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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Universita’ di Perugia
15 Aprile 2010
Ruolo delle correlazioni superconduttive in conduttori mesoscopici: utilizzo per l’implementazione di rilevatori
quantistici
Francesco GiazottoNEST Istituto Nanoscienze-CNR & Scuola Normale Superiore
Pisa, Italia
Collaboration
J. T. PeltonenM. MeschkeJ. P. PekolaLow Temperature Laboratory, Helsinki University of Technology, 02015TKK, Finland
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
Andreev reflection in SN contacts
BdG equationsAndreev reflection
BTK, PRB 25, 4515 (1982)
Proximity effect and supercurrent
S SN
Metallic contact between a normal metal and a superconductor
SS SSNN
Electron-hole correlations: proximity effect
Supercurrent Andreev bound states (ABS)
Reflected hole
Incident electron
SuperconductorSuperconductorNormal metal Normal metal (Semiconductor)(Semiconductor)
Cooper pair
Andreev reflection
Proximity effect in SNS systems: basic formalism
Diffusive mesoscopic N wire:quasi-1D geometryLφ >L >> leD = diffusion coefficient∆ = superconducting order parameterφ = macroscopic phase of the order parameterETh = D/L2 Thouless energy
LDOS properties:
N(-E) = N(E)Eg for |E| ≤ Eg
Eg(φ = 0) ≈ 3.2ETh for ∆>>ETh
Eg(φ = π) = 0
Usadel equations
LDOS
Modification of the LDOS in SNS systems due to proximity effect
J. C. Hammer et al., PRB 76, 064514 (2007)
Phase dependence
J. C. Cuevas et al., PRB 73, 184505 (2006)
Length and position dependence
Al/Cu SN structure with tunnel probes
Spatial spectroscopy of PE probed with tunnel junctions
Phase-dependence of PE probed with STM spectroscopy
Al/Ag SNS proximity SQUIDs
Experiment to theory comparison
Phase-dependence of PE probed with STM spectroscopy
H. le Sueur et al., PRL 100, 197002 (2008)
Phase-evolution of PE
Full phase-control of the minigap
amplitude
I) φ-tuning of specific heat: quantum control of a thermodynamic variable
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)
Electron entropy Electron specific heat
II) φ-tuning of e-ph interaction: quantum control of relaxation
T. T. Heikkila and F. G., PRB 79, 094514 (2009)
Sensitivity through proximity
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
SQUIPT: fabrication details and configurations
Shadow-mask evaporation27 nm Al @ 25°Oxidation 4.4 mbar 5’ (tunnel junctions)27 nm Cu @ -25°60 nm Al @ 60° (clean SN interfaces)