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Universita’ di Perugia 15 Aprile 2010

Ruolo delle correlazioni superconduttive in conduttori mesoscopici: utilizzo per l’implementazione di rilevatori quantistici. Francesco Giazotto NEST Istituto Nanoscienze-CNR & Scuola Normale Superiore Pisa, Italia. Universita’ di Perugia 15 Aprile 2010. Collaboration. J. T. Peltonen

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Universita’ di Perugia 15 Aprile 2010

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  1. Ruolo delle correlazioni superconduttive in conduttori mesoscopici: utilizzo per l’implementazione di rilevatori quantistici Francesco Giazotto NEST Istituto Nanoscienze-CNR & Scuola Normale Superiore Pisa, Italia Universita’ di Perugia 15 Aprile 2010

  2. Collaboration J. T. Peltonen M. Meschke J. P. Pekola Low Temperature Laboratory, Helsinki University of Technology, 02015TKK, Finland

  3. Outline • Part I: Andreev reflection and proximity effect in superconducting hybrid systems – impact on the density of states • Basic concepts of electron transport in hybrid systems: AR and PE • Proximity-induced modification of the DOS • Probing the proximized DOS: experiments with tunnel junctions and STM spectroscopy • Consequences • Part II: Superconducting quantum interference proximity transistor (SQUIPT) • Theoretical behavior of the SQUIPT • Structure fabrication details • Experimental results and comparison with theory • Advantages • Future perspectives

  4. Andreev reflection BdG equations BTK, PRB 25, 4515 (1982) Andreev reflection in SN contacts

  5. Metallic contact between a normal metal and a superconductor N S S Andreev reflection Normal metal (Semiconductor) Superconductor Electron-hole correlations: proximity effect Cooper pair Incident electron S N S Reflected hole Supercurrent Andreev bound states (ABS) Proximity effect and supercurrent

  6. Proximity effect in SNS systems: basic formalism Diffusive mesoscopic N wire: quasi-1D geometry L >L >> le D = diffusion coefficient  = superconducting order parameter  = macroscopic phase of the order parameter ETh = D/L2 Thouless energy Usadel equations LDOS properties: N(-E) = N(E) Eg for |E|  Eg Eg( = 0)  3.2ETh for >>ETh Eg( = ) = 0 LDOS

  7. Modification of the LDOS in SNS systems due to proximity effect Length and position dependence Phase dependence J. C. Cuevas et al., PRB 73, 184505 (2006) J. C. Hammer et al., PRB 76, 064514 (2007)

  8. Spatial spectroscopy of PE probed with tunnel junctions Al/Cu SN structure with tunnel probes

  9. Phase-dependence of PE probed with STM spectroscopy Al/Ag SNS proximity SQUIDs

  10. Phase-dependence of PE probed with STM spectroscopy Full phase-control of the minigap amplitude Phase-evolution of PE Experiment to theory comparison H. le Sueur et al., PRL 100, 197002 (2008)

  11. I) -tuning of specific heat: quantum control of a thermodynamic variable Electron entropy Electron specific heat H. Rabani, F. Taddei, F. G. and R. Fazio, JAP 105, 093904 (2009); H. Rabani, F. Taddei, R. Fazio, and F. G., PRB 78, 012503 (2008)

  12. II) -tuning of e-ph interaction: quantum control of relaxation T. T. Heikkila and F. G., PRB 79, 094514 (2009)

  13. Sensitivity through proximity

  14. SQUIPT: a novel quantum interferometer Active manipulation of the DOS of a proximity N metal Phase control (through magnetic flux) SQUIPT Detection (through tunnel junctions) High sensitivity for flux detection

  15. SQUIPT: fabrication details and configurations Fabrication details Shadow-mask evaporation 27 nm Al @ 25 Oxidation 4.4 mbar 5’ (tunnel junctions) 27 nm Cu @ -25 60 nm Al @ 60 (clean SN interfaces) Geometry and materials details L  1.5 m Probe width  200 nm N wire width  240 nm SN overlapping  250 nm Rt  50-70 k LG  40 pH IJ  3 A  = 200 eV

  16. SQUIPT (theo): prediction of its behavior in the current-bias mode A-type configuration quasiparticle current Usadel equations

  17. SQUIPT (theo): current-voltage characteristic vs  N-region DOS Low-temperature I-V characteristic • Calculation parameters • from the samples: • T = 0.1 Tc • Tc = 1.3 K • ETh = 4 eV • D = 110 cm2/s (Cu) • = 200 eV • Rt = 50 k  to V transformer modulation amplitude

  18. SQUIPT (theo): voltage modulation and transfer function Voltage modulation V() Transfer function V/ • Features: • nonmonotonic behavior in I • change of concavity • Features: • nonmonotonic behavior in I • change of sign

  19. A-type SQUIPT (exp): current-voltage characteristic vs  Rt = 50 k T = 68 mK Rt = 50 k T = 53 mK Theory Coherent modulation of the N DOS

  20. A-type SQUIPT (exp): Josephson coupling in the proximity metal Rt = 50 k T = 68 mK Rt = 50 k T = 53 mK 0  0.17 Oe A  120 m2 IJ 17 pA

  21. A-type SQUIPT (exp): voltage modulation vs  Rt = 50 k T = 54 mK theory Change of concavity exp  50-60% theory • device parameters • non ideal phase-biasing V  7V @ 1 nA

  22. A-type SQUIPT (exp): transfer function theory Rt = 50 k T = 54 mK V/  30 V/0 @ 1 nA

  23. B-type SQUIPT (exp): voltage modulation vs  and transfer function Rt = 70 k T = 53 mK Rt = 70 k T = 53 mK V/  60 V/0 @ 0.6 nA doubled response in B-type SQUIPT V  12V @ 1 nA

  24. A-type SQUIPT (exp): temperature dependence Rt = 50 k I = 1 nA Rt = 50 k I = 1 nA change of concavity between 376 mK and 411 mK

  25. SQUIPT: dissipation and flux sensitivity Power dissipation lowered Pdiss= VI 100 fW increasing the probing junction resistance 4-5 orders of magnitude smaller in the SQUIPT DC SQUIDS Ultralow dissipation cryogenic applications Flux sensitivity NEF = <V2N>1/2/|V/|1/2 NPre 1.2 nV/Hz1/2 NEF  2  10-5 0/Hz1/2 NEF  4  10-7 0/Hz1/2 with Nb (1.5 meV) and L = 150 nm

  26. SQUIPT: advantages • simple DC readout scheme, similar to DC SQUID • current- or voltage-biased measurements • flexibility in farication parameters and materials • (semiconductors NWs, carbon nanotubes, graphene) • Nb or V to enhance response and operating temperature • ultralow dissipation (1-100 fW) • implementation in series or parallel array for enhanced output • implementation with S coolers to “actively” tune the working temperature

  27. SQUIPT: future perspectives (i) Short junction limit (<<ETh) Al and L = 150 nm (ii) V SNS junction SQUIPT C. Pascual Garcia and F. G., APL 94, 132508 (2009) (iii) Noise? Both theory and experiment

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