A. Zibold

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.

Spin-waves in the mid-infrared spectrum of antiferromagnetic YBa2Cu3O6.0

The mid-infrared spin-wave spectrum of antiferromagnetic YBa2Cu3O6.0 was determined by infrared transmission and reflection measurements (k∥c) at T = 10 K. Excitation of single magnons of the optical branch was observed at Eop = 178.0 meV. Two further peaks at 346 meV ( ≈ 1.94Eop) and 470 meV ( ≈ 2.6Eop) both belong to the two-magnon spectrum. Linear spin-wave theory is in good agreement with the measured two-magnon spectrum, and allows to determine the exchange constant J to be about 120 meV, whereas the intrabilayer coupling J12 is approximately 0.5J.

Charged magnons and magneto-elastic polarons in the mid- infrared spectrum of YBa2Cu3O6

Abstract The optical conductivity of undoped and low-doped YBa 2 Cu 3 O 6 is studied in detail in the mid-infrared range. Full isotope substitution of 16 O with 18 O is used to (a) study the optical phonons in the far-infrared and (b) identify the rich structure emerging in the mid-infrared from multiphonon and bimagnon-plus-phonon absorption. Substitution of Zn in the CuO 2 planes allows direct observation of bimagnon excitations. Very light chemical doping with oxygen introduces localized charge carriers in the form of magneto-elastic polarons, i.e., holes which are strongly coupled both to the vibrational and magnetic degrees of freedom.

Optical anisotropy in the Pb2Sr2(Y, Ca)Cu3O8+δ superconductor

Abstract We present ellipsometric and reflectance measurements of the polarization dependence of the optical response of Pb 2 Sr 2 (Y, Ca)Cu 3 O 8+δ for light polarized parallel and perpendicular to the Cu 2 O planes on a single crystal. A plasma edge can only be observed in the a -direction, indicating a carrier concentration of 0.28 holes per unit cell which is comparable to the carrier concentration in YBa 2 Cu 3 O 6.3 . In the region between 1.7 and 5 eV several structures are present in the optical spectra and can be analyzed in terms of a band structure calculation by Mattheiss et al. Most noteworthy in this energy region is the absence of a sharp excitation at 4.1 eV suggesting that the Ba-admixture to the final state is important for the appearance of this transition in YBa 2 Cu 3 O 6 .

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.

Ordering of chain oxygen in single-domain crystals of YBCO

Abstract We have performed polarized in-situ reflection measurements on YBa 2 Cu 3 O x single domain crystals (x=6.5) in a temperature range between room temperature and 150°C. The energy range was varied from 150 meV to 6 eV. Using the in-plane anisotropy of the optical properties, we were able to separate the contribution of Cu-O layers and Cu-O chains to the electronic transport properties. Heating up the samples we observed a strong decrease of the chain length, the charge carrier concentration and the Cu + concentration. These results are in agreement with investigations previously performed on quenched YBa 2 Cu 3 O x (x=6.5) samples during room temperature annealing.