Andreev and Josephson transport in InAs nanowire-based quantum dots

Superconducting proximity effects are of fundamental interest

and underlie recent proposals for experimental realization of

topological states. Here we study superconductor-quantum dot-

superconductor (S-QD-S) junctions formed by contacting short-

channel InAs nanowire transistors with Nb leads . When the

carrier density is low, one or more quantum dots form in the

nanowire due to spatial potential fluctuations. Low-temperature

electrical transport shows clear signatures of proximity

superconductivity, such as regions of negative differential

conductance, Multiple Andreev Reflections (MAR) and

spectroscopic features hinting at the formation of Andreev Bound

States (ABS). These features can coexist with the Coulomb

diamond structure resulting from the dot charging energy. The

theory of Andreev and Josephson transport in S-QD-S structures

is invoked in order to elucidate the experimental data. Particular

attention is devoted to an intermediate coupling regime, wherein

the superconducting energy gap Δ is on the same order of

magnitude as the tunnel coupling strength Γ, but smaller

than the Coulomb charging energy of the dot U. In this model, a

rich interplay exists between U, which favours a spin-doublet

ground state for the quantum dot, Δ, which favours a BCS-

like singlet ground state, and Kondo correlations in the dot,

which favour a Yu-Shiba-Rusinov-like singlet ground state. A

quantum phase transition can occur from the doublet to the BCS-

like singlet ground state, marking a 0-π transition in the

Josephson current of the junction. The significance of these

results to the search for topological states in semiconductor

nanowire junctions is discussed.


http://indico.cern.ch/event/355942/session/19...bution/731