K. Widder

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.

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.

Growth and characterization of (Y, Pr)-123 single crystals: influence of Al contamination and post-annealing under oxygen pressure

Abstract (Y 1− x Pr x )Ba 2 Cu 3− z Al z O 7−δ (0 ≤ x ≤ 1) single crystals of large surface area (up to 9 × 8 mm 2 ) and thickness between 0.3 and 1.5 mm were grown in Al 2 O 3 crucibles using a self-flux method. The crystals were characterized by means of AC susceptibility, X-ray diffraction, energy dispersive X-ray spectrometry, optical reflectance measurements and Raman spectroscopy. To ensure that the crystals are fully oxygenated, post-annealing of the crystals was performed under high oxygen pressure (100–200 bar) in a home-made autoclave. With increasing Pr content a strong reduction of T c , a softening of the Raman B 1g mode, a hardening of the Raman A 1g mode, a red shift of the plasma edge and a strong decrease of the in-plane optical conductivity in the infra-red region were observed. The Raman data indicate that the valency of Pr is close to +3. EDX measurements as well as Raman measurements show, that the Al contamination of the crystals (which results from crucible corrosion) increases with increasing Pr content. The complete disappearance of the free carrier concentration which was determined by optical reflectivity measurements is affected by the Al contamination.

Charge carrier distribution in Y1−zCazBa2Cu3Ox

Abstract We have performed polarized reflection measurements on the (001) plane of various single domain crystals of Y1−zCazBa2Cu3Ox with different Ca and 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. It was found that the additional carriers introduced as a result of the Ca doping only appear in the CuO layers, while the doping of the chains is even reduced because of an increase of oxygen vacancies with increasing Ca doping for samples with an oxygen content close to seven. The superconducting transition temperature Tc could be correlated with the free carrier concentration in the CuO layers, indicating a comparable phenomenon of overdoping in the 1-2-3 compound as it has been found in other high Tc cuprates.

Dielectric Function of YNi2B2C between 10 meV and 50 eV

We have performed reflectance and EELS measurements on the quaternary superconductor YNi2B2C. Linking together the results of these two types of experiments we derived the dielectric function in the energy range between 10 meV and 50 eV. The data were analysed by using a generalized Drude-Lorentz model, where retardation effects resulting from the electron-phonon interaction were taken into account to describe the free-carrier response. For the d.c. coupling constant a value of λtr ~ 1.2 was obtained.

Optical investigation of Y1−zPrzBa2Cu3O7−δ single-domain crystals. A comparison between impurity-free and Al doped samples

Abstract We have determined the polarized reflectance spectra of the (001) plane of different single-domain Y 1− z Pr z Ba 2 Cu 3 O 7−δ crystals, grown in a non-corrosive BaZrO 3 crucible. Separating the conductivity of the CuO chains and CuO layers we are able to determine the influence of Pr doping onto the two different kinds of CuO networks. For a comparison we also present data from crystals grown in Al 2 O 3 crucibles. For an explanation of those results we propose a quite different state of oxygen order in the chain-plane.

Optical investigations of oxygen ordering and persistent photo-doping in tetragonal YBCO

Abstract We present optical measurements of the tetragonal phase of YBCO. The measurements give strong evidence that the doping for low oxygen content is driven by the clustering of oxygen atoms in the chain plane, giving rise to metallic islands with local orthorhombic structure. By increasing the oxygen content, a percolation transition to macroscopic metallic behaviour takes place at an oxygen content between x =6.1 and x =6.2. The state oxygen order in the chains can be changed by either annealing the sample or by a long-time illumination with visible laser light. From the optical investigation of this persistent photo-conductivity effect, it can be concluded that the additional conductivity due to illumination is most likely due to oxygen order and not to a charge transfer and trapping of electrons at some defects.

Optical investigation of the metal-insulator transition in Y1−xPrxBa2Cu3O7−δ

Abstract We have performed reflectance measurements on (001) surfaces of several Y 1−x Pr x Ba 2 Cu 3 O 7−δ single crystals in an energy range from 50 meV up to 6 eV. The Pr content x was varied between x=0 and x=1. Using a Kramers-Kronig transformation and fitting a Drude-Lorentz model to the reflectance spectra, we determined the dynamic conductivity. On the basis of optical sum rules we discuss the controverse question if the supression of the metallic and superconducting behaviour with increasing Pr content is the result of a localization of the free carriers or the compensation of the oxygen doping.

Dielectric properties of YNi2B2C

The dielectric function of the quarternary superconductor YNi2B2C has been derived in an energy range from WmeV up to 50eV by linking together optical reflectance and electron energy loss measurements, obtained from polycrystalline samples at room temperature. The low energy part of the spectrum was additionally investigated in a temperature range from 10K up to 300K. In order to determine the anisotropy of this compound, the reflectance spectra of a single crystal were taken, using polarized light with polarization along the crystallographic axes. The single crystal data exhibit a normal Drude-like metallic behaviour, while the polycrystalline data can only be interpreted by means of a strong energy dependent scattering rate, which has been interpreted as resulting from strong electron-phonon coupling.

Influence of the Oxygen Content on the Optical Conductivity Function of YBa2Cu3Ox (6 ≤ × ≤ 7)

We have performed polarized reflection measurements on twinned single crystals of the system YBa 2 Cu 3 O x in the energy range between 50 meV and 6 eV and in the stoichiometry range between x=6 and x=7 at room temperature. By fitting a Lorentz-Drude model and by Kramers-Kronig analysis the spectral distributions of the components of the conductivity function parallel and perpendicular to the ab-plane were determined, showing contributions of infrared active phonons, free carrier absorption and interband transitions.