106 research outputs found
Indications of a Metallic Antiferromagnetic Phase in the 2 Dimensional Model
We present mean-field and quantum Monte Carlo results that suggest the
existence of an itinerant antiferromagnetic ground state in the half-filled
model in two dimensions. In particular, working at we
found that antiferromagnetic long range order develops at , while a study of the density of states and the response to an
external magnetic field indicates that the system becomes insulating at a
larger coupling .Comment: 4 pages, RevTex 3.0, 5 figures embedded in the text, modifications in
the text, as well as added data to Fig.
The Phase Separation Scenario for Manganese Oxides
Recent computational studies of models for manganese oxides have revealed a
rich phase diagram, not anticipated in early calculations in this context
performed in the 1950's and 60's. In particular, the transition between the
antiferromagnetic insulator state of the hole-undoped limit and the
ferromagnetic metal at finite hole-density was found to occur through a
mixed-phase process. When extended Coulomb interactions are included, a
microscopically charge inhomogeneous state should be stabilized. These phase
separation tendencies, also present at low electronic densities, influence the
properties of the ferromagnetic region by increasing charge fluctuations.
Experimental data reviewed here using several techniques for manganites and
other materials are consistent with this scenario. Similarities with results
previously discussed in the context of cuprates are clear from this analysis,
although the phase segregation tendencies in manganites seem stronger.Comment: Accepted for publication in Science, 10 pages, Revtex, with 5 figures
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Effect of Adiabatic Phonons on Striped and Homogeneous Ground States
The effects of adiabatic phonons on a spin-fermion model for high T_c
cuprates are studied using numerical simulations. In the absence of
electron-phonon interactions (EPI), stripes in the ground state are observed
for certain dopings while homogeneous states are stabilized in other regions of
parameter space. Different modes of adiabatic phonons are added to the
Hamiltonian:breathing, shear and half-breathing modes. Diagonal and
off-diagonal electron-phonon couplings are considered. It is observed that
strong diagonal EPI generate stripes in previously homogeneous states, while in
striped ground states an increase in the diagonal couplings tends to stabilize
the stripes, inducing a gap in the density of states (DOS) and rendering the
ground state insulating. The off-diagonal terms, on the other hand, destabilize
the stripes creating inhomogeneous ground states with a pseudogap at the
chemical potential in the DOS. The breathing mode stabilizes static diagonal
stripes; while the half-breathing (shear) modes stabilize dynamical (localized)
vertical and horizontal stripes. The EPI induces decoherence of the
quasi-particle peaks in the spectral functions.Comment: latex, 9 pages,13 figure
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