Indications of a Metallic Antiferromagnetic Phase in the 2 Dimensional U−t−t′U-t-t' Model

We present mean-field and quantum Monte Carlo results that suggest the existence of an itinerant antiferromagnetic ground state in the half-filled $U-t-t'$ model in two dimensions. In particular, working at $t'/t=-0.2$ we found that antiferromagnetic long range order develops at $U_{c_1}/t\approx 2.5$, while a study of the density of states $N(\omega)$ and the response to an external magnetic field indicates that the system becomes insulating at a larger coupling $4<U_{c_2}/t<6$.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 embedde

Robust D-wave Pairing Correlations in a Hole-Doped Spin-Fermion Model for Cuprates

Pairing Correlations are studied numerically in the hole-doped spin-fermion model for cuprates. Simulations performed on up to 12x12 clusters provide robust indications of D-wave superconductivity away from half-filling. The pairing correlations are the strongest in the direction perpendicular to the dynamic stripe-like inhomogeneities that appear in the ground state at some densities. An optimal doping, where the correlations reach a maximum value, was observed at about 25% doping, in qualitative agreement with hight T_{c} cuprates' experiments. On the other hand, pairing correlations are suppressed by static stripe inhomogeneities.Comment: 4 pages, 4 figure