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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 $\infty$ 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.Comment: Accepted in Phys. Rev. Letter

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 randomnly 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.PACS numbers: 75.30.Kz Complex phenomena such as "colossal" magnetoresistance (CMR) in manganites and high temperature superconductivity (HTS) in cuprates have challenged our understanding of correlated electrons [1]. Recent developments unveiled a previously mostly ignored aspect of doped transition-metal-oxides (TMO): these systems are intrinsically inhomogeneous, even in the best crystals. (i) The evidence in the CMR context is overwhelming. Experiments and theory provide a picture where competing ferromagnetic (FM) and charge-ordered (CO) states form microscopic and/or mesoscopic coexisting clusters [2,3]. Exciting recent experiments [4] identified features referred to as a "quantum critical point" (QCP) [5] -defined as the drastic reduction of ordering temperatures near the zero temperature (T=0) transition between ordered states -by modifying the A-site cation mean-radius r A by chemical substitution at fixed hole density (left inset of Fig. 1). The paramagnetic state in the QCP region -where the Curie temperature T C is the lowest -is crucial to understand CMR phenomenology, producing the largest CMR ratio [1,2,3]. (ii) In the HTS context, scanning tunneling microscopy (STM) studies of superconducting (SC) Bi2212 revealed a complex surface with nm-size coexisting clusters [6]. Underdoped cuprates also appear to be inhomogeneous [7]. In addition, a "colossal" proximity effect (CPE) was reported on underdoped YBa 2 Cu 3 O 6+x over large distances [8].In this paper, the competition between two ordered states in the presence of quenched disorder is investigated. These states are assumed sufficiently "different" that their low-T transition in the clean limit has firstorder characteristics. The approach has similarities with the classical work of Imry and Ma [9]. From the general considerations, doped TMOs are here considered, with intrinsic disorder caused by chemical substitution. For Mn-oxides, a possible rationalization of the CMR effect is discussed, with predictions including a scale T * for cluster formation -the analog of the Griffiths temperature [10] but in the regime of competing orders. For underdoped Cu-oxides, a similar inhomogeneous picture is proposed. The calculations are mainly carried out using a two dimensional (2D) toy model of Ising spins, but ...

The theoretical need to study the properties of the Fe-based high-T_c superconductors with reliable many-body techniques requires us to determine the minimum number of orbital degrees of freedom that will capture the physics of these materials. While the shape of the Fermi surface (FS) obtained with the local density approximation (LDA) can be reproduced by a two-orbital model, it has been argued that the bands that cross the chemical potential result from the strong hybridization of three of the Fe 3d orbitals. For this reason, a three-orbital Hamiltonian obtained with the Slater-Koster formalism by considering the hybridization of the As p orbitals with the Fe d_xz,d_yz, and d_xy orbitals is discussed here. This model reproduces qualitatively the FS shape and orbital composition obtained by LDA calculations for undoped pnictides when four electrons per Fe are considered. Within a mean-field approximation, its magnetic and orbital properties in the undoped case are described. With increasing Coulomb repulsion, four regimes are obtained: (1) paramagnetic, (2) magnetic (pi,0) spin order, (3) the same (pi,0) spin order but now including orbital order, and finally (4) a magnetic and orbital ordered insulator. The spin-singlet pairing operators allowed by the lattice and orbital symmetries are also constructed. It is found that for pairs of electrons involving up to diagonal nearest-neighbors sites, the only fully gapped and purely intraband spin-singlet pairing operator is given by Delta(k)=f(k)\sum_{alpha} d_{k,alpha,up}d_{-k,alpha,down} with f(k)=1 or f(k)=cos(k_x)cos(k_y) which would arise only if the electrons in all different orbitals couple with equal strength to the source of pairing

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Computational studies of models for manganese oxides show the generation of large coexisting metallic and insulating clusters with equal electronic density, in agreement with the recently discovered micrometer-sized inhomogeneities in manganites. The clusters are induced by disorder on exchange and hopping amplitudes near first-order transitions of the nondisordered strongly coupled system. The random-field Ising model illustrates the qualitative aspects of our results. Percolative characteristics are natural in this context. The conclusions are general and apply to a variety of compounds.

The density-of-states (DOS) and one-particle spectral function $\rm A({\bf k}, \omega)$ of the one- and two-orbital models for manganites, the latter with Jahn-Teller phonons, are evaluated using Monte Carlo techniques. Unexpectedly robust pseudogap (PG) features were found at low- and intermediate-temperatures, particularly at or near regimes where phase-separation occurs as $\rm T$$\to$0. The PG follows the chemical potential and it is caused by the formation of ferromagnetic metallic clusters in an insulating background. It is argued that PG formation should be generic of mixed-phase regimes. The results are in good agreement with recent photoemission experiments for $\rm La_{1.2} Sr_{1.8} Mn_2 O_7$.Comment: Accepted for publication in Phys. Rev. Lett., 4 pages, Revtex, with 4 figures embedde