W. König

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 Cuue5f8O stretching vibration (502 cm −1 , x=0) and the Baue5f8O vibrations (154 cm −1 , x=0) decrease in frequency when oxygen is removed, while the bending-stretching modes of the Cuue5f8O 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 Cuue5f8O 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 Cuue5f8O planes of the superconductor.

Lattice vibrations of Y1-xPrxBa2Cu3O7 : theory and experiment

Abstract We report measurements of the far-infrared reflectance of Y 1− x Pr x Ba 2 Cu 3 O 7 thin films. Detailed understanding of the experimental lineshapes is achieved with the proper treatment of the SrTiO 3 substrate, the finite film thicknesses, the background contribution of free carriers and the mid-infrared band, and the phonon modes. Shell-model calculations predict the frequencies of the infrared-active phonons to be very similar for YBa 2 Cu 3 O 7 and PrBa 2 Cu 3 O 7 with the exception of rare-earth-related vibrations. The experimental frequencies of the in-plane as well as z -polarized infrared-active phonons of PrBa 2 Cu 3 O 7 agree satisfactorily with the calculated ones. An in-plane mode at 270 cm -1 , assigned to vibrations confined to the Y/Pr and neighboring oxygen ions, is found to be sensitive to the rare-earth-ion substitution. We report also the frequencies of the Raman-active B 2g and B 3g phonons of PrBa 2 Cu 3 O 7 , thus giving a nearly complete picture of all k =0 eigenmode frequencies in the RBa 2 Cu 3 O 7 crystal structure.

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 Cu2ue5f8O2 and Cu2ue5f8O3 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.

Raman and infrared results on YBa2Cu3O7−x type materials

Abstract We report both Raman and infrared results on the same semiconducting YBa2Cu3O7-x material. With the appropriate heat treatment, this material can be reversibly changed from an orthorhombic superconductor (x≈0.0) to a semiconductor (x>0.5). In the semiconducting material it is easier to measure the phonon features using either technique, and we observe almost the number of modes allowed. By simultaneously considering the modes observed by both techniques, using group theory to sort out the modes in both phases, and using projection operators to determine the symmetry adapted vectors, we arrive at an understanding of some of the modes. We find few differences when comparing our results to other measurements in the superconducting phase. The two general conclusions that we arrive at are: 1) splittings of the modes due to lowering the symmetry from tetragonal to the superconducting orthorhombic phase are not observed; 2) no frequency renormalizations, of the type that have been discussed theoretically, are observed.

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.

Phonon characterization of Bi2 (Sr1-xCax)2 CuO6+δ by infrared and Raman spectroscopy

Abstract We report the preparation, x-ray diffraction, electrical, and magnetic measurements as well as infrared and Raman spectra of nearly single phase Biue5f8Caue5f8Srue5f8Cuue5f8O ( BCSCO-1 ) samples. The phase has an orthorhombic crystal structure with unit cell parameters a = 5.388 A, b = 5.381 A, c = 24.408 A, and Tc larger than 50 K. Eight ir- and five Raman modes were observed. The temperature dependence of photon features exhibits an anomalous behavior for the ir-active phonon at ∼490 cm−1. The mode assignment is given on the basis of the vibrational properties of the initial oxides and of CuO5 pyramides in YBa2Cu3O6.

Theory of the Energy Levels and Precise Two-Photon Spectroscopy of Atomic Hydrogen and Deuterium

In last few years significant progress has been achieved both in the experimental technique and the theoretical methods for the determination of the energy levels of simple hydrogenic systems. We review recent two-photon spectroscopic measurements performed in Garching and the relevant theoretical predictions for the hydrogen energy levels. Good agreement is achieved when all theoretical contributions are included, showing the importance of recently calculated higher order corrections. .

Phonons in Nd2−xCexCuO4

We have measured far infrared reflection and Raman spectra on high quality single crystals and ceramic samples of the Nd2−xCexCuO4 system. The TO and LO frequencies of all infrared-active phonons were determined from a Kramers-Kronig analysis of the infrared reflection spectra. The frequencies of the detected Raman-active phonons are 228 cm−1 (A1g Nd), 328 cm−1 (B1g O2) and 480 cm−1 (Eg 02). A resonantly enhanced peak at 580 cm−1 is observed for Ce-doped samples only with polarization properties like an A1g phonon. We have also performed lattice-dynamical calculations with parameters extracted from those of perovskites and metallic oxides. The frequencies obtained agree well with our experimental findings. We present the eigenvectors of all zone center phonons. Finally, we also detected a feature at 165 cm−1 in the ab-polarized infrared spectra in Nd2CuO4 which might be of magnetic origin.

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.