2,220 research outputs found
Geometrical optics method in the theory of channeling of high energy particles in crystals
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
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
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
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
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
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}
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
We present optical conductivity data for Lu(NiCo)BC 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 MgB
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
MgB
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