509 research outputs found
Mesoscopic Fluctuations in Quantum Dots in the Kondo Regime
Properties of the Kondo effect in quantum dots depend sensitively on the
coupling parameters and so on the realization of the quantum dot -- the Kondo
temperature itself becomes a mesoscopic quantity. Assuming chaotic dynamics in
the dot, we use random matrix theory to calculate the distribution of both the
Kondo temperature and the conductance in the Coulomb blockade regime. We study
two experimentally relevant cases: leads with single channels and leads with
many channels. In the single-channel case, the distribution of the conductance
is very wide as fluctuates on a logarithmic scale. As the number of
channels increases, there is a slow crossover to a self-averaging regime.Comment: 4 pages, 3 figure
Multi-parameter scaling of the Kondo effect in quantum dots with an even number of electrons
We address a recent theoretical discrepancy concerning the Kondo effect in
quantum dots with an even number of electrons where spin-singlet and -triplet
states are nearly degenerate. We show that the discrepancy arises from the fact
that the Kondo scaling involves many parameters, which makes the results depend
on concrete microscopic models. We illustrate this by the scaling calculations
of the Kondo temperature, , as a function of the energy difference between
the singlet and triplet states . decreases with
increasing , showing a crossover from a power law with a universal
exponent to that with a nonuniversal exponent. The crossover depends on the
initial parameters of the model.Comment: 8 pages, 3 figure
Flux-quantum-modulated Kondo conductance in a multielectron quantum dot
We investigate a lateral semiconductor quantum dot with a large number of
electrons in the limit of strong coupling to the leads. A Kondo effect is
observed and can be tuned in a perpendicular magnetic field. This Kondo effect
does not exhibit Zeeman splitting. It shows a modulation with the periodicity
of one flux quantum per dot area at low temperatures. The modulation leads to a
novel, strikingly regular stripe pattern for a wide range in magnetic field and
number of electrons.Comment: 4 pages, 5 figure
Magnetotransport through a strongly interacting quantum dot
We study the effect of a magnetic field on the conductance through a strongly
interacting quantum dot by using the finite temperature extension of Wilson's
numerical renormalization group method to dynamical quantities. The quantum dot
has one active level for transport and is modelled by an Anderson impurity
attached to left and right electron reservoirs. Detailed predictions are made
for the linear conductance and the spin-resolved conductance as a function of
gate voltage, temperature and magnetic field strength. A strongly coupled
quantum dot in a magnetic field acts as a spin filter which can be tuned by
varying the gate voltage. The largest spin-filtering effect is found in the
range of gate voltages corresponding to the mixed valence regime of the
Anderson impurity model.Comment: Revised version, to appear in PRB, 4 pages, 4 figure
Nonequilibrium Kondo Effect in a Multi-level Quantum Dot near singlet-triplet transition
The linear and nonlinear transport through a multi-level lateral quantum dot
connected to two leads is investigated using a generalized finite-
slave-boson mean field approach. For a two-level quantum dot, our calculation
demonstrates a substantial conductance enhancement near the degeneracy point of
the spin singlet and triplet states, a non-monotonic temperature-dependence of
conductance and a sharp dip and nonzero bias maximum of the differential
conductance. These agree well with recent experiment observations. This
two-stage Kondo effect in an out-of-equilibrium situation is attributed to the
interference between the two energy levels.Comment: 4 pages, 3 figure
Kondo effect induced by a magnetic field
We study peculiarities of transport through a Coulomb blockade system tuned
to the vicinity of the spin transition in its ground state. Such transitions
can be induced in practice by application of a magnetic field. Tunneling of
electrons between the dot and leads mixes the states belonging to the ground
state manifold of the dot. Remarkably, both the orbital and spin degrees of
freedom of the electrons are engaged in the mixing at the singlet-triplet
transition point. We present a model which provides an adequate theoretical
description of recent experiments with semiconductor quantum dots and carbon
nanotubes
Fano Resonances in Electronic Transport through a Single Electron Transistor
We have observed asymmetric Fano resonances in the conductance of a single
electron transistor resulting from interference between a resonant and a
nonresonant path through the system. The resonant component shows all the
features typical of quantum dots, but the origin of the non-resonant path is
unclear. A unique feature of this experimental system, compared to others that
show Fano line shapes, is that changing the voltages on various gates allows
one to alter the interference between the two paths.Comment: 8 pages, 6 figures. Submitted to PR
G protein signaling-biased agonism at the k-opioid receptor is maintained in striatal neurons
Biased agonists of G protein-coupled receptors may present a means to refine receptor signaling in a way that separates side effects from therapeutic properties. Several studies have shown that agonists that activate the k-opioid receptor (KOR) in a manner that favors G protein coupling over b-Arrestin2 recruitment in cell culture may represent a means to treat pain and itch while avoiding sedation and dysphoria. Although it is attractive to speculate that the bias between G protein signaling and b-Arrestin2 recruitment is the reason for these divergent behaviors, little evidence has emerged to show that these signaling pathways diverge in the neuronal environment. We further explored the influence of cellular context on biased agonism at KOR ligand-directed signaling toward G protein pathways over b-Arrestin-dependent pathways and found that this bias persists in striatal neurons. These findings advance our understanding of how a G protein-biased agonist signal differs between cell lines and primary neurons, demonstrate that measuring [35S]GTPgS binding and the regulation of adenylyl cyclase activity are not necessarily orthogonal assays in cell lines, and emphasize the contributions of the environment to assessing biased agonism
Singlet-triplet transition in a lateral quantum dot
We study transport through a lateral quantum dot in the vicinity of the
singlet-triplet transition in its ground state. This transition, being sharp in
an isolated dot, is broadened to a crossover by the exchange interaction of the
dot electrons with the conduction electrons in the leads. For a generic set of
system's parameters, the linear conductance has a maximum in the crossover
region. At zero temperature and magnetic field, the maximum is the strongest.
It becomes less pronounced at finite Zeeman splitting, which leads to an
increase of the background conductance and a decrease of the conductance in the
maximum
Kondo effect in multielectron quantum dots at high magnetic fields
We present a general description of low temperature transport through a
quantum dot with any number of electrons at filling factor . We
provide a general description of a novel Kondo effect which is turned on by
application of an appropriate magnetic field. The spin-flip scattering of
carriers by the quantum dot only involves two states of the scatterer which may
have a large spin. This process is described by spin-flip Hubbard operators,
which change the angular momentum, leading to a Kondo Hamiltonian. We obtain
antiferromagnetic exchange couplings depending on tunneling amplitudes and
correlation effects. Since Kondo temperature has an exponential dependence on
exchange couplings, quantitative variations of the parameters in different
regimes have important experimental consequences. In particular, we discuss the
{\it chess board} aspect of the experimental conductance when represented in a
grey scale as a function of both the magnetic field and the gate potential
affecting the quantum dot
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