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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

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

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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

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

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

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Proximity effect in SNS systems: basic formalism Diffusive mesoscopic N wire: quasi-1D geometry L >L >> l e D = diffusion coefficient = superconducting order parameter = macroscopic phase of the order parameter E Th = D/L 2 Thouless energy LDOS properties: N (- E ) = N ( E ) E g for | E | E g E g ( = 0) 3.2 E Th for >> E Th E g ( = ) = 0 Usadel equations LDOS

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

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

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

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

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

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

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Sensitivity through proximity

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

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SQUIPT: fabrication details and configurations Shadow-mask evaporation 27 nm 25 Oxidation 4.4 mbar 5’ (tunnel junctions) 27 nm -25 60 nm 60 (clean SN interfaces) Fabrication details Geometry and materials details L 1.5 m Probe width 200 nm N wire width 240 nm SN overlapping 250 nm R t k L G 40 pH I J 3 A = 200 eV

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

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SQUIPT (theo): current-voltage characteristic vs Calculation parameters from the samples: T = 0.1 T c T c = 1.3 K E Th = 4 eV D = 110 cm 2 /s (Cu) = 200 eV R t = 50 k Low-temperature I - V characteristic modulation amplitude to V transformer N-region DOS

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

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A-type SQUIPT (exp): current-voltage characteristic vs R t = 50 k T = 68 mK Coherent modulation of the N DOS R t = 50 k T = 53 mK Theory

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A-type SQUIPT (exp): Josephson coupling in the proximity metal R t = 50 k T = 68 mK I J 17 pA R t = 50 k T = 53 mK 0 0.17 Oe A 120 m 2

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A-type SQUIPT (exp): voltage modulation vs R t = 50 k T = 54 mK V 7 1 nA Change of concavity theory exp 50-60% theory device parameters non ideal phase-biasing

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A-type SQUIPT (exp): transfer function R t = 50 k T = 54 mK V / 30 V/ 1 nA theory

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B-type SQUIPT (exp): voltage modulation vs and transfer function R t = 70 k T = 53 mK V 12 1 nA V / 60 V/ 0.6 nA R t = 70 k T = 53 mK doubled response in B-type SQUIPT

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

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SQUIPT: dissipation and flux sensitivity P diss = VI 100 fW increasing the probing junction resistance lowered DC SQUIDS4-5 orders of magnitude smaller in the SQUIPT Ultralow dissipation cryogenic applications Power dissipation Flux sensitivity NEF = 1/2 /| V/ | 1/2 N Pre 1.2 nV/Hz 1/2 NEF 2 0 /Hz 1/2 NEF 4 0 /Hz 1/2 with Nb ( 1.5 meV) and L = 150 nm

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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

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

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