900 research outputs found
Quantum Transport Through a Stretched Spin--1 Molecule
We analyze the electronic transport through a model spin-1 molecule as a
function of temperature, magnetic field and bias voltage. We consider the
effect of magnetic anisotropy, which can be generated experimentally by
stretching the molecule. In the experimentally relevant regime the conductance
of the unstretched molecule reaches the unitary limit of the underscreened
spin- 1 Kondo effect at low temperatures. The magnetic anisotropy generates an
antiferromagnetic coupling between the remaining spin 1/2 and a singular
density of quasiparticles, producing a second Kondo effect and a reduced
conductance. The results explain recent measurements in spin-1 molecules
[Science 328 1370 (2010)].Comment: 5 pages, 3 figures, minor changes, accepted for publication in EP
Impact of capacitance and tunneling asymmetries on Coulomb blockade edges and Kondo peaks in non-equilibrium transport through molecular quantum dots
We investigate theorerically the non-equilibrium transport through a
molecular quantum dot as a function of gate and bias voltage, taking into
account the typical situation in molecular electronics. In this respect, our
study includes asymmetries both in the capacitances and tunneling rates to the
source and drain electrodes, as well as an infinitely large charging energy on
the molecule. Our calculations are based on the out-of-equilibrium
Non-Crossing-Approximation (NCA), which is a reliable technique in the regime
under consideration. We find that Coulomb blockade edges and Kondo peaks
display strong renormalization in their width and intensity as a function of
these asymmetries, and that basic expectations from Coulomb blockade theory
must be taken with care in general, expecially when Kondo physics is at play.
In order to help comparison of theory to experiments, we also propose a simple
phenomenological model which reproduces semi-quantitatively the Coulomb
blockade edges that were numerically computed from the NCA in all regimes of
parameters.Comment: 9 pages, 8 figure
Non-Fermi liquid behavior in transport through Co doped Au chains
We calculate the conductance as a function of temperature through Au
monoatomic chains containing one Co atom as a magnetic impurity, and connected
to two conducting leads with a 4-fold symmetry axis. Using the information
derived from {\it ab initio} calculations, we construct an effective model
\Heff that hybridizes a 3d quadruplet at the Co site with two 3d
triplets through the hopping of 5d and 5d electrons of Au. The
quadruplet is split by spin anisotropy due to spin-orbit coupling. Solving
\Heff with the numerical renormalization group (NRG) % Wb: reverted my own
change we find that at low temperatures and the ground
state impurity entropy is , a behavior similar to the two-channel
Kondo model. Stretching the chain leads to a non Kondo phase, with the physics
of the underscreened Kondo model at the quantum critical point.Comment: Accepted in Physical Review Letter
Thermopower of an SU(4) Kondo resonance under an SU(2) symmetry-breaking field
We calculate the thermopower of a quantum dot described by two doublets
hybridized with two degenerate bands of two conducting leads, conserving
orbital (band) and spin quantum numbers, as a function of the temperature
and a splitting of the quantum dot levels which breaks the SU(4)
symmetry. The splitting can be regarded as a Zeeman (spin) or valley (orbital)
splitting. We use the non-crossing approximation (NCA), the slave bosons in the
mean-field approximation (SBMFA) and also the numerical renormalization group
(NRG) for large . The model describes transport through clean C
nanotubes %with weak disorder and in Si fin-type field effect transistors,
under an applied magnetic field. The thermopower as a function of temperature
displays two dips that correspond to the energy scales given by the
Kondo temperature and and one peak when reaches the
charge-transfer energy. These features are much more pronounced than the
corresponding ones in the conductance, indicating that the thermopower is a
more sensitive probe of the electronic structure at intermediate or high
energies. At low temperatures () is a constant that
increases strongly near the degeneracy point . We find that the SBMFA
fails to provide an accurate description of the thermopower for large .
Instead, a combination of Fermi liquid relations with the quantum-dot
occupations calculated within the NCA gives reliable results for .Comment: 8 pages, 7 figure
Universal transport signatures in two-electron molecular quantum dots: gate-tunable Hund's rule, underscreened Kondo effect and quantum phase transitions
We review here some universal aspects of the physics of two-electron
molecular transistors in the absence of strong spin-orbit effects. Several
recent quantum dots experiments have shown that an electrostatic backgate could
be used to control the energy dispersion of magnetic levels. We discuss how the
generically asymmetric coupling of the metallic contacts to two different
molecular orbitals can indeed lead to a gate-tunable Hund's rule in the
presence of singlet and triplet states in the quantum dot. For gate voltages
such that the singlet constitutes the (non-magnetic) ground state, one
generally observes a suppression of low voltage transport, which can yet be
restored in the form of enhanced cotunneling features at finite bias. More
interestingly, when the gate voltage is controlled to obtain the triplet
configuration, spin S=1 Kondo anomalies appear at zero-bias, with non-Fermi
liquid features related to the underscreening of a spin larger than 1/2.
Finally, the small bare singlet-triplet splitting in our device allows to
fine-tune with the gate between these two magnetic configurations, leading to
an unscreening quantum phase transition. This transition occurs between the
non-magnetic singlet phase, where a two-stage Kondo effect occurs, and the
triplet phase, where the partially compensated (underscreened) moment is akin
to a magnetically "ordered" state. These observations are put theoretically
into a consistent global picture by using new Numerical Renormalization Group
simulations, taylored to capture sharp finie-voltage cotunneling features
within the Coulomb diamonds, together with complementary out-of-equilibrium
diagrammatic calculations on the two-orbital Anderson model. This work should
shed further light on the complicated puzzle still raised by multi-orbital
extensions of the classic Kondo problem.Comment: Review article. 16 pages, 17 figures. Minor corrections and extra
references added in V
Unusual Kondo physics in a Co impurity atom embedded in noble-metal chains
We analyze the conduction bands of the one dimensional noble-metal chains
that contain a Co magnetic impurity by means of ab initio calculations. We
compare the results obtained for Cu and Ag pure chains, as well as O doped Cu,
Ag and Au chains with those previously found for Au pure chains. We find
similar results in the case of Cu and Au hosts, whereas for Ag chains a
different behavior is obtained. Differences and similarities among the
different systems are analyzed by comparing the electronic structure of the
three noble-metal hosts. The d-orbitals of Cu chains at the Fermi level have
the same symmetry as in the case of Au chains. These orbitals hybridize with
the corresponding ones of the Co impurity, giving rise to the possibility of
exhibiting a two-channel Kondo physics.Comment: Accepted in IEEE Trans. Magn. - April 201
The Rotating-Wave Approximation: Consistency and Applicability from an Open Quantum System Analysis
We provide an in-depth and thorough treatment of the validity of the
rotating-wave approximation (RWA) in an open quantum system. We find that when
it is introduced after tracing out the environment, all timescales of the open
system are correctly reproduced, but the details of the quantum state may not
be. The RWA made before the trace is more problematic: it results in incorrect
values for environmentally-induced shifts to system frequencies, and the
resulting theory has no Markovian limit. We point out that great care must be
taken when coupling two open systems together under the RWA. Though the RWA can
yield a master equation of Lindblad form similar to what one might get in the
Markovian limit with white noise, the master equation for the two coupled
systems is not a simple combination of the master equation for each system, as
is possible in the Markovian limit. Such a naive combination yields inaccurate
dynamics. To obtain the correct master equation for the composite system a
proper consideration of the non-Markovian dynamics is required.Comment: 17 pages, 0 figures
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