1,301 research outputs found
Spectral Densities of Response Functions for the O(3) Symmetric Anderson and Two Channel Kondo Models
The O(3) symmetric Anderson model is an example of a system which has a
stable low energy marginal Fermi liquid fixed point for a certain choice of
parameters. It is also exactly equivalent, in the large U limit, to a localized
model which describes the spin degrees of freedom of the linear dispersion two
channel Kondo model. We first use an argument based on conformal field theory
to establish this precise equivalence with the two channel model. We then use
the numerical renormalization group (NRG) approach to calculate both
one-electron and two-electron response functions for a range of values of the
interaction strength U. We compare the behaviours about the marginal Fermi
liquid and Fermi liquid fixed points and interpret the results in terms of a
renormalized Majorana fermion picture of the elementary excitations. In the
marginal Fermi liquid case the spectral densities of all the Majorana fermion
modes display a |omega| dependence on the lowest energy scale, and in addition
the zero Majorana mode has a delta function contribution. The weight of this
delta function is studied as a function of the interaction U and is found to
decrease exponentially with U for large U. Using the equivalence with the two
channel Kondo model in the large U limit, we deduce the dynamical spin
susceptibility of the two channel Kondo model over the full frequency range. We
use renormalized perturbation theory to interpret the results and to calculate
the coefficient of the ln omega divergence found in the low frequency behaviour
of the T=0 dynamic susceptibility.Comment: 26 pages, 18 figures, to be published in Eur. Phys. J.
Convergence acceleration and stabilization for dynamical-mean-field-theory calculations
The convergence to the self-consistency in the dynamical-mean-field-theory
(DMFT) calculations for models of correlated electron systems can be
significantly accelerated by using an appropriate mixing of hybridization
functions which are used as the input to the impurity solver. It is shown that
the techniques and the past experience with the mixing of input charge
densities in the density-functional-theory (DFT) calculations are also
effective in DMFT. As an example, the increase of the computational
requirements near the Mott metal-insulator transition in the Hubbard model due
to critical slowing down can be strongly reduced by using the modified
Broyden's method to numerically solve the non-linear self-consistency equation.
Speed-up factors as high as 3 were observed in practical calculations even for
this relatively well behaved problem. Furthermore, the convergence can be
achieved in difficult cases where simple linear mixing is either not effective
or even leads to divergence. Unstable and metastable solutions can also be
obtained. We also determine the linear response of the system with respect to
the variations of the hybridization function, which is related to the
propagation of the information between the different energy scales during the
iteration.Comment: 9 pages, 8 figure
Mapping Itinerant Electrons around Kondo Impurities
We investigate single Fe and Co atoms buried below a Cu(100) surface using
low temperature scanning tunneling spectroscopy. By mapping the local density
of states of the itinerant electrons at the surface, the Kondo resonance near
the Fermi energy is analyzed. Probing bulk impurities in this well-defined
scattering geometry allows separating the physics of the Kondo system and the
measuring process. The line shape of the Kondo signature shows an oscillatory
behavior as a function of depth of the impurity as well as a function of
lateral distance. The oscillation period along the different directions reveals
that the spectral function of the itinerant electrons is anisotropic.Comment: 5 pages, 4 figures, accepted by Physical Review Letter
Slave-boson approach to the infinite-U Anderson-Holstein impurity model
The infinite- Anderson-Holstein impurity model is studied with a focus on
the interplay between the strong electron correlation and the weak
electron-phonon interaction. The slave boson method has been employed in
combination with the large degeneracy expansion (1/N) technique. The charge and
spin susceptibilities and the phonon propagator are obtained in the
approximation scheme where the saddle point configuration and the Gaussian 1/N
fluctuations are taken into account. The spin susceptibility is found not to be
renormalized by electron-phonon interaction, while the charge susceptibility is
renormalized.
From the renormalized charge susceptibility the Kondo temperature is found to
increase by the electron-phonon interaction. It turns out that the bosonic 1/N
Gaussian fluctuations play a very crucial role, in particular, for the phonon
propagator.Comment: 12pages, 3 figures. Published in Physical Review
The Fermi edge singularity in the SU(N) Wolff model
The low temperature properties of the SU(N) Wolff impurity model are studied
via Abelian bosonization. The path integral treatment of the problem allows for
an exact evaluation of low temperature properties of the model. The single
particle Green's function enhances due to the presence of local correlation.
The basic correlation function such as the charge or spin correlator are also
influenced by the presence of impurity, and show local Fermi liquid behaviour.
The X-ray absorption is affected by the presence of local Hubbard interaction.
The exponent is decreased (increased) for repulsive (attractive) interactions.Comment: 7 pages, 4 figure
Resonating bipolarons
Electrons coupled to local lattice deformations end up in selftrapped
localized molecular states involving their binding into bipolarons when the
coupling is stronger than a certain critical value. Below that value they exist
as essentially itinerant electrons. We propose that the abrupt crossover
between the two regimes can be described by resonant pairing similar to the
Feshbach resonance in binary atomic collision processes. Given the
intrinsically local nature of the exchange of pairs of itinerant electrons and
localized bipolarons, we demonstrate the occurrence of such a resonance on a
finite-size cluster made out of metallic atoms surrounding a polaronic ligand
center.Comment: 7 pages, 4 figures, to be published in Europhysics Letter
Non-equilibrium transport theory of the singlet-triplet transition: perturbative approach
We use a simple iterative perturbation theory to study the singlet-triplet
(ST) transition in lateral and vertical quantum dots, modeled by the
non-equilibrium two-level Anderson model. To a great surprise, the region of
stable perturbation theory extends to relatively strong interactions, and this
simple approach is able to reproduce all experimentally-observed features of
the ST transition, including the formation of a dip in the differential
conductance of a lateral dot indicative of the two-stage Kondo effect, or the
maximum in the linear conductance around the transition point. Choosing the
right starting point to the perturbation theory is, however, crucial to obtain
reliable and meaningful results
Faithful fermionic representations of the Kondo lattice model
We study the Kondo lattice model using a class of canonical transformations
that allow us to faithfully represent the model entirely in terms of fermions
without constraints. The transformations generate interacting theories that we
study using mean field theory. Of particular interest is a new manifestly
O(3)-symmetric representation in terms of Majorana fermions at half-filling on
bipartite lattices. This representation suggests a natural O(3)-symmetric trial
state that is investigated and characterized as a gapped spin liquid.Comment: 11 pages, 2 figures, minor update
Conductance through an array of quantum dots
We propose a simple approach to study the conductance through an array of
interacting quantum dots, weakly coupled to metallic leads. Using a mapping to
an effective site which describes the low-lying excitations and a slave-boson
representation in the saddle-point approximation, we calculated the conductance
through the system. Explicit results are presented for N=1 and N=3: a linear
array and an isosceles triangle. For N=1 in the Kondo limit, the results are in
very good agreement with previous results obtained with numerical
renormalization group (NRG). In the case of the linear trimer for odd , when
the parameters are such that electron-hole symmetry is induced, we obtain
perfect conductance . The validity of the approach is discussed in
detail.Comment: to appear in Phys. Rev.
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