Giant nonadiabatic effects in layer metals: raman spectra of intercalated graphite explained

The occurrence of nonadiabatic effects in the vibrational properties of metals has been predicted since the 1960s, but hardly confirmed experimentally. We report the first fully ab initio calculations of nonadiabatic frequencies of a number of conventional (hcp Ti and Mg) and layered metals (MgB2, CaC6, and other intercalated graphites). Nonadiabatic effects can be spectacularly large (up to 30% of the phonon frequencies) in both cases, but they can only be experimentally observed in the Raman spectra of layered compounds. In layered metals nonadiabatic effects are crucial to explaining the observed Raman shifts and linewidths. Moreover, we show that those quantities can be used to extract the electron momentum-relaxation time.     

Optical properties of the semiconductor 7, 7′, 8, 8′, tetracyanoquinodimethane (TCNQ)

Summary The reflectivity and absorption spectra of 7, 7′, 8, 8′ tetracyanoquinodimethane thin films, in the visible and near-ultraviolet range, have been measured. Samples have been grown and characterized by means of elemental microanalysis and differential scanning calorimetry (DSC) in order to obtain the purity degree. The experimental optical spectra are interpreted on the basis of a calculation of the electronic molecular energy levels by the complete neglect of differential-overlap method in a new parametrization scheme. Work partially supported by the Ministero della Pubblica Istruzione through the G.N.S.M.     

Thermomodulated A-band absorption of KBr: Tl

A marked oscillator-strength decrease of the A band in KBr: Tl has been pointed out by means of thermally-modulated optical measurements throughout the temperature range 88–380°K. The experimental results suggest the existence of a clear correlation between the states responsible of the absorption bands and are discussed in order to clarify the role of the thermal perturbation upon Tl⁺ energy levels.     

Optical properties of the layer compound GaTe

The optical constants of layer compound GaTe have been obtained in the energy range 1.0 to 6.0 eV from near normal incidence reflectivity spectrum. Data have been processed to yield the real and imaginary parts of the refraction index and of the dielectric constant. Band parameters have been also empirically estimated.     

Multiple ionic-plasmon resonances in naturally occurring multiwall nanotubes: infrared spectra of chrysotile asbestos

Chrysotile asbestos is formed by densely packed bundles of multiwall hollow nanotubes. Each wall in the nanotubes is a cylindrically wrapped layer of Mg(3)Si(2)O(5)(OH)(4). We show by experiment and theory that the infrared spectra of chrysotile present multiple ionic-plasmon resonances in the Si-O stretching bands. These collective charge excitations are universal features of the nanotubes that are obtained by cylindrically wrapping an anisotropic material. The multiple plasmons can be observed if the width of the resonances is sufficiently small as in chrysotile.     

Doping in carbon nanotubes probed by Raman and transport measurements

In situ Raman experiments together with transport measurements have been carried out on carbon nanotubes as a function of gate voltage. In metallic tubes, a large increase in the Raman frequency of the G(-) band, accompanied by a substantial decrease of its linewidth, is observed with electron or hole doping. In addition, we see an increase in the Raman frequency of the G(+) band in semiconducting tubes. These results are quantitatively explained using ab initio calculations that take into account effects beyond the adiabatic approximation. Our results imply that Raman spectroscopy can be used as an accurate measure of the doping of both metallic and semiconducting nanotubes.     

Non-factorization of the semi-inclusive neutrino-proton cross section

In a neutrino-proton experiment performed with the bubble chamber BEBC, which was exposed to the wide-band neutrino beam of the CERN SPS, the semi-inclusive reaction νp→μ^?h^± + X is studied where h^± is a hadron of a chosen type. Non-factorization of the semi-inclusive cross section is investigated in terms of double moments with respect to Bjorken-x and the energy fraction z and comparisons are made with predictions of perturbative QCD in next-to-leading order.