89 research outputs found

### Anharmonic effects in magnetoelastic chains

We describe a new mechanism leading to the formation of rational
magnetization plateau phases, which is mainly due to the anharmonic spin-phonon
coupling. This anharmonicity produces plateaux in the magnetization curve at
unexpected values of the magnetization without explicit magnetic frustration in
the Hamiltonian and without an explicit breaking of the translational symmetry.
These plateau phases are accompanied by magneto-elastic deformations which are
not present in the harmonic case.Comment: 5 pages, 3 figure

### 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

### Spiral Magnets as Gapless Mott Insulators

In the large $U$ limit, the ground state of the half-filled, nearest-neighbor
Hubbard model on the triangular lattice is the three-sublattice
antiferromagnet. In sharp contrast with the square-lattice case, where
transverse spin-waves and charge excitations remain decoupled to all orders in
$t/U$, it is shown that beyond leading order in $t/U$ the three Goldstone modes
on the triangular lattice are a linear combination of spin and charge. This
leads to non-vanishing conductivity at any finite frequency, even though the
magnet remains insulating at zero frequency. More generally, non-collinear spin
order should lead to such gapless insulating behavior.Comment: 10 pages, REVTEX 3.0, 3 uuencoded postscript figures, CRPS-94-0

### 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.

### Excitations with fractional spin less than 1/2 in frustrated magnetoelastic chains

We study the magnetic excitations on top of the plateaux states recently
discovered in spin-Peierls systems in a magnetic field. We show by means of
extensive density matrix renormalization group (DMRG) computations and an
analytic approach that one single spin-flip on top of $M=1-\frac2N$
($N=3,4,...$) plateau decays into $N$ elementary excitations each carrying a
fraction $\frac1N$ of the spin. This fractionalization goes beyond the
well-known decay of one magnon into two spinons taking place on top of the M=0
plateau. Concentrating on the $\frac13$ plateau (N=3) we unravel the
microscopic structure of the domain walls which carry fractional
spin-$\frac13$, both from theory and numerics. These excitations are shown to
be noninteracting and should be observable in x-ray and nuclear magnetic
resonance experiments.Comment: 6 pages, 5 figures. Accepted to be published in Phys. Rev.

### Incommensurate Phase of a Triangular Frustrated Heisenberg Model Studied via Schwinger-Boson Mean-Field Theory

We study a triangular frustrated antiferromagnetic Heisenberg model with
nearest-neighbor interaction $J_{1}$ and third-nearest-neighbor interactions
$J_{3}$ by means of Schwinger-boson mean-field theory. It is shown that an
incommensurate phase exists in a finite region in the parameter space for an
antiferromagnetic $J_{3}$ while $J_{1}$ can be either positive or negtaive. A
detailed solution is presented to disclose the main features of this
incommensurate phase. A gapless dispersion of quasiparticles leads to the
intrinsic $T^{2}$-law of specific heat. The local magnetization is
significantly reduced by quantum fluctuations (for S=1 case, a local
magnetization is estimated as $m= \approx0.6223$). The magnetic
susceptibility is linear in temperature at low temperatures. We address
possible relevance of these results to the low-temperature properties of
NiGa$_{2}$S$_{4}$. From a careful analysis of the incommensurate spin wave
vector, the interaction parameters for NiGa$_{2}%$S$_{4}$ are estimated as,
$J_{1}\approx-3.8755$K and $J_{3}\approx14.0628$K, in order to account for the
experimental data.Comment: 9pages, 3figure

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