J. Kircher

Infrared optical response of YBa2Cu3O7 and PrBa2Cu3O7 An ellipsometric study

Abstract Ellipsometry was used to study (110) and (001) oriented films of YBa 2 Cu 3 O 7 and PrBa 2 Cu 3 O 7 in the mid-and near-infrared spectral regions. Below a photon energy of 0.1 eV, the in-plane component of the dielectric tensor of YBa 2 Cu 3 O 7 is dominated by a Drude term with a squared plasma energy of (3.0 ± 0.3) eV 2 . This “oscillator strength”, and the Lorentzian broadening energy of (0.104 ± 0.005) eV at room temperature, are confirmed by the changes induced in Pr-substituted material, and by low-temperature measurements in the near-infrared. The deviation from the Drude behavior observed above 0.1 eV is accounted for by a broad absorption band with an oscillator strength of (3.6 ± 0.1) eV 2 in YBa 2 Cu 3 O 7 which shifts to higher energies and takes over almost all of the oscillator strength of the Drude term when Y is substituted by Pr. The response to electric fields perpendicular to the planes is much weaker, with an upper bound of 0.63 eV 2 for the squared plasma energy. The in-plane loss function computed from the measured dielectric function follows the Drude-like lineshape, modified by the bound-state absorption band, down to the lowest energy reached in our measurements (0.058 eV).

c-Polarized vibrational modes in La2CuO4 and La1.87Sr0.13CuO4 determined by ellipsometry

Abstract We have measured the c -axis linear optical response of large, high-quality single crystals of La 2 CuO 4 and La 1.87 Sr 0.13 CuO 4 between 30 and 700 cm −1 in the 10–300 K temperature range with a far-infrared Fourier ellipsometer. The response is, like that of an ionic insulator, mainly of phononic origin. Temperature dependent phonon parameters, i.e., TO frequency, oscillator strength, and line width are discussed for all observed modes. The parameters obtained for La 2 CuO 4 at 300 K are compared with different phonon calculations for the tetragonal phase. The eigenvectors resulting from these calculations agree for the 235 cm −1 and 500 cm −1 mode whereas discrepancies exist for the weak low frequency mode, which has not been observed by far infrared spectroscopy so far. The analysis of the measured oscillator strengths in terms of effective charges based on the different proposed eigenvectors favor the assignment of this mode to the motion of Cu together with the in-plane oxygen ions against La.

Interband transitions in YBa2Cu3O7

Abstract Using rotating analyzer ellipsometry we have measured all three components of the dielectric tensor of YBa 2 Cu 3 O 7 from the near-infrared to the vacuum-ultraviolet (0.7–24 eV). In the low-energy region the spectra for a − b polarization can be well modelled by a Drude free-carrier response with a frequency independent scattering rate. Below 10 eV we observe strongly anisotropic interband transitions involving the partially filled valence band and the conduction band. At higher energies the anisotropy becomes smaller. In that region we see two prominent double structures around 16 and 20 eV that we assign to transitions from lower-lying bands with O s and Ba p character to states close to the Fermi level.

Are transverse phonons important for Γ - X-intervalley scattering?

Abstract We employ the rigid pseudo-ion method to calculate the q -dependence of intervalley deformation potentials for GaP from the Γ-point to the X -valley. We find that the fast transverse acoustic phonon (which cannot couple to the symmetry point X ) becomes as important along the XW -direction as the longitudinal acoustic phonon (which is allowed at X ). We have measured the temperature dependence of the lifetime broadenings of the E 0 -gap of GaP with spectroellipsometry in order to obtain an effective intervalley phonon energy from which the relative contributions of the different phonon branches can be determined, but difficulties arise as intervalley and intravalley (Frohlich) exciton-phonon interaction both contribute to the broadenings.

Electronic Raman scattering in YBa2Cu4O8

We report on the electronic Raman scattering in YBa2Cu4O8 and find that, In contrast to the other superconducting cuprates, the continuum in the normal state is strongly temperature ⪤pendent in all polarizations. This temperature dependence is found to follow a Bose-Einstein-like form. We conclude that the absence of the Bose-Einstein factor in the electronic continuum is not essential to high-Tc superconductivity, as is sometimes implied. Additionally, we report on the rearrangement of the continuum which occurs belowTc.

Temperature dependence of the dielectric function and the interband critical-point parameters of GaP

We have used a rotating analyser type of spectroscopic ellipsometer to measure the dielectric function ϵ(ω) of GaP from 10 to 640 K in the 1.6−5.6 eV photon energy region. By performing a line shape analysis of the observed structures in d2ϵdω2 with the Aspnes equation, the interband critical-point (CP) parameters (strength, threshold energy, broadening, and excitonic phase angle) and their temperature dependence have been determined. Special emphasis is put on the E0′, E0′ + Δ0′, and E2 CPs. We determine the spin-orbit splitting Δ0′ of Γc15 to be 160 ± 10 meV. The observed decrease in energy of the CPs (after correction for the effect of thermal expansion) and the corresponding increase in broadening with increasing temperature agree reasonably well woth results of a calculation which takes into account the Debye-Waller and self-energy terms of the deformation potential type electron-phonon interaction. These calculations are based on an emperical pseudopotential band structure and a phonon shell model fitted to neutron scattering data and evaluate the strength of the deformation potential interaction using perturbation theory (rigid pseudoion method).

Raman scattering in high-Tc superconductors: electronic excitations

Since the discovery of the high Tc superconductors Raman scattering has proven to be an excellent technique to characterize them and to investigate basic physical properties relevant to the elusive mechanism responsible for their superconductivity. We discuss here several aspects of the technique as applied to superconductivity, including scattering by lattice vibrations, magnetic excitations, and electronic excitations, with particular emphasis on the latter, both in the normal and the superconducting state.

Mid- and near-IR ellipsometry of Y1−xPrxBa2Cu3O7 epitaxial films

We have determined the optical response of Y1−xPrxBa2Cu3O7 for several compositions x from 0 to 1 using multiple-angle-of-incidence ellipsometry. The spectral range from 58 to 1800 meV was covered with two Fourier transform ellipsometers and a conventional dispersive rotating-analyser instrument. Epitaxial (001) films with very smooth surfaces were grown by pulsed laser deposition from ceramic targets on strontium titanate. The gradual reduction in the superconducting transition temperature with x increasing from 0 to 0.5 is found to be accompanied by pronounced changes in the normal-state optical response. We exploit the advantage of both real and imaginary parts of the response functions being obtained simultaneously at each photon energy. This enables us to investigate specific spectral features of the dielectric function, conductivity, and loss function.

Theoretical and experimental investigation of the linear optical response of YBa2Cu4O8: The relevance of LDA calculations

Abstract We report results of ellipsometric measurements of the dielectric tensor of YBa 2 Cu 4 O 8 as well as calculations employing density functional theory in the local density approximation (LDA). For the computation of the one-electron eigenvalues and states the linear-muffin-tin-orbital method (LMTO) was used. Measured and calculated results show good agreement. In particular, the calculation predicts different band structures for YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 in the vicinity of the Fermi energy which lead to significant differences in the optical spectra. One such difference is a region of small ϵ 2 in the near infrared which is a fingerprint of the high (relative to the top of the highest fully occupied band) Fermi energy as compared to the situation in YBa 2 Cu 3 O 7 . Our experimental results confirm the theoretical findings.

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 .