604 research outputs found
Climbing the Entropy Barrier: Driving the Single- towards the Multichannel Kondo Effect by a Weak Coulomb Blockade of the Leads
We study a model proposed recently in which a small quantum dot is coupled
symmetrically to several large quantum dots characterized by a charging energy
E_c. Even if E_c is much smaller than the Kondo temperature T_K, the
long-ranged interactions destabilize the single-channel Kondo effect and induce
a flow towards a multi-channel Kondo fixed point associated with a rise of the
impurity entropy with decreasing temperature. Such an ``uphill flow'' implies a
negative impurity specific heat, in contrast to all systems with local
interactions. An exact solution found for a large number of channels allows us
to capture this physics and to predict transport properties.Comment: 4 pages, 3 figures. Recent references and new title added in
published versio
A brief review of recent advances on the Mott transition: unconventional transport, spectral weight transfers, and critical behaviour
Strongly correlated metals close to the Mott transition display unusual
transport regimes, together with large spectral weight transfers in optics and
photoemission. We briefly review the theoretical understanding of these
effects, based on the dynamical mean-field theory, and emphasize the key role
played by the two energy scales associated with quasiparticle coherence scale
and with the Mott gap. Recent experimental results on two-dimensional organic
compounds and transition metal oxides are considered in this perspective. The
liquid-gas critical behaviour at the Mott critical endpoint is also discussed.
Transport calculations using the numerical renormalization group are presented.Comment: Review article. 9 pages, 5 figures. Proceedings of the Vth
International Conference on Crystalline Organic Metals, Superconductors and
Magnets (ISCOM 2003
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
Interplay of Kondo effect and strong spin-orbit coupling in multi-hole ultraclean carbon nanotubes
We report on cotunneling spectroscopy magnetoconductance measurements of
multi-hole ultraclean carbon nanotube quantum dots in the SU(4) Kondo regime
with strong spin-orbit coupling. Successive shells show a gradual weakening of
the Kondo effect with respect to the spin-orbital splittings, leading to an
evolution from SU(4) to SU(2) symmetry with a suppressed conductance at half
shell filling. The extracted energy level spectrum, overally consistent with
negligible disorder in the nanotube, shows in the half filled case large
renormalizations due to Coulombian effects.Comment: 5 pages, 4 figures, 1 supplementary fil
The Kondo effect in bosonic spin liquids
In a metal, a magnetic impurity is fully screened by the conduction electrons
at low temperature. In contrast, impurity moments coupled to spin-1 bulk
bosons, such as triplet excitations in paramagnets, are only partially
screened, even at the bulk quantum critical point. We argue that this
difference is not due to the quantum statistics of the host particles but
instead related to the structure of the impurity-host coupling, by
demonstrating that frustrated magnets with bosonic spinon excitations can
display a bosonic version of the Kondo effect. However, the Bose statistics of
the bulk implies distinct behavior, such as a weak-coupling impurity quantum
phase transition, and perfect screening for a range of impurity spin values. We
discuss implications of our results for the compound Cs2CuCl4, as well as
possible extensions to multicomponent bosonic gases.Comment: 4 pages, 3 figures. The weak coupling RG flow was corrected and
expanded in last versio
Gate-tuned high frequency response of carbon nanotube Josephson junctions
Carbon nanotube (CNT) Josephson junctions in the open quantum dot limit
exhibit superconducting switching currents which can be controlled with a gate
electrode. Shapiro voltage steps can be observed under radiofrequency current
excitations, with a damping of the phase dynamics that strongly depends on the
gate voltage. These measurements are described by a standard RCSJ model showing
that the switching currents from the superconducting to the normal state are
close to the critical current of the junction. The effective dynamical
capacitance of the nanotube junction is found to be strongly gate-dependent,
suggesting a diffusive contact of the nanotube.Comment: 14 pages, 8 figure
Singular dynamics and pseudogap formation in the underscreened Kondo impurity and Kondo lattice models
We study a generalization of the Kondo model in which the impurity spin is
represented by Abrikosov fermions in a rotation group SU(P) larger than the
SU(N) group associated to the spin of the conduction electrons, thereby forcing
the single electronic bath to underscreen the localized moment. We demonstrate
how to formulate a controlled large N limit preserving the property of
underscreening, and which can be seen as a ``dual'' theory of the multichannel
large N equations usually associated to overscreening. Due to the anomalous
scattering on the uncompensated degrees of freedom, the Fermi liquid
description of the electronic fluid is invalidated, with the logarithmic
singularities known to occur in the S=1 SU(2) Kondo impurity model being
replaced by continuous power laws at N=\infty. The present technique can be
extended to tackle the related underscreened Kondo lattice model in the large N
limit. We discover the occurence of an insulating pseudogap regime in place of
the expected renormalized metallic phase of the fully screened case, preventing
the establishement of coherence over the lattice. This work and the recent
observation of a similar weakly insulating behavior on transport in CeCuAs_2
should give momentum for further studies of underscreened impurity models on
the lattice.Comment: 9 pages, 3 figures. Several modifications in published version,
including new title, further details on the interpretation of the formalism
and possible experimental connection
Slave-rotor mean field theories of strongly correlated systems and the Mott transition in finite dimensions
The multiorbital Hubbard model is expressed in terms of quantum phase
variables (``slave rotors'') conjugate to the local charge, and of auxiliary
fermions, providing an economical representation of the Hilbert space of
strongly correlated systems. When the phase variables are treated in a local
mean-field manner, similar results to the dynamical mean-field theory are
obtained, namely a Brinkman-Rice transition at commensurate fillings together
with a ``preformed'' Mott gap in the single-particle density of states. The
slave- rotor formalism allows to go beyond the local description and take into
account spatial correlations, following an analogy to the superfluid-insulator
transition of bosonic systems. We find that the divergence of the effective
mass at the metal- insulator transition is suppressed by short range magnetic
correlations in finite-dimensional systems. Furthermore, the strict separation
of energy scales between the Fermi- liquid coherence scale and the Mott gap
found in the local picture, holds only approximately in finite dimensions, due
to the existence of low-energy collective modes related to zero-sound.Comment: 16 pages, 12 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
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