I. Pollini

Photoemission study of the bulk and surface electronic structure of Bi(111)

Angle-resolved energy distributions for photoelectrons emitted from the Bi(111) single crystal face are presented at 16.85 eV photon energy. The variation of peak-energy positions as a function of the electron emission angle shows four main dispersing features located between the Fermi level and 5 eV. The bulk electronic structure is found in qualitative agreement with the pseudopotential calculation performed by Golin along the TMU and TQW symmetry lines of the Brillouin zone. The experimental features located at around 0.3 and 3 eV below the Fermi level are sampled in the two-dimensional Brillouin zone along the and symmetry directions (dispersion about 1 eV) and are assigned to electronic surface states.

Quantum conductance in single-and double-wall carbon nanotube networks

The electrical conductance of single- and double-wall carbon nanotube systems was measured by a mechanically controllable break-junction technique using freestanding nanotubes not subject to any chemical modification. For metallic single-wall carbon nanotubes, two channels with transmission coefficient of 0.88 contribute to ballistic electronic transport at room temperature. In double-wall carbon nanotubes, one metallic channel contributes to ballistic electronic transport and additional field- and temperature-dependent two-channel contributions were observed.

Optical and electron energy loss experiments in ionic CrBr3 crystals

The fundamental reflectance of CrBr3 crystals has been measured over the energy range 2-31 eV from 300 to 30 K using synchrotron radiation. The high-frequency dielectric tensor elements epsilon xxT(E) have been determined by Kramers-Kronig analysis. Energy losses of 50 keV electrons incident along the c axis of the crystal have also been measured up to 30 eV. The results have been described in terms of single-particle excitations (charge transfer, inter-band excitons and transitions) and collective valence electron plasma oscillations. The assignments of direct inter-band transitions both at the critical points Gamma and Z and along symmetry lines of the Brillouin zone ( Lambda line) have been made by considering the intersecting-sphere model calculation of the band structure of CrBr3. The fundamental energy gap, assigned to Gamma 3- to Gamma 1+ transitions, at the zone centre is observed around 8.00 eV. Plasma resonance has been observed at 19.8 eV in the electron energy loss spectrum in fine agreement with the maximum of the optical energy loss function. Finally, the fractional ionic character fiDT=0.79 has been evaluated in the framework of the Phillips dielectric theory.

Determination of the optical constants of absorbing isotropic materials from multiangle reflectance with polarized synchrotron radiation

An experimental method for determining the optical constants n and k of absorbing isotropic materials is presented. The calculation of the optical properties is based on the measurement of the oblique-incidence reflectance at different angles, and precise evaluation of the degree of polarization of the ultraviolet synchrotron radiation. The technique is applied to the calculation of the optical constants of thin films of thallium in the energy region of absorption (k ≠ 0), near the 5d-shell excitation threshold (between 900 and 1000 A). The obtained values of n and k present an average error of better than ±0.005.

Valence band photoelectron spectra of CrBr3 insulators

The valence band structure of CrBr3 has been obtained by ultraviolet and X-ray photoelectron spectroscopy. The comparison of the X-ray photoemission valence band spectrum with published band calculations for the ground state within a one-electron formalism shows satisfactory agreement between observed and calculated structures. The contribution of the Cr 3d electrons relative to that of the Br 4p electrons in the photoemission energy distribution has been evaluated and compared with the ratio of photoionization cross sections for Kr 4p6 (calculated) and Cu 3d10 (experimental) at 40.8 eV. The degree of the p/d hybridization has then been estimated to be around 10-15%.