W. Bauhofer

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.

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 .

Raman scattering in the high Tc superconductors MBa2Cu3O7-x

Abstract We report Raman scattering measurements of MBa 2 Cu 3 O 7 ( M = Y , Sm , Eu ) polycrystalline superconductors in the region of Cu-O stretching vibrations. Four peaks are seen. On the basis of the factor group analysis given here they are assigned to two Raman allowed and two ir-allowed LO modes. The latter are probably induced by disorder (e.g. O-vacancies).

Struktur und eigenschaften von Gd2XC (X Cl,Br,I)☆

Zusammenfassung Die Verbindungen Gd 2 XC (X  Cl, Br, I) wurden erstmals prapariert (Umsetzung von GdX 3 , Gd, Graphit in zugeschweiβten Ta-Kapseln bei 1250 bis 1500 K). Nach Rontgenuntersuchungen (Gd 2 IC, Einkristall, P 6 3 / mmc , a = 380,10(3). pm, c = 1479,2(2) pm, 135 symmetrieunabhangige Reflexe, R = 0,048; Gd 2 BrC, Pulver, isotyp mit Gd 2 IC, a = 378,58(5) pm, c = 1420,9(2) pm; Gd 2 ClC, Pulver, R 3 m , a = 369,02(2) pm, c = 2030,8(1) pm) liegen Gd-C-Gd-Schichten vor, die uber Schichten von X-Atomen verknupft sind. Die Umgebung der Cl-Atome ist trigonal antiprismatisch, die der Br- und I-Atome trigonal prismatisch. Die unterschiedlichen Anordnungen werden auf hohere Polarisierbarkeit von Br − und I − zuruckgefuhrt (starkere X p-d-Mischung). Die Strukturtypen werden mit Extended-Huckel-Rechnungen analysiert. Alle Verbindungen sind metallische Leiter; Gd 2 ClC ordnet bei 32 K antiferromagnetisch, Gd 2 BrC bei 110 K und Gd 2 IC bei 182 K ferromagnetisch.

Study of the ferroelectric phase transition of TlGaSe2 by dielectric, calorimetric, infrared and X-ray diffraction measurements

AbstractThe real part of the dielectric constant ε′, the heat capacitycp, the infrared reflectivity, and the X-ray diffraction of TlGaSe2 have been measured in the temperature range from 12 K (30K) to 300 K. Both ε′ andcp show two anomalies at about 110 K and 120 K. A study of the hysteresis loop as well as an investigation of the dielectric dispersion in the microwave region show that the phase below 110 K is ferroelectric. The crystal structure remains nearly unchanged in the course of the phase transition. The loss of the

Dynamic percolation in an insulator-conductor composite: Polyethyleneoxide-polypyrrole, as studied by a.c. conductivity measurements

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Preparation of lanthanum hexaboride by electrolysis and measurements of the Raman-active phonons

symmetry (C2/c→Cc) results from small positional shifts of the T1 atoms in the ab plane accompanied by a discontinuity in the axial ratios. We suggest, that the ferroelectricity is caused by the stereochemically active electron lone pair configuration of the Tl+ ion. Thus TlGaSe2 may provide the first example for ferroelectricity caused by this mechanism.