H. Rietschel

Phonon density-of-states for the high-T c superconductor La1.85Sr0.15CuO4 and its non-superconducting reference La2CuO4

From inelastic neutron-scattering experiments, we determined the generalized phonon density-of-states for the high-Tc superconductor La1.85Sr0.15CuO4 and its semiconducting reference La2CuO4 atT=300K andT=6K. The spectra differ only little from each other. In particular, we do not observe any pronounced soft-mode behaviour either on cooling or when comparing the superconducting sample to the non-superconducting reference. We find, however, modest but distinct indications of coupling between electrons and oxygen breathing-modes at high frequencies.

Thermal expansion, sound velocities, specific heat and pressure derivative of Tc in YBa2Cu3O7

We report measurements, between helium and room temperature, of the thermal expansion α(T), sound velocitiesvL(T) andvT(v) and specific heatC(T) of YBa2Cu3O7 samples, all cut from the same sintered pellet. A linear term in α(T) is resolved at low temperatures. It is compared with a corresponding term in the low temperature specific heat and, using the bulk modulus from our sound propagation measurements, a Grüneisen parameter of usual size (1.2±0.4) is derived-in conformity with the possible existence of non-superconducting carriers well belowTc. Longtime drift effects in the length of the sample between 30 K and 60 K point to the vicinity of a structural instability. The Debye temperature derived fromC(T) amounts to about 450 K atT=Tc=91 K and forT→0 approaches 350 K, in agreement with the value deduced fromvL(T) andvT(T). The superconducting transition is indicated by anomalies with idealized discontinuities Δα and ΔC. A thermodynamic relation between Δα and ΔC yields the relative pressure derivative ofTc,tp=Tc−1(∂Tc/∂p)p→0=+(0.7±0.2)·10−7 kbar−1.

Raman scattering study of C120, a C60 dimer

Abstract A modified preparation method and results of a Raman scattering study for C 120 , a dimer of C 60 , are reported. The Raman features of C 120 are compared with those of the dimer-like oxides C 120 O and C 120 O 2 , phototransformed C 60 and with calculations.

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.

Strong changes in the phonon spectra of 123 superconductors by varying oxygen concentration

By means of inelastic neutron scattering we have studied the phonon densities of states (PDOS) ofY based 123 superconductors with oxygen concentrations varying between O7 and O6. We find drastic changes in the PDOS above 40 meV which develop in a systematic manner if we switch from superconducting to semiconducting samples. Model calculations clearly show that this cannot be explained solely by structural changes but is likely to reflect strong differences in the electron-phonon coupling.

Magnetic properties of the high-Tc superconductor La1.85Sr0.15CuO4

For the high-Tc superconductor La1.85Sr0.15CuO4, we present results for the low-field magnetization and the lower and upper critical fieldsBc1(T) andBc2(T). ForBc1(0) we find a value of about 20 mT. Extrapolating our resistive data for the transition midpoint taken in fields up to 12 T, we deduce a value of about 50 T forBc2(0). TheBc2 values obtained by inductive measurements are significantly lower. We explain this by a largeBc2 anisotropy due to the layered structure of these compounds. Losses in the Meissner state observed in magnetization and susceptibility measurements disappear after powdering the sample. This is taken as strong evidence for intergrain Josephson coupling in the bulk sample.

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.

Selected aspects of the lattice dynamics in high-Tc oxide superconductors

Abstract We present selected results of inelastic neutron-scattering experiments on single-crystalline La 2 CuO 4 . For this compound we find very unusual features in the nuclear scattering law S( K , ω) such as strong asymmetries in S( K , ω) for K = τ ±q and in particular an unexpected splitting of the breathing mode. For YBa 2 Cu 3 O 7−δ , very recent measurements seem to exhibit similar extra modes. A possible explanation for these pronounced anharmonicities would be hybridization of lattice modes with electronic excitations thus indicating very strong electron-lattice coupling.

Dependence of the low temperature specific heat of YBa2Cu3O6+δ on preparation conditions

Specific heats, C, of 3 samples YBa 2 Cu 3 O 6 and 3 samples YBa 2 Cu 3 O 7 prepared by different methods have been measured between 1.4 К and 40 K. Regarding the amount of secondary phases in the samples best results have attained with a new “direct” preparation method. A residual amount of BaCuO 2 in the samples prevents unambiguous conclusions as to the nature of the specific heat term linear in temperature.