285 research outputs found
Pair-tunneling resonance in the single-electron transport regime
We predict a new electron pair-tunneling (PT) resonance in non-linear
transport through quantum dots with positive charging energies exceeding the
broadening due to thermal and quantum fluctuations. The PT resonance shows up
in the single-electron transport (SET) regime as a peak in the derivative of
the non-linear conductance when the electrochemical potential of one electrode
matches the average of two subsequent charge addition energies. For a single
level quantum dot (Anderson model) we find the analytic peak shape and the
dependence on temperature, magnetic field and junction asymmetry and compare
with the inelastic cotunneling peak which is of the same order of magnitude. In
experimental transport data the PT resonance may be mistaken for a weak SET
resonance judging only by the voltage dependence of its position. Our results
provide essential clues to avoid such erroneous interpretation of transport
spectroscopy data.Comment: 5 pages, 2 figures, published versio
Non-linear thermoelectrics of molecular junctions with vibrational coupling
We present a detailed study of the non-linear thermoelectric properties of a
molecular junction, represented by a dissipative Anderson-Holstein model. A
single orbital level with strong Coulomb interaction is coupled to a localized
vibrational mode and we account for both electron and phonon exchange with both
electrodes, investigating how these contribute to the heat and charge
transport. We calculate the efficiency and power output of the device operated
as a heat to electric power converter and identify the optimal operating
conditions, which are found to be qualitatively changed by the presence of the
vibrational mode. Based on this study of a generic model system, we discuss the
desirable properties of molecular junctions for thermoelectric applications.Comment: 8 pages, 5 figure
Transport signature of pseudo-Jahn-Teller dynamics in a single-molecule transistor
We calculate the electronic transport through a molecular dimer, in which an
excess electron is delocalized over equivalent monomers, which can be locally
distorted. In this system the Born-Oppenheimer approximation breaks down
resulting in quantum entanglement of the mechanical and electronic motion. We
show that pseudo Jahn-Teller (pJT) dynamics of the molecule gives rise to
conductance peaks that indicate this violation. Their magnitude, sign and
position sharply depend on the electro-mechanical properties of the molecule,
which can be varied in recently developed three-terminal junctions with
mechanical control. The predicted effect depends crucially on the degree of
intramolecular delocalization of the excess electron, a parameter which is also
of fundamental importance in physical chemistry.Comment: 6 pages, 3 figure
Kinetic Equations for Transport Through Single-Molecule Transistors
We present explicit kinetic equations for quantum transport through a general
molecular quantum-dot, accounting for all contributions up to 4th order
perturbation theory in the tunneling Hamiltonian and the complete molecular
density matrix. Such a full treatment describes not only sequential,
cotunneling and pair tunneling, but also contains terms contributing to
renormalization of the molecular resonances as well as their broadening. Due to
the latter all terms in the perturbation expansion are automatically
well-defined for any set of system parameters, no divergences occur and no
by-hand regularization is required. Additionally we show that, in contrast to
2nd order perturbation theory, in 4th order it is essential to account for
quantum coherence between non-degenerate states, entering the theory through
the non-diagonal elements of the density matrix. As a first application, we
study a single-molecule transistor coupled to a localized vibrational mode
(Anderson-Holstein model). We find that cotunneling-assisted sequential
tunneling processes involving the vibration give rise to current peaks i.e.
negative differential conductance in the Coulomb-blockade regime. Such peaks
occur in the cross-over to strong electron-vibration coupling, where inelastic
cotunneling competes with Franck-Condon suppressed sequential tunneling, and
thereby indicate the strength of the electron-vibration coupling. The peaks
depend sensitively on the coupling to a dissipative bath, thus providing also
an experimental probe of the Q-factor of the vibrational motion.Comment: 17 pages, 5 figures. Final published version. Some important typos
correcte
Nonlinear thermoelectric response due to energy-dependent transport properties of a quantum dot
Quantum dots are useful model systems for studying quantum thermoelectric
behavior because of their highly energy-dependent electron transport
properties, which are tunable by electrostatic gating. As a result of this
strong energy dependence, the thermoelectric response of quantum dots is
expected to be nonlinear with respect to an applied thermal bias. However,
until now this effect has been challenging to observe because, first, it is
experimentally difficult to apply a sufficiently large thermal bias at the
nanoscale and, second, it is difficult to distinguish thermal bias effects from
purely temperature-dependent effects due to overall heating of a device. Here
we take advantage of a novel thermal biasing technique and demonstrate a
nonlinear thermoelectric response in a quantum dot which is defined in a
heterostructured semiconductor nanowire. We also show that a theoretical model
based on the Master equations fully explains the observed nonlinear
thermoelectric response given the energy-dependent transport properties of the
quantum dot.Comment: Cite as: A. Svilans, et al., Physica E (2015),
http://dx.doi.org/10.1016/j.physe.2015.10.00
Spin-dependent electronic hybridization in a rope of carbon nanotubes
We demonstrate single electron addition to different strands of a carbon
nanotube rope. Anticrossings of anomalous conductance peaks occur in quantum
transport measurements through the parallel quantum dots forming on the
individual strands. We determine the magnitude and the sign of the
hybridization as well as the Coulomb interaction between the carbon nanotube
quantum dots, finding that the bonding states dominate the transport. In a
magnetic field the hybridization is shown to be selectively suppressed due to
spin effects.Comment: 4 pages, 4 figure
Using polymer electrolyte gates to set-and-freeze threshold voltage and local potential in nanowire-based devices and thermoelectrics
We use the strongly temperature-dependent ionic mobility in polymer
electrolytes to 'freeze in' specific ionic charge environments around a
nanowire using a local wrap-gate geometry. This enables us to set both the
threshold voltage for a conventional doped substrate gate and the local
disorder potential at temperatures below 200 Kelvin, which we characterize in
detail by combining conductance and thermovoltage measurements with modeling.
Our results demonstrate that local polymer electrolyte gates are compatible
with nanowire thermoelectrics, where they offer the advantage of a very low
thermal conductivity, and hold great potential towards setting the optimal
operating point for solid-state cooling applications.Comment: Published in Advanced Functional Materials. Includes colour versions
of figures and supplementary informatio
Majorana bound states in a coupled quantum-dot hybrid-nanowire system
Hybrid nanowires combining semiconductor and superconductor materials appear
well suited for the creation, detection, and control of Majorana bound states
(MBSs). We demonstrate the emergence of MBSs from coalescing Andreev bound
states (ABSs) in a hybrid InAs nanowire with epitaxial Al, using a quantum dot
at the end of the nanowire as a spectrometer. Electrostatic gating tuned the
nanowire density to a regime of one or a few ABSs. In an applied axial magnetic
field, a topological phase emerges in which ABSs move to zero energy and remain
there, forming MBSs. We observed hybridization of the MBS with the end-dot
bound state, which is in agreement with a numerical model. The ABS/MBS spectra
provide parameters that are useful for understanding topological
superconductivity in this system.Comment: Article and Supplementary Materia
Probing Transverse Magnetic Anisotropy by Electronic Transport through a Single-Molecule Magnet
By means of electronic transport, we study the transverse magnetic anisotropy
of an individual Fe single-molecule magnet (SMM) embedded in a
three-terminal junction. In particular, we determine in situ the transverse
anisotropy of the molecule from the pronounced intensity modulations of the
linear conductance, which are observed as a function of applied magnetic field.
The proposed technique works at temperatures exceeding the energy scale of the
tunnel splittings of the SMM. We deduce that the transverse anisotropy for a
single Fe molecule captured in a junction is substantially larger than the
bulk value.Comment: 18 pages with 16 figures; version as publishe
- …