Insulator to Metal Transition Induced by Disorder in a Model for Manganites

The physics of manganites appears to be dominated by phase competition among ferromagnetic metallic and charge-ordered antiferromagnetic insulating states. Previous investigations (Burgy {\it et al.}, Phys. Rev. Lett. {\bf 87}, 277202 (2001)) have shown that quenched disorder is important to smear the first-order transition between those competing states, and induce nanoscale inhomogeneities that produce the colossal magnetoresistance effect. Recent studies (Motome {\it et al.} Phys. Rev. Lett. {\bf 91}, 167204 (2003)) have provided further evidence that disorder is important in the manganite context, unveiling an unexpected insulator-to-metal transition triggered by disorder in a one-orbital model with cooperative phonons. In this paper, a qualitative explanation for this effect is presented. It is argued that the transition occurs for disorder in the form of local random energies. Acting over an insulating states made out of a checkerboard arrangement of charge, with ``effective'' site energies positive and negative, this form of disorder can produce lattice sites with an effective energy near zero, favorable for the transport of charge. This explanation is based on Monte Carlo simulations and the study of simplified toy models, measuring the density-of-states, cluster conductances using the Landauer formalism, and other observables. The applicability of these ideas to real manganites is discussed.Comment: 14 pages, 23 figures, submitted to Physical Review

Dynamical Mean-Field Study of the Ferromagnetic Transition Temperature of a Two-Band Model for Colossal Magnetoresistance Materials

The ferromagnetic (FM) transition temperature (Tc) of a two-band Double-Exchange (DE) model for colossal magnetoresistance (CMR) materials is studied using dynamical mean-field theory (DMFT), in wide ranges of coupling constants, hopping parameters, and carrier densities. The results are shown to be in excellent agreement with Monte Carlo simulations. When the bands overlap, the value of Tc is found to be much larger than in the one-band case, for all values of the chemical potential within the energy overlap interval. A nonzero interband hopping produces an additional substantial increase of Tc, showing the importance of these nondiagonal terms, and the concomitant use of multiband models, to boost up the critical temperatures in DE-based theories.Comment: 4 pages, 4 eps figure

Fragility of the A-type AF and CE Phases of Manganites: An Exotic Insulator-to-Metal Transition Induced by Quenched Disorder

Using Monte Carlo simulations and the two eg-orbital model for manganites, the stability of the CE and A-type antiferromagnetic insulating states is analyzed when quenched disorder in the superexchange JAF between the t2g localized spins and in the on-site energies is introduced. At vanishing or small values of the electron-(Jahn-Teller)phonon coupling, the previously hinted "fragility" of these insulating states is studied in detail, focusing on their charge transport properties. This fragility is here found to induce a rapid transition from the insulator to a (poor) metallic state upon the introduction of disorder. A possible qualitative explanation is presented based on the close proximity in energy of ferromagnetic metallic phases, and also on percolative ideas valid at large disorder strength. The scenario is compared with previously discussed insulator-to-metal transitions in other contexts. It is argued that the effect unveiled here has unique properties that may define a new class of giant effects in complex oxides. This particularly severe effect of disorder must be present in other materials as well, in cases involving phases that arise as a compromise between very different tendencies, as it occurs with striped states in the cuprates.Comment: 13 pages, 17 figures, RevTex 4, submitted for publicatio

The Crossover from Impurity to Valence Band in Diluted Magnetic Semiconductors: The Role of the Coulomb Attraction by Acceptor

The crossover between an impurity band (IB) and a valence band (VB) regime as a function of the magnetic impurity concentration in models for diluted magnetic semiconductors (DMS) is studied systematically by taking into consideration the Coulomb attraction between the carriers and the magnetic impurities. The density of states and the ferromagnetic transition temperature of a Spin-Fermion model applied to DMS are evaluated using Dynamical Mean-Field Theory (DMFT) and Monte Carlo (MC) calculations. It is shown that the addition of a square-well-like attractive potential can generate an IB at small enough Mn doping $x$ for values of the $p-d$ exchange $J$ that are not strong enough to generate one by themselves. We observe that the IB merges with the VB when $x >= x_c$ where $x_c$ is a function of $J$ and the Coulomb attraction strength $V$. Using MC calculations, we demonstrate that the range of the Coulomb attraction plays an important role. While the on-site attraction, that has been used in previous numerical simulations, effectively renormalizes $J$ for all values of $x$, an unphysical result, a nearest-neighbor range attraction renormalizes $J$ only at very low dopings, i.e., until the bound holes wave functions start to overlap. Thus, our results indicate that the Coulomb attraction can be neglected to study Mn doped GaSb, GaAs, and GaP in the relevant doping regimes, but it should be included in the case of Mn doped GaN that is expected to be in the IB regime.Comment: 8 pages, 4 Postscript figures, RevTex

Study of the One- and Two-Band Models for Colossal Magnetoresistive Manganites Using the Truncated Polynomial Expansion Method

Considerable progress has been recently made in the theoretical understanding of the colossal magnetoresistance (CMR) effect in manganites. The analysis of simple models with two competing states and a resistor network approximation to calculate conductances has confirmed that CMR effects can be theoretically reproduced using non-uniform clustered states. In this paper, the recently proposed Truncated Polynomial Expansion method (TPEM) for spin-fermion systems is tested using the double-exchange one-band, with finite Hund coupling $J_{\rm H}$, and two-band, with infinite $J_{\rm H}$, models. Two dimensional lattices as large as 48$\times$48 are studied, far larger than those that can be handled with standard exact diagonalization (DIAG) techniques for the fermionic sector. The clean limit (i.e. without quenched disorder) is here analyzed in detail. Phase diagrams are obtained, showing first-order transitions separating ferromagnetic metallic from insulating states. A huge magnetoresistance is found at low temperatures by including small magnetic fields, in excellent agreement with experiments. However, at temperatures above the Curie transition the effect is much smaller confirming that the standard finite-temperature CMR phenomenon cannot be understood using homogeneous states. By comparing results between the two methods, TPEM and DIAG, on small lattices, and by analyzing the systematic behavior with increasing cluster sizes, it is concluded that the TPEM is accurate to handle realistic manganite models on large systems. Our results pave the way to a frontal computational attack of the colossal magnetoresistance phenomenon using double-exchange like models, on large clusters, and including quenched disorder.Comment: 14 pages, 17 figure

Antibiotic resistance and biofilm forming abilities of Listeria monocytogenes and effect of subMIC concentration of white vinegar on these virulence factors

In the present study, antibiotic resistance profiles and biofilm forming abilities of 9 Listeria monocytogenes isolates obtained from out of 30 retail meat samples were determined, and the effect of commercial white vinegar on these virulence factors in isolates exposed to subMIC concentrations were investigated. All isolates were found to be resistant to cefotixin and oxacillin, 8 isolates (26.6%) to clindamycin, 1 isolate (3.3%) to rifampicin, and 1 (3.3%) isolate was found to show intermediate resistance against clindamycin. Biofilm formation was determined for all the isolates at 22 °C and 37 °C (24 h, 48 h and 72 h). MIC values of white vinegar samples were determined at 3.12% for all isolates. MIC/2 and MIC/4 concentrations of white vinegar increased the biofilm forming capacity of the isolates by 21.2% and 17.1%, respectively. After exposure to MIC/2 concentration of white vinegar for seven days, the antibiotic resistance status of the isolates to tetracycline, rifampicin, and clindamycin changed, and the biofilm forming abilities significantly decreased at 4 °C and 37 °C for 48 h and at 37 °C for 72 h ( P < 0.05). The results showed that the use of subMIC concentrations of white vinegar should be avoided in routine sanitation applications

VACCINATION PRACTICES OF ADULT FAMILIAL MEDITERRANEAN FEVER PATIENTS IN TURKEY.

[Abstract Not Available

The Relationship Between Serum Adiponectin, Tumor Necrosis Factor−Alpha, Leptin Levels and Insulin Sensitivity in Childhood and Adolescent Obesity: Adiponectin is a Marker of Metabolic Syndrome

Objective: This study aimed (a) to investigate the relationship between the degree of obesity and serum adiponectin, tumor necrosis factor (TNF)−α, leptin, insulin levels and the lipid profile; (b) to clarify the relationship between insulin resistance/glucose tolerance and adipocytokine levels; and (c) to investigate the value of adipocytokine levels as a marker of metabolic syndrome (MS)

A private contributions game for joint replenishment

We study a non-cooperative game for joint replenishment by n firms that operate under an EOQ-like setting. Each firm decides whether to replenish independently or to participate in joint replenishment, and how much to contribute to joint ordering costs in case of participation. Joint replenishment cycle time is set by an intermediary as the lowest cycle time that can be financed with the private contributions of participating firms. We characterize the behavior and outcomes under undominated Nash equilibria. © Springer-Verlag 2011