2,197 research outputs found

    Geometrical optics method in the theory of channeling of high energy particles in crystals

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    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 figure

    On quantum spreading of a localized stationary flow of high energy particles

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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

    Spectral method in axial channeling theory

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    The energy quantization of transverse particle motion in continuous potentials of atomic chains and planes can occur when fast charged particles travel in crystals. In the proposed paper, the energy levels of electrons moving in the mode of axial channeling in a system of parallel atomic chains have been found. The energy eigenvalues were determined numerically using the so-called spectral method, which shows itself to good advantage in the problem of the plane channeling of charged particles in crystal

    Quantum and classical effects at scattering of high energy charged particles in thin crystals

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    The present work reviews the results concerning quantum scattering theory of ultrarelativistic electrons in ultrathin crystals and itscomparison with analogous classical results. It deals with an intermediate range of thicknesses, large enough for that the particlemotion could not be considered as rectilinear but small enough for that the channeling regime of motion was not established. Thequantum theory is based both upon the representation of the scattering amplitude as an integral over the surface surrounding thetarget, and on the so-called operator method of determination of the wave function as a solution of a Schro..dinger-like equation.The latter method implies a wide use of the Fourier technique, both in calculation of each next step in the wave packet evolution,and in moving from the spatial coordinates to the angular ones. The authors compare the quantum differential scattering cross-sec-tions with the classical ones in the considered range of crystal thicknesses and show their resemblances, distinctions and the evo-lution of these distinctions with the change of the particle energy. The simplest variant of quantum scattering theory based uponthe eikonal approximation of quantum mechanics is considered. In the paper the quantum differential scattering cross-section wascalculated and its affinity with the classical one was demonstrated. In the preparation of these lecture notes the material of the paper[4] was used

    Spectral distribution in the reflection of parametric X-rays

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    The parametric X-ray radiation (PXR) spectra are measured on condition when the angular size of PXR cone is smaller than the angular resolution of the experiment. The PXR is generated under interaction of 50GeV proton beam with silicon crystal in Bragg geometry. The comparison of experimental data with results of developed theoretical model is presented and discusse

    Positrons vs electrons channeling in silicon crystal: energy levels, wave functions and quantum chaos manifestations

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    The motion of fast electrons through the crystal during axial channeling could be regular and chaotic. The dynamical chaos in quantum systems manifests itself in both statistical properties of energy spectra and morphology of wave functions of the individual stationary states. In this report, we investigate the axial channeling of high and low energy electrons and positrons near direction of a silicon crysta

    The oxygen isotope effect in the ab-plane reflectance of underdoped YBa_2Cu_3O_{7-delta}

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    We have measured the effect of oxygen isotope substitution on the ab-plane reflectance of underdoped YBCO. The frequency shift of the transverse optic phonons due to the substitution of O-16 by O-18 yields an isotope effect of the expected magnitude for copper-oxygen stretching modes with alpha=0.5 +- 0.1. The reflectance shoulder at 400 - 500 cm^-1 shows a much smaller exponent of alpha=0.1 +- 0.1 in the normal state and alpha=0.23+- 0.1 in the superconducting state. These observations suggest that the shoulder is of electronic origin and not due to a phonon mode as has been suggested recently.Comment: 4 pages 2 figure

    Infrared and optical properties of pure and cobalt-doped LuNi_2B_2C

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    We present optical conductivity data for Lu(Ni1x_{1-x}Cox_x)2_2B2_2C over a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials show evidence of being good Drude metals with the infrared data in reasonable agreement with dc resistivity measurements at low frequencies. An absorption threshold is seen at approximately 700 cm-1. In the cobalt-doped material we see a superconducting gap in the conductivity spectrum with an absorption onset at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong coupling. The pure material is in the clean limit and no gap can be seen. We discuss the data in terms of the electron-phonon interaction and find that it can be fit below 600 cm-1 with a plasma frequency of 3.3 eV and an electron-phonon coupling constant lambda_{tr}=0.33 using an alpha^{2}F(omega) spectrum fit to the resistivity.Comment: 10 pages with 10 embedded figures, submitted to PR

    Surface effects in multiband superconductors. Application to MgB2_2

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    Metals with many bands at the Fermi level can have different band dependent gaps in the superconducting state. The absence of translational symmetry at an interface can induce interband scattering and modify the superconducting properties. We dicuss the relevance of these effects to recent experiments in MgB2_2
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