Attenuation of a Stoneley wave and higher Lamb modes due to the scattering by two-dimensional irregularities of the walls of a fluid-filled borehole

Attenuation of Stoneley waves and higher Lamb modes propagating along an irregular surface of a fluid-filled borehole is investigated. This problem generalizes the problem on the attenuation of Rayleigh waves by an irregular surface of an empty borehole [10]. The technique used to evaluate the attenuation coefficient is based on the perturbation method (surface irregularity heights are considered to be small in comparison with the wavelength) and the mean field method. As a result, an expression is obtained for the partial coefficients of the eigenmode attenuation due to the scattering of eigenmodes by the irregularities of the borehole walls into the same or other eigenmodes, as well as into the bulk longitudinal and transverse waves. The frequency-dependent behavior of the partial attenuation coefficients of both Stoneley waves and higher modes is analyzed against the ratio between the irregularity correlation length and the borehole radius for different correlation functions of irregularities.     

The new frozen spin target at MAMI

The new frozen spin polarized target for experiments at the polarized beam of the real photon facility A2 of the MAMI accelerator is described. The A2-collaboration at the Mainz Microtron MAMI is measuring photon absorption cross section using circularly and linearly polarized photons up to the energy of 1.5 GeV. The photons are produced in the’ Bremsstrahlungs’ process. In the years 2005/2006 the Crystal Ball detector with its unique capability to cope with multi photon final states was set up in Mainz. Since 2010 the experimental apparatus has been completed by a polarized target. The horizontal dilution refrigerator of the Frozen-Spin Target has been constructed and is operated in close cooperation with the Joint Institute for Nuclear Research in Dubna, Russia. The system offers the opportunity to provide longitudinally and transversely polarized protons and deuteron. In this paper the operation experience of this new Frozen-Spin Target and first results from the runs in 2010 and 2011 are presented.     

Studying acoustic-wave fields in langasite-family crystals using the BESSY II synchrotron radiation source

The process of the excitation and propagation of pseudosurface acoustic waves in crystals of the langasite family is studied via X-ray diffractometry for the first time. The investigations are carried out using the BESSY II synchrotron radiation source in the double-crystal X-ray diffractometer scheme. The process of pseudosurface acoustic wave propagation is studied based on an analysis of the diffraction spectra of acoustically modulated crystals. Both the velocities of the pseudosurface acoustic waves and the power flow angles of the acoustic energy are measured for the first time. The pseudosurface acoustic wave is shown to be flowing. Surface and pseudosurface acoustic waves generated by an interdigital transducer in the Z cut of a La₃Ga_5.5Ta_0.5O₁₄ crystal are compared.     

Pulsed laser deposition of Co and growth of CoSi2 on Si(111)

Monolayers of Co were prepared by pulsed laser deposition (PLD) and thermal deposition (TD) on Si(111) substrates. The surface structure and morphology were studied as a function of annealing temperature with scanning tunneling microscopy and low-energy electron diffraction (LEED). Comparing PLD to TD in the monolayer regime, we find surface phases with lower Co content in the top layer for annealing temperatures below 500 °C, indicating an implantation of Co into the Si substrate. The implanted Co leads to an increased intermixing of Co and Si and a higher density of nucleation centers for Co multilayers produced by PLD compared to TD. Multilayer PLD films therefore show an improved film quality, which is detected by an increased intensity of the LEED reflectivity. PACS 68.37.Ef; 61.14.Hg; 68.55.-a     

Schottky barrier formation in the low metal coverage limit

A theoretical discussion of the state of the art in the field of Schottky barrier formation is presented. The thin metal layer limit is analyzed for Al and In on GaAs(110), and the adsorption sites are determined as a function of the metal coverage. Intrinsic and extrinsic charge neutrality levels are also discussed for the In-case.     

Determination of the second order susceptibility tensor in banana-shaped liquid crystals

Optical second harmonic generation (SHG) measurements have been carried out on the B2 phases of three liquid crystal materials composed of conventional banana-shaped molecules (P-10-O-PIMB, P-8-O-PIMB and P-8-PIMB). Quantitative values of the second order susceptibilities d_ij have been deduced from SHG measurements at oblique incidences on polydomain racemic samples under an electric field. The d_ij coefficients are in the range 1-10 pm V^-1.     

The revised structure of mortonin

The structure 1 previously proposed tor mortonin, is revised to 2 based on spectroscopic and chemical evidence, and on biogenetic considerations.     

Additivity of the hydrogen bridges in several polymers of formamide

The purpose of the study was to examine the energy additivity of the hydrogen bridges in formamide polymers. Consideration was given to both the linear polymers observed in polypeptides, and to the cyclic polymers observed in the crystal structure. Additivity was found in several geometries. The linear polymers were considered as far as the tetramer, and the cyclic as far as the heptamer.Work supported in part by the Instituto Fisica, Universidad Nacional Autónoma de Mexico and Conacyt, México.