Quadrupolar XMCD at the Fe K -edge in Fe phthalocyanine film on Au: Insight into the magnetic ground state

The observation of an anomalous quadrupolar signal in x-ray magnetic circular dichroism (XMCD) at the Fe K-edge of iron phthalocyanine (FePc) films is reported. All ground states previously suggested for FePc are incompatible with the experimental data. Based on ab initio molecular orbital multiplet calculations of the isolated FePc molecule, we propose a model for the magnetic ground state of the FePc film that explains the XMCD data and reproduces the observed values of the orbital moments in the perpendicular and planar directions

Molecular tilting and columnar stacking of Fe phthalocyanine thin films on Au(111)

Scanning tunneling microscopy and x-ray absorption spectroscopic results at the Fe K edge of Fe phthalocyanine (FePc) thin films grown on Au substrates, together with theoretical calculations, allow us to refine the structure of the film. In particular, we show that the columnar stacking of the FePc molecules is different from that found in bulk a and ß phases. Moreover, the molecules do not lay parallel to the surface of the substrate. These structural findings are relevant to understand magnetism of FePc films

Structural relationships in high temperature superconductors

The recent discovery of two types of metallic copper oxide compounds which are superconducting to above 90/sup 0/K has renewed interest in the search for new high temperature superconducting materials. It is significant that both classes of compounds, La/sub 2-x/Sr/sub x/CuO/sub 4-y/ and YBa/sub 2/Cu/sub 3/O/sub 7-delta/ are intimately related to the extensively studied perovskite family. Both compounds contain highly oxidized, covalently bonded Cu-O sublattices, however, they differ in geometry. In this paper we discuss the relationship of these features to the superconducting properties. 30 refs., 6 figs

The influence of the boundary resistivity on the proximity effect

We apply the theory of Takahashi and Tachiki in order to explain theoretically the dependence of the upper critical magnetic field of a S/N multilayer on the temperature. This problem has been already investigated in the literature, but with a use of an unphysical scaling parameter for the coherence length. We show explicitely that, in order to describe the data, such an unphysical parameter is unnecessary if one takes into account the boundary resisitivity of the S/N interface. We obtain a very good agreement with the experiments for the multilayer systems Nb/Cu and V/Ag, with various layer thicknesses.Comment: 12 pages, 5 figure

Preparation and characterization of superlattices

During this last year we were involved in a number of studies geared towards understanding the relationship between interfacial disorder and X-ray diffraction from superlattices. In addition, we have performed a number of physical properties studies in order to relate structure to physical properties. These studies include magnetism and superconducting studies in a variety of different thin film systems. A major development was the implementation of a structural refinement program capable of determining the important structural parameters in superlattices and thin films. This program is being supplied to many research groups for the study of related problems. We are currently devoted to detailed, comparative studies of magnetic samples prepared by Molecular Beam Epitaxy and sputtering. The unique facilities available to us not only allow preparing samples by the two different methods but also are capable of determining interfacial structure using in-situ characterization techniques such as Reflection High Energy Electron Diffraction (RHEED), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). We have developed a data acquisition system capable of acquiring electron diffraction in real time during growth. Part of the work done here is a good example of a collaboration between a University (UCSD), a national lab (ANL) and industrial organizations (Amoco, IBM)

LASER IRRADIATION EFFECTS IN SUPERCONDUCTORS

Nous avons étudié le comportement d'un SQUID (Superconducting Quantum Interference Device) à couche mince et d'une boucle supraconductrice bi-métallique sous irradiation d'une lumière laser. Nous trouvons que les résultats expérimentaux peuvent être expliqués par un simple échauffement. Cette interprétation ne suppose pas l'existence d'une répartition de quasi-particules qui soit simultanément hors-équilibre et non-thermique. Nous trouvons également que les effets thermiques empêchent l'observation d'un comportement collectif à basse température suggéré par Bari et Sukhanov.We have studied the behaviour of a thin film Superconducting Quantum Interference Device (SQUID) and of a bi-metallic superconducting loop under laser light irradiation. We find that the experimental data can be explained by simple thermal heating without assuming the existence of non-thermal non-equilibrium quasiparticle distribution. We also find that the observation of collective behaviour at low temperature as suggested by Bari and Sukhanov will be hindered by thermal effects

Layered Ultrathin Coherent Structures (LUCS)

A new class of superconducting materials, Layered Ultrathin Coherent Structures (LUCS) are described. These materials are produced by sequentially depositing ultrathin layers of materials using high rate magnetron sputtering or thermal evaporation. Strong evidence is presented that layers as thin as 10 A can be prepared in this fashion. Resistivity data indicates that the mean free path is layer thickness limited. A strong disagreement is found between the experimentally measured transition temperatures T/sub c/ and the T/sub c/'s calculated using the Cooper limit approximation. This is interpreted as a change in the band structure or the phonon structure of the material due to layering or to surfaces

UPPER CRITICAL FIELD OF Mo-Ni HETEROSTRUCTURES

Upper critical field and its anisotropy have been measured on two very short wavelength Mo-Ni heterostructures of different degrees of perfection, lambda = 13.8A (disordered structure) and lambda = 16.6A (layered structure). In both cases the parallel critical field has an unexpected temperature dependence, a large and temperature dependent anisotropy, and over 60% enhancement over the Clogston-Chandrasekhar limit. Data are fit to the Werthamer-Helfand-Hohenberg theory and the spin-orbit scattering times are found to be 1.79 x 10 T s and 2 x 10 T s, respectively