M. Bauer

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.

Raman and infrared studies of the oxygen deficient semiconducting phase of the superconducting cuprate perovskites

Abstract We report Raman and far infrared spectra of the semiconducting (O6) defect perovskites MBa2Cu3O6 and compare them to the spectra found in the high Tc superconductors (O7). Corresponding peaks are observed in both types of materials, differences between the spectra are pointed out and explained. We use the results of a lattice dynamical calculation (shell model) for the assignment of the observed modes and display the phonon eigenvectors calculated at k ≈ 0. The agreement between the calculated and observed frequencies is satisfactory.

Infrared and Raman spectra of the new superconducting cuprate perovskites MBa2Cu3O7, M =Nd, Dy, Er, Tm

Abstract We report far-infrared and Raman studies of several ceramic superconducting cuprate perovskites MBa 2 Cu 3 O 7 with M=Nd, Dy, Er, Tm. Systematic studies of the phonon-related peaks show that the frequencies of optical phonons related to vibrations of M atoms decrease slightly with increasing atomic number of the rare earth, thus confirming the trend observed earlier for M=Y, Sm, Eu, Gd, Ho. The temperature dependence of phonon features exhibits an anomalous behavior for the optical phonons at ∼310 cm −1 and 280 cm −1 . Such anomalous temperature dependence of phonons attributed to the bond bending vibrations of Cu2O2 and Cu2O3 bonds, seems to be a universal property of all superconducting cuprates. Possible origins of the phonons softening and the systematics of the variation of phonon frequencies with the ionic radius of M are discussed.

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.

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.

Determination of the gap distribution in YBa2Cu3O7 using a far-infrared reflection-Fabry-Pérot device

This paper reports on far-infrared measurements of YBa2Cu3O7 films oriented with the c-axis perpendicular to the surface, by using a silicon reflection Fabry-Pérot interferometer as a multireflection device. From these we could derive the dielectric function, the refractive index, the field penetration depth and the surface impedance of the material. The one order of magnitude higher sensitivity of the method compared to a direct reflectance measurement allowed to find an almost continuous gap distribution in the 70–215 cm−1 region together with a separate gap at about 330 cm−1. A quasizero gap absorption is found down to 20 cm−1 even at low temperatures (10 K).

Far-infrared investigations on YBa2Cu3O7 films with the use of a reflection Fabry-Perot-interferometer

Abstract We report on far-infrared measurements of an YBa2Cu3O7−δ film with the c-axis oriented perpendicular to the surface, by using a silicon-reflection Fabry-Perot interferometer as a multireflection device. The much higher sensitivity of the method compared to a direct reflectance measurement allowed the determination of a continuous distribution of superconducting energy gaps with two regions from 75–215 cm-1 and from 330–360 cm-1. A quasi-zero gap absorption is found down to 20 cm-1, even at low temperatures (10 K).

Observation of an energy gap in the far infrared spectra of iron-doped YBa2Cu3Ox superconductors

We have measured the far infrared reflectance of superconducting YBa2(Cu1−cFec)3Ox ceramics with an iron concentration c in the range of O < c < 0.06 at temperatures between 10 and 300 K. For c larger than 0.01, the energy gap structure shifts into a frequency range, where the reflectance is no longer dominated by infrared active phonon modes, thus allowing an accurate determination of the gap value. Although the decrease of the transition temperature Tc is only moderate for such small iron concentrations, we observe a strong reduction of the energy gap Δ with increasing c, i.e. a decrease in the ratio 2ΔkBTc. The results are qualitatively similar to the predictions of the Abrikosov-Gorkov theory for conventional BCS-superconductors with paramagnetic impurities. We cannot, however, conclude that paramagnetic pair breaking dominates the gap supression, since samples without iron doping also show this behaviour when the oxygen content is reduced (x≤6.85).