L. Genzel

Systematic Raman and infrared studies of the superconductor YBa2Cu3O7-x as a function of oxygen concentration (0≦x≦1)

Abstract We have studied the variation of Raman and infrared spectra of the new high T c superconductors with oxygen content. For the material YBa 2 Cu 3 O 7-x we varied x between 0 and 1, a range over which the material is known to lose its superconducting properties. We find that the Raman peaks associated with CuO stretching vibration (502 cm −1 , x=0) and the BaO vibrations (154 cm −1 , x=0) decrease in frequency when oxygen is removed, while the bending-stretching modes of the CuO frames (334 and 438 cm −1 , x=0) harden under the same conditions. In the infrared spectra the phonon peaks behave similarly although the vibrations associated with CuO frames appear to be stationary when oxygen is removed. The vibrations of the peak frequencies are explained with changes in the unit cell parameters associated with oxygen removal and with the decreasing electronic shielding resulting from the transition to a semiconducting material. Our results are indicative of conduction in the CuO planes of the superconductor.

Optical conductivity of BCS superconductors with arbitrary purity

Abstract An explicit expression for the complex optical and AC conductivity of a homogeneous BCS superconductor with arbitrary electron mean free path is given. This compact expression and a fast self-contained FORTAN program may be used to fit experimental data. For comparison, we give also the complex AC conductivity of high- T c superconductors containing an elastically pinned, viscously moving flux-line lattice with flux creep.

Temperature dependence of the Raman and infrared phonons of MBa2Cu3O7−δ-type superconductors

An anomalous temperature dependence of the ∼340 cm-1 Raman phonons of YBa2Cu3O7-δ has been recently reported by Macfarlane et al. Here we show that this phenomenon, a phonon softening with decreasing T for T < Tc, is common to many materials of the family (M = Y, Sm, Eu, Ho, Gd). We also show the ir-active Davidov counterpart of this phonon, and another ir-active phonon at slightly lower frequency, also exhibit similar effects. These phonons are attributed to bending vibrations of the Cu2-02 and Cu3-03 bonds. All other ir and Raman phonons observed exhibit a normal temperature dependence (i.e., hardening with decreasing T even for T < Tc). The 310 cm-1 phonon of the tetragonal non-superconducting YBa2Cu3O6 also shows a normal behavior vs. T.

Far-infrared spectroscopy of the high Tc superconductor YBa2Cu3O7-δ

Abstract We report measurements of the far-infrared reflectivity of polycrystalline samples of YBa 2 Cu 3 O 7-δ which have their superconducting transition temperatures in the 90 K range. The measurements performed between 300 K and 10 K reveal temperature-dependent structures attributed to optical phonons in the 150 cm -1 – 350 cm -1 range, and excitation of the electrons across the energy gap. Complicated structures make unambiguous fitting with the Mattis-Bardeen theory difficult, but it seems that the energy gap is around 3.5 kT c for T⪡T c . Additionally, our results indicate the possibility of a second gap at an energy corresponding to 5.4 kT c .

The temperature-dependence of the far-infrared spectra of l-alanine

Abstract Far-i.r. spectra in the region 20–600 cm−1 are reported for solid l -alanine as a function of temperature between 6 and 300 K. Understanding of the temperature-sensitive as well as the temperature-insensitive bands is aided by the normal coordinate analysis of l -alanine. Bands containing contributions from hydrogen bonds, skeletal deformation and NH3+ torsion are found to be highly sensitive to temperature changes. Although the hydrogen bonds are stronger at lower temperatures, no evidence of amide formation is detected even at 6 K. Thus the zwitterion form seems to be more stable than the amide form in the temperature range studied here.

Effect of isotopic substitution of oxygen on Tc and the phonon frequencies of high Tc superconductors

Abstract The Raman and infrared spectra of YBa2Cu318Oβ16O7−β and TmBa2Cu318Oβ16O7−β with various amounts β of 16O substituted by 18O have been measured. The observed phonon frequency shifts of the ir and Raman active modes are compared with predictions from a recently proposed model for the lattice dynamics of these compounds. The application of this model allows a detailed study of the frequency shifts induced by istope substitution of various inequivalent oxygen positions (OI, OII, III, OIV). The resistive transitions of these samples have also been measured. No isotope effect is ound if only one out of seven 16O is replaced by 18O. For 85% substition, a shift of ΔTc = 0.3±0.1 K is observed in agreement with previous determinations.

High Tc superconductors MBa2Cu3O7−x with M = Mixed Rare earths and Y: Low frequency phonon peak engineering

Abstract We report far infrared reflectivity and Raman spectra of the high T c superconductor MBa 2 Cu 3 O 7 with M = Y 0.5 Sm 0.5 , Y 0.5 Ho 0.5 , and Sm 0.5 Ho 0.5 . By mixing the rare earths Sm and Ho with Y we can achieve a region in the infrared around the superconducting energy gap free of the sharp phonon peak observed otherwise. We obtain a gap for M = Y 0.5 Ho 0.5 at ∼220 cm −1 . Also we propose a connection between the softening of a phonon observed in Raman scattering near ∼330 – 340 cm −1 and the quality of the superconductor as described by its transition temperature and width.

Far Infrared Metal Mesh Filters and Fabry-Perot Interferometry

The use of metal meshes is becoming increasingly important for applications in the far infrared. This paper reviews the important aspects of metal meshes, metal mesh filters, Fabry-Perot interferometers and their applications. The article includes the following: an introductory description and historical review of far-infrared metal meshes, the optical properties of metal meshes, beam splitters and filters based on inherent optical properties of meshes, Fabry-Perot interferometers, multimesh filters, applications of metal mesh filters and Fabry-Perot interferometers for far-infrared lasers, for plasma diagnostics and for astronomy.

Far-infrared study of optical phonons in an YBa2Cu3O6 crystal

Abstract We have performed polarized reflectance measurements on a highly oriented multicrystalline block of YBa 2 Cu 3 O 6 for temperatures between 300 K and 10 K in the frequency range between 50 cm -1 and 10000 cm -1 . With the electric field E parallel to the c-axis of the crystal, the reflectance exhibits all of the five theoretically expected infrared active A 2 u modes. Polarization perpendicular to the c-axis reveals five of the six predicted E u modes. A Kramers-Kronig analysis of the spectra allows a comparison of the experimentally obtained eigenfrequencies and oscillator strengths with lattice dynamical calculations. The agreement between experiment and theory is good. The strength of all observed infrared modes appears to be normal, in contrast to the superconducting phase YBa 2 Cu 3 O 7 , where some of the infrared active phonons exhibit unusually high oscillator strengths.

Raman and far infrared studies of YBa2Cu3O7−δ prepared exclusively with 18O

Abstract We present Raman and far infrared spectra of YBa 2 Cu 3 O 7−δ prepared by oxidizing the metallic constituents with 18 O. This method guarantees the isotopic purity of the material, in contrast to preparation based on annealing the 16 O compound in 18 O. The superconducting transition temperature of the material so obtained is T c = 91.9 K (midpoint) thus convincingly excluding a significant isotope shift. The Raman and far infrared phonon frequencies are lowered in energy, especially those involving mainly oxygen motion. The results agree well with those of a lattice dynamical calculation.