24 research outputs found
Conductance of nano-systems with interactions coupled via conduction electrons: Effect of indirect exchange interactions
A nano-system in which electrons interact and in contact with Fermi leads
gives rise to an effective one-body scattering which depends on the presence of
other scatterers in the attached leads. This non local effect is a pure
many-body effect that one neglects when one takes non interacting models for
describing quantum transport. This enhances the non-local character of the
quantum conductance by exchange interactions of a type similar to the
RKKY-interaction between local magnetic moments. A theoretical study of this
effect is given assuming the Hartree-Fock approximation for spinless fermions
in an infinite chain embedding two scatterers separated by a segment of length
L\_c. The fermions interact only inside the two scatterers. The dependence of
one scatterer onto the other exhibits oscillations which decay as 1/L\_c and
which are suppressed when L\_c exceeds the thermal length L\_T. The
Hartree-Fock results are compared with exact numerical results obtained with
the embedding method and the DMRG algorithm
Effect of flux-dependent Friedel oscillations upon the effective transmission of an interacting nano-system
We consider a nano-system connected to measurement probes via non interacting
leads. When the electrons interact inside the nano-system, the coefficient
|ts(E_F)|^2 describing its effective transmission at the Fermi energy E_F
ceases to be local. This effect of electron-electron interactions upon
|ts(E_F)|^2 is studied using a one dimensional model of spinless fermions and
the Hartree-Fock approximation. The non locality of |ts(E_F)|^2 is due to the
coupling between the Hartree and Fock corrections inside the nano-system and
the scatterers outside the nano-system via long range Friedel oscillations.
Using this phenomenon, one can vary |ts(E_F)|^2 by an Aharonov-Bohm flux
threading a ring which is attached to one lead at a distance Lc from the
nano-system. For small distances Lc, the variation of the quantum conductance
induced by this non local effect can exceed 0.1 (e^2/h)
The spectral weight of the Hubbard model through cluster perturbation theory
We calculate the spectral weight of the one- and two-dimensional Hubbard
models, by performing exact diagonalizations of finite clusters and treating
inter-cluster hopping with perturbation theory. Even with relatively modest
clusters (e.g. 12 sites), the spectra thus obtained give an accurate
description of the exact results. Thus, spin-charge separation (i.e. an
extended spectral weight bounded by singularities) is clearly recognized in the
one-dimensional Hubbard model, and so is extended spectral weight in the
two-dimensional Hubbard model.Comment: 4 pages, 5 figure
Metal-insulator transition and charge ordering in the extended Hubbard model at one-quarter filling
We study with exact diagonalization the zero temperature properties of the
quarter-filled extended Hubbard model on a square lattice. We find that
increasing the ratio of the intersite Coulomb repulsion, , to the band width
drives the system from a metal to a charge ordered insulator. The evolution of
the optical conductivity spectrum with increasing is compared to the
observed optical conductivity of several layered molecular crystals with the
theta and beta'' crystal structures.Comment: 5 pages, 3 figure
Spectroscopic signatures of spin-charge separation in the quasi-one-dimensional organic conductor TTF-TCNQ
The electronic structure of the quasi-one-dimensional organic conductor
TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The
experimental spectra reveal significant discrepancies to band theory. We
demonstrate that the measured dispersions can be consistently mapped onto the
one-dimensional Hubbard model at finite doping. This interpretation is further
supported by a remarkable transfer of spectral weight as function of
temperature. The ARPES data thus show spectroscopic signatures of spin-charge
separation on an energy scale of the conduction band width.Comment: 4 pages, 4 figures; to appear in PR
Strong-Coupling Expansion for the Hubbard Model
A strong-coupling expansion for models of correlated electrons in any
dimension is presented. The method is applied to the Hubbard model in
dimensions and compared with numerical results in . Third order expansion
of the Green function suffices to exhibit both the Mott metal-insulator
transition and a low-temperature regime where antiferromagnetic correlations
are strong. It is predicted that some of the weak photoemission signals
observed in one-dimensional systems such as should become stronger as
temperature increases away from the spin-charge separated state.Comment: 4 pages, RevTex, 3 epsf figures include
Residual conductance of correlated one-dimensional nanosystems: A numerical approach
We study a method to determine the residual conductance of a correlated
system by means of the ground-state properties of a large ring composed of the
system itself and a long non-interacting lead. The transmission probability
through the interacting region and thus its residual conductance is deduced
from the persistent current induced by a flux threading the ring. Density
Matrix Renormalization Group techniques are employed to obtain numerical
results for one-dimensional systems of interacting spinless fermions. As the
flux dependence of the persistent current for such a system demonstrates, the
interacting system coupled to an infinite non-interacting lead behaves as a
non-interacting scatterer, but with an interaction dependent elastic
transmission coefficient. The scaling to large lead sizes is discussed in
detail as it constitutes a crucial step in determining the conductance.
Furthermore, the method, which so far had been used at half filling, is
extended to arbitrary filling and also applied to disordered interacting
systems, where it is found that repulsive interaction can favor transport.Comment: 14 pages, 10 EPS figure
Role of Collective Mode for Optical Conductivity and Reflectivity in Quarter-Filled Spin-Density-Wave State
Taking account of a collective mode relevant to charge fluctuation, the
optical conductivity of spin-density-wave state has been examined for an
extended Hubbard model with one-dimensional quarter-filled band. We find that,
within the random phase approximation, the conductivity exhibits several peaks
at the frequency corresponding to the excitation energy of the commensurate
collective mode. When charge ordering appears with increasing inter-site
repulsive interactions, the main peak with the lowest frequency is reduced and
the effective mass of the mode is enhanced indicating the suppression of the
effect of the collective mode by charge ordering. It is also shown that the
reflectivity becomes large in a wide range of frequency due to the huge
dielectric constant induced by the collective mode.Comment: 11 pages, 16 figure
Dynamical properties of the spin-Peierls compound \alpha'--NaV2O5
Dynamical properties of the novel inorganic spin-Peierls compound
\alpha'--NaV2O5 are investigated using a one-dimensional dimerized Heisenberg
model. By exact diagonalizations of chains with up to 28 sites, supplemented by
a finite-size scaling analysis, the dimerization parameter \delta is determined
by requiring that the model reproduces the experimentally observed spin gap
\Delta. The dynamical and static spin structure factors are calculated. As for
CuGeO3, the existence of a low energy magnon branch separated from the
continuum is predicted. The present calculations also suggest that a large
magnetic Raman scattering intensity should appear above an energy threshold of
1.9 \Delta. The predicted photoemission spectrum is qualitatively similar to
results for an undimerized chain due to the presence of sizable short-range
antiferromagnetic correlations.Comment: 4 pages, latex, minor misprints corrected and a few references adde
Critical Properties in Dynamical Charge Correlation Function for the One-Dimensional Mott Insulator
Critical properties in the dynamical charge correlation function for the
one-dimensional Mott insulator are studied. By properly taking into account
{\it the final-state interaction} between the charge and spin degrees of
freedom, we find that the edge singularity in the charge correlation function
is governed by massless spinon excitations, although it is naively expected
that spinons do not directly contribute to the charge excitation over the
Hubbard gap. We obtain the momentum-dependent anomalous critical exponent by
applying the finite-size scaling analysis to the Bethe ansatz solution of the
half-filled Hubbard model.Comment: 7 pages, REVTe