14 research outputs found
Transport through anisotropic magnetic molecules with partially ferromagnetic leads: Spin-charge conversion and negative differential conductance
We theoretically investigate inelastic transport through anisotropic magnetic
molecules weakly coupled to one ferromagnetic and one nonmagnetic lead. We find
that the current is suppressed over wide voltage ranges due to spin blockade.
In this system, spin blockade is associated with successive spin flips of the
molecular spins and depends on the anisotropy energy barrier. This leads to the
appearance of a window of bias voltages between the Coulomb blockade and spin
blockade regimes where the current is large and to negative differential
conductance at low temperatures. Remarkably, negative differential conductance
is also present close to room temperature. Spin-blockade behavior is
accompanied by super-Poissonian shot noise, like in nonmagnetic quantum dots.
Finally, we show that the charge transmitted through the molecule between
initial preparation in a certain spin state and infinite time very strongly
depends on the initial spin state in certain parameter ranges. Thus the
molecule can act as a spin-charge converter, an effect potentially useful as a
read-out mechanism for molecular spintronics.Comment: 8 pages with 5 figures, version as publishe
Transport through a quantum dot with excitonic dot-lead coupling
We study the effect of a dot-lead interaction on transport through a quantum
dot hybridized to two semi-infinite Luttinger-liquid leads. A bosonization
approach is applied to treat the interaction between charge fluctuations on the
dot and the dynamically generated image charge in the leads. The nonequilibrium
distribution function of the dot and the tunneling current are computed within
a master-equation approach. The presence of the excitonic dot-lead coupling is
found to enhance transport in the vicinity of the Coulomb-blockade threshold.
This behavior is in contrast to the usual power-law suppression of electronic
tunneling which is found if this interaction is ignored.Comment: 9 pages, 2 figure
Spin amplification, reading, and writing in transport through anisotropic magnetic molecules
Inelastic transport through a single magnetic molecule weakly coupled to
metallic leads is studied theoretically. We consider dynamical processes that
are relevant for writing, storing, and reading spin information in molecular
memory devices. Magnetic anisotropy is found to be crucial for slow spin
relaxation. In the presence of anisotropy we find giant spin amplification: The
spin accumulated in the leads if a bias voltage is applied to a molecule
prepared in a spin-polarized state can be made exponentially large in a
characteristic energy divided by temperature. For one ferromagnetic and one
paramagnetic lead the molecular spin can be reversed by applying a bias voltage
even in the absence of a magnetic field. We propose schemes for reading and
writing spin information based on our findings.Comment: 5+ pages with 5 figure
Effect of a Coulombic dot-lead coupling on the dynamics of a quantum dot
The effect of a Coulombic coupling on the dynamics of a quantum dot
hybridized to leads is determined. The calculation treats the interaction
between charge fluctuations on the dot and the dynamically generated image
charge in the leads. A formally exact solution is presented for a dot coupled
to a Luttinger liquid and an approximate solution, equivalent to treating the
lead dynamics within a random phase approximation, is given for a dot coupled
to a two- or three-dimensional metallic lead. The leading divergences arising
from the long-ranged Coulomb interaction are found to cancel, so that in the
two- and three-dimensional cases the quantum-dot dynamics is equivalent to that
obtained by neglecting both the dot-lead Coulomb coupling and the Coulomb
renormalization of the lead electrons, while in the one-dimensional case the
dot-lead mixing is enhanced relative to the non-interacting case. Explicit
results are given for the short-time dynamics.Comment: 8 pages, 2 figures, version as publishe
Cotunneling and non-equilibrium magnetization in magnetic molecular monolayers
Transport and non-equilibrium magnetization in monolayers of magnetic
molecules subject to a bias voltage are considered. We apply a master-equation
approach going beyond the sequential-tunneling approximation to study the
Coulomb-blockade regime. While the current is very small in this case, the
magnetization shows changes of the order of the saturation magnetization for
small variations of the bias voltage. Inelastic cotunneling processes manifest
themselves as differential-conductance steps, which are accompanied by much
larger changes in the magnetization. In addition, the magnetization in the
Coulomb-blockade regime exhibits strong signatures of sequential tunneling
processes de-exciting molecular states populated by inelastic cotunneling. We
also consider the case of a single molecule, finding that cotunneling processes
lead to the occurrence of magnetic sidebands below the Coulomb-blockade
threshold. In the context of molecular electronics, we study how additional
spin relaxation suppresses the fine structure in transport and magnetization.Comment: 8 pages, 8 figures, version as publishe
Resonant and Kondo tunneling through molecular magnets
Transport through molecular magnets is studied in the regime of strong
coupling to the leads. We consider a resonant-tunneling model where the
electron spin in a quantum dot or molecule is coupled to an additional local,
anisotropic spin via exchange interaction. The two opposite regimes dominated
by resonant tunneling and by Kondo transport, respectively, are considered. In
the resonant-tunneling regime, the stationary state of the impurity spin is
calculated for arbitrarily strong molecule-lead coupling using a
master-equation approach, which treats the exchange interaction perturbatively.
We find that the characteristic fine structure in the differential conductance
persists even if the hybridization energy exceeds thermal energies. Transport
in the Kondo regime is studied within a diagrammatic approach. We show that
magnetic anisotropy gives rise to a splitting of the Kondo peak at low bias
voltages.Comment: 13 pages, 5 figures, version as publishe
Cotunneling through a magnetic single-molecule transistor based on N\atC60
We present an experimental and theoretical study of a magnetic
single-molecule transistor based on N@C60 connected to gold electrodes.
Particular attention is paid to the regime of intermediate molecule-lead
coupling, where cotunneling effects manifest themselves in the Coulomb-blockade
regime. The experimental results for the differential conductance as a function
of bias, gate voltage, and external magnetic field are in agreement with our
analysis of the tunneling rates and provide evidence of magnetic signatures in
single-N@C60 devices arising from an antiferromagnetic exchange interaction
between the C60 spin and the nitrogen spin.Comment: Accepted for publication in PRB Rapid Com, 4 pages, 4 figures, with
supplementary information (6 pages, 3 figures
Quantum Noise Interference and Back-action Cooling in Cavity Nanomechanics
We present a theoretical analysis of a novel cavity electromechanical system
where a mechanical resonator directly modulates the damping rate kappa of a
driven electromagnetic cavity. We show that via a destructive interference of
quantum noise, the driven cavity can effectively act like a zero-temperature
bath irrespective of the ratio kappa / omega_M, where omega_M is the mechanical
frequency. This scheme thus allows one to cool the mechanical resonator to its
ground state without requiring the cavity to be in the so-called `good cavity'
limit kappa << omega_M.Comment: 4+ pages, 2 figures. Error in second last paragraph correcte
Theory for transport through a single magnetic molecule: Endohedral N@C60
We consider transport through a single N@C60 molecule, weakly coupled to
metallic leads. Employing a density-matrix formalism we derive rate equations
for the occupation probabilities of many-particle states of the molecule. We
calculate the current-voltage characteristics and the differential conductance
for N@C60 in a break junction. Our results reveal Coulomb-blockade behavior as
well as a fine structure of the Coulomb-blockade peaks due to the exchange
coupling of the C60 spin to the spin of the encapsulated nitrogen atom.Comment: 5 pages, 4 figures, v2: version as publishe