320 research outputs found
Transport through quantum dots in mesoscopic circuits
We study the transport through a quantum dot, in the Kondo Coulomb blockade
valley, embedded in a mesoscopic device with finite wires. The quantization of
states in the circuit that hosts the quantum dot gives rise to finite size
effects. These effects make the conductance sensitive to the ratio of the Kondo
screening length to the wires length and provide a way of measuring the Kondo
cloud. We present results obtained with the numerical renormalization group for
a wide range of physically accessible parameters.Comment: 4 pages, 5 figure
Many Body Effects on the Transport Properties of Single-Molecule Devices
The conductance through a molecular device including electron-electron and
electron-phonon interactions is calculated using the Numerical Renormalization
Group method. At low temperatures and weak electron-phonon coupling the
properties of the conductance can be explained in terms of the standard Kondo
model with renormalized parameters. At large electron-phonon coupling a charge
analog of the Kondo effect takes place that can be mapped into an anisotropic
Kondo model. In this regime the molecule is strongly polarized by a gate
voltage which leads to rectification in the current-voltage characteristics of
the molecular junction.Comment: 4 pages, 4 figures, minor changes, added reference
Electronic Transport through Magnetic Molecules with Soft Vibrating Modes
The low-temperature transport properties of a molecule are studied in the
field-effect transitor geometry. The molecule has an internal mechanical mode
that modulates its electronic levels and renormalizes both the interactions and
the coupling to the electrodes. For a soft mechanical mode the spin
fluctuations in the molecule are dominated by the bare couplings while the
valence changes are determined by the dressed energies. In this case, the
transport properties present an anomalous behavior and the Kondo temperature
has a weak gate voltage dependence. These observations are in agreement with
recent experimental data.Comment: 4 pages, 3 figures, accepted in PRB R
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
Transport through side-coupled double quantum dots: from weak to strong interdot coupling
We report low-temperature transport measurements through a double quantum dot
device in a configuration where one of the quantum dots is coupled directly to
the source and drain electrodes, and a second (side-coupled) quantum dot
interacts electrostatically and via tunneling to the first one. As the interdot
coupling increases, a crossover from weak to strong interdot tunneling is
observed in the charge stability diagrams that present a complex pattern with
mergings and apparent crossings of Coulomb blockade peaks. While the weak
coupling regime can be understood by considering a single level on each dot, in
the intermediate and strong coupling regimes, the multi-level nature of the
quantum dots needs to be taken into account. Surprisingly, both in the strong
and weak coupling regimes, the double quantum dot states are mainly localized
on each dot for most values of the parameters. Only in an intermediate coupling
regime the device presents a single dot-like molecular behavior as the
molecular wavefunctions weight is evenly distributed between the quantum dots.
At temperatures larger than the interdot coupling energy scale, a loss of
coherence of the molecular states is observed.Comment: 9 pages, 5 figure
Enhanced Kondo Effect in an Electron System Dynamically Coupled with Local Optical Phonon
We discuss Kondo behavior of a conduction electron system coupled with local
optical phonon by analyzing the Anderson-Holstein model with the use of a
numerical renormalization group (NRG) method. There appear three typical
regions due to the balance between Coulomb interaction and
phonon-mediated attraction . For , we
observe the standard Kondo effect concerning spin degree of freedom. Since the
Coulomb interaction is effectively reduced as , the
Kondo temperature is increased when is increased. On
the other hand, for , there occurs the Kondo effect
concerning charge degree of freedom, since vacant and double occupied states
play roles of pseudo-spins. Note that in this case, is decreased
with the increase of . Namely, should be maximized for
. Then, we analyze in detail the Kondo behavior
at , which is found to be explained by the polaron
Anderson model with reduced hybridization of polaron and residual repulsive
interaction among polarons. By comparing the NRG results of the polaron
Anderson model with those of the original Anderson-Holstein model, we clarify
the Kondo behavior in the competing region of .Comment: 8 pages, 8 figure
Carbapenem-resistant Pseudomonas aeruginosa with acquired bla(vim) metallo-beta-lactamase determinants, Italy.
6nonenoneROSSOLINI G.M.; RICCIO M.L.; CORNAGLIA G.; PAGANI L.; LAGATOLLA C.; SELAN L. AND FONTANA R.Rossolini, G. M.; Riccio, M. L.; Cornaglia, G.; Pagani, L.; Lagatolla, Cristina; Selan, L. AND FONTANA R
Nuclear magnetic resonance probes for the Kondo scenario for the 0.7 feature in semiconductor quantum point contact devices
We propose a probe based on nuclear relaxation and Knight shift measurements
for the Kondo scenario for the "0.7 feature" in semiconductor quantum point
contact (QPC) devices. We show that the presence of a bound electron in the QPC
would lead to a much higher rate of nuclear relaxation compared to nuclear
relaxation through exchange of spin with conduction electrons. Furthermore, we
show that the temperature dependence of this nuclear relaxation is very
non-monotonic as opposed to the linear-T relaxation from coupling with
conduction electrons. We present a qualitative analysis for the additional
relaxation due to nuclear spin diffusion (NSD) and study the extent to which
NSD affects the range of validity of our method. The conclusion is that nuclear
relaxation, in combination with Knight shift measurements, can be used to
verify whether the 0.7 feature is indeed due to the presence of a bound
electron in the QPC.Comment: Published version. Appears in a Special Section on the 0.7 Feature
and Interactions in One-Dimensional Systems. 16 page
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
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