Matthias Mayr

Colossal effects in transition metal oxides caused by intrinsic inhomogeneities.

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.

Giant Cluster Coexistence in Doped Manganites and Other Compounds

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.

Resistivity of Mixed-Phase Manganites.

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.

Global versus local ferromagnetism in a model for diluted magnetic semiconductors studied with Monte Carlo techniques

Recent advances in experimental techniques have allowed us for the introduction of magnetic elements into semiconducting hosts beyond the solubility limit. This has stimulated the exciting research area of diluted magnetic semiconductors ~DMS! with the prospect of manipulating the charge as well as the spin degrees of freedom, and its possible technological applications in such novel fields as ‘‘spintronics.’’ Prominent among these new compounds is the III-V system Ga12xMnxAs, where ferromagnetic ~FM! transition temperatures as high as 110 K were obtained. 1 Although some of the basic ingredients of the physics of these materials such as the local antiferromagnetic ~AF! exchange between the Mn spins and the charge carriers are known, the origin of such fairly high critical temperatures is still in question. The ferromagnetism itself can be understood as being carrier induced, in a similar fashion as the FM state in the double-exchange ~DE! model for manganites at intermediate doping. 2 However, alternative descriptions based on Ruderman-Kittel-KasuyaYosida ~RKKY! interactions between the impurity spins are claimed to lead to transition temperatures comparable to those observed. 3 Yet it is not obvious whether RKKY can

Percolative transitions with first-order characteristics in the context of colossal magnetoresistance manganites

The unusual magnetotransport properties of manganites are widely believed to be caused by mixed-phase tendencies and concomitant percolative processes. However, dramatic deviations from ‘‘standard’’ percolation have been unveiled experimentally. Here, a semiphenomenological description of Mn oxides is proposed based on coexisting clusters with smooth surfaces, as suggested by Monte Carlo simulations of realistic models for manganites, also briefly discussed here. The present approach produces fairly abrupt percolative transitions and even first-order discontinuities, in agreement with experiments. These transitions may describe the percolation that occurs after magnetic fields align the randomly oriented ferromagnetic clusters believed to exist above the Curie temperature in Mn oxides. In this respect, part of the manganite phenomenology could belong to a new class of percolative processes triggered by phase competition and correlations.

A -type antiferromagnetic and C -type orbital-ordered states in LaMnO 3 using cooperative Jahn-Teller phonons

The effect of Jahn-Teller phonons on the magnetic and orbital structure of

Wigner crystallization in Na3Cu2O4 and Na8Cu5O10 chain compounds

{\mathrm{LaMnO}}_{3}

Phase Fluctuations in Strongly Coupled d-Wave Superconductors

is investigated using a combination of relaxation and Monte Carlo techniques on three-dimensional clusters of

Effect of Inhomogeneity on s-wave Superconductivity in the Attractive Hubbard Model

{\mathrm{MnO}}_{6}

PSEUDOGAP FORMATION IN AN ELECTRONIC SYSTEM WITH D-WAVE ATTRACTION AT LOW DENSITY

octahedra. In the physically relevant region of parameter space for