Superconducting Transition Edge Sensors & Topological Defects Formation

1. Superconducting Transition Edge Sensors& Topological Defects Formation Dept. of Physics Slovak Technical University Ilkovičova 3 812 19 Bratislava Slovak Republic Valko Pavol valko@elf.stuba.sk http://www.valko.net

2. What are TES • sensor • low TC superconductor • native (tungsten) • proximity structure (Al-Ag) • special phase (b-Ta) • deposited as thin film • sputtering, evaporation • strip form • lithography, shadow mask • absorber • with low heat capacity • dielectric crystals (Al2O3) • semiconductors (Si, Ge) • superconductors (Nb, etc.)

3. Motivation & Method • already extensively studied as radiation detectors with “fast” phase transition guarantied (to achieve high count rate) • possibility to cause local or global heating of tested samples by choosing source of energy (laser, radiation, particles...) • spontaneous magnetic flux could modify the state of sample,i.e. different resistance at fixed temperature might be a signal • “missing energy” like signals should be observable for high resolution detectors • broad range of superconductors (native, composite, anisotropic, heavy-fermions, ...) available for tests at various temperatures, geometries ....

4. b-Tantalum with a particles • b-Tantalum sputtered (200 nm thick) film on silicon (500 mm Si) absorber • 5.4 MeV a particles (241Am source) used as localized “heater” releasing energy over less than 4 mm path (most of it near the end point) • affected “heated” region of superconductor is of similar size right above a track end-point • superconductor is directly heated by phonons propagating spherically from a track end-point (ignoring focusing properties of crystal) • only small part of superconductor heated above critical temperature, followed by fast cooling (quasiparticle - phonon system might follow various energy spread processes ) • pulse amplitudes recorded only

5. b-Tantalum experimental results typical pulse shape typical a spectrum • “hotspot” cooling rate > 50 K/s (from pulse rise time) • energy spectra with large tails (possible signal) • no clear “missing” energy signal seen

6. a-Tungsten tests with X rays • experiment with “global” sample heating • tungsten film (200 nm) sputtered on heated sapphire substrate (20 x 10 x 0.5 mm3) and lithographically structured to 1 x 0.5 mm2 area • 55Fe source X-rays used as “heat source” (5.8 and 6.4 keV ) with sample hit rate of 0.7 Hz • X-ray induced events originated directly from in metal film (17%) and dielectric crystal (83%) • whole traces recorded for eachevent • search of “satellite” peaks at fixed bias point • temperature scan over sensor R-T transition

7. a-Tungsten experimental results typical X-ray spectrum temperature scan over transition • observed “satellite” peak for events originating from tungsten film • observed “multiple” transitions during R-T scan • both effects could be associated with defect creation

8. Possible ways to improve • to perform 3He + n like defect creation experiments in bulk superconductors searching for “missing energy” using TES or STJ as energy deficit sensors • native short coherence length (or artificially reduced) superconductors are preferred • nuclear fission would be ideal local energy source • large localized energy depositions are required to achieve comparable density of deposited energy vs. condensation energy in “realistic” superconductors • dedicated TES experiments with fast SQUID read-out testing localised energy release deep in bulk • similar experiments with a particles, extremely pure Nb and STJ’s arrays were already performed (Gaitskell et. al., 1991)

9. THE END • Major concerns • effect of residual and bias current induced magnetic fields • large and shallow pinning centres • properties of thin superconductor films (d~ξ) near (at) critical temperature • intermediate (mixed) state resistance dynamics • interactions of quasiparticles and phonons in superconductors • Links to previous observations • possible source of observed “extra” noise in TES detectors (deKorte et al.) • multi decay time constants of observed pulses