G. Roth

Crystallographic study of tetragonal, superconducting YBa2(Cu0,93Fe0.05)3O7

Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal- and defect-structure of YBa2(Cu0.93Fe0.05)3O7. Crystals of this compound are superconducting (Tc≈80 K) and appeart to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa2Cu3O7 phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site too, leading to the approximate formula YBa2(Cu0.97Fe0.03)2(Cu0.86Fe0.1□0.04)O7. High resolution electron microscopy reveals the existence of orthorhombic micro-domains, 20–30 Å in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.

Phonon density-of-states for high-Tc (Y, RE) Ba2Cu3O7 superconductors and non-superconducting reference systems

From inelastic neutron scattering experiments, we determined the generalized phonon density-of-states (PDOS) for the high-Tc superconductors MEBA2Cu3O7 (ME=Y, Y0.5Pr0.5, Nd) and the non-superconducting references YBa2Cu3O6 and PrBa2Cu3O7. A detailed study of the Y-O7 compound at temperatures between 300 K and 6 K gave no evidence of any pronounced soft-mode behaviour. Moderate changes only occur in the PDOS between Nd and Pr samples and Y and Y0.5Pr0.5 samples, respectively, but drastic differences show up in the Y-O7 and Y-O6 spectra. Thus, we could not find correlations betweenTc-values and particular features in the PDOS although there is some evidence that electron-phonon coupling is strong in this class of materials.

Crystal structure and specific heat of YBa2(Cu1-xZnx)3O7-†(x=0.01 ... 0.1, †=0;0.9)

Abstract Properties of a series of Zn-doped YBa2Cu3O7-samples prepared under identical conditions have been studied by powder neutron diffraction (20 K, 270 K), powder X-ray and electron diffraction (290 K), by measurements of specific heat Cp (1.4 K–300 K) and Meissner effect (4 K–100 K). Tc decreases linearly at about 10 K /at% Zn. The Meissner signal is still present for high Zn contents. In contrast to this, the transition in Cp vanishes at x≈0.03. Zn is quantitatively dissolved even at x=0.1. The Cu(1)-(chain-) site incorporates about 50% of the total Zn content, while the two equivalent Cu(2)-(plane-) sites contain 25% each. The occupancy of the O(1)-(chain-) site decreases with increasing Zn content according to the approximate formula YBa2(Cu1-xZnx)3O7-x/2. This leads to an almost constant average copper oxidation state, independently confirmed by iodometric titration. Our findings are discussed in terms of a transition to gapless superconductivity upon Zn-doping.

Is Nd2−xCexCuO4 an electron-superconductor?

We present results from electron energy-loss spectroscopy on Nd2−xCexCuO4. There is no clear evidence for Cu1+ but for holes at the oxygen sites. The valency of Ce is found to be close to 3.4. Thus our results do not corroboraten-type superconductivity in this compound.

Neutron scattering study of intermolecular excitations in solid C60 and C70

The spectrum of intermolecular excitations in solid C60 and C70 is investigated by neutron time-of-flight spectroscopy. The quasielastic scattering above the ordering transition is studied in detail and compared to a rotational diffusion model. Below the transition the scattering spectra have essentially a three peak structure with a prominent libronic excitation near ±2 meV for both fullerenes. A detailed analysis gives evidence for a broader distribution of librational modes with an additional pronounced maximum near 5 meV.

Copper deficiency in YBa2Cu3O7−δ

Abstract Combined information from single crystal X-ray as well as powder and single crystal neutron diffraction on YBa 2 Cu 3 O 6 and YBa 2 Cu 3 O 7 samples strongly suggests that a considerable amount of copper deficiency (several percent), presumably due to vacancies, exists on the Cu(1) site in these compounds. The observation that even O 7 -samples containing as much as 9% vacancies on the Cu(1) site are still superconducting at liquid nitrogen temperature sheds a new light on the importance of the Cu(1)-O(1) chains in the 1:2:3 superconductors.

Lattice dynamics of MeBaCuO superconductors

Abstract From inelastic neutron scattering experiments we determined the generalized phonon density-of-states (PDOS) for the high-T c superconductors ME Ba 2 Cu 3 O 7 (ME = Y, Y .5 Pr .5 ,Nd) and the non-superconducting references YBa 2 Cu 3 O 6 and PrBa 2 Cu 3 O 7 . No indications for a pronounced temperature dependence of spectra or correlations between transition temperatures and particular features in the PDOS were found, although, there is this class of material.

Crystallographic study of the tetragonal high-Tc-superconductor YBa2(Cu0.95Fe0.05)3O7

Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal and defect-structure of YBa 2 (Cu 0.95 Fe 0.05 ) 3 O 7 , Crystals of this compound are superconducting (T c 80 K) and appear to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa2Cu307 phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site. too leading to the approximate formula YBa 2 (Cu 0.97 Fe 0.03 ) 2 (Cu 0.86 Fe ·1 [1 0.04 )O 7 . High resolution electron microscopy reveals the existence of micro-domains, 20–30 d in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.