laboratoire pierre aigrain
électronique et photonique quantiques
 
laboratoire pierre aigrain
 

Seminaire, 16 Septembre 2013

Vladimir Manucharyan (Harvard University)
Superconducting proximity effect in InAs semiconducting nanowires

We present tunneling spectroscopy of individual quasiparticle bound
states in an Al/InAs/Al mesoscopic Josephson junction. The device is a
broken superconducting Al loop bridged by a short section of a
crystalline semiconducting InAs nanowire. The loop is pierced with a
magnetic flux to control the superconducting phase-difference across
the junction ; the electron density in the nanowire can be gate-tuned
down to a full depletion ; spectroscopy is performed using a separate
normal metal lead.

At a large electron density in the nanowire, the tunneling spectrum shows a phase-tunable minigap, characteristic of the proximity effect in
diffusive metals. Reducing the density, we resolve a discrete sub-gap
resonance corresponding to the Andreev bound state for a quasiparticle
in the junction region. We argue that the corresponding BCS-like
ground state of the nanowire consists of only a single Cooper pair,
which "leaked" into the nanowire from the leads. At densities closer
to depletion, Coulomb interaction switches on and turns the junction
(at an odd occupancy) into a spin 1/2 quantum impurity. The sub-gap
resonance then has a qualitatively different gate and phase dependence
and can be explained by the formation of a Kondo-like singlet between
a quasiparticle in Al and the impurity.

Our experiments reveal how superconductivity can be proximity-induced
in systems containing only a few electronic states, and may find
applications in quantum information processing with individual
Bogolyubov quasiparticles and in proximity-engineering of topological
superconductivity.