[en] The uniform spin susceptibility, χ_s , is determined from the Gd³⁺ high frequency ESR Knight shift in the underdoped high temperature superconductor, Gd:YBa₂ Cu₄O ₈ (T_c=82 K) between 8 to 260K. Measurements at two fields (5.4 and 8.1T) permit high precision at low temperatures. In the range of 0.1T_c to 0.4T_c , a linear temperature dependence of χ_s is observed with a large slope. The corresponding maximum d-wave superconducting gap is Δ₀=190 K , 1.1 times the weakly coupled d-wave limit. The results support suggestions that the normal state gap observed below 180K and the superconducting gap below T_c have nodes with the same symmetry. copyright 1997 The American Physical Society

[en] This paper reports on the transition temperature of YBa₂Cu₃O_y ceramic samples studied as a function of oxygen content by DC electrical resistivity measurements. Two series of samples had been prepared by equilibrating pieces from the fully oxidized orthorhombic and fully reduced tetragonal phase at low (series I 420 degrees C) and high (series II 600 degrees C) temperatures. The T_c vs. oxygen content dependence was found to show a continuous change in series II and a 55 K plateau around y = 6.6-6.7 in series I. A current dependence was observed in series I in the range y = 6.5-6.6 where room temperature x-ray data may indicate the occurrence of more than one orthorhombic phases

[en] In this paper we give a brief description of SRS Station 16.3, and outline a novel approach to instrument control software. Operation of the Station is illustrated with three examples: resonant magnetic x-ray diffraction, high-resolution measurements of Kossel lines, and observations of resonant peak shifts in dynamical diffraction. (author)

[en] Photopolymerization of C₆₀ was performed on a gram scale via a new monomer transport method. After subsequent extractions of the raw polymer in organic solvents, 100mg dimer and 10mg mixture of soluble oligomers were yielded. The structures of the soluble fractions and the insoluble polymer were studied by optical spectroscopy and x-ray diffraction methods. According to IR and UV-VIS spectroscopic data, the obtained photodimer is identical to the (2+2) cycloadduct C₁₂₀ prepared previously. In contrast to previous crystallization studies, a fast precipitation of the dimer from toluene gives rise to the formation of a solvent-free microcrystalline solid. The crystal structure of the pure dimer is fcc (a=14.05 A) consisting of closely packed balls with the cycloadduct bonds oriented randomly along the 12 nearest neighbor directions. A linear relationship is found to exist between the fcc lattice parameter (a) and the average number of bonded neighbors (n_B) in various photopolymerized structures. For insoluble polymers n_B≅2 is estimated, supporting previous photopolymer models of small ring or branched clusters

[en] Muon spin relaxation (μ⁺SR) measurements of CsC₆₀ and RbC₆₀ have revealed magnetic ordering at 20 K and 15 K respectively, with static internal fields at the muon site of ∼7 G for both samples. No magnetic ordering is seen in KC₆₀ down to 2.5 K. An apparent small increase in muon spin relaxation which had been seen at 70 K in RbC₆₀ is ascribed to an extrinsic effect, originating in muons which stopped in the aluminum sample holder

[en] Tunneling and optical transmission studies have been performed on superconducting samples of Rb₃C₆₀. At temperatures much below the superconducting transition temperature T_c, the energy gap is 2Δ=5.2±0.2 meV, corresponding to 2Δ/k_BT_c=4.2. The low temperature density of states and the temperature dependence of the optical conductivity resemble BCS behavior, although there is an enhanced open-quote open-quote normal state close-quote close-quote contribution. The results indicate that this fulleride material is an s-wave superconductor, but the superconductivity cannot be described in the weak coupling limit. copyright 1996 The American Physical Society

[en] We report structural studies on new C₆₀ clathrates formed with ether as a guest molecule. Single crystal x-ray diffraction shows that in C₆₀-ether two different pseudo tetragonal unit cells exist, which correspond to different compositions. The unit cells are based on the same relative displacements of the square-planar layers as found in the C₆₀-n-pentane clathrate. However, the stacking sequence of the layers is differf the layers is different and reciprocal space maps reveal a significant stacking disorder. Model calculations suggest that the disorder comes mainly from the random sequence of the relative displacements. (orig.)

[en] Molecular-dynamics calulations have been carried out to investigate the phase transitions of pure C₆₀ crystals in the solid phase. We employed a constant pressure method which allows changing of the volume and the shape of the unit cell. The interaction between molecules is based on a pairwise, additive, atom-atom Lennard-Jones potential. The calculations yield a stable fcc structure above transition temperature (T_c). At about T_c the rapid change of the volume and the order parameters versus temperature shows the well known first order phase transition. Some physical parameters have been derived and their values agrees well with measured data. Calulations have been extended to C₆₀-clathrate systems. Results show a somewhat different phase transition than in pure fcc C₆₀. (orig.)

[en] X-ray diffraction studies show that the stable phase of the alkali fullerene RbC₆₀ is orthorhombic (o-RbC₆₀) below 350 K. C₆₀ molecules form chains along a with an unusually short spacing of 9.12 A and magnetic properties suggest that o-RbC₆₀ is a quasi-1D metal with a transition to a spin density wave ground state at 50 K. The high temperature fcc phase of RbC₆₀ may be stabilized below 300 K by quenching from 500 K; it is paramagnetic above 300 K and transforms into a nonmagnetic ground state beween 300 and 250 K

[en] Among the future X-ray sources linac-based X-ray free electron lasers are close to realization. They will produce extremely short (<100 fs) and intense hard X-ray pulses (∼10¹² photon/pulse). The unique features of this beam will allow the study of the atomic structure and the different physical and chemical processes in solids at a level which is not accessible today. However, to understand the experimental results a detailed picture of what happens during the burst in the sample is necessary. In several suggested applications, small samples containing only 10³-10⁶ atoms are the possible candidates. In order to have an understanding about the atomic motions, we performed model calculations on the dynamics of particles of a cluster in an intense hard X-ray pulse. The movement of the particles was followed by non-relativistic classical dynamics. The main processes: photo-absorption, Auger process, inelastic and elastic scattering of electrons were taken into account by their respective cross sections. Here, we report our findings on clusters containing various elements. The results show that the clusters disintegrate via Coulomb explosion, similar to small clusters in intense laser beam. However, the dynamics of the explosion is significantly different. We investigate the effect of cluster size, pulse length, atom density and composition on the explosion. We discuss the consequences of our results to single-molecule imaging by the free-electron laser pulses, recently proposed by Neutze et al. [R. Neutze, R. Wouts, D. Spoel, E. Weckert, J. Hajdu, Nature, 406 (2000) 752]. The critical evaluation of these results gives guidelines and sets important limits on experimental conditions for future experiments aiming for single-molecule structure solution