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The study addresses the quantum spreading of a localized stationary flow of high energy particles. Results demonstrate that as particle energy increases, the spreading speed of the particle wave packet diminishes rapidly. Concurrently, increasing the energies stabilizes the initially localized packet, preserving its transverse form in a vacuum over extended distances. This allows substantial simplifications when using various approximate methods to calculate the wave function in an external field.

The process of scattering of fast charged particles in thin crystals is considered in the transitional range of thicknesses, between those at which the channeling phenomenon is not developed and those at which it is realized. The possibility is shown of application of the methods of geometrical optics for description of the scattering process. The dependence is studied of the total scattering cross-section of ultrarelativistic positrons on target thickness in this range of crystal thicknesses. In the case of ultrarelativistic particles channeling the possibility is shown of the existence of an effect analogical to the Ramsauer-Townsend effect of conversion into zero of the total scattering cross-section at some values of crystal thickness. The important role is outlined of the Morse-Maslov index that enters into the wave function expression in the geometrical optics method., Comment: 6 pages, 2 figures

Quantum theory is proposed of high energy electrons scattering in ultrathin crystals. This theory is based upon a special representation of the scattering amplitude in the form of the integral over the surface surrounding the crystal, and on the spectral method of determination of the wave function. The comparison is performed of quantum and classical differential scattering cross-sections in the transitional range of crystal thicknesses, from those at which the channeling phenomenon is not developed up to those at which it is realized. It is shown that in this thickness range the quantum scattering cross-section, unlike the classical one, contains sharp peaks corresponding to some specific scattering angles, that is connected with the diffraction of the incident plane wave onto the periodically distributed crystal atomic strings. It is shown that the value of the scattering cross-section in the peaks varies periodically with the change of the target thickness. We note that this must lead to a new interference effect in radiation that is connected with the rearrangement of incident wave packet in transitional area of crystal thicknesses., Comment: 5 pages, 3 figures

On the basis of a microscopic model of self-consistent field, the thermodynamics of the many-particle Fermi system at finite temperatures with account of three-body interactions is built and the quasiparticle equations of motion are obtained. It is shown that the delta-like three-body interaction gives no contribution into the self-consistent field, and the description of three-body forces requires their nonlocality to be taken into account. The spatially uniform system is considered in detail, and on the basis of the developed microscopic approach general formulas are derived for the fermion's effective mass and the system's equation of state with account of contribution from three-body forces. The effective mass and pressure are numerically calculated for the potential of "semi-transparent sphere" type at zero temperature. Expansions of the effective mass and pressure in powers of density are obtained. It is shown that, with account of only pair forces, the interaction of repulsive character reduces the quasiparticle effective mass relative to the mass of a free particle, and the attractive interaction raises the effective mass. The question of thermodynamic stability of the Fermi system is considered and the three-body repulsive interaction is shown to extend the region of stability of the system with the interparticle pair attraction. The quasiparticle energy spectrum is calculated with account of three-body forces., Comment: 21 pages, 5 figures

The mean square radius of the proton charge distribution was studied in the framework of the relativistic quasipotential quark model in assumption of the SU(6)-symmetry. It was shown that the proton charge radius is represented as function f(m/\gamma)/M^2 for point quarks (m is a quark mass, \gamma is the scale of the coupling energy, M is the nucleon mass, f is undimentional function, which does not depended on M). It was shown, that in the ultra relativistic region the mean square radius of the bound system can have negative value. To describe simultaneously nucleon magnetic moments and proton radius in the oscillator model it is need to suppose that quark has the negative mean square radius = -1.875/m^2., Comment: 7 pages, 1 figures, Submitted to "Izvestiya VUZov. Fizika" (Russian Physics Journal)

C++ package {\tt BtoVVana} is aimed to extract physics parameters in decays of the scalar neutral B-meson to two vector particles in the intermediate state, and two leptons and two pseudoscalar mesons in the final state. Improved angular moments method is implemented in the package {\tt BtoVVana}: the method uses the time dependent weighting functions and time-integrated observables with variable upper time limit. These observables allow to keep full time informative contents of the decay. By using the package {\tt BtoVVana} it was shown that the statistical errors of extraction of the observables strongly depends on the choice of the angular weighting functions. The best angular weighting functions, which are the linear combinations of the amplitude angular functions, are implemented in the package {\tt BtoVVana}. The first version of the program contains algorithms to extract physics parameters in the case of untagged decays. The package {\tt BtoVVana} and program codes for the channels $B_s^0\to J/\psi\phi$ and $B_d^0\to J/\psi K^*$ in the Monte Carlo generator {\tt SIMUB} were written in parallel and mutually tested with high precision., Comment: 41 pages, 3 figures, 8 Tables, Submit. to Comp.Phys.Commun., January, 2005

Calculation of the nucleon magnetic moments was performed in the quasipotential quark model. The main contribution into the nucleon magnetic moments is determined by the SU(6) symmetrical spin-isospin part of the wave function and is equal to 3 and -2 (in the units of nuclear magnetons) for proton and neutron, respectively. Corrections are explained by relative quark motion and are equal to $-4a/3$ and $+a$ for proton and neutron, respectively, where the dimensionless value $a$ depends on the dimensionless ratio of the oscillator parameter and quark mass in case of the oscillator interaction. It is shown that the independence of the nucleon magnetic moments on nucleon mass is a consequence of relativistic kinematics. The invariant dimensionless parameter is introduced to estimate the violation of current conservation. By means of this parameter it is shown that the current leakage is neglegibly small in the model., Comment: 7 pages, Submitted to "Izvestiya VUZov.Fizika" ("Russian Physics Journal")

The structure of the nucleon wave function as a bound system of the constituent quarks was considered in framework of the quasipotential method of description of the bound states with a fixed number of particles. In the impulse approximation the wave function is reduced to the three-quark component of the vector state in the Fock-momentum space. Spin structure of the wave function was studied by the decomposition method of the irreducible representations product of inhomogeneous Lorentz group. Physical distinction of SU(6) symmetric solution is determined by uniqueness of this solution in the nonrelativistic limit. The effective mass approximation for the relativistic theory was numerically studied, in which dispersion of the average quark momentum is small as compare to a big average quark momentum. Relativistic generalization of the nonrelativistic three-particle oscillator was proposed., Comment: 7 pages, Submitted to "Izvestiya VUZov.Fizika" ("Russian Physics Journal")

The title compound is investigated by specific heat measurements in the normal and superconducting state supplemented by upper critical field transport, susceptibility and magnetization measurements. From a detailed analysis including also full potential electronic structure calculations for the Fermi surface sheets, Fermi velocities and partial densities of states the presence of both strong electron-phonon interactions and considerable pair-breaking has been revealed. The specific heat and the upper critical field data can be described to first approximation by an effective single band model close to the clean limit derived from a strongly coupled predominant hole subsystem with small Fermi velocities. However, in order to account also for Hall-conductivity and thermopower data in the literature, an effective general two-band model is proposed. This two-band model provides a flexible enough frame to describe consistently all available data within a scenario of phonon mediated s-wave superconductivity somewhat suppressed by sizeable electron-paramagnon or electron-electron Coulomb interaction. For quantitative details the relevance of soft phonons and of a van Hove type singularity in the electronic density of states near the Fermi energy is suggested., Comment: 21 pages, 26 figures (high quality version of figures 2,11 available at http://www.ifw-dresden.de/imw/21/coworker/waelte.priv/)

The performance of the method of angular moments on the $\Delta\Gamma_s$ determination from analysis of untagged decays $B^0_s(t),\bar{B}^0_s(t)\to J/\psi (\to l^+l^-) \phi (\to K^+K^-)$ is examined by using the SIMUB generator. The results of Monte Carlo studies with evaluation of measurement errors are presented. The method of angular moments gives stable results for the estimate of $\Delta\Gamma_s$ and is found to be an efficient and flexible tool for the quantitative investigation of the $B^0_s\to J/\psi \phi$ decay. The statistical error of the ratio $\Delta\Gamma_s/\Gamma_s$ for values of this ratio in the interval [0.03, 0.3] was found to be independent on this value, being 0.015 for $10^5$ events., Comment: 34 pages, 5 figures, accepted for publication in Comp. Phys. Comm. at 27/07/2003

In Monte Carlo simulation of $pp$ and $pn$ interactions, it is shown that the charge asymmetry of like-sign lepton pairs can be observed as a manifestation of $B-\bar{B}$ production asymmetry. In this way, the $B-\bar{B}$ production asymmetry could be studied experimentally without full reconstruction of $B$-mesons., Comment: 9 pages, 11 figures

We calculate the tunneling density of states (DOS) of MgB2 for different tunneling directions, by directly solving the real-axis, two-band Eliashberg equations (EE). Then we show that the numeric inversion of the standard single-band EE, if applied to the DOS of the two-band superconductor MgB2, may lead to wrong estimates of the strength of certain phonon branches (e.g. the E_2g) in the extracted electron-phonon spectral function alpha^(2)F(omega). The fine structures produced by the two-band interaction turn out to be clearly observable only for tunneling along the ab planes in high-quality single crystals. The results are compared to recent experimental data., Comment: 2 pages, 2 figures, proceedings of M2S-HTSC-VII conference, Rio de Janeiro (May 2003)

In the present work we calculate the tunneling density of states (DOS) of MgB% $_{2}$ for different tunneling directions by directly solving the two-band Eliashberg equations (EE) in the real-axis formulation. This procedure reveals the fine structures of the DOS due to the optical phonons. Then we show that the numeric inversion of the standard \emph{single-band} EE (the only available method), when applied to the \emph{two-band} DOS of MgB$_{2}$, may lead to wrong estimates of the strength of certain phonon branches (e.g. the $E_{2g}$) in the extracted electron-phonon spectral function $\alpha^{2}F(\omega)$. The fine structures produced by the two-band interaction at energies between 20 and 100 meV turn out to be clearly observable only for tunneling along the $ab$ planes, when the extracted $\alpha ^{2}F(\omega)$ contains the combination $\alpha ^{2}F_{\sigma \sigma}(\omega)$\textbf{+}$\alpha ^{2}F_{\sigma \pi }(\omega)$, together with a minor $\alpha ^{2}F_{\pi \pi}(\omega )$\textbf{+}$\alpha ^{2}F_{\pi \sigma} (\omega)$ component. Only in this case it is possible to extract information on the $\sigma$-band contribution to the spectral functions. For any other tunneling direction, the $\pi$-band contribution (which does not determine the superconducting properties of MgB$_{2}$) is dominant and almost coincides with the whole $\alpha^2F(\omega)$ for tunneling along the c axis. Our results are compared with recent experimental tunneling and point-contact data., Comment: 5 pages, 3 figures. Submitted to Phys. Rev. B (Brief Reports)

The performance of the method of angular moments on the $\Delta\Gamma_s$ determination from analysis of untagged decays $B^0_s(t),\bar{B}^0_s(t)\to J/\psi(\to l^+l^-)\phi(\to K^+K^-)$ is examined. The results of Monte Carlo studies with evaluation of measurement errors are presented. The method of angular moments gives stable results for the estimate of $\Delta\Gamma_s$ and is found to be an efficient and flexible tool for the quantitative investigation of the $B^0_s\to J/\psi\phi$ decay. The statistical error of the ratio $\Delta\Gamma_s/\Gamma_s$ for values of this ratio in the interval [0.03, 0.3] was found to be independent on this value, being 0.015 for $10^5$ events., Comment: 19 pages, 3 figures, submitted to "Particle and Nuclei, Letters"

We consider electronic structure and superconductivity aspects in field-doped polyacenes (PA) and C60. Within a modified Thomas-Fermi approach for typical experimental values of the surface charge density the injected charge is confined to a monolayer. The electron-phonon coupling constant for internal modes is estimated using the work of Devos et al. (Phys. Rev. B 58, 8236 (1998)) and the density of states N(0) estimated from a 2D-one-band model derived from a full potential LDA band structure calculation for bulk anthracene. The large differences in the reported Tc-values for n-doped PA and C60 are ascribed to enhanced empirical Coulomb pseudopotentials for polyacenes., Comment: 6pages, 5figures, Slihtly extended version (one figure added) contribution to the Proceedings of 2002 Int. Conf. on Physics and Chemistry of Molecular and Oxide Superconductors, August 13-19, 2002, Hsinchu, Taiwan, R.O.C. (J. Low Temp. Physics, in press)

Polycrystalline samples of the recently discovered MgCNi3 superconductor were investigated by transport, ac susceptibility, dc magnetization and specific heat measurements in magnetic fields up to 16 T. Consistent results were obtained for the temperature dependence of the upper critical field H_c2(T) from resistance, ac susceptibility and specific heat measurements. A WHH like temperature dependence of H_c2(T) and the quadratic relationship H_c2(0) ~ T_c^2 point to an effective predominant single band behavior near the quasi clean limit. Evidence for strong electron-phonon and electron-paramagnon coupling was found analyzing the specific heat data. The corresponding s- and p-wave scenarios are briefly discussed using calculated densities of states of different Fermi surface sheets., Comment: 8 pages, 11 ps-figures included

The main statement of the nucleon constituent quark model with a fixed number of particles as basic ansatz are considered in a framework of the single-time (quasipotential) approach to the bound state problem. The scaling law breacking for the proton form factor is investigated for Q^2 = 0 -- 2 Gev^2., Comment: 6 pages, 1 figures, VIth International School-Seminar ``Actual Problems of Particle Physics'', July 30 - August 8, 1999, Gomel, Belarus

The strong coupling Schossmann-Schachinger theory for the upper critical field is considered methodically from the user point of view. We check the accuracy of approximate formulas for the upper critical field. In particular, we explain in detail as a recently proposed convenient expression (Shulga et al. cond-mat/0103154) can be justified. The connection of Hc2(0) with the critical temperature Tc is considered and the similarity with the Allen-Dynes Tc-formula is shown explicitly., Comment: 5 pages, 2 figures, methodical remarks to cond-mat/0103154

We present the SIMUB package developed at Dubna for MC generation of B meson production and decays. The starting version of the package includes lepton modes of B decays, in particular, semileptonic decays B->D^*l\nu and "golden" mode B->J/\psi(\mu\mu)\phi(KK) with taking into account all theoretical refinements including B^0-Bbar^0 oscillations and angular correlations., Comment: 16 pages, 3 figures, submitted to VI-th International School-Seminar "Actual Problems of Particle Physics", August 7-16, 2001, Gomel, Belarus, see http://gomelschool.hep.by

The upper critical field Hc2(T) for MgB2 is analyzed in terms of single and multi-band Eliashberg models. The relatively high value of Hc2(0) in the range of 14 to 18 Tesla can be understood, if a strongly coupled subgroup of heavy quasiparticles is involved in the superconductivity. The sizable coupling to both low- and high-frequency bosons is essential. This picture is supported by LDA calculations of Fermi velocity distribution over different sheets of the Fermi surface with special emphasis on the hole tubes. The possible origin of anharmonic soft modes is briefly discussed. Similarities and differences with transition metal borocarbides are figured out., Comment: 4 pages, 2 figures

The interpretation of optical conductivity in the normal and superconducting states is considered in the frame of the standard Isotropic Single wide Band (ISB) model using the theory proposed by Nam. The exact analytical inversion of the normal state Nam equations is performed and applied for the recovery of the reliable information from the experimental data. The Allen formula is derived in the strong coupling approximation and the phenomenological "generalised" Drude formula in a high temperature approximation. It is shown, that the reconstruction of the shape of the spectral function from the normal state optical data at T>Tc is not unique. A novel adaptive method of the exact numerical inversion of the superconducting state equations is presented. It is shown that, the fine structure of the spectral function can be recovered from optical data in the case of s-wave pairing symmetry. In contrast, in the d-wave case the resulting image is approximately the convolution of the input electron-boson interaction function and the wide Gauss distribution function. A simplified visual accessibility procedure is proposed for ''by eye`` analyses of the optical reflectivity of the ISB metals with s- and d- pairing symmetry., Comment: 26 pages, 9 figures, the lecture for Nato-ASI ''Material Science, Fundamental Properties and Future Electronic Applications of High-Tc Superconductors``, Albena, Bulgaria, September 1998, Kluwer Academic Publishers, Dortrecht, pp. 323-360 (2001)

Resistivity measurements from 4.2 K up to 300 K were made on YNi_{2}B_{2}C single crystals with Tc=15.5 K. The resulting rho(T) curve shows a perfect Bloch-Grueneisen (BG) behavior, with a very small residual resistivity which indicates the low impurity content and the high cristallographic quality of the samples. The value lambda_{tr}=0.53 for the transport electron-phonon coupling constant was obtained by using the high-temperature constant value of d(rho)/dT and the plasma frequency reported in literature. The BG expression for the phononic part of the resistivity rho_{ph}(T) was then used to fit the data in the whole temperature range, by approximating alpha^{2}_{tr}F(Omega) with the experimental phonon spectral density G(Omega) multiplied by a two-step weighting function to be determined by the fit. The resulting fitting curve perfectly agrees with the experimental points. We also solved the real-axis Eliashberg equations in both s- and d-wave symmetries under the approximation alpha^{2}F(Omega)= alpha^{2}_{tr}F(Omega). We found that the value of lambda_{tr} here determined in single-band approximation is quite compatible with Tc and the gap Delta experimentally observed. Finally, we calculated the normalized tunneling conductance, whose comparison with break-junction tunnel data gives indication of the possible s-wave symmetry for the order parameter in YNi_{2}B_{2}C., Comment: 6 pages, 5 figures. Proceedings of SATT10 Conference, to be published in Int. J. Mod. Phys. B

In this work, we present the results of precise measurements of the resistivity of YNi$_2$B$_2$C single crystals with $T_{\rm c}$ = 15.5 K as a function of the temperature, and analyze the experimental data in the framework of the Bloch-Gr\"{u}neisen theory and electron-phonon coupling. The transport electron-phonon spectral function that best fits the resistivity data is then inserted in the real-axis Eliashberg equations, which are directly solved to determine the normalized tunneling conductance both in s- and d-wave symmetry., Comment: M2S-HTSC-VI Conference paper (2 pages, 2 figures), using Elsevier style espcrc2.sty

We present new upper critical field Hc2(T) data in a broad temperature region from 0.3K to Tc for LuNi2B2C and YNi2B2C single crystals with well characterized low impurity scattering rates. The absolute values for all T, in particular Hc2(0), and the sizeable positive curvature (PC) of Hc2(T) at high and intermediate T are explained quantitatively within an effective two-band model. The failure of the isotropic single band approach is discussed in detail. Supported by de Haas van Alphen data, the superconductivity reveals direct insight into details of the electronic structure. The observed maximal PC near Tc gives strong evidence for clean limit type II superconductors., Comment: 4 pages, 2 figures, Phys. Rev. Lett. accepted

We calculate the optical conductivity of superconductors in the strong-coupling limit. In this anomalous limit the typical energy scale is set by the coupling energy, and other energy scales such as the energy of the bosons mediating the attraction are negligibly small. We find a universal frequency dependence of the optical absorption which is dominated by bound states and differs significantly from the weak coupling results. A comparison with absorption spectra of superconductors with enhanced electron-phonon coupling shows that typical features of the strong-coupling limit are already present at intermediate coupling., Comment: 10 pages, revtex, 4 uuencoded figures