  We theoretically investigate electronic transport through a junction where a
quantum dot (QD) is tunnel coupled on both sides to semiconductor nanowires
with strong spin-orbit interaction and proximity-induced superconductivity. The
results are presented as stability diagrams, i.e., the differential conductance
as a function of the bias voltage applied across the junction and the gate
voltage used to control the electrostatic potential on the QD. A small applied
magnetic field splits and modifies the resonances due to the Zeeman splitting
of the QD level. Above a critical field strength, Majorana bound states (MBS)
appear at the interfaces between the two superconducting nanowires and the QD,
resulting in a qualitative change of the entire stability diagram, suggesting
this setup as a promising platform to identify MBS. Our calculations are based
on a nonequilibrium Green's function description and is exact when Coulomb
interactions on the QD can be neglected. In addition, we develop a simple
pictorial view of the involved transport processes, which is equivalent to a
description in terms of multiple Andreev reflections, but provides an
alternative way to understand the role of the QD level in enhancing transport
for certain gate and bias voltages. We believe that this description will be
useful in future studies of interacting QDs coupled to superconducting leads
(with or without MBS), where it can be used to develop a perturbation expansion
in the tunnel coupling.
