151 research outputs found
Classical Antiferromagnetism in Kinetically Frustrated Electronic Models
We study the infinite U Hubbard model with one hole doped away half-filling,
in triangular and square lattices with frustrated hoppings that invalidate
Nagaoka's theorem, by means of the density matrix renormalization group. We
find that these kinetically frustrated models have antiferromagnetic ground
states with classical local magnetization in the thermodynamic limit. We
identify the mechanism of this kinetic antiferromagnetism with the release of
the kinetic energy frustration as the hole moves in the established
antiferromagnetic background. This release can occurs in two different ways: by
a non-trivial spin-Berry phase acquired by the hole or by the effective
vanishing of the hopping amplitude along the frustrating loops.Comment: 12 pages and 4 figures, with Supplementary Material. To be published
in Phys. Rev. Let
Anderson impurity in the one-dimensional Hubbard model on finite size systems
An Anderson impurity in a Hubbard model on chains with finite length is
studied using the density-matrix renormalization group (DMRG) technique. In the
first place, we analyzed how the reduction of electron density from
half-filling to quarter-filling affects the Kondo resonance in the limit of
Hubbard repulsion U=0. In general, a weak dependence with the electron density
was found for the local density of states (LDOS) at the impurity except when
the impurity, at half-filling, is close to a mixed valence regime. Next, in the
central part of this paper, we studied the effects of finite Hubbard
interaction on the chain at quarter-filling. Our main result is that this
interaction drives the impurity into a more defined Kondo regime although
accompanied in most cases by a reduction of the spectral weight of the impurity
LDOS. Again, for the impurity in the mixed valence regime, we observed an
interesting nonmonotonic behavior. We also concluded that the conductance,
computed for a small finite bias applied to the leads, follows the behavior of
the impurity LDOS, as in the case of non-interacting chains. Finally, we
analyzed how the Hubbard interaction and the finite chain length affect the
spin compensation cloud both at zero and at finite temperature, in this case
using quantum Monte Carlo techniques.Comment: 9 pages, 9 figures, final version to be published in Phys. Rev.
A test of the bosonic spinon theory for the triangular antiferromagnet spectrum
We compute the dynamical structure factor of the spin-1/2 triangular
Heisenberg model using the mean field Schwinger boson theory. We find that a
reconstructed dispersion, resulting from a non trivial redistribution of the
spectral weight, agrees quite well with the spin excitation spectrum recently
found with series expansions. In particular, we recover the strong
renormalization with respect to linear spin wave theory along with the
appearance of roton-like minima. Furthermore, near the roton-like minima the
contribution of the two spinon continuum to the static structure factor is
about 40 % of the total weight. By computing the density-density dynamical
structure factor, we identify an unphysical weak signal of the spin excitation
spectrum with the relaxation of the local constraint of the Schwinger bosons at
the mean field level. Based on the accurate description obtained for the static
and dynamic ground state properties, we argue that the bosonic spinon theory
should be considered seriously as a valid alternative to interpret the physics
of the triangular Heisenberg model.Comment: 6 pages, 5 figures, extended version including: a table with ground
state energy and magnetization; and the density-density dynamical structure
factor. Accepted for publication in Europhysics Letter
Quantum dot with ferromagnetic leads: a density-matrix renormalization group study
A quantum dot coupled to ferromagnetically polarized one-dimensional leads is
studied numerically using the density matrix renormalization group method.
Several real space properties and the local density of states at the dot are
computed. It is shown that this local density of states is suppressed by the
parallel polarization of the leads. In this case we are able to estimate the
length of the Kondo cloud, and to relate its behavior to that suppression.
Another important result of our study is that the tunnel magnetoresistance as a
function of the quantum dot on-site energy is minimum and negative at the
symmetric point.Comment: 4 pages including 5 figures. To be published as a Brief Report in
Phys. Rev.
Schwinger-boson approach to quantum spin systems: Gaussian fluctuactions in the "natural" gauge
We compute the Gaussian-fluctuation corrections to the saddle-point
Schwinger-boson results using collective coordinate methods. Concrete
application to investigate the frustrated J1-J2 antiferromagnet on the square
lattice shows that, unlike the saddle-point predictions, there is a quantum
nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by
considering the corrections to the spin stiffness on large lattices and
extrapolating to the thermodynamic limit, which avoids the infinite-lattice
infrared divergencies associated to Bose condensation. The very good agreement
of our results with exact numerical values on finite clusters lends support to
the calculational scheme employed.Comment: 4 pages, Latex, 3 figures included as eps files,minor correction
Interplay between spatial anisotropy and further exchange interactions in the triangular Heisenberg model
We investigate the interplay between spatial anisotropy and further exchange
interactions in the spin- Heisenberg antiferromagnetic model on a
triangular lattice. We use the Schwinger boson theory by including Gaussian
fluctuations above the mean-field approach. The phase diagram exhibits a strong
reduction of the long range collinear and incommensurate spirals regions with
respect to the mean-field ones. This reduction is accompanied by the emergence
of its short range order counterparts, leaving an ample room for -flux and
nematic spin liquid regions. Remarkably, within the neighborhood of the
spatially isotropic line, there is a range where the spirals are so fragile
that only the commensurate N\'eel ones survive. The good
agreement with recent variational Monte Carlo predictions gives support to the
rich phase diagram induced by spatial anisotropy.Comment: 8 pages, 8 figure
Quasiparticle excitations in frustrated antiferromagnets
We have computed the quasiparticle wave function corresponding to a hole
injected in a triangular antiferromagnet. We have taken into account
multi-magnon contributions within the self consistent Born approximation. We
have found qualitative differences, under sign reversal of the integral
transfer t, regarding the multi-magnon components and the own existence of the
quasiparticle excitations. Such differences are due to the subtle interplay
between magnon-assisted and free hopping mechanisms. We conclude that the
conventional quasiparticle picture can be broken by geometrical frustration
without invoking spin liquid phases.Comment: 5 pages, 4 figures, presented at " At the Frontiers of the condensed
Matter II, Buenos Aires. June, 2004 ". To be published in Physica
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