4,355 research outputs found
Broken discrete and continuous symmetries in two dimensional spiral antiferromagnets
We study the occurrence of symmetry breakings, at zero and finite
temperatures, in the J_1-J_3 antiferromagnetic Heisenberg model on the square
lattice using Schwinger boson mean field theory. For spin-1/2 the ground state
breaks always the SU(2) symmetry with a continuous quasi-critical transition at
J_3/J_1=0.38, from N\'eel to spiral long range order, although local spin
fluctuations considerations suggest an intermediate disordered regime around
0.35 < J_3/J_1 < 0.5, in qualitative agreement with recent numerical results.
At low temperatures we find a Z_2 broken symmetry region with short range
spiral order characterized by an Ising-like nematic order parameter that
compares qualitatively well with classical Monte Carlo results. At intermediate
temperatures the phase diagram shows regions with collinear short range orders:
for J_3/J_11 a novel phase
consisting of four decoupled third neighbour sublattices with N\'eel (\pi,\pi)
correlations in each one. We conclude that the effect of quantum and thermal
fluctuations is to favour collinear correlations even in the strongly
frustrated regime.Comment: 17 pages, accepted for publication in Journal of Physics: condensed
Matte
Effects of semiclassical spiral fluctuations on hole dynamics
We investigate the dynamics of a single hole coupled to the spiral
fluctuations related to the magnetic ground states of the antiferromagnetic
J_1-J_2-J_3 Heisenberg model on a square lattice. Using exact diagonalization
on finite size clusters and the self consistent Born approximation in the
thermodynamic limit we find, as a general feature, a strong reduction of the
quasiparticle weight along the spiral phases of the magnetic phase diagram. For
an important region of the Brillouin Zone the hole spectral functions are
completely incoherent, whereas at low energies the spectral weight is
redistributed on several irregular peaks. We find a characteristic value of the
spiral pitch, Q=(0.7,0.7)\pi, for which the available phase space for hole
scattering is maximum. We argue that this behavior is due to the non trivial
interference of the magnon assisted and the free hopping mechanism for hole
motion, characteristic of a hole coupled to semiclassical spiral fluctuations.Comment: 6 pages, 5 figure
Low temperature properties of the triangular-lattice antiferromagnet: a bosonic spinon theory
We study the low temperature properties of the triangular-lattice Heisenberg
antiferromagnet with a mean field Schwinger spin-1/2 boson scheme that
reproduces quantitatively the zero temperature energy spectrum derived
previously using series expansions. By analyzing the spin-spin and the boson
density-density dynamical structure factors, we identify the unphysical spin
excitations that come from the relaxation of the local constraint on bosons.
This allows us to reconstruct a free energy based on the physical excitations
only, whose predictions for entropy and uniform susceptibility seem to be
reliable within the temperature range $0< T <0.3J, which is difficult to access
by other methods. The high values of entropy, also found in high temperature
expansions studies, can be attributed to the roton-like narrowed dispersion at
finite temperatures.Comment: 16 pages, 5 figure
RVB signatures in the spin dynamics of the square-lattice Heisenberg antiferromagnet
We investigate the spin dynamics of the square-lattice spin-1/2 Heisenberg
antiferromagnet by means of an improved mean field Schwinger boson calculation.
By identifying both, the long range N\'eel and the RVB-like components of the
ground state, we propose an educated guess for the mean field triplet
excitation consisting on a linear combination of local and bond spin flips to
compute the dynamical structure factor. Our main result is that when this
triplet excitation is optimized in such a way that the corresponding sum rule
is fulfilled, we recover the low and high energy spectral weight features of
the experimental spectrum. In particular, the anomalous spectral weight
depletion at found in recent inelastic neutron scattering experiments
can be attributed to the interference of the triplet bond excitations of the
RVB component of the ground state. We conclude that the Schwinger boson theory
seems to be a good candidate to adequately interpret the dynamic properties of
the square-lattice Heisenberg antiferromagnet.Comment: 6 pages with 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
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
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