonic component. The analytically established transition energy E ₀ ^3/2;±1/2 is used to calculate biaxial strains in epitaxial films. Fiz. Tverd. Tela (St. Petersburg) 41, 725–731 (April 1999)     

Up-conversion and population of excited erbium levels in LiY1−x ErxF4 (x=0.003–1) crystals under CW InGaAs laser-diode pumping

Formation of the inverse population of working levels of 3-μm laser transition in LiY_1−x Er_xF₄ (x=0.003–1) crystals under CW InGaAs laser-diode pumping (0.967–0.982 μm) was investigated. Dependences of population of the ⁴ I _11/2 and ⁴ I _13/2 levels on the dopant concentration and pump power were studied theoretically and experimentally. Relative changes in populations of the studied levels were experimentally monitored by measuring the steady-state spectra of IR crystal luminescence in the wavelength range corresponding to ⁴ I _11/2→⁴ I _13/2 (2.7–2.8 μm), ⁴ I _11/2→⁴ I _15/2 (0.96–1.04 μm), and ⁴ I _13/2→⁴ I _15/2 (1.45–1.65 μm) transitions. Theoretical and experimental estimates of the rates of intracenter and intercenter relaxation processes (migration, self-quenching, and up-conversion) with allowance for statistics of coupling of impurity centers in the system were used to determine the energy-transfer mechanisms, elucidate the predominant mechanisms, and obtain microparameters and concentration dependences of the energy-transfer rates and nonlinear coupling. Dependences of the steady-state population of the levels of laser transition ⁴ I _11/2→⁴ I _13/2 on the dopant concentration and pumping power density were calculated within the context of rate balance equations for the scheme with the five lowest excited states of erbium. Good agreement between theory and experiment was obtained. __________ Translated from Optika i Spektroskopiya, Vol. 92, No. 1, 2002, pp. 73–88. Original Russian Text Copyright ? 2002 by Tkachuk, Razumova, Mirzaeva, Malyshev, Gapontsev.     

Model of current and optical conductivity switching in YBa2Cu3O7−x films

A model is proposed for the thermal and electrical responses of films of the high-T _c superconducting material YBa₂Cu₃O7_7−x to current and optical pulses. Numerical calculations are compared with experimental data for current pulses of duration 100 μs and laser pulses of duration 0.1 ns; this yields improved data on the thermal conductivity of thin YBa₂Cu₃O7_7−x films (1.5–2 W/m·K) and thermal resistance of the film-substrate contact (5×10⁻⁸m²·K/W) in the neighborhood of the superconducting transition. This model can be used for optimizing the film structure parameters and control regimes for switching elements for pulses lasting longer than 0.1 ns. Zh. Tekh. Fiz. 69, 77–82 (October 1999)     

An anomaly in the magnetic susceptibility of the Mg1−x CuxO solid solutions with 0.01≤x≤0.20

The problem of localized superconductivity has motivated the preparation of Mg_1−x Cu_xO solid solutions with NaCl structure and 0.01≤x≤0.20, as well as a study of the magnetization and magnetic susceptibility χ in the 2–400 K temperature range and in magnetic fields of up to 5 T. The temperature dependence of χ is described for all compositions by the Curie-Weiss law, χ = C/(T − θ), where the constant C is close to the value calculated for each composition for μ_eff = 1.7–1.9μ_B, and θ is close to zero. For T < 30 K, χ(T) deviates for all compositions toward lower χ, which can be attributed to magnetic ordering of exchange-coupled clusters in the solid solution. At T∼320–330 K, an anomaly of a diamagnetic type, i.e., a decrease of χ by 6–30% of its paramagnetic value, has been observed for all compositions against the background of the generally paramagnetic χ(T). A discussion is presented of alternative reasons for this anomaly and of its possible connection with localized superconductivity. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 4, 2000, pp. 701–703. Original Russian Text Copyright ? 2000 by Samokhvalov, Arbuzova, Viglin, Naumov, Smolyak, Korolev, Lobachevskaya.     

Synthesis, structure, and dielectric properties of KH2PO4 fractal aggregates

Macroscopic fractal aggregates of KH₂PH₄ (KDP) measuring up to 500 μm have been obtained. The fractal structure forms as a result of the precipitation of KDP particles from a supersaturated aqueous solution in the presence of a temperature gradient followed by a diffusioncontrolled mechanism of aggregation. The electron-microscopic analysis performed has shown that the fractals are formed predominantly from crystallites of the tetragonal modification measuring ∼1 μm. The dielectric constant (ɛ) of fractal KH₂PO₄ has been measured in the temperature range 80–300 K. A characteristic anomaly has been discovered on the ɛ(T) curve in the vicinity of 122 K, which attests to a ferroelectric phase transition. The absolute value of ɛ is significantly smaller than the components ɛ ₁₁ and ɛ ₃₃ for KH₂PO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 2059–2061 (November 1999)     

Oscillation phenomena in polyacetylene: R 1×S 1 Gross-Neveu model with a chemical potential

The phase structure of the two-dimensional Gross-Neveu model in a spacetime of the form R ¹×S ¹ and in the presence of a chemical potential μ is investigated. The phase portrait of the model is constructed in the parameter plane (μ, γ), where γ=1/L and L is the length of the circle S ¹. In the portrait there exist two massive phases, with spontaneously broken chiral invariance, as well as infinitely many massless symmetric phases. Such a vacuum structure leads to oscillations of the critical curve μ _c (γ) of chiral phase transitions. In addition, the particle number density in the vacuum state of the model oscillates if μ<μ _c (γl). Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 431–436 (10 September 1998)     

Thermal and Auger processes in p-n junctions based on GaInAs/InAs and InAsSbP/InAs heterostructures

A study is made of the excess-energy relaxation processes and the mechanisms responsible for overheating of the active zone of infrared emitters made from nonisoperiodic structures with stressed InGaAs layers and from nearly isoperiodic InAsSbP structures and emitting in the wavelength range λ=2.5–5.0 μm are investigated. The relationship between the overheat ΔT of the active zone of the structure and Auger processes is established for In_1−x Ga_xAs infrared emitters. It is shown that the efficiency of Auger recombination decreases as x increases in the interval 0–0.09, promoting a sharp reduction in ΔT. At x>0.09 the efficiency of CHHS Auger processes decreases exponentially, but an increase in the density of dislocations due to the appreciable value (∼6.9%) of the lattice mismatch parameter causes ΔT to increase, but slowly. Zh. Tekh. Fiz. 67, 68–71 (September 1997) Deceased.     

Determination of parameters of the dipole moment of the CO2 molecule

Analytic expressions are obtained for vibrational transition moments of the first, second, and third orders using the eigenfunctions in the second-order perturbation theory. These expressions can be used to solve the inverse electrooptical problem for the ¹²C¹⁶O₂ molecule. The resonance interactions were taken into account by solving secular equations. The mixing coefficients for the eigenfunctions were calculated with an accuracy of 0.1%. The experimental data on purely vibrational transition moments 〈μ〉² used in the solution were obtained by averaging the data available in the literature with the weights that are inversely proportional to the error, resulting in mean-square deviations of (0.1–10)%. Five parameters of the dipole moment of the Σ _u symmetry were calculated using 29 values of the transition moments; five parameters of the π_u symmetry were calculated using 27 values of the transition moments. The accuracy of the solution of the inverse problem is characterized by the quantity Q=[Σ(δμ _i ^(theor) /δμ _i ^(exp) )²/(n-m)]^1/2, where δμ _i ^(theor) is the deviation of calculations from the experiment, δμ _(exp) ⁱ is the experimental error, n is the number of the experimental data used, and m is the number of parameters smaller than unity. __________ Translated from Optika i Spektroskopiya, Vol. 88, No. 5, 2000, pp. pp719–732. Original Russian Text Copyright ? 2000 by Kolomiĭtsova, Lyaptsev, Shchepkin.     

Autosolitons in electron-hole plasma in p-Ge

The self-organization of an electron-hole plasma (EHP) heated by an electric field in pure p-Ge samples at T = 77 K has been studied experimentally. The derived current-voltage characteristics (CVCs) and the distributions of the electric field and IR emission of the hot carriers along the samples show that the segments of a steep rise or the S-shaped segments of the CVCs in samples with n-p junctions are related to the formation of longitudinal thermal-diffusion autosolitons (AS_‖); as a result, thin (d = 2–20 μm in diameter), melted-through current channels appear. Such AS_‖ are formed at high EHP densities (n ≥ 1 × 10¹⁶ cm⁻³), when the electron-hole scattering is dominant, and at electron temperatures T _e = (2–4.5)T ₀ (T ₀ is the lattice temperature). The saturation segments and the N-shaped segments in the CVCs are attributable to the generation of transverse thermal-diffusion high-field autosolitons (AS_⊥) in the form of narrow strata with electric field strengths = 1–20 kV cm⁻¹. High-field AS_⊥ are formed at EHP densities n = 5 × 10¹³−1 × 10¹⁶ cm⁻³, when the electron-phonon scattering is dominant, and at electron temperatures T _e ∼ Θ ≥ 5T ₀ (Θ is the Debye temperature). The generated longitudinal and transverse autosolitons have high temperatures (T _e ≥ 1000 K) and reduced carrier densities and can exist simultaneously in different parts of the sample. Original Russian Text ? M.N. Vinoslavskiĭ, P.A. Belevskii, A.V. Kravchenko, 2006, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2006, Vol. 129, No. 3, pp. 477–492.     

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.     

Structure of the free exciton luminescence band of heteroepitaxial ZnSe/GaAs layers

The exciton reflectance and photoluminescence spectra of epitaxial ZnSe/GaAs layers with a thickness of 2–4 μm are investigated in the temperature range 10–120 K. It is shown that one of the causes of the formation of the doublet structure of the A _n=1 photoluminescence band is interference of the exciton radiation at the boundaries of the near-surface dead layer. Fiz. Tverd. Tela (St. Petersburg) 40, 881–883 (May 1998)     

Magnetic structure of Er<Subscript>5</Subscript>Ge<Subscript>3</Subscript> at 4.2 K

A symmetry analysis of the possible magnetic structures of Er₅Ge₃ in the ground state is performed using the results of measurements of elastic magnetic neutron scattering at 4.2 K. It is shown that the minimum discrepancy factor R _m ≈9.5% corresponds to a modulated collinear magnetic structure in which the magnetic moments of erbium atoms are oriented along the a ₃ axis of the unit cell of the crystal structure and induce an antiferromagnetic longitudinal spin wave (AFLSW). The magnetic structure is characterized by the wave vector k=2π(0, 0, μ /a ₃) (where μ≈0.293) and the modulation period λ≈3.413a ₃. The magnetic ordering temperature T _N ≈38 K is determined from the temperature dependence of the intensity of magnetic reflections. __________ Translated from Fizika Tverdogo Tela, Vol. 45, No. 9, 2003, pp. 1653–1659. Original Russian Text Copyright ? 2003 by Vokhmyanin, Dorofeev.     

Erbium electroluminescence excitation in amorphous hydrogenated silicon under thermally stimulated deep-center tunneling ionization

A study of the electroluminescence of erbium-doped, amorphous hydrogenated silicon, a-Si:H 〈Er〉, is reported. It has been found that the electroluminescence intensity at the wavelength λ=1.54 μm corresponding to the ⁴ I _13/2→⁴ I _15/2 intra-4f shell transition in Er passes through a maximum near room temperature. The unusual temperature and field dependences of the electroluminescence indicate electric-field induced multi-phonon tunneling emission of electrons from deep centers. The electroluminescence of Er³⁺ ions is due to their becoming excited as conduction-band electrons are captured by neutral dangling bonds (D ⁰ centers), which form when erbium is incorporated into the amorphous matrix. This Auger process transforms the center from its neutral state, D ⁰, to a negatively charged state, D ⁻, and the energy released in the capture is transferred by Coulomb interaction into the erbium-ion 4f shell. The steady-state current through the electroluminescent structure is supported by the reverse process of multi-phonon tunneling-electron emission from the D ⁻ center to the conduction band. The proposed theoretical model is in a good agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 210–217 (February 1999)     

Stereochemical studies of oligomers XXIX. Reinvestigation of the structure of N-m-tolyl phthalimide

C₁₅H₁₁NO₂, Mr 237.3, monoclinic, space groupC _c, a=8.539(2),b=19.865(4),c=7.599(2)?,β=111.44(2)°,V=1199.8 ?³,Z=4,D _c=1.31 gcm⁻³,λ(CuKα)=1.5418 ?,μ=6.74cm⁻¹,F(000)=496, room temperature. The structure was solved by direct methods with SHELX-86 and refined down to agreement valueR=0.046 for 1117 reflections above 2σ(I). The angle between the plane of the phthalimide group, which shows a little bent [1.2(2)°] between its two rings, and the tolyl group is 56.1(1)°. The packing of the molecules is stabilized by van der Waal’s forces only. Part XXVIII: Bocelli and Rizzoli (1990)     

Weak ferromagnetism in copper metaborate CuB2O4

CuB₂O₄ single crystals have been grown and their magnetic and resonance properties have been investigated for the first time. The temperature dependence of the susceptibility was found to contain features at T=21 and 10 K. The CuB₂O₄ single crystal transformed at T=21 K to a weakly ferromagnetic state. The sharp drop in susceptibility at T<10 K is caused by a transition of the magnetic system of CuB₂O₄ to an antiferromagnetic state. The effective magnetic moment of the Cu₂₊ ion, determined from the high-temperature part of the magnetic susceptibility, is 1.77 μ _B. The room-temperature g factors are, respectively, 2.170 and 2.133 for magnetic field parallel and perpendicular to the c axis of the crystal. The antiferromagnetic resonance parameters in the weakly ferromagnetic and antiferromagnetic phases were measured. Fiz. Tverd. Tela (St. Petersburg) 41, 1267–1271 (July 1999)