J. Fink

Plasmons in the Heavy Alkali Metals: Strong Deviations from RPA

The q-dependence of plasmon energy and width for the heavy alkali metals K, Rb, and Cs were measured by electron energy loss spectroscopy. For Rb, we found almost no plasmon dispersion while for Cs, the small-q dispersion turned out to be even negative. These results can be explained qualitatively but not quantitatively by recent electron liquid models including exchange and correlations. Going from K to Rb and Cs, the q-dependence of the plasmon width changes from quadratic to linear.

Electronic Structure of Undoped and Doped Polyphenylenevinylene

Poly(phenylenevinylene) (PPV) has a molecular structure between that of polyacetylene and polyphenylene. It is at present the only non-degenerate ground state polymer which can be prepared in a highly oriented form. Therefore, it is of particular interest for studies of the changes of the electronic structure upon doping. According to theoretical models [1] the non-degenerate ground state implies the formation of polarons and of bipolarons at lower doping concentrations. At higher concentrations the transition to the metallic state should be observable. In this contribution, we have studied the electronic structure of undoped and n-type doped PPV by electron energy-loss spectroscopy (EELS).

Electronic Structure of Doped Highly Oriented Polyacetylene

Despite very active investigations of the electronic structure of conducting polymers during the last 10 years, there is a continuous debate on the semiconductor-metal transition and on the nature of the highly conducting state of the prototype polyacetylene (PA). Various models have been proposed. The metallic droplet model explained the transition by a percolation by an increasing number of small metallic regions [1], In the soliton model, the charged defects (“solitons”) form at sufficiently high concentrations a soliton band,and the metallic state is reached due to a disorder-induced quenching of the Peierls distortion [2]. Recently, a first order transition from a soliton lattice to a polaron lattice has been discussed [3]. In this contribution we give a first report on investigations of the band structure of heavily doped highly oriented PA by electron energy loss spectroscopy (EELS). This method gives more information compared to optical spectroscopy because nonvertical transitions can be excited, which give information on the dispersion of the bands.

Electronic Structure of Conducting Polymers by Electron Energy-Loss Spectroscopy

The evolution of the band structure of conducting polymers upon doping is the basis for the understanding of the mechanism leading to the high conductivities observed in these materials. Experimentally, many investigations around the energy range of the fundamental gap have been performed by optical spectroscopy. In addition, the density of occupied states of many conducting polymers has been studied by photoelectron spectroscopy. Another powerful method for investigations of the electronic structure of conducting polymers is electron energy-loss spectroscopy (EELS). It covers a large energy range from 0.1 to ∿2000 eV. Thus, it is possible to study not only excitations of π and ≡ electrons but also of core electrons similar to X-ray absorption spectroscopy (XANES, EXAFS) using synchrotron radiation. Moreover, EELS allows one to perform excitations with varying momentum transfer, thus giving information on the dispersion of bands which cannot be obtained by optical spectroscopy.

Electron energy loss spectra of high-Tc A15 materials

Abstract Using transmission electron energy loss spectroscopy, we have studied the structures near core excitation edges in various A15 compounds. These structures reflect the unoccupied parts of the electronic densities of states N(E). In all materials investigated (Nb3Sn, Nb3Ge and Nb3Al), we have observed pronounced peaks immediately above EF. The peak heights decrease very much for nonstoichiometric samples with low superconducting transition temperatures Tc. Our spectra agree well with tight-binding N(E) curves based on the APW bandstructure work of Klein et al. In particular, we observe larger peaks in Nb3Al and Nb3Ge than in Nb3Sn. This result is consistent with the theoretical prediction that EF shifts from the top part of the high N(E) region in Nb3Sn towards the center and bottom parts in Nb3Ge and Nb3Al.