L. Fàbrega

EURECA - A European-Japanese micro-calorimeter array

The EURECA (EURopean-JapanEse Calorimeter Array) project aims to demonstrate the science performance and technological readiness of an imaging X-ray spectrometer based on a micro-calorimeter array for application in future X-ray astronomy missions, like Constellation-X and XEUS. The prototype instrument consists of a 5 × 5 pixel array of TES-based micro-calorimeters read out by by two SQUID-amplifier channels using frequency-domain-multiplexing (FDM). The SQUID-amplifiers are linearized by digital base-band feedback. The detector array is cooled in a cryogenfree cryostat consisting of a pulse tube cooler and a two stage ADR. A European-Japanese consortium designs, fabricates, and tests this prototype instrument. This paper describes the instrument concept, and shows the design and status of the various sub-units, like the TES detector array, LC-filters, SQUID-amplifiers, AC-bias sources, digital electronics, etc. Initial tests of the system at the PTB beam line of the BESSY synchrotron showed stable performance and an X-ray energy resolution of 1.58 eV at 250 eV and 2.5 eV @ 5.9 keV for the read-out of one TES-pixel only. Next step is deployment of FDM to read-out the full array. Full performance demonstration is expected mid 2009.

Improvement of superconducting (Hg,Re)-1223 ceramics synthesised by the sealed quartz tube technique

We analyse the effect of several synthesis conditions and parameters on the superconducting properties of (Hg,Re)-1223 ceramics obtained by the sealed quartz tube technique. The incorporation of liquid mercury results in an increase of the superconducting volume fraction, probably as a consequence of the displacement of the reaction towards the superconducting phase formation. On the other hand, the oxygen content of the precursor is shown to have a significant effect on the sample compactness, and therefore on the intergrain connectivity of the superconductor. Finally, wrapping the unreacted superconductor pellet in a gold foil has also beneficial effects on the quality of the synthesised samples.

Effects of Stress and Morphology on the Resistivity and Critical Temperature of Room-Temperature-Sputtered Mo Thin Films

We report on the structural and electrical characterization of Mo thin films deposited at room temperature by RF magnetron sputtering. The effect of RF power on the morphology and residual stress of the films is analyzed. The films are under compressive stress and consist of densely packed columns with a lateral size on the order of 20 nm. The stress, critical temperature, and resistivity of the films are found to rise when increasing the ejected ion energy during the sputtering process. The changes in critical temperature and resistivity are discussed in terms of the observed morphology and stress changes.

Mo-Based Proximity Bilayers for TES: Microstructure and Properties

We report on the fabrication and characterization of Mo films, Mo/Au and Mo/Cu bilayers for Transition Edge Sensors (TES). The fabrication conditions (at room temperature) have been varied to achieve layers with the required properties for TES applications. The dependence of their functional properties (i.e. electrical resistivity and superconducting critical temperature) on microstructure (grain size, stress) is investigated.

Effects of morphology and strain on the dielectric response of multiferroic CoFe2O4–BaTiO3 nanocomposite thin films

The dielectric response (permittivity and losses) of epitaxial CoFe2O4–BaTiO3 thin film nanocomposites deposited under different conditions has been measured. Exhaustive microstructural analyses have allowed tracking the evolution of structure and morphology as a function of thickness, deposition temperature and rate. It is found that the dielectric permittivity of the nanocomposite increases with the grain size of the BaTiO3 matrix and when reducing its unit cell volume, while losses are quite insensitive to changes in morphology and crystal structure of the ferroelectric phase. We analyze the consequences of these results in view of the functional optimization of nanocomposites.

Magnetic irreversibility and surface barriers in grain-aligned Re-doped Hg-1223

Hysteresis cycles are studied on a grain-aligned sample of the Re-doped Hg-1223 superconductor as a function of temperature. They are used to analyse the origin of the magnetic irreversibility, and extract the irreversibility line and critical currents. Comparison with samples without Re shows that the Re-substitution enhances both (i) the irreversible contribution arising from surface barriers at high temperatures, and (ii) bulk pinning at low temperatures. Possible mechanisms for the Re-doping leading to these effects are discussed.

Field-Induced Diamagnetic Fluctuations at Low Temperature in Pr1.85Ce0.15CuO4-y Superconductor

Magnetization measurements on a Pr1.85Ce0.15CuO4-y single crystal (Tc = 19.2 K) have provided evidence of strong field-induced diamagnetic fluctuations near the super-conducting transition; the critical zone represents an important fraction (> 33%) of the reversible diamagnetic regime, at any field above 0.05T. The high-field magnetization displays scaling behaviour and the observed scaling functions are in excellent agreement with the predictions for 2D superconducting systems.

Superconductivity and magnetoresistance in YBa2Cu3O7/SrTiO3/La2/3Sr1/3MnO3 heterostructures

Abstract We report on the design, growth and basic transport characterization of YBa 2 Cu 3 O 7 /SrTiO 3 /La 2/3 Sr 1/3 MnO 3 heterostructures for spin-polarized quasiparticle injection experiments. Trilayers with planar and cross strip geometry have been grown by pulsed-laser ablation, eventually using shadow masks. We have analyzed the microstructure and basic physical properties of the heterostructures.

THE RE-DOPED HIGH TC SUPERCONDUCTOR HGBA2CA2CU3OX : MAGNETIC IRREVERSIBILITY VERSUS ANISOTROPY

We report on magnetization measurements on grain-aligned Re-doped Hg-1223 superconductor (Tc≈133 K). We analyze the anisotropic magnetic hysteresis Mirr as a function of temperature and magnetic field, and determine the irreversibility line Hirr(T) for magnetic fields applied parallel and perpendicular to the c axis of the structure. The similar values of both Mirr and Hirr(T) along the two main crystallographic directions reveal a marked decrease of the superconducting anisotropy, due to the Re substitution. This anisotropy reduction might be at the origin of the rise of the magnetic irreversibility already reported in these compounds, as compared to those without Re.

Characterization of a Mo/Au Thermometer for ATHENA

The first dark characterization of a thermometer fabricated with our Mo/Au bilayers to be used as a transition edge sensor is presented. High-quality, stress-free Mo layers, whose thickness is used to tune the critical temperature (<i>T</i>_C) down to 100 mK, are deposited by sputtering at room temperature (<i>RT</i>) on Si₃N₄ bulk and membranes, and protected from degradation with a 15-nm sputtered Au layer. An extra layer of high-quality Au is deposited by ex situ e-beam to ensure low residual resistance. The thermometer is patterned on a membrane using standard photolithographic techniques and wet etching processes, and is contacted through Mo paths, displaying a sharp superconducting transition (α ≈ 600). Results show a good coupling between Mo and Au layers and excellent <i>T</i>_C reproducibility, allowing to accurately correlate <i>d</i>_Mo and <i>T</i>_C. Since <i>d</i>_Au is bigger than ξ_M for all analyzed samples, bilayer residual resistance can be modified without affecting <i>T</i>_C . Finally, first current to voltage measurements at different temperatures are measured and analyzed, obtaining the corresponding characterization parameters.