K. Lüders

Observation of a lambda-shaped ultrasonic attenuation peak in superconducting UPt3

Abstract We report measurements of the temperature dependence of the ultrasonic attenuation in superconducting UPt3. In the vicinity of the superconducting transition temperature a pronounced longitudinal attenuation peak is observed. Such a peak does not appear for transverse ultrasound. Our experimental results can be explained by a “correlation hump” of the density-density correlation function.

Superconductivity and NMR investigations of KTl1.5-doped C60

Doping of C60 fullerene with KTl1.5 results in superconductivity withTconset=17.6 K as shown by a dc magnetization measurement. Preliminary13C NMR studies on a doped and undoped sample are reported. Compared to original C60 the linewidth increases by a factor of 3, the resonance frequency is shifted by 42 ppm, and the relaxation rateT1−1 is an order of magnitude larger in the intercalated material at room temperature. Both observations support the picture of a metallic sample.

NQR investigations of the high-Tc superconductor Ba2YCU3O7

Abstract NQR measurements of 63,65Cu in both the normal and superconducting state are reported for the Cu1 and Cu2 sites in Ba2YCu3O7. An anomalous behaviour in the vicinity and below Tc is observed: after a steep decrease at Tc the quadrupole frequency increases with decreasing temperature.

Superconductivity of Ba doped C60 — susceptibility results and upper critical field

Abstract A series of BaxC60 samples were prepared with stoichiometries ranging from x = 3 to x = 6. The lattice structure was analysed by using X-ray diffraction (XRD) investigations and the superconducting properties were determined by applying acsusceptibility and dc-magnetization (SQUID) techniques in external magnetic dc fields up to 2 T. Beyond x=3, XRD-results reveal the presence of more than one phase in the samples and, in contrast to the Ba3C60 sample, superconductivity around Tc = (6.8±0.2) K occurs. Three intrinsic BaxC60 phases (x = 3, 4, 6) are identified, and superconductivity is associated to the x = 4 composition which exhibits the largest superconducting volume fraction (∼ 22 %) in this series. Furthermore first estimates of the upper critical field of Ba4C60 are presented. The slope of the upper critical field ( ∂ B c2 ∂ T ) near Tc is one order of magnitude smaller than in alkali-metal doped C60 compounds. The whole Bc2(T)-curve is compared with the Werthamer-Helfand-Hohenberg (WHH) theory in the dirty and clean limit. We find zero temperature values for the upper critical field Bc2(0) = (2.4±0.3) T and for the corresponding Ginzburg-Landau coherence length ξ GL (0) = (116±7) A .

Magnetic penetration depth in oriented YBaCuO powder samples

Abstract For oriented YBa2Cu3O7-δ powder samples we report for what we believe to be the first time on measurements of the temperature dependence of the magnetic penetration depth and of the lower critical magnetic field Bc1(T). The results are found to agree with theoretical expressions derived for weak-coupled BCS-type superconductors in the clean local limit. From the lower critical field values Bc1∣(0) = (340 ± 50)G and Bc1⊥(0) = (113 ± 30)G the magnitude of the anisotropic penetration depth has been determined yielding λ ∣ (0) = (1400 ± 200) A and λ ⊥ (0) = (6100 ± 1000) A .

7Li and 133Cs NMR parameters in C6Li and C8Cs intercalated oriented graphites

The absolute values of the Knight shift in C6Li and C8Cs, prepared from highly oriented pyrolytic graphite, are found to be very different. Comparison to the corresponding values of Li and Cs metal, however, shows that the Knight shift reductions are similar. Nevertheless, the origins of these reductions are different in accordance with the different electronic structures of these compounds.

Nuclear quadrupole interactions and charge localization in HgBa2CuO4 + δ with different oxygen content

Abstract Variation of the oxygen content δ in HgBa 2 CuO 4 + δ (Hg-1201) provides a good opportunity to study the influence of oxygen doping on nuclear quadrupole interactions (NQI) and charge localisation in different regions of the ( T c − δ ) phase diagram. We performed 63,65 Cu Nuclear Quadrupole Resonance (NQR) studies on a series of HgBa 2 CuO 4 + δ samples with different oxygen content δ. Based on the experimental NQR spectra of the Hg-1201 compounds and the known copper NQR frequency for the undoped Nd 2 CuO 4 a phenomenological method is developed to analyze the NQI in high temperature superconductors (HTSC) including both, the on-site electric field gradient (EFG) and the lattice contribution in the EFG. Using this method we estimated the Sternheimer antishielding factor γ ∞ = −16.2 for Cu 2+ and the hole concentration n x 2 − y 2 on the copper 3d x 2 − y 2 orbital. The n x 2 − y 2 value rises monotonously with increasing doping level over the whole Hg-1201 sample series and is the main factor causing the increase of quadrupole frequency v Q . No saturation effect on the copper hole doping process has been found at high oxygen content level.

NMR investigations on oriented grains of superconducting YBa2Cu3O7

Oriented powdered samples of YBa2Cu3O7 were prepared by exposing the mechanically vibrated powder at 4.5 K to a magnetic field of 1.2 T. NMR experiments on 63,65Cu were performed and the results are compared with those of non-oriented samples. The data confirm the assignment of νQ1 ≅ 22 MHz to the Cu(1) and νQ2 ≅ 31.5 MHz to the Cu(2) sites. By means of the less common low-field NMR technique (νLarmor < νQuadrupole) the asymmetry parameter at the Cu(2) sites was determined to be η < 0.01, and at 100 K the Knight shift in the (a,b) plane was found to be K⊥ = (0.35 ± 0.1)% which is about a factor 2 smaller than the values reported in literature so far. Using published NMR data for 63Cu in Cu2O and in antiferromagnetic YBa2Cu3O6, our results are indicative for fluctuations of the Cu(1) ions between the ionic states Cu+ and Cu3+, and of the Cu(2) ions between Cu+ and Cu2+.

The structure of fullerene black and the incorporation of C60 investigated by 13C NMR

13C NMR measurements were performed on different fullerene black materials in order to gain knowledge about the fullerene black structure, the incorporation of C60 molecules in the fullerene black, the generation process of the fullerenes in the arc reactor, the influence of solvents and the amount of fullerenes not removable by solvents. Our results are consistent with the model that fullerene black is an amorphous carbon material containing a high amount of bent and spheroidal carbon fragments. The C60 molecules are dispersed on the large fullerene black surface and rotate rapidly. A weak signal in the fullerene black at about 70 ppm could arise from polyynic carbon chains, one of the discussed intermediate compounds in the fullerene generation process. X-ray diffractometry shows that the treatment of the fullerene black raw material with tetrahydrofuran (THF) leads to structural relaxation and a rearrangement of carbon sheets. 13C NMR is rather insensitive to this modified texture of the fullerene black matrix indicating that the local carbon structure remains largely unchanged by solvent treatments. In the extracted fullerene black materials we did not find fullerene signals proving that the amount of possibly entrapped fullerenes is very small and that the conventional fullerene extraction with toluene can be regarded as complete.

Evidence of local magnetic order in the superconducting phase of hydrogen doped YBa2Cu3O7 using nuclear magnetic resonance

Abstract Nuclear magnetic resonance absorption traces were obtained for the 1H line in H0.11YBa2Cu3O7 as a function of temperature. The superconducting temperature Tc is 93K. A spectral splitting was observed which we attribute to local antiferromagnetism. The Neel temperature is approximately 320K.