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
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
