69 research outputs found
Phase diagram and dynamic response functions of the Holstein-Hubbard model
We present the phase diagram and dynamical correlation functions for the
Holstein-Hubbard model at half filling and at zero temperature. The
calculations are based on the Dynamical Mean Field Theory. The effective
impurity model is solved using Exact Diagonalization and the Numerical
Renormalization Group. Excluding long-range order, we find three different
paramagnetic phases, metallic, bipolaronic and Mott insulating, depending on
the Hubbard interaction U and the electron-phonon coupling g. We present the
behaviour of the one-electron spectral functions and phonon spectra close to
the metal insulator transitions.Comment: contribution to the SCES04 conferenc
Numerical Renormalization Group Study of the O(3)-symmetric Anderson Model
We use the numerical renormalization group method to study the O(3)-symmetric
version of the impurity Anderson model of Coleman and Schofield. This model is
of general interest because it displays both Fermi liquid and non-Fermi liquid
behaviour, and in the large limit can be related to the compactified two
channel Kondo model of Coleman, Ioffe and Tsvelik. We calculate the
thermodynamics for a parameter range which covers the full range of behaviour
of the model. We find a non-Fermi liquid fixed point in the isotropic case
which is unstable with respect to channel anisotropy.Comment: 10 pages, LaTeX, 8 figures includes as eps-file
Renormalization group approach to Fermi Liquid Theory
We show that the renormalization group (RG) approach to interacting fermions
at one-loop order recovers Fermi liquid theory results when the forward
scattering zero sound (ZS) and exchange (ZS) channels are both taken into
account. The Landau parameters are related to the fixed point value of the
``unphysical'' limit of the forward scattering vertex. We specify the
conditions under which the results obtained at one-loop order hold at all order
in a loop expansion. We also emphasize the similarities between our RG approach
and the diagrammatic derivation of Fermi liquid theory.Comment: 4 pages (RevTex) + 1 postcript file, everything in a uuencoded file,
uses epsf (problem with the figure in the first version
Theory of Transition Temperature of Magnetic Double Perovskites
We formulate a theory of double perovskite coumpounds such as SrFeReO
and SrFeMoO which have attracted recent attention for their possible
uses as spin valves and sources of spin polarized electrons. We solve the
theory in the dynamical mean field approximation to find the magnetic
transition temperature . We find that is determined by a subtle
interplay between carrier density and the Fe-Mo/Re site energy difference, and
that the non-Fe same-sublattice hopping acts to reduce . Our results
suggest that presently existing materials do not optimize
A planar diagram approach to the correlation problem
We transpose an idea of 't Hooft from its context of Yang and Mills' theory
of strongly interacting quarks to that of strongly correlated electrons in
transition metal oxides and show that a Hubbard model of N interacting electron
species reduces, to leading orders in N, to a sum of almost planar diagrams.
The resulting generating functional and integral equations are very similar to
those of the FLEX approximation of Bickers and Scalapino. This adds the Hubbard
model at large N to the list of solvable models of strongly correlated
electrons.
PACS Numbers: 71.27.+a 71.10.-w 71.10.FdComment: revtex, 5 pages, with 3 eps figure
Microscopic theory for quantum mirages in quantum corrals
Scanning tunneling microscopy permits to image the Kondo resonance of a
single magnetic atom adsorbed on a metallic surface. When the magnetic impurity
is placed at the focus of an elliptical quantum corral, a Kondo resonance has
been recently observed both on top of the impurity and on top of the focus
where no magnetic impurity is present. This projection of the Kondo resonance
to a remote point on the surface is referred to as quantum mirage. We present a
quantum mechanical theory for the quantum mirage inside an ideal quantum corral
and predict that the mirage will occur in corrals with shapes other than
elliptical
Electronic correlations in organometallic complexes
We investigate an effective model for organometallic complexes (with
potential uses in optoelectronic devices) via both exact diagonalisation and
the configuration interaction singles (CIS) approximation. This model captures
a number of important features of organometallic complexes, notably the
sensitivity of the radiative decay rate to small chemical changes. We find that
for large parameter ranges the CIS approximation accurately reproduces the low
energy excitations and hence the photophysical properties of the exact
solution. This suggests that electronic correlations do \emph{not} play an
important role in these complexes. This explains why time-dependent density
functional theory works surprisingly well in these complexes.Comment: 11 pages, 6 figure
The Two Dimensional Kondo Model with Rashba Spin-Orbit Coupling
We investigate the effect that Rashba spin-orbit coupling has on the low
energy behaviour of a two dimensional magnetic impurity system. It is shown
that the Kondo effect, the screening of the magnetic impurity at temperatures T
< T_K, is robust against such spin-orbit coupling, despite the fact that the
spin of the conduction electrons is no longer a conserved quantity. A proposal
is made for how the spin-orbit coupling may change the value of the Kondo
temperature T_K in such systems and the prospects of measuring this change are
discussed. We conclude that many of the assumptions made in our analysis
invalidate our results as applied to recent experiments in semi-conductor
quantum dots but may apply to measurements made with magnetic atoms placed on
metallic surfaces.Comment: 22 pages, 1 figure; reference update
Kondo effect in a Luttinger liquid: nonuniversality of the Wilson ratio
Using a precise coset Ising-Bose representation, we show how backscattering
of electrons off a magnetic impurity destabilizes the two-channel Kondo fixed
point and drives the system to a new fixed point, in agreement with previous
results. In addition, we verify the scaling proposed by Furusaki and Nagaosa
and prove that the other possible critical fixed point, namely the local Fermi
liquid class, is not completely universal when backscattering is included
because the Wilson ratio is not well-defined in the spinon basis.Comment: 4 pages, RevTeX; to appear in Physical Review
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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