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 ...

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 resistivity rho(dc) of manganites is studied using a random resistor-network, based on phase separation between metallic and insulating domains. When percolation occurs, both as chemical composition or temperature vary, results in good agreement with experiments are obtained. Similar conclusions are reached using quantum calculations and microscopic considerations. Above the Curie temperature, it is argued that ferromagnetic clusters should exist in Mn oxides. Small magnetic fields induce large rho(dc) changes and a bad-metal state with (disconnected) insulating domains.

Phenomenological models for the antiferromagnetic (AF) vs. d-wave superconductivity competition in cuprates are studied using conventional Monte Carlo techniques. The analysis suggests that cuprates may show a variety of different behaviors in the very underdoped regime: local coexistence or first-order transitions among the competing orders, stripes, or glassy states with nanoscale superconducting (SC) puddles. The transition from AF to SC does not seem universal. In particular, the glassy state leads to the possibility of "colossal" effects in some cuprates, analog of those in manganites. Under suitable conditions, non-superconducting Cu-oxides could rapidly become superconducting by the influence of weak perturbations that align the randomly oriented phases of the SC puddles in the mixed state. Consequences of these ideas for thin-film and photoemission experiments are discussed.

We report susceptibility, specific heat, and neutron diffraction measurements on NaCu2O2, a spin-1/2 chain compound isostructural to LiCu2O2, which has been extensively investigated. Below 12 K, we find a long-range ordered, incommensurate magnetic helix state with a propagation vector similar to that of LiCu2O2. In contrast to the Li analogue, substitutional disorder is negligible in NaCu2O2. We can thus rule out that the helix is induced by impurities, as was claimed on the basis of prior work on LiCu2O2. A spin Hamiltonian with frustrated longer-range exchange interactions provides a good description of both the ordered state and the paramagnetic susceptibility.PACS numbers: 75.10. Pq, 75.40.Cx, 75.25.+z Copper oxides are excellent model systems for lowdimensional spin-1/2 quantum antiferromagnets. In particular, copper oxides with magnetic backbones comprised of chains of CuO 4 squares have been shown to exhibit quasi-one-dimensional behavior. Two classes of copper oxide spin chain materials are known. Compounds in which adjacent squares share their corners are excellent realizations of the one-dimensional (1D) spin-1/2 Heisenberg Hamiltonian [1, 2, 3]. Linear Cu-O-Cu bonds along the spin chains give rise to a large antiferromagnetic nearest-neighbor exchange coupling. In compounds built up of edge-sharing squares, on the other hand, the Cu-O-Cu bond angle is nearly 90 • , so that the nearest-neighbor coupling is more than an order of magnitude smaller [4]. Because of the anomalously small nearest-neighbor coupling, longer-range frustrating exchange interactions have a pronounced influence on the physical properties of these materials. Edge-sharing copper oxides thus provide uniquely simple model systems to test current theories of spin correlations in frustrated quantum magnets.At low temperatures, the ground state of edge-sharing copper oxides is either a 3D-ordered antiferromagnet [5,6,7] or a spin-Peierls state [8], depending on whether interchain exchange interactions or spin-phonon interactions are dominant. In the former case, the magnetic order is almost always collinear. An interesting exception was recently discovered in LiCu 2 O 2 [9, 10, 11], which undergoes a transition to a magnetic helix state at low temperatures. While such a state is expected for classical spin models with frustrating interactions, quantum models predict a gapped spin liquid state in the range of exchange parameters that was claimed to describe the spin system in LiCu 2 O 2 . Since the ionic radii of Li + and Cu 2+ are similar, chemical disorder was identified as a possible solution to this puzzle. Indeed, a chemical analysis of the sample used in the neutron scattering study of Ref. [11] showed that about 16% of the Cu 2+ ions in the spin chains were replaced by nonmagnetic Li + impurities. Since even much lower concentrations of nonmagnetic impurities are found to induce magnetic long-range order in other quasi-1D spin-gap systems, the authors of Ref.[11] attributed the unexpected helix state to the Here we report magnet...

A lattice spin-fermion model for diluted magnetic semiconductors (DMS) is investigated numerically, improving on previously used mean-field (MF) approximations. Curie temperatures are obtained varying the Mn-spin x and hole n densities, and the impurity-hole exchange J in units of the hopping amplitude t. Optimal values are found in the subtle intermediate regime between itinerant and localized carriers. Our main result is the behavior of the Curie temperature at large J/t, where a "clustered" state is observed and ferromagnetism is suppressed. Formal analogies between DMS and manganites are also discussed.PACS numbers: 75.50. Pp,75.10.Lp,75.30.Vn Diluted magnetic semiconductors based on III-V compounds are attracting considerable attention due to their combination of magnetic and semiconducting properties, that may lead to spintronic applications [1,2]. Ga 1−x Mn x As is the most studied of these compounds with a maximum Curie temperature T C ≈110 K at low doping x, and with a carrier concentration p=(n/x)<1 due to the presence of As antisite defects [1] or Mn intersticials [3]. It is widely believed that this ferromagnetism is carrier-induced, with holes introduced by doping mediating the interaction between S=5/2 Mn-spins. This Zener mechanism operates in other materials as well [4].In spite of the excitement around 110 K DMS, room temperature ferromagnetism should be achieved for potential applications, with logic and memory operations in a single device. For this reason, a goal of the present effort is to analyze the dependence of T C on the parameters x, p, and J/t, helping in setting realistic expectations for DMS potential technological applications. This goal can only be achieved with good control over the many-body aspects of the problem, and for this purpose lattice Monte Carlo (MC) techniques are crucial, improving on previously employed MF approximations. Our results lead to an optimistic view in this respect, since T C is found to increase linearly with x up to x∼0.25.Our effort builds upon previous important DMS theoretical studies [2,5,6,7,8,9,10]. However, to analyze whether T C can be substantially increased from current values, techniques as generic as possible are necessary. In particular, both the strong interactions and disorder must be considered accurately, with computational studies currently providing the best available tools. For these reasons, our work differs from previous approaches in important qualitative aspects: (1) Some groups use a continuum six-band description of DMS [5]. (2) Other theories assume carriers strongly bounded to impurity sites [6], and employ Hartree-Fock approximations. (3) Dynamical MF theory (DMFT) [7] may not capture the percolative character of DMS, with a random impurity distribution and cluster picture [8,9]. (4) Other approaches use MF uniform states [2], or introduce a reduced basis in simulations [10]. While the previous work is important in describing current DMS materials, our goal is to establish the phase diagram of a DMS model avoiding MF appr...

The effect of Jahn-Teller phonons on the magnetic and orbital structure of LaMnO3 is investigated using a combination of relaxation and Monte Carlo techniques on three-dimensional clusters of MnO6 octahedra. In the physically relevant region of parameter space for LaMnO3, and after including small corrections due to tilting effects, the A-type antiferromagnetic and C-type orbital structures were stabilized, in agreement with experiments.PACS numbers: 75.30. Kz, 75.50.Ee,