Competing Ferromagnetic and Charge-Ordered States in Models for Manganites: the Origin of the CMR Effect

The one-orbital model for manganites with cooperative phonons and superexchange coupling $J_{\rm AF}$ has been investigated via large-scale Monte Carlo (MC) simulations. Results for two-orbitals are also briefly discussed. Focusing on electronic density $n$=0.75, a regime of competition between ferromagnetic (FM) metallic and charge-ordered (CO) insulating states was identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects were observed. The CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state.Comment: 4 pages, 4 figures, accepted for publication in the Phys. Rev. Let

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

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

Determination of Flower Characteristics of Some Kiwifruit Genotypes (Actinidia Spp.) Obtained with Breeding Program

All Actinidia species are dioecious, male and female flowers grow on separate kiwifruit plants. In breeding studies, it is generally desirable to obtain female individuals. However, male plants are also of great importance for pollination. Therefore, it is necessary to examine the flower characteristics of the genotypes obtained by breeding studies. This research was conducted in the kiwifruit breeding plot of Yalova Atatürk Horticultural Central Research Institute for two years. Genotypes obtained from cultivars belonging to Actinidia deliciosa and Actinidia chinensis were used in the research. At the time of flowering, phenological observations of male and female genotypes, which are prominent in the population, have been made and the developmental stages of the flowers have been determined. At least 10 flowers of each genotype were used to determine the morphological characteristics.Number of leaves, number of petals, number of male organs, number of filaments, number of female organs, number of female organs and number of stylus were examined in order to determine flower characteristics. When the data obtained as a result of two years are evaluated; significant differences have been obtained particularly in terms of flowering time, flowering period, the number of stylus, the filament size, the number of female organs and the number of male organs. Female cultivars/genotypes tend to flowering later than male cultivars/genotypes, and female cultivars/genotypes have shorter filament length than male cultivars/genotypes. It has also been clearly observed that ovaries are not functional in male types

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

Study of Short-distance Spin and Charge Correlations and Local Density-of-States in the CMR regime of the One-Orbital Model for Manganites

The metal-insulator transition, and the associated magnetic transition, in the colossal magnetoresistance (CMR) regime of the one-orbital model for manganites is here studied using Monte Carlo (MC) techniques. Both cooperative oxygen lattice distortions and a finite superexchange coupling among the $t_{\rm 2g}$ spins are included in our investigations. Charge and spin correlations are studied. In the CMR regime, a strong competition between the ferromagnetic metallic and antiferromagnetic charge-ordered insulating states is observed. This competition is shown to be important to understand the resistivity peak that appears near the critical temperature. Moreover, it is argued that the system is dynamically inhomogeneous, with short-range charge and spin correlations that slowly evolve with MC time, producing the glassy characteristics of the CMR state. The local density-of-states (LDOS) is also investigated, and a pseudogap (PG) is found to exist in the CMR temperature range. The width of the PG in the LDOS is calculated and directly compared with recent scanning-tunneling-spectroscopy (STS) experimental results. The agreement between our calculation and the experiment suggests that the depletion of the conductance at low bias observed experimentally is a reflection on the existence of a PG in the LDOS spectra, as opposed to a hard gap. The apparent homogeneity observed via STS techniques could be caused by the slow time characteristics of this probe. Faster experimental methods should unveil a rather inhomogeneous state in the CMR regime, as already observed in neutron scattering experiments.Comment: 13 pages, 10 figure

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

Bilayer manganites: polarons in the midst of a metallic breakdown

The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistant (CMR) effect, is still under heavy debate. By combining new photoemission and tunneling data, we show that in La{2-2x}Sr{1+2x}Mn2O7 the polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong electron-lattice interactions result in vanishing coherent quasi-particle spectral weight at the Fermi level for all locations in k-space. The incoherence of the charge carriers offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopic data. The stacking number N is the key factor for true metallic behavior, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in the bilayer system.Comment: 7 pages, 2 figures, includes supplementary informatio