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

Clinical and Demographic Characteristics and Two-Year Efficacy and Safety Data of 508 Multiple Sclerosis Patients with Fingolimod Treatment

Introduction: Fingolimod is the first oral immunomodulatory treatment used as secondary care therapy in the treatment of multiple sclerosis for the last 10 years. The objective of our study is to reveal the experiences of the first generic fingolimod active ingredient treatment in different centers across Turkey. Method: The first generic fingolimod efficacy and safety data of patients followed-up in 29 different clinical multiple sclerosis units in Turkey were analyzed retrospectively. Data regarding efficacy and safety of the patients were transferred to the data system both before the treatment and on the 6th, 12th and 24th month following the treatment. The data were analyzed using the IBM SPSS 20.00. P value of <0.05 was considered to be statistically significant. Results: A total of 508 multiple sclerosis patients, 331 of whom were women, were included in the study. Upon comparing the Expanded Disability Status values before and after the treatment, a significant decrease was observed, especially at month 6 and thereafter. Since bradycardia occurred in 11 of the patients (2.3%), the first dose had to be longer than 6 hours. During the observation of the first dose, no issues that could prevent the use of the drug occured. Side effects were seen in 49 (10.3%) patients during the course of fingolimod treatment. Respectively, the most frequent side effects were bradycardia, hypotension, headache, dizziness and tachycardia. Conclusion: The observed results regarding efficacy and safety were similar to clinical trial data in the literature and real life data in terms of the first equivalent with fingolimod active ingredient. © 2023, Turkish Neuropsychiatric Society. All rights reserved.Thanks to SANOVEL for funding the publication process

A Real Space Description of Field Induced Melting in the Charge Ordered Manganites: II. the Disordered Case

We study the effect of A site disorder on magnetic field induced melting of charge order (CO) in half doped manganites using a Monte-Carlo technique. Strong A-site disorder destroys CO even without an applied field. At moderate disorder, the zero field CO state survives but has several intriguing features in its field response. Our spatially resolved results track the broadening of the field melting transition due to disorder and explain the unusual dependence of the melting scales on bandwidth and disorder. In combination with our companion paper on field melting of charge order in clean systems we provide an unified understanding of CO melting across all half doped manganites.Comment: 9 pages, pdflatex, 10 embedded png fig

Etlik piliç kümeslerin su hatlarında Campylobacter coli varlığı

This study aimed to determine the presence of Campylobacter coli in water lines of commercial broiler houses. The samples were taken in the period of three sequential focks in two houses. There was a distance of approximately 25 km between the houses and drinking water was supplied by groundwater in one house while the municipal water system with polyvinyl chloride plastic pipe was the source of drinking water in the other. C. coli was isolated and identified in drinking water and nipple swab samples, and fresh fecal dropping samples in both houses. The results of this study suggest that C. coli could be found in water-line of commercial broiler houses and drinking water might be a source for fock's colonization by C. coli

A Real Space Description of Magnetic Field Induced Melting in the Charge Ordered Manganites: I. The Clean Limit

We study the melting of charge order in the half doped manganites using a model that incorporates double exchange, antiferromagnetic superexchange, and Jahn-Teller coupling between electrons and phonons. We primarily use a real space Monte Carlo technique to study the phase diagram in terms of applied field $(h)$ and temperature $(T)$, exploring the melting of charge order with increasing $h$ and its recovery on decreasing $h$. We observe hysteresis in this response, and discover that the `field melted' high conductance state can be spatially inhomogeneous even without extrinsic disorder. The hysteretic response plays out in the background of field driven equilibrium phase separation. Our results, exploring $h$, $T$, and the electronic parameter space, are backed up by analysis of simpler limiting cases and a Landau framework for the field response. This paper focuses on our results in the `clean' systems, a companion paper studies the effect of cation disorder on the melting phenomena.Comment: 16 pages, pdflatex, 11 png fig