H. P. Geserich

Anisotropy of the reflectance spectrum and of the dielectric function of YBa2Cu3O7 within the (001) plane

Abstract We have performed polarized reflection measurements on nontwinned domains of the (001) surface of YBa 2 Cu 3 O 7 single crystals in the energy range between 0.05 and 6 eV. From the spectra, obtained for the polarization direction parallel to the a- and to the b-axis, respectively, we could determine for the first time the tensor components of the dielectric function of YBa 2 Cu 3 O 7 parallel to both axis. The evaluation was done by fitting Lorentz-Drude models and by Kramers-Kronig analysis. Using the experimental data, the contributions of the CuO planes and of the CuO chains to the electronic transport properties of YBa 2 Cu 3 O 7 could be separated.

Optical investigations of the electrical anisotropy of α- and β-(BEDT-TTF)2I3

The anisotropy of the electrical transport properties at room temperature of the α- and β-phase of (BEDT-TTF)2I3 was determined from the polarized reflectance spectra.

Dielectric function of YBa2Cu3O7-δ between 50 meV and 50 eV

Electronic excitations with polarization parallel to the CuO2-planes in single-crystalline YBa2Cu3O7 and YBa2Cu3O6 have been investigated by optical reflectance and by highenergy electron energy-loss spectroscopy in transmission in the energy ranges from 50 meV to 6 eV and 0.2 eV to 150 eV, respectively. From a combination of these experimental data the dielectric function, the reflectivity and the optical conductivity have been obtained in a wide energy range. The observed spectra are interpreted in terms of optical phonons, free-carrier absorption, transitions across the charge-transfer gap, further interband transitions, low-lying core-level excitations, excitonic transitions and valence conservingd-d transitions within the Cu 3d shell combined with O 2p intraband transitions. The charge carrier plasmon and the 4 eV-excitation in YBa2Cu3O6 show quadratic dispersions in momentum transfer. From the dispersion constant of the plasmon a mean value of the Fermi velocity of the charge carriers parallel to the CuO2 planes has been derived.

Optical reflectivity of (SN)x-single crystals

Abstract The optical reflectivity of (SN)x single crystals was measured in the visible region and the near infrared with light polarized parallel and perpendicular to the polymer chain axis. Whereas the reflectivity for light polarized perpendicular to the chain axis is approximately constant in this region, the reflectance for light polarized parallel to the chain axis exhibits a pronounced plasma edge at 2.72 eV. A simple Drude model was used to analyse the data. Differences between these results and those of previous film measurements are discussed.

Optical investigation of the metal-insulator transition in NdBa2Cu3Ox A comparison with YBa2Cu3Ox

Abstract We have performed reflectance measurements on the (001) surfaces of twinned single crystals of NdBa 2 Cu 3 O x . The oxygen concentration x was varied between 6 ≤ x ≤ 7. The results were compared to former investigations on YBa 2 Cu 3 O x . The different dependence of the electronic transport properties on the oxygen content of both compounds can be explained by the different distributions of oxygen amongst the CuO chains.

Polarized reflectance spectra of a (001) surface of YBa2Cu3O7 before and after detwinning

Abstract We have performed reflection measurements on a (001) surface of a YBa 2 Cu 3 O 7 single crystal, using polarized light in the energy range between 50 meV and 6 eV. The crystal was grown with the typical twinned structure of this compound, showing a reflectance spectrum which was independent of the direction of polarization. The spectrum was characterized by free carrier absorption with a plasma edge near 1 eV. By applying uniaxial stress at 450°C the twinned structure was removed. The crystal surface was transformed into a single untwinned domain, resulting in a strong splitting of the plasma edge with respect to the direction of polarization. By fitting a Lorentz-Drude model to the polarized reflectance spectra of the single domain crystal, R ∥ a and R ∥ b , the tensor components of the dielectric function could be determined whereas the reflectance spectrum of the twinned surface represents an average of the components of both crystallographic directions.

Optical anisotropy and electron-phonon coupling in Bi2Sr2CaCu2O8 single crystals

We have determined the polarized reflectance spectra R‖c, R‖c and R‖c of the high temperature superconductor Bi2Sr2CaCu2O8 in the normal state. By fitting a Lorentz-Drude model or by performing a Kramers-Kronig analysis, respectively, the spectral distribution of the tensor components of the real and the imaginary part of the dielectric function were obtained. The optical results reveal a low electrical anisotropy within the (001) plane and a very high anisotropy between the in-plane conductivity and the conductivity perpendicular to this plane. The spectrum R‖c gives evidence for the interaction between infrared-active phonons and the low-energy part of interband transitions.

Ordering of chain oxygen in YBa2Cu3Ox. Optical investigations on single-domain crystals

Abstract We have determined the polarized reflectance spectra of (001) surfaces of YBa 2 Cu 3 O x single-domain crystals with intermediate oxygen content. Using the in-plane anisotropy of the optical properties, we could separate the contribution of the Cu-O layers and the Cu-O chains to the electronic transport properties. Taking the spectra in situ while the samples were heated up from room temperature to 150°C, thermodynamical equilibrium states were established. We observe a strong decrease of the free carrier and the Cu 1+ concentration and the chain length with increasing temperature. Comparing the total number of free carriers with the number of Cu 1+ ions, we find a direct correlation in accordance with a simple chemical model.

Optical investigation of the metal-insulator transition regime in single-domain crystals of YBa2Cu3Ox

Abstract We have determined the polarized reflectance spectra of YBa 2 Cu 3 O x crystals with single-domain (001) surfaces. The oxygen content of the crystals was varied between the superconducting limit x = 7 and the semiconducting limit x = 6. By fitting a Lorentz—Drude model as well as by performing a Kramers—Kronig transformation the spectral distribution of the in-plane tensor components of the cnergy loss function and of the conductivity function were obtained. Using the in-plane anisotropy of the optical properties we were able to separate the contribution of the CuO layers and of the CuO chains to the electronic transport properties. An analysis of the free-carrier concentration shows that there is no sharp metal—insulator transition if x is varied from x = 7 to x = 6. Rather, we find a broad transition regime extending from x = 6.5 to x = 6, where different consecutive localization processes of the free carriers are observed.

Optical properties of orientated (SN)x-films in the visible and near infrared region

Abstract Optical reflectance and transmittance measurements between 0.3 and 5.5 eV were performed on (SN) x -films which showed a parallel orientation of the crystallites with respect to their polymer chain axis. The optical behaviour below 3 eV for light polarized parallel to the chain axis and in particular a strong absorption peak between 1 and 2 eV is explained to be a Maxwell-Garnett resonance of the free carriers. The influence of the free carriers is also observed for light polarized perpendicular to the chains and is analysed by the same model. From the anisotropy of the transport properties the interchain coupling is inferred.