Measurement of the proton structure function F 2 at low x and low Q 2 at HERA

We report on a measurement of the proton structure functionF ₂ in the range 3.5×10⁻⁵≤x≤4×10⁻³ and 1.5 GeV²≤Q ²≤15GeV² at theep collider HERA operating at a centre-of-mass energy of √s=300GeV. The rise ofF ₂ with decreasingx observed in the previous HERA measurements persists in this lowerx andQ ² range. TheQ ² evolution ofF ₂, even at the lowestQ ² andx measured, is consistent with perturbative QCD. supported by EU HCM contract ERB-CHRX-CT93-0376     

Diffraction at HERA, Color Opacity and Nuclear Shadowing in DIS off nuclei

The QCD factorization theorem for diffractive processes in DIS is used to derive formulae for the leading twist contribution to the nuclear shadowing of parton distributions in the low thickness limit (due to the coherent projectile (photon) interactions with two nucleons). Based on the current analyzes of diffraction at HERA we find that the average strength of the interactions which govern diffraction in the gluon sector at x≤ 10⁻³, Q ₀= 2 GeV is ∼50mb. This is three times larger than in the quark sector and suggests that applicability of DGLAP approximation requires significantly larger Q ₀ in the gluon sector. We use this information on diffraction to estimate the higher order shadowing terms due to the photon interactions with N≥ 3 nucleons which are important for the scattering of heavy nuclei and to calculate nuclear shadowing and Q ² dependence of gluon densities. For the heavy nuclei the amount of the gluon shadowing: G _A(x,Q ₀ ²) /AG _N(x,Q ₀ ²)_{|x ≤ 10−3}∼ 0.25–0.4 is sensitive to the probability of the small size configurations within wave function of the gluon “partonometer” at the Q ₀ scale. At this scale for A∼ 200 the nonperturbative contribution to the gluon density is reduced by a factor of 4–5 at x≤ 10⁻³ unmasking PQCD physics in the gluon distribution of heavy nuclei. We point out that the shadowing of this magnitude would strongly modify the first stage of the heavy ion collisions at the LHC energies, and also would lead to large color opacity effects in eA collisions at HERA energies. In particular, the leading twist contribution to the cross section of the coherent J/ψ production off A≥ 12 nuclei at s ⁻²≥ 70 GeV is strongly reduced as compared to the naive color transparency expectations. The Gribov black body limit for F _2A(x,Q ²) is extended to the case of the gluon distributions in nuclei and shown to be relevant for the HERA kinematics of eA collisions. Properties of the final states are also briefly discussed. Received: 12 March 1999     

Dielectric spectroscopy of PbZr1 ? xTixO3 solid solutions (0.495 ≤ x ≤ 0.51) in a temperature range of 100–300 K at frequencies from 1 × 10?2 to 2 × 107 Hz

The behavior of the relative permittivity ɛ/ɛ₀ of PbZr_{1 − x} Ti _x O₃ (PZT) solid solutions (0.495 ≤ x ≤ 0.51) in the temperature range of 100–300 K at frequencies from 1 × 10⁻² to 2 × 10⁷ Hz was investigated. Diffuse, strongly relaxing maxima at T = 230−260 K (x = 0.495−0.505) and 150–160 K (x = 0.510) were observed in the PZT studied. The relaxation processes are well described by the Vogel-Fulcher law, and the dielectric spectra are approximated by the Cole-Cole formula.     

EPR studies of Cu2+ and V4+ ions in phosphate glasses

Two lead-phosphate glass systems doped with both copper and vanadium ions in different ratios were studied by EPR (electron paramagnetic resonance) method. EPR spectra and parameters (g_‖ = 2.44, g_⊥ = 2.08 andA _‖ = 117.6 · 10⁻⁴ cm⁻¹) obtained for x(CuO · V₂O₅)(l−x)[2P₂O₅ · PbO] glasses withx ≤ 10 mol% suggest a tetrahedral (Td) coordination of Cu²⁺ ions and not a tetragonally elongated octahedron as has been assumed in previous works. The ground state of the paramagnetic electron is thed _xy copper orbital with a 4p_z contribution of 6%. For 20 ≤x ≤ 40 mol% a broad line (ΔB = 307 G) characteristic for clustered ions appears atg = 2.18. The V⁴⁺ ions are evidenced only in the spectra of x(CuO · 2V₂O₅)(1 −x)[2P₂O₅ · PbO] glasses and the resonance parameters suggest a pentacoordinated C_4v local symmetry for these ions. The hyperfine structures characteristic for Cu²⁺ and V⁴⁺ ions disappear for 10 ≤x ≤ 40 mol% due to the mixed exchange Cu²⁺−V⁴⁺ pair formation in these glasses.     

The exponent λ (x, Q2) of the proton structure functionF2(x, Q2) at lowx

The exponent λ of the structure function F₂ ∼x ^−λ is calculated using the solution of the DGLAP equation for gluon at lowx reported recently by the present authors. The quantity λ is calculated both as a function ofx at fixedQ ² and as a function ofQ ² at fixedx and compared with the most recent data from H1     

Transmission spectra of epitaxial layers of Pb1 ? xEuxTe (0 ≤ x ≤ 0.37) solid solutions in the frequency range 7–4000 cm?1

The spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.37) solid solution layers grown on BaF₂ and Si substrates have been investigated over a wide frequency range 7–4000 cm⁻¹ at temperatures of 5–300 K. Apart from the phonon and impurity absorption lines, the absorption in a local mode in PbEuTe layers of substrates and buffer layers has been observed in the frequency range 110–114 cm⁻¹. As the temperature decreases from 300 to 5 K, the transverse phonon mode softens from 33.0 to 19.5 cm⁻¹.     

Dispersion of the Voigt effect in the magnetic semiconductors Cd1−x MnxTe

Spectral measurements of the Voigt birefringence Δn were performed for the cubic magnetic semiconductor Cd_1−x Mn _x Te (0≤x≤0.52) in order to investigate how the exchange interaction of Mn²⁺ ions with itinerant electrons depends on the electron wave vector. It was determined that Δn/x ² is independent of x and the magnetic field direction, i.e., the effect is due to the Mn²⁺ ions and is isotropic. Below the band gap edge the dispersion of the birefringence Δn can be described well in all samples by the unusual dependence Δn∼(E _g −ℏω) ^−3.5. This can be explained by a decrease of the exchange interaction of Mn²⁺ ions with itinerant electrons with increasing distance from the center of the Brillouin zone. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 569–573 (25 April 1998)     

Order parameters and dielectric relaxation in betaine proton glasses

Mixed crystals of betaine, phosphite and betaine phosphate have been investigated using broadband di-electric spectroscopy at frequencies 10⁻¹Hz≤ν≤10⁹Hz and temperatures 1.5 K≤T≤300 K for several betaine phosphate concentrationsx. For 0.2≤x≤0.65 an orientational glass state is found at low temperatures. The broad susceptibility spectra were analyzed using the concepts of distributions of relaxation times and of distributions of energy barriers. A critical comparison of the different approaches is given. In the mixed crystals that show antiferroelectric order at low temperatures, charge transport phenomena are studied. The static permittivity of the proton glass-forming crystals is analyzed, in terms of effectively one- and three-dimensional Ising models that incorporate random fields and random bonds.     

A μSR and neutron polarisation analysis study of Mn moment delocalisation in Fe substituted YMn2

Neutron polarisation analysis measurements reveal antiferromagnetic spin correlations persisting to temperatures of 120 K in Pauli paramagnetic Y(Mn_1−x Fe _x )₂, 0.03≤x≤0.05. The mean moment at the Mn(Fe) site is found to be 0.2μ _B. Transverse field μSR is characterised by weak exponential damping with a rate of 0.02 μs⁻¹ at 300 K increasing according to the power lawT ^−0.75 to only 0.16μ _S ⁻¹ at 12 K. It is suggested that these results are consistent with a slowing down of longitudinal spin fluctuations at the Mn site as temperature decreases.     

Thermal stability and electrical conductivity of multiferroics BiFeO<Subscript>3</Subscript>/REEs

Ceramic samples of bismuth ferrite and solid solutions of Bi_{1 − x} A _x FeO₃ type (where A = Lu, Yb, Tm, Er, Ho, Dy, Tb, Gb, Eu, Sm, Nd, Pr, La; 0.05 ≤ x ≤ 0.20; Δx = 0.05) were prepared. Spectra of the real part of electrical conductivity were studied within the range 10⁻⁴–10⁻⁶ Hz. The dependence of the samples’ thermal stability and electrical conductivity on the size of the substituting ions was established.     

Low temperature photoconductivity of Pb1?xSnxTe?In? films at millimeter wavelengths

Photoconductivity of single crystal Pb_1−xSn_xTe〈In〉, (0.24≤x≤0.25), films at 4.2K in the wavelength region 4÷5 mm and magnetic fields up to 8 T was studied. Preparation technology of the films with high photoconductivity (P_FEP ≤ 10⁻¹² W×Hz^1/2) and photoresponsiveness (τ ≤ 10⁻⁶ s), as well as the measurements procedure are described in the paper. Also the mechanism of photoconductivity is discussed.     

Morphotropic phase boundary, weak ferromagnetism, and strong piezoelectric effect in Bi1 ? xCaxFeO3 ? x/2 compounds

The crystal structure and magnetic and piezoelectric properties of the Bi_{1 − x} Ca _x FeO_{3 − x/2} system (x ≤ 0.2) are studied. The crystal-structure transformations occur in the following sequence: rhombohedral (R3c) polar phase (x ≤ 0.06) → modulated polar phase (0.07 ≤ x ≤ 0.1) → modulated antipolar phase (0.11 ≤ x ≤ 0.14). The modulated polar and antipolar phases are weakly ferromagnetic with a spontaneous magnetization of 0.25 G cm³/g (x = 0.09). In the polar weak ferromagnet with x = 0.09, a uniform piezoelec- tric response 2.5 times stronger than in the initial BiFeO₃ compound is detected by the piezoelectric force microscopy.     

Near IR laser light visualizators using nonlinear GaSe and other layered crystallites

Two methods of preparation of the devices for visualization of pulsed and continuous near-IR (near infrared) are described and the results of conversion of pulsed and continuous IR (800–1360 nm) laser radiation into the visible range of spectra (400–680 nm) by using a transparent substrate covered with the particles (including nanoparticles) of effective nonlinear materials of GaSe _x S_{1 − x} (0.2 ≤ x ≤ 0.8) are presented. Converted light can be detected in transmission or reflection geometry as a visible spot corresponding to the real size of the incident laser beam. Developed device structures can be used for checking if the laser is working or not, for optical adjustment, for visualization of distribution of laser radiation over the cross of the beam and for investigation of the content of the laser radiation. Low energy (power density) limit for visualization of the IR laser pulses with 2–3 ps duration for these device structures are: between 4.6–2.1 μJ (3 × 10⁻⁴−1 × 10⁻⁴ W/cm²) at 1200 nm; between 8.4–2.6 μJ (4.7 × 10⁻⁴−1.5 × 10⁻⁴ W/cm²) at 1300 nm; between 14.4–8.1 μJ (8.2 × 10⁻⁴–4.6 × 10⁻⁴ W/cm²) at 1360 nm. Threshold damage density is more than 10 MW/cm² at λ = 1060 nm, pulse duration τ = 35 ps. The results are compared with commercially existing laser light visualizators.     

Truncated first moment of the parton distribution – A modified approach

We derive the LO DGLAP evolution equation for the full Mellin moments of the first moment of the nonsinglet parton distribution truncated at x₀. This “moment of moment” approach allows one to determine the small-x₀ behaviour of the truncated first moment. We compare our predictions to results obtained from x-space solutions for parton distributions with use of the Chebyshev-polynomial method and to solutions of the evolution equations for the truncated moments proposed by other authors. The comparison is performed for different input parametrisations for 10^-5≤x₀≤0.1 and 1≤Q²≤100 GeV². We give an example of an application to the determination of the contribution to the Bjorken sum rule. PACS 12.38.Bx; 11.55.Hx     

Elastic J/ψ production at HERA

Cross sections for elastic production of J/ψ mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb^-1. Results are presented for photon virtualities Q² up to 80 GeV². The dependence on the photon-proton centre of mass energy W_γp is analysed in the range 40≤W_γp≤305 GeV in photoproduction and 40≤W_γp≤160 GeV in electroproduction. The W_γp dependences of the cross sections do not change significantly with Q² and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q². Differential cross sections dσ/dt, where t is the squared four-momentum transfer at the proton vertex, are measured in the range |t|<1.2 GeV² as functions of W_γp and Q². Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/ψ production and decay angular distributions are consistent with s-channel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q² and is found to be described by perturbative QCD based models.     

Structural,electrical and optical properties of boron doped ZnO thin films using LSMCD method at room temperature

Zn_1−x B _x O (0≤x≤0.04) thin films were deposited by the liquid source misted chemical vapor deposition (LSMCD) method. The thin films were polycrystalline with grain sizes of 16 nm to 22 nm. The structural, optical, and electrical properties were investigated by X-ray diffraction, UV-visible spectrophotometry, Raman spectroscopy, and Hall effect measurement. Also scanning electron (SEM) and atomic force microscopy (AFM) techniques were used in order to determine the morphological and topological characteristics of the films. The optimal result of Zn_1−x B _x O films was obtained at x=0.02, with a low resistivity of ≈10⁻² Ω cm, and a high transmittancy of 85% in the visible light spectrum (300 nm ∼ 800 nm).     

57Fe Mössbauer study of ternary alloy systems Fe(Sb1−x Te x )2 for 0.5≤x≤1.0 at 4.2K

Summary We report on low-temperature (4.2 K)⁵⁷Fe M?ssbauer studies of the ternary alloy system Fe(Sb_1−x Te _x )₂ for 0.5≤x≤1.0. The M?ssbauer spectra are electric-quadrupole doublets for all the specimens, indicating a non-magnetic character. The quadrupole splitting decreases with the increase in tellurium concentration. FeTe₂ has also been studied in external magnetic fields up to 6T. These experiments show that the electric-field gradient in this alloy is negative and has an asymmetry parameter of η ≈0.7. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

Incorporation of Sn into epitaxial GaAs grown from the liquid phase

The incorporation of Sn into LPE GaAs was studied as a function of the atomic fractionx _Sn ^l of Sn in the liquid (1.6×10⁻⁴≤x _Sn ^l ≤0.54), the growth temperatureT _K and the cooling rate α. The diffusion coefficient of As in Ga for moderate Sn-doping was deduced from the growth velocities to beD _As (760° C)=(3.3±1.0)×10⁻⁵ cm²/s. The epitaxial layers were analyzed after van der Pauw with special emphasis on the sources of experimental error. With the aid of current mobility theories the concentrations of the ionized donors and acceptors were derived. From their dependence onx _Sn ^l , on α and onT _K combined with the Schottky-barrier model of Sn incorporation it can be concluded that the melt and the growing crystal surface were in thermal equilibrium. The diffusion coefficient of Sn in GaAs is about 8×10⁻¹⁴ cm²/s at 760° C. The distribution coefficient for Sn increases from 4.4×10⁻⁵ to 12.3×10⁻⁵ in the temperature range from 690 to 800° C. The total Sn incorporationx _Sn ^s was measured using the atomic absorption spectroscopy for the first time down tox _Sn ^s =10¹⁷/cm³. From these data it can be concluded that up tox _Sn ^l =0.54 the dopant Sn is incorporated as donor and as acceptor only and that within the experimental scatter there is no indication of incorporation as a neutral species.