28 research outputs found
Transport through a molecular quantum dot in the polaron crossover regime
We consider resonant transport through a molecular quantum dot coupled to a
local vibration mode. Applying the non-equilibrium Green function technique in
the polaron representation, we develop a non-perturbative scheme to calculate
the electron spectral function of the molecule in the regime of intermediate
electron-phonon coupling. With increasing tunneling coupling to the leads,
correlations between polaron clouds become more important at relatively high
temperature leading to a strong sharpening of the peak structure in the
spectral function. The detection of such features in the current-voltage
characteristics is briefly discussed
Transport Through Andreev Bound States in a Graphene Quantum Dot
Andreev reflection-where an electron in a normal metal backscatters off a
superconductor into a hole-forms the basis of low energy transport through
superconducting junctions. Andreev reflection in confined regions gives rise to
discrete Andreev bound states (ABS), which can carry a supercurrent and have
recently been proposed as the basis of qubits [1-3]. Although signatures of
Andreev reflection and bound states in conductance have been widely reported
[4], it has been difficult to directly probe individual ABS. Here, we report
transport measurements of sharp, gate-tunable ABS formed in a
superconductor-quantum dot (QD)-normal system, which incorporates graphene. The
QD exists in the graphene under the superconducting contact, due to a
work-function mismatch [5, 6]. The ABS form when the discrete QD levels are
proximity coupled to the superconducting contact. Due to the low density of
states of graphene and the sensitivity of the QD levels to an applied gate
voltage, the ABS spectra are narrow, can be tuned to zero energy via gate
voltage, and show a striking pattern in transport measurements.Comment: 25 Pages, included SO
Green function techniques in the treatment of quantum transport at the molecular scale
The theoretical investigation of charge (and spin) transport at nanometer
length scales requires the use of advanced and powerful techniques able to deal
with the dynamical properties of the relevant physical systems, to explicitly
include out-of-equilibrium situations typical for electrical/heat transport as
well as to take into account interaction effects in a systematic way.
Equilibrium Green function techniques and their extension to non-equilibrium
situations via the Keldysh formalism build one of the pillars of current
state-of-the-art approaches to quantum transport which have been implemented in
both model Hamiltonian formulations and first-principle methodologies. We offer
a tutorial overview of the applications of Green functions to deal with some
fundamental aspects of charge transport at the nanoscale, mainly focusing on
applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references,
submitted to Springer series "Lecture Notes in Physics
Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device
We report on the observation of excitation of Majorana fermions in a Nb-InSb
nanowire quantum dot-Nb hybrid system. The InSb nanowire quantum dot is formed
between the two Nb contacts by weak Schottky barriers and is thus in the regime
of strong couplings to the contacts. Due to the proximity effect, the InSb
nanowire segments covered by superconductor Nb contacts turn to superconductors
with a superconducting energy gap . Under an applied magnetic field
larger than a critical value for which the Zeeman energy in the InSb nanowire
is , the entire InSb nanowire is found to be in a nontrivial
topological superconductor phase, supporting a pair of Majorana fermions, and
Cooper pairs can transport between the superconductor Nb contacts via the
Majorana fermion states. This transport process will be suppressed when the
applied magnetic field becomes larger than a second critical value at which the
transition to a trivial topological superconductor phase occurs in the system.
This physical scenario has been observed in our experiment. We have found that
the measured zero-bias conductance for our hybrid device shows a conductance
plateau in a range of the applied magnetic field in quasi-particle Coulomb
blockade regions.Comment: 7 pages, 4 figures, supplementary materials of 3 pages and 1 figur