We present a low-energy muon-spin-rotation study of the magnetic and superconducting properties of YBa2Cu3O7-delta/PrBa2Cu3O7-delta trilayer and bilayer heterostructures. By determining the magnetic-field profiles throughout these structures, we show that a finite superfluid density can be induced in otherwise semiconducting PrBa2Cu3O7-delta layers when juxtaposed to YBa2Cu3O7-delta "electrodes," while the intrinsic antiferromagnetic order is unaffected.

We report on the preparation and characterization of epitaxial ACuO2 (A = Sr, Ca, Ba) thin films and multilayers with the so- called infinite layer (IL) structure, by rf magnetron sputtering. Films and multilayers without Ba have a remarkable crystal quality, whereas those containing this large ion are often multiphased and unstable. In spite of the excellent crystalline quality of these samples, obtaining thin films having both IL structure and displaying superconducting properties has not succeeded; our pure IL samples display semiconducting behavior, and the different procedures tried in order to dope them—annealings, introduction of disorder or cation vacancies, artificial layering—have failed. These results support that the pure IL structure ACuO2 (A = alkaline earth) cannot superconduct.

The atomic structures of ultrathin YBa2Cu3O7-x (YBCO) films on SrTiO3(001) (STO) and (LaxSr1-x)(AlyTa1-y)O-3(001) (LSAT) were investigated with sub-Angstrom resolution using surface x-ray diffraction and the phase-retrieval direct-method difference map using the constraints of atomicity and film shift (DCAF). The model-independent electron densities which emerge from random initializations in DCAF are exceedingly stable. The films grow with a well-defined stacking sequence even when grown on substrates with mixed terrace termination. Only very minor out-of-plane deviations from bulk YBCO are observed in the film structures, although they are perfectly strained to the substrate and are therefore tetragonal. The films are superconducting, with critical temperatures for growth on STO and LSAT of 43 K and 70 K, respectively. These results have important implications for reliable structure determination of technologically relevant complex-metal oxide surfaces and interfaces.

Manufacturing superconducting circuits out of ultrathin films is a challenging task when it comes to patterning complex compounds, which are likely to be deteriorated by the patterning process. With the purpose of developing high-T(c) superconducting photon detectors, we designed a novel route to pattern ultrathin YBCO films down to the nanometric scale. We believe that our method, based on a specific use of a focused-ion beam, consists of locally implanting Ga(3+) ions and/or defects instead of etching the film. This protocol could be of interest for engineering high-T(c) superconducting devices (SQUIDS, SIS/SIN junctions and Josephson junctions), as well as to treat other sensitive compounds.

Heterostructures consisting of magnetic and superconducting layers juxtaposed to each other are ideal systems to investigate the interplay of the two order parameters and possible interlayer coupling. We used the low energy muon spin rotation technique to study magnetic field distributions at different implantation depths in tri-layered films composed of 75nm thick YBa2Cu3O7-δ layers enclosing a 50nm thick barrier layer of PrBa2Cu3O7-ɛ. The PrBa2Cu3O7-ɛ layer shows the known antiferromagnetic ordering of the Cu and Pr moments, whereas our measurements indicate the formation of a regular vortex lattice in the YBa2Cu3O7-δ layers if an external field is applied perpendicular to the film.

We present recent results on our development of single photon detectors, including: gated and free-running InGaAs/InP avalanche photodiodes; hybrid detection systems based on sum-frequency generation and Si APDs; and SSPDs (superconducting single photon detectors), for telecom wavelengths; as well as SiPM (Silicon photomultiplier) detectors operating in the visible regime.

In this work we map tunnel conductance curves with nanometric spatial resolution, tracking polaronic quasiparticle excitations when cooling across the insulator-to-metal transition in La0.7Ca0.3MnO3 films. In the insulating phase the spectral signature of polarons, a depletion of conductance at low bias flanked by peaks, is detected all over the scanned surface. These features are still observed at the transition and persist on cooling into the metallic phase. Polaron-binding energy maps reveal that polarons are not confined to regions embedded in a highly-conducting matrix but are present over the whole field of view both above and below the transition temperature.

X-ray diffraction and transport measurements on a series of La0.67Ca0.33MnO3 films grown on (110)-cut NdGaO3 substrates are presented. Contrary to widespread belief assuming strain-free growth, this work shows the presence of strain in a 42nm film. On increasing thickness structural relaxation occurs, reaching a bulklike state for 500nm. No evidence of coexistence of strained and relaxed regions is found. The evolution of lattice parameters toward bulk values is accompanied by an increase of the metal-to-insulator transition temperature and a decrease of the polaron activation energy. Therefore, strain effects cannot always be neglected in La0.67Ca0.33MnO3 films grown on small-mismatch NdGaO3.

In this work we map tunnel conductance curves with nanometric spatial resolution, tracking polaronic quasiparticle excitations when cooling across the insulator-to-metal transition in La0.7Ca0.3MnO3 films. In the insulating phase the spectral signature of polarons, a depletion of conductance at low bias flanked by peaks, is detected all over the scanned surface. These features are still observed at the transition and persist on cooling into the metallic phase. Polaron-binding energy maps reveal that polarons are not confined to regions embedded in a highly-conducting matrix but are present over the whole field of view both above and below the transition temperature.

MER Michel DECROUX, Alfred MANUEL Postdocs Louis ANTOGNAZZA, Morten ESKILDSEN, Isabelle JOUMARD, Edmond KOLLER, Olivier KUFFER, Martin KUGLER, Ivan MAGGIO-APRILE, Serge REYMOND, Shukichi TANAKA PhD students Laurent BESSON, Cédric DUBOIS, Bart HOOGENBOOM, Pascal REINERT, Emmanuel TREBOUX Diploma students Estelle DE CHAMBRIER, Daniel GUTIERREZ RIOS Technicians Paul-Emile BISSON, Jean-Gabriel BOSCH, Arthur STETTLER

We report results of a time resolved X-ray Absorption Spectroscopy (XAS) experiment on the oxidation process of epitaxial Y-1(Nd0.05Ba1.95)Cu3Ox superconducting thin films. For the first time Cu K-edge XAS technique has been used to explore local structural changes around the Cu ions during the oxidation process of a high critical temperature superconducting film. The results show that during the oxygenation of a tetragonal Y-1(Nd0.05Ba1.95)Cu3Ox additional local transitions appear in the orthorhombic I phase, which are not linked to long range order transformations as shown by in situ X-ray diffraction experiment. New questions concerning the dynamic microscopic process leading to the oxygenation and eventually to superconductivity of the complex R1+xBa2-xCu3Ox (R = Y or rare earth) compounds arose from these results.

We have investigated the resistive response of high Tc thin films submitted to a high density of current. For this purpose, current pulses were applied into bridges made of Nd(1.15)Ba(1.85)Cu3O7 and Bi2Sr2CaCu2O8. By recording the time dependent voltage, we observe that at a certain critical current j*, a highly dissipative domain develops somewhere along the bridge. The successive formation of these domains produces stepped I-V characteristics. We present evidences that these domains are not regions with a temperature above Tc, as for hot spots. In fact this phenomenon appears to be analog to the nucleation of phase-slip centers observed in conventional superconductors near Tc, but here in contrast they appear in a wide temperature range. Under some conditions, these domains will propagate and destroy the superconductivity within the whole sample. We have measured the temperature dependence of j* and found a similar behavior in the two investigated compounds. This temperature dependence is just the one expected for the depairing current, but the amplitude is about 100 times smaller.

The I–V characteristics of YBa2Cu3O7 (YBCO) superconducting lines have been measured up to very high current densities using current pulses, ranging from 10μs to 1 ms. When the current exceeds 2–3 times the critical current, an abrupt transition into a highly dissipative state occurs, whereas the temperature of the superconducting line is still well below Tc. This indicates that this transition is driven by the current. By increasing the pulse length, the propagation velocity of this initial dissipative state is determined. This velocity is an important parameter for the thin films resistive fault current limiter (FCL).

The use of a high-intensity and well collimated X-ray beam from a third-generation synchrotron source, combined with an area detector with online readout capabilities, allows high-quality powder patterns to be obtained with exposure times of only a few seconds. Powder measurements of a rare-earth manganate perovskite (Nd(0.43)Sr(0.57)MnO(3)) were performed in the temperature range 105-200 K, and the data were of sufficient quality to be able to extract, via Rietveld refinement, accurate values for the lattice constants and atomic parameters. The temperature dependence of the Mn-O-Mn bond angles and Mn-O distances obtained in this manner illustrate the well known correlation between the distortion of the MnO(6) octahedra and the onset of orbital ordering. The success of this study, for which only two hours of synchrotron beam time was required, indicates the enormous improvements in efficiency and throughput which area detectors are bringing to powder diffraction experiments.

The Ginzburg number of superconducting Chevrel phases MxMo6S8 with small coherence length (10−3 to 10−5) is intermediate between those obtained for conventional low Tc materials (10−8) and those of high Tc (10−1) indicating that these phases may display features in the dynamics of the vortices similar to those observed in high Tc superconductors. In this work we present a detailed study of I–V measurements close to the Bc2 line carried out on quasi epitaxial thin films of Cu2Mo6S8. The non-linear I–V curves show a scaling behaviour making possible to determine a transition temperature between an unpinned vortex state and a vortex glass state. However, the temperature range of the unpinned vortex state is much wider than expected.

Flux flow instabilities at high current densities have been recently reported to occur in high temperature superconductors. Such flux flow instabilities were already predicted by Larkin and Ovchinnikov. However, other mechanisms may possibly also account for this behaviour, as for instance hot spots or thermal instabilities. To investigate the mechanism at the origin of these instabilities, we have carried out detailed I- V experiments, using short current pulses, on YBCO thin films grown on sapphire substrates. When the current density exceeds 2-3 times the conventional critical current density, we observe an extremely fast transition from a low dissipative state into a highly dissipative state. We present some indications that this transition does not originate from temperature instabilities but is driven by the current.

We have investigated vortex dynamics in DyBa2Cu3O7/(Sr1-xCax)RuO3 multilayers in parallel and perpendicular applied magnetic fields. In both configurations, we have measured activation energies U for flux motion and determined vortex coupling through either ferromagnetic (SrRuO3) or nonmagnetic (CaRuO3) barriers. Coupled motion of pancake vortices belonging to different DyBa2Cu3O7 layers occurs for nonmagnetic barriers (CaRuO3) as large as several hundreds of Angstrom. In the case of ferromagnetic barriers (SrRuO3), 50 Angstrom was found to be sufficient to decouple the vortices between the barrier. The ferromagnetism present in the SrRuO3 barrier is believed to be responsible for such behavior.