Surface superconductivity and twinning-plane superconductivity in aluminum

The critical supercooling field H _sc is measured in aluminum single crystals and twinned bicrystals in a temperature range slightly below T _c0 (T _c0 ? 0.055 K < T < T _c0), where T _c0 is the critical superconducting transition temperature. It is found that, even in this small temperature range, the H _sc(H _c) dependence, which is considered to be identical to the H _c3(H _c) dependence for single crystals, is substantially nonlinear. The H _sc(H _c) dependences of the twinned bicrystals and single crystals are shown to be significantly different. The qualitative features of the phase diagram of the twinned aluminum bicrystals coincide with those of the phase diagram of twinning-plane superconductivity obtained earlier for tin in [1]. These findings allow the conclusion that the phenomenon of twinning-plane superconductivity also exists in face-centered cubic crystal lattices.     

Features of multiplicity distributions in a bootstrap model of inclusive spectra

Using a bootstrap model of inclusive spectra we derive an integral equation satisfied by the generating function for multiplicity distributions. The (semi-asymptotic) solution of this equations has the form Ψ(λ, s) = Ψ(λ, s₀)(s/s₀)^b(λ) where s is the usual energy variable and b(λ) satisfies an eigenvalue equation and is completely determined by the leading particle distribution. Closed formulae for the binomial moments and for correlation coefficients are also given, and in addition we discuss some general features of the bootstrap model. As a phenomenological application we discuss the rate of variation with energy of multiplicity moments. Our results are expected to be representative for multiperipheral-like models.     

Some aspects of interactions between HTSC-films and substrates

Different aspects of the interaction between YBa₂Cu₂O_y(YBCO) films and (100) ZrO₂〈Y〉 (YSZ) substrates have been investigated. It was determined using X-ray diffraction methods that the structural mismatch between the film and the substrate leads to a film deformation throughout its thickness. At the same time a strained layer appears in the substrate, whose thickness is proportional to the film thickness. The surface morphology changes of YBCO films which take place with variation of the growth temperaturet_sin the vicinity of the optimum temperature lead to changes of the film grain structure probably connected with nucleation centers. Tl₂Ba₂CaCu₂O_y(TBCCO) films on YSZ substrates were also synthesized. It was found that the dependence of the TBCCO film surface morphology changes with annealing temperature and the dependence of YBCO film surface morphology changes ont_sare similar.     

Low-temperature deformation of nanocrystalline niobium

The strain characteristics of nanocrystalline niobium are measured in the temperature range 4.2–300 K. It is shown that the development of a strong local deformation with clearly delineated macroscopic slip bands occurs at 4.2 K and 10 K. The thermal effects at a stress jump observed upon transition of the sample (or a niobium strip placed close to the sample) from the superconducting state to the normal state are estimated. It is demonstrated that the temperature dependence of the yield point σ_s(T) can be divided into three portions: two portions (T<10 K and T>70 K) with a slight change in σ_s and the third portion with a strong dependence σ_s(T). The strain characteristics of polycrystals with nano-and larger-sized grains are compared with those of single crystals.     

Surface recombination velocity in a silicon optical waveguide

Measurements of the varying infrared absorption by free carriers in a silicon waveguide during intermittent electron bombardment are used to study the surface recombination velocitys. The same values for s are obtained for heat-treated and untreated samples, which supports the theory that electron beam irradiation in itself has a dehydrating effect. Other experiments show thats is not dependent on the energy of the impinging electrons in the range 16–23 keV. Finally, the temperature dependence ofs in gold-doped silicon is estimated. Ifs is written in the forms=s ₀(293/T)^x,x is found to be 2.1±0.2 in the temperature interval 261K≦T≦309K.     

Spin density functional theory of the temperature-dependent spin susceptibility: Pd and Pt

The finite temperature spin density functional (SDF) formalism is used to derive a variational expression for the temperature-dependent spin susceptibility χ_p(T) of an inhomogeneous electron gas. The use of a simple trial function in the variational expression results in a Stoner form for χ_p(T), i.e., χ_p(T) ≥ χ_s(T)/[1 - I(T) χ_s (T)] where χ_s(T) is the single-particle spin susceptibility including exchange-correlation (XC) effects on the band structure within the framework of the density functional formalism and I(T) is the SDF analog of the Stoner parameter. It should be emphasized that this form for χ_p(T) is derived for a general XC free energy functional F_xc[n, m; T]. χ_s(T) has been calculated self-consistently, including relativi stic effects, using the local approximation for F_xc for Pd and Pt. These results have been used to investigate the temperature dependence of I(T) required to explain their experimental susceptibilities. It is found that when the spin-orbit interaction is included in the calculation of χ_s(T), the temperature variation of I(T) is stronger than that of χ_s(T).     

Constraints on gluon evolution at smallx

The BFKL and the unified angular-ordered equations are solved to determine the gluon distribution at smallx. The impact of kinematic constraints is investigated. Predictions are made for observables sensitive to the gluon at smallx. In particular comparison is made with measurements at the HERA electron-proton collider of the proton structure functionF ₂(x, Q ²) as a function of lnQ ², the charm componentF _c ² (X, Q ²) and diffractiveJ/Ψ photoproduction.     

The dc magnetic field,temperature and film thickness dependence of microwave surface resistance of YBCO and DyBCO thin films

We investigated the dc magnetic field and temperature dependences of the microwave surface resistance (R_s) of YBa₂Cu₃O_y (YBCO) and DyBa₂Cu₃O_y (DyBCO) superconducting thin films. The YBCO and DyBCO thin films, each with a thickness of 300, 500, or 700 nm, were deposited on MgO (1 0 0) substrates by the thermal co-evaporation method. The R_s was measured using the dielectric resonator method. A dc magnetic field of up to 5.0 T was applied parallel to the c-axis of the superconducting thin films. The results showed that the R_s value had almost the same temperature dependence at various thicknesses in a zero-external field. The R_s of the YBCO and DyBCO thin films increased with the applied dc magnetic field. The DyBCO thin films showed weaker magnetic field dependence of R_s than the YBCO thin films. The R_s ratio (defined as R_s(5 T)/R_s(0 T)) linearly increased with the film thickness. These results show that pinning strength decreased with an increasing film thickness.     

Possible explanation of magnetic dependence of the bragg peak intensity in the modulated phase of HoNo6S8

The possible explanation for the observed, by Lynn et al. [1], existence of the maximum in the magnetic field dependence of the Bragg peak intensity I_Q(H) in the modulated phase of HoMo₆S₈ is given. It is based on the so-called EX-model - the dominance of the exchange interaction between superconducting electrons and localized moments in the formation and destruction of the coexistence phase. In this approach the maximum in I_Q(H) results from the (zero wave-vector) electronic spin susceptibility dependence, in superconducting state, on the spin-orbit scattering time τ_So. The in [1] observed anisotropic character of I_Q(H), for H|Q and H⊥Q, is also explained.     

Surface demagnetizing field as a source of uniaxial surface anisotropy in GdCoMo thin amorphous films

Surface magnetic anisotropy energy was studied for (Gd_0.26Co_0.74)_0.96Mo_0.04 and (Gd_0.29Co_0.71)_0.96Mo_0.04 thin amorphous films by means of microwave spectroscopy at the X-band within the temperature range 4–295 K. Excitations of surface spin waves were observed in the spin wave resonance spectra. The experiment was performed in a rotating external magnetic field. The angular dependence of the resonance field for the uniform mode (spin wave vector k=0) and the surface mode made it possible to determine the surface uniaxial anisotropy constant K_s and its temperature dependence. An inhomogeneity of the saturation magnetization M_s within a close-to-surface layer of thickness d can generate the surface anisotropy energy with anisotropy constant K_s given by the formula: K_s=4πM^b_s (M^b_s−M^surf_s)d, where the indexes b and surf correspond to the bulk and surface values, respectively. The temperature dependence of K_s calculated by means of the formula agrees qualitatively with temperature dependence of K_s found in the experiment.     

The semihard processes γγ→ΨX, γγ→ΨΨ, γγ→ϱΨ

The cross sections for Ψ-production in the semihard collisions are calculated at s ⪢ |t|, m_Ψ² and at not too large a jet effective mass m_X²≲m_Ψ² + |t| for the process γγ→ΨX. We found these cross sections in the lowest nontrivial QCD order (two-gluon exchange in the t-channel): σ(γγ→ΨX) ≈ 10³⁵ cm², σ(γγ→ΨΨ)=2×10⁻³⁶ cm². The polarization of Ψ in these processes is transversal (contrary to the longitudinal one of light mesons ϱ⁰, ω, ϕ in similar processes).     

Investigation of slow surface states on a real germanium surface by the optical excitation technique

The slow states (SS) charging ΔQ_s under the action of light quanta of different energy (2 ? hv ? 4.6 eV) has been investigated on a real germanium surface. The considerable influence on the optical SS charging of the preparation method as well as of adsorption-desorption processes has been revealed. On the basis of the spectral dependencies ΔQ_s(hv) the conclusion has been made about the existence of the adsorption-sensitive system of “fluctuation” electron states near the edges of energy bands of the oxide layer. The photocharging method has been shown the construction of the whole energy scheme of the semiconductor-dielectric heterojunction (including the band gap of the dielectric layer). The possible origin of the deep traps in oxide layer which are responsible for the optical charging of a real germanium surface, has been discussed.     

Studies of vibrational surface modes in ionic crystals: III. Vibrational contributions to the surface thermodynamic functions for the (001) face of LiF,MgO, NaF,NaCl, Nal,RbF, and RbCl

The temperature dependence of the vibrational contributions to surface specific heat, surface entropy, surface energy, and surface Helmholtz free energy have been calculated for the (001) face of seven crystals having the rocksalt structure. The calculations assume a perfect, unrelaxed surface and make use of shell models fitted to bulk phonon spectra determined from inelastic neutron scattering. In terms of the bulk zero-temperature Debye temperature θ₀, the surface specific heat C_v^s exhibits an effective power law behavior, T^α, from at least T = 0.02 θ₀ up to 0.05 θ₀ in most cases (and up to 0.07 θ₀ for NaF), with α ≈ 2.5 in most cases — in contrast with the result of α = 2 in a Debye-like model. (Below 0.02 θ₀, results derived for our 15-layer films depart significantly from intrinsic surface effects because of the finite thickness.) C_v^s attains a maximum at a temperature T(C_max^s) ranging from 0.14 θ₀ to 0.20 θ₀, in contrast with the result T(C_max^s) = 0.21 θ₀ for the Debye-like model. The peak value C_max^s ranges from 0.34 k_BA_SUC to 0.41 k_BA_SUC, where A_SUC is the area of the surface unit cell. The shap the peak in C_v^s differs characteristically between that class of crystals in which there is some overlap of the acoustical and optical bulk bands and that class in which there is an appreciable absolute gap between the acoustical and optical bulk bands; in the latter class the peak is flattened on the low side of the maximum, with the maximum pushed to somewhat higher temperature. On those points of comparison with the rather sparse existing data for surface-excess heat capacity in which the value of specific surface area is not required (e.g., the value of T(C^s_max)), the agreement ranges from encouraging to equivocal. On those comparisons which require the surface area of the experimental samples (e.g., the magnitude of C^s_max) the agreement ranges from only fair to bad. Further experimental work is needed, and great care in surface area determinations is necessary.     

Calculable observables in quantum chromodynamics: (II). Heavy quark production

It is argued that the color-averaged inclusive cross section dσ/d³p for the production of a heavy quark of given momentum p and given flavor is infrared-singularity free and therefore calculable in perturbative QCD if it is evaluated away from heavy quark thresholds and if the typical reaction energy Q is such that α_s(Q)ln[Q/m(Q)] is small, α_s and m being the strong coupling constant and the heavy quark mass respectively. An interpretation of this cross section is proposed in terms of heavy flavor hadronic production.     

Magnetostriction and other magnetic properties of Co-Ni based amorphous alloys

The magnetostriction coefficients λ_s of (Co_1−xNi_x)₇₅Si₁₅B₁₀ amorphous alloys have been determined in the range 0 ⩽ x ⩽ 0.41. The dependence of the magnetostriction on temperature T has been related to that of the magnetic polarization J_s in a phenomenological way as λ_s(T) = α[J_s(T)]³ + β[J_s(T)]². The compositional dependence of the coefficients α and β has been obtained.From the studies of the magnetostriction in the region close to the Curie temperature it has been possible to evaluate the magnetostriction critical exponent K.     

Light scattering by an inhomogeneous spherical particle with intermediate layers

A model of a radially inhomogeneous multilayer spherical particle with a continuously varying refractive index in the intermediate layers between the shells of the particle and between the particle and the surroundings is proposed. Such a particle scatters light much like a dust particle with a rough and ragged surface of the layers, which is simulated with the help of the discrete dipole approximation method. For dust particles whose surface shape deviates from the spherical one, the refractive index profile and the behavior of the extinction Q _ext(x) and absorption Q _abs(x) efficiency factors with increasing thickness of the intermediate layers are studied. Properties of such particles in dependence on the number of layers are also studied. It is revealed that, as the number of shells increases, the order of the relative position of substances ceases to play a role, as is also the case for a multilayer spherical particle without intermediate layers. It is shown that, upon an increase in the number of shells at the same percentages of substances in the intermediate layers, the difference of the values of Q _ext(x) and Q _abs(x) from the corresponding values calculated with the model without intermediate layers decreases.     

Origin of the Q 2-dependence of the DIS structure functions

We consider in detail Q ²-dependence of the DIS structure functions. Very often this dependence is claimed to be originated by the Q ²-dependence of the QCD coupling. This leads to the small-x asymptotics of the structure functions with Q ²-dependent intercepts. We demonstrate that the DGLAP parametrization α _s = α _s (Q ²) is an approximation valid in the region of large x (where 2pq can be approximated by Q ²) only, providing the factorization scale is also large. Outside this region, the DGLAP parametrization fails, so α _s should be replaced by an effective coupling which is independent of Q ² at small x. As a consequence, intercepts of the structure functions are independent of Q ² . Nevertheless, the small-x asymptotics of the structure functions explicitly depend on Q ² , even when the coupling does not depend on it. We also consider the structure functions at small Q ² and give a comment on power-Q ² corrections to the structure functions at large and small Q ² .     

Temperature dependence of the spontaneous magnetization of Cu x Ni1-x Fe0.6Cr1.4O4 chromite ferrites with a frustrated magnetic structure

The behavior of the spontaneous magnetization σ _s (T) of samples in the Cu _x Ni_1-x Fe_0.6Cr_1.4O₄ system (x = 0, 0.1, 0.2, 0.3, 0.4) with variations in temperature is studied. The conclusion is drawn that the degree of frustration of magnetic interactions in the A sublattice increases with increasing x, with the result that the dependence σ _s (T) transforms from an N-type curve to an anomalous dependence of a new type.     

Effect of gamma irradiation on internal friction in lithium tetraborate

The effect of gamma irradiation on the mechanical properties of lithium tetraborate Li₂B₄O₇ in the single-crystal and vitreous states is investigated. It is found that, after irradiation of the Li₂B₄O₇ single crystal, the temperature range of the dissipative process initially occurring at 380–420 K becomes broader and the fine structure of the peak in the temperature dependence of the internal friction Q^?1(T) undergoes a substantial trans-formation. After irradiation of the vitreous Li₂B₄O₇ sample, the increase in the internal friction, which is characteristic of the onset of the α relaxation in this material, is not observed in the dependence Q^?1(T) up to a temperature of 570 K. It is shown that the mechanical properties of the irradiated samples are almost completely recovered after annealing at 570 K for 1 h.