P. Wachter

The blue bronze K0.3MoO3: A new one-dimensional conductor

Abstract The optical reflectivity of the blue bronze K 0.3 MoO 3 has been measured on single crystals for photon energies between 0.03 and 12 eV at temperatures from 10 to 300 K using polarized light. The data are interpreted that this compound is a one-dimensional conductor for temperatures above 180 K and that the metal-semiconductor transition at 180 K is due to a Peierls type transition, leading to a gap of 0.15 eV in the density of states.

Evidence of anisotropic metallic behaviour in the optical properties of carbon nanotubes

We have conducted reflectivity measurements on carbon nanotubes from the far-infrared up to the ultraviolet, in order to achieve the complete excitation spectrum. We have analysed our experimental findings in terms of the Maxwell-Garnett (MG) effective medium model, where small conducting particles are dispersed in a dielectric host-medium. The fair agreement between our experimental data and the phenomenological MG-approach suggests that carbon nanotubes have an intrinsic and anisotropic metallic behaviour.

First observation of a negative elastic constant in intermediate valent TmSe

Abstract The sound velocities v L , v T 1 and v T 2 have been measured at 15 MHz on Tm 3+ 0.87 Se and Tm 2.8+ 0.99 Se between 300 K and 4.2 K and the elastic constants c ij have been derived. In intermediate valent Tm 2.8+ 0.99 Se c 12 turned out to be negative. This sign is interpreted as being typical for intermediate valent compounds. Also for the first time experimental evidence is given for crystal field effects in Tm 3+ 0.87 Se. Strong elastic nonlinearities are observed in intermediate valent Tm 2.8+ 0.99 Se with uniaxial pressure.

Negative elastic constants in intermediate valent SmxLa1−xS

Abstract On cubic SmS-crystals doped with La surface acoustic waves have been measured using inelastic light scattering (Brillouin scattering). The angular dispersion of the velocity of the surface acoustic phonons in the (100)-oriented plane allowed us to calculate the whole elastic tensors (C11, C44 and C12) and Poissons ratio. C12 and Poissons ratio became negative which is typical for intermediate-valent materials as SmxLa1−xS (x ∈ 0.90, 0.75, 0.65) and can be explained by their special electronic structure. Furthermore the valence of all measured crystals has been derived.

Search for cold-fusion events

Abstract In a recent electrochemical investigation M. Fleischmann and S. Pons [1] found some excess heat, which — according to the authors — was due to the cold fusion of deuterium nuclei. Using a very similar experimental arrangement we looked for the characteristic neutrons and gamma rays from the dd fusion reaction. None were found. In addition, LaNi5, a substance which is known to absorb a very large amount of hydrogen, was loaded with deuterium gas. Again no nuclear fusion processes could be observed.

The optical response of carbon nanotubes

Abstract We have conducted reflectivity measurements on carbon nanotubes from the far-infrared up to the ultraviolet (from 20 cm −1 up to 3× 10 4 cm −1 ), in order to achieve the complete excitation spectrum. We have analysed our experimental findings in terms of the Maxwell-Garnett (MG) effective medium model, where small conducting particles are dispersed in a dielectric host-medium. We establish that carbon nanotubes have an intrinsic and anisotropic metallic behaviour.

Evidence of valence fluctuations below the verwey transition of magnetite (Fe3O4) and its relation to high TC superconductors

Abstract For the first time to our knowledge we present a new optical effect in the reflectivity spectrum of synthetic single crystals of magnetite (Fe 3 O 4 ). It consists of many absorption lines in the far-infrared that will be interpreted as an evidence of tunnel modes between different valencies of iron. These will be brought in connection with superconducting fluctuations between Cu 2+ and Cu 3+ ions in the new high T C superconductors. Together with new experimental evidence for the mixed valence situation in these high T C superconductors the model of non-magnetic, exchange induced bipolarons will be promoted.

Optical properties of the red bronze K0.33MoO3

Abstract The optical reflectivity of the red bronze K 0.33 MoO 3 has been measured on single crystals in the spectral energy range between 0.03 and 12 eV at temperatures from 4 K to 300 K using polarized light. The optical constants have been determined by means of a Kramers-Kronig analysis; the data are interpreted that this compound is a 0.5 eV energy gap semiconductor with very strong anisotropy in the infrared and visible energy range.

A comparative study of FMR and Brillouin light scattering on FeNiB alloys

Abstract The magnetization and the g-factor of amorphous Fe40Ni40B20 alloys have been determined by both microwave ferromagnetic resonance, operating at different frequencies, and Brillouin light scattering. All measurements have been performed at the same spot of the same sample in order to compare both techniques. We have found systematic differencies in the magnetization M from the results of FMR and Brillouin scattering experiments. The magnetization determined by Brillouin scattering tends to be 5% lower than that determined by FMR.

The blue shift of the plasma edge of a ferromagnetic semimetal

Abstract Magnetic measurements on nearly stoichiometric TbN single crystals reveal metamagnetic behavior with a T N of 31 K. Excess carriers due to nonstoichiometry make the crystals ferromagnetic with T C = 48 K, a fact which has been revealed by elastic neutron scattering measurements in zero applied field. The compound is a self-compensated semimetal, in which the valence p band of nitrogen has a maximum at the Γ point of the Brillouin zone and the conduction d band has a minimum at the X point. A slight overlap of both bands yields about 10 21 carriers cm −3 . The optical reflectivity and the thermal expansion coefficient have been measured on the ferromagnetic compositions. The plasma edge of the free carriers exhibits a blue shift upon cooling below T C , following a spin correlation function. This is mainly due to an exchange splitting of the conduction d band.