Phase diagram for Ca_{1-x}Y_xMnO_3 type crystals

We present a simple model to study the electron doped manganese perovskites. The model considers the competition between double exchange mechanism for itinerant electrons and antiferromagnetic superexchange interaction for localized electrons. It represents each Mn^{4+} ion by a spin 1/2, on which an electron can be added to produce Mn^{3+}; we include a hopping energy t, a strong intratomic interaction exchange J (in the limit J/t>>1), and an interatomic antiferromagnetic interaction K between the local spins. Using the Renormalized Perturbation Expansion and a Mean Field Approximation on the hopping terms and on the superexchange interaction we calculate the free energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic, and novel spin glass phases can be determined as functions of the parameters characterizing the system. The model results can be expressed in terms of t and K for each value of the doping x in phase diagrams. The magnetization m and canting angle can also be calculated as fuctions of temperature for fixed values of doping and model parameters.Comment: 4 figure

Financial frictions, occupational choice and economic inequality

We develop a quantitative theory of entrepreneurship, income inequality, and financial frictions disciplined with household data from Brazil. The theory extends Lucas (1978) by modeling heterogeneity in two skills: -working and managerial skills. Consistently with the evidence, the theory implies three occupational categories: workers, employers, and self-employed entrepreneurs. We find that the removal of financial frictions decreases self-employment rates from 24% to 11% (with small effects on the number of employers), increases aggregate output by 48%, and has non- trivial effects on the distribution of income. We also find that while most households benefit from a reform that eliminates enforcement problems, the majority of employers (about two thirds) lose from the reform. By depressing the demand for labor, limited enforcement depresses the equilibrium wage rate, increasing the profits of employers. Our theory thus suggests that employers in Brazil may have a vested interested in maintaining a status quo with low enforcement.Erosa acknowledges financial support from the European Commission through Marie Curie International Reintegration Grants PIRG03- GA-2008-23109

Intermediate Valence Model for the Colossal Magnetoresistance in Tl_{2}Mn_{2}O_{7}

The colossal magnetoresistance exhibited by Tl_{2}Mn_{2}O_{7} is an interesting phenomenon, as it is very similar to that found in perovskite manganese oxides although the compound differs both in its crystalline structure and electronic properties from the manganites. At the same time, other pyrochlore compounds, though sharing the same structure with Tl_{2}Mn_{2}O_{7}, do not exhibit the strong coupling between magnetism and transport properties found in this material. Mostly due to the absence of evidence for significant doping into the Mn-O sublattice, and the tendency of Tl to form conduction bands, the traditional double exchange mechanism mentioned in connection with manganites does not seem suitable to explain the experimental results in this case. We propose a model for Tl_{2}Mn_{2}O_{7} consisting of a lattice of intermediate valence ions fluctuating between two magnetic configurations, representing Mn-3d orbitals, hybridized with a conduction band, which we associate with Tl. This model had been proposed originally for the analysis of intermediate valence Tm compounds. With a simplified treatment of the model we obtain the electronic structure and transport properties of Tl_{2}Mn_{2}O_{7}, with good qualitative agreement to experiments. The presence of a hybridization gap in the density of states seems important to understand the reported Hall data.Comment: 8 pages + 5 postscript fig

Council in the Classroom: New Ways of Knowing, Being and Interacting

This paper examines my learnings on Council, an ancient form of communication, currently being revisited and remolded to suit modern needs. In this vein I explored its application in a school environment. I describe here a year-long study of Council that took me to both Argentina and the United States. The focus is primarily on a different form of classroom management and its impact on students and their learning. In chapter one, I give an introduction to Council, outlining its history, and providing a brief explanation of the philosophy behind it. In chapter two, I look at my experience as a student in a graduate program where Council is used for checking in with the group. In chapter three, I explore how I used Council in a monocultural setting in Argentina as a school administrator for conflict resolution and as an English teacher as a teaching tool for the opening and closing of class. In chapter four, is my exploration as an English instructor of the use of Council to facilitate group communication for practicing listening and speaking skills in the United States in a multicultural classroom. In chapter five the paper concludes with a summary of my learnings through these experiences with Council and how these have impacted my teaching

Effect of disorder on the magnetic and transport properties of La_{1-x}Sr_{x}MnO_{3}

We study a simplified model of the electronic structure of compounds of the type of La$_{1-x}$Sr$_x$MnO$_3$. The model represents each Mn$^{4+}$ ion by a spin S=1/2, on which an electron can be added to produce Mn$^{3+}$. We include two strong intratomic interactions in the Hamiltonian: exchange ($J$% ) and Coulomb ($U$). Finally, to represent the effect of Sr substitution by La in a simple way, we include a distribution of diagonal energies at the Mn sites. Then we use Green function techniques to calculate a mobility edge and the average density of states. We find that according to the amount of disorder and to the concentration of electrons in the system, the Fermi level can cross the mobility edge to produce a metal to insulator transition as the magnetization decreases (increase of temperature). If the disorder is large, the system remains insulating for all concentrations. Concentrations near zero or one favor the insulating state while intermediate values of concentration favor the metallic state.Comment: 11 pages, 4 figures available upon request, accepted for publication in Solid State Communication

Anomalous Shift of Chemical Potential in the Double-Exchange Systems

Double-exchange system is investigated by the dynamical mean-field theory. We show that the chemical potential shifts as a function of temperature and magnetization, which is anomalously large. We also discuss the influences of dynamic Jahn-Teller effect to the shift of the chemical potential. Measurement of the shift of the chemical potential casts a constraint to theoretical approaches for the magnetoresistance phenomena in ($R$,$A$)MnO$_3$ such as double-exchange effects and dynamic Jahn-Teller effects. We also propose a method to measure the shift of $\mu$.Comment: Reference added, to be published in J. Phys. Soc. Jpn. Vol. 66 No.8 (Aug. 1997

The effect of Coulomb interaction at ferromagnetic-paramagnetic metallic perovskite junctions

We study the effect of Coulomb interactions in transition metal oxides junctions. In this paper we analyze charge transfer at the interface of a three layer ferromagnetic-paramagnetic-ferromagnetic metallic oxide system. We choose a charge model considering a few atomic planes within each layer and obtain results for the magnetic coupling between the ferromagnetic layers. For large number of planes in the paramagnetic spacer we find that the coupling oscillates with the same period as in RKKY but the amplitude is sensitive to the Coulomb energy. At small spacer thickness however, large differences may appear as function of : the number of electrons per atom in the ferromagnetics and paramagnetics materials, the dielectric constant at each component, and the charge defects at the interface plane emphasizing the effects of charge transfer.Comment: tex file and 7 figure

Electron-Doped Manganese Perovskites: The Polaronic State

Using the Lanczos method in linear chains we study the ground state of the double exchange model including an antiferromagnetic super-exchange in the low concentration limit. We find that this ground state is always inhomogeneous, containig ferromagnetic polarons. The extention of the polaron spin distortion, the dispersion relation and their trapping by impurities, are studied for diferent values of the super exchange interaction and magnetic field. We also find repulsive polaron polaron interaction.Comment: 4 pages, 6 embedded figure