Adriana Moreo

Recent computational studies of models for manganese oxides have revealed a rich phase diagram, which was not anticipated in early calculations in this context performed in the 1950s and 1960s. 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 charged 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 by applying 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 appear stronger.

The Kondo lattice Hamiltonian with ferromagnetic Hund's coupling as a model for manganites is investigated. The classical limit for the spin of the (localized) ${t}_{2g}$ electrons is analyzed on lattices of dimension 1, 2, 3, and $\ensuremath{\infty}$ using several numerical methods. The phase diagram at low temperature is presented. A regime is identified where phase separation occurs between hole undoped antiferromagnetic and hole-rich ferromagnetic regions. Experimental consequences of this novel regime are discussed. Regions of incommensurate spin correlations have also been found. Estimations of the critical temperature in 3D are compatible with experiments.

The influence of quenched disorder on the competition between ordered states separated by a first-order transition is investigated. A phase diagram with features resembling quantum-critical behavior is observed, even using classical models. The low-temperature paramagnetic regime consists of coexisting ordered clusters, with randomly oriented order parameters. Extended to manganites, this state is argued to have a colossal magnetoresistance effect. A scale T(*) for cluster formation is discussed. This is the analog of the Griffiths temperature, but for the case of two competing orders, producing a strong susceptibility to external fields. Cuprates may have similar features, compatible with the large proximity effect of the very underdoped regime.

We study the normal state of the 2D attractive Hubbard model using quantum Monte Carlo simulations. We show that singlet pairing correlations develop above ${\mathit{T}}_{\mathit{c}}$, and the normal state of a short coherence length superconductor deviates from a canonical Fermi liquid. In the intermediate U regime, the spin susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathit{s}}$ is strongly temperature dependent, and the low-frequency spectral weight, as measured by the NMR relaxation rate 1/${\mathit{T}}_{1}$T, is shown to track ${\mathrm{\ensuremath{\chi}}}_{\mathit{s}}$. This provides a simple, qualitative explanation for the spin-gap behavior observed in several high-${\mathit{T}}_{\mathit{c}}$ systems.

Using a Lanczos technique we study the frustrated spin-(1/2 Heisenberg model on square lattices of 16 and 20 sites. Frustration is introduced by an interaction along the diagonals of the plaquettes with coupling ${J}_{2}$\ensuremath{\ge}0. For large ${J}_{2}$ we found that the ground state breaks (spontaneously) the lattice rotational symmetry. For intermediate values of ${J}_{2}$, the squares of order parameters associated with spin-Peierls and ``twisted'' states have a peak while a similar quantity for a chiral state shows no interesting structure.