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Title: Colossal Magnetoresistance Observed in Monte Carlo Simulations of the One- and Two-Orbital Models for Manganites

Abstract

he one- and two-orbital double-exchange models for manganites are studied using Monte Carlo computational techniques in the presence of a robust electron-phonon coupling (but neglecting the antiferromagnetic exchange $$J_{\rm AF}$$ between the localized spins). The focus in this effort is on the analysis of charge transport. Our results for the one-orbital case confirm and extend previous recent investigations that showed the presence of robust peaks in the resistivity vs. temperature curves for this model. Quenched disorder substantially enhances the magnitude of the effect, while magnetic fields drastically reduce the resistivity. A simple picture for the origin of these results is presented. It is also shown that even for the case of just one electron, the resistance curves present metallic and insulating regions by varying the temperature, as it occurs at finite electronic density. Moreover, in the present study these investigations are extended to the more realistic two-orbital model for manganites. The transport results for this model show large peaks in the resistivity vs. temperature curves, located at approximately the Curie temperature, and with associated large magnetoresistance factors. Overall, the magnitude and shape of the effects discussed here closely resemble experiments for materials such as $$\rm La_{0.70} Ca_{0.30} Mn O_{3}$$, and they are in qualitative agreement with the current predominant theoretical view that competition between a metal and an insulator, enhanced by quenched disorder, is crucial to understand the colossal magnetoresistance (CMR) phenomenon.

Authors:
 [1];  [2];  [2];  [2]
  1. Florida State University
  2. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Center for Computational Sciences
Sponsoring Org.:
USDOE Laboratory Directed Research and Development (LDRD) Program; ORNL other overhead
OSTI Identifier:
931279
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review B; Journal Volume: 73
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; MANGANESE OXIDES; CHARGE TRANSPORT; CURIE POINT; ELECTRON-PHONON COUPLING; MAGNETORESISTANCE; MONTE CARLO METHOD; MATHEMATICAL MODELS

Citation Formats

Sen, Cengiz, Alvarez, Gonzalo, Aliaga, Horacio A, and Dagotto, Elbio R. Colossal Magnetoresistance Observed in Monte Carlo Simulations of the One- and Two-Orbital Models for Manganites. United States: N. p., 2006. Web. doi:10.1103/PhysRevB.73.224441.
Sen, Cengiz, Alvarez, Gonzalo, Aliaga, Horacio A, & Dagotto, Elbio R. Colossal Magnetoresistance Observed in Monte Carlo Simulations of the One- and Two-Orbital Models for Manganites. United States. doi:10.1103/PhysRevB.73.224441.
Sen, Cengiz, Alvarez, Gonzalo, Aliaga, Horacio A, and Dagotto, Elbio R. Sun . "Colossal Magnetoresistance Observed in Monte Carlo Simulations of the One- and Two-Orbital Models for Manganites". United States. doi:10.1103/PhysRevB.73.224441.
@article{osti_931279,
title = {Colossal Magnetoresistance Observed in Monte Carlo Simulations of the One- and Two-Orbital Models for Manganites},
author = {Sen, Cengiz and Alvarez, Gonzalo and Aliaga, Horacio A and Dagotto, Elbio R},
abstractNote = {he one- and two-orbital double-exchange models for manganites are studied using Monte Carlo computational techniques in the presence of a robust electron-phonon coupling (but neglecting the antiferromagnetic exchange $J_{\rm AF}$ between the localized spins). The focus in this effort is on the analysis of charge transport. Our results for the one-orbital case confirm and extend previous recent investigations that showed the presence of robust peaks in the resistivity vs. temperature curves for this model. Quenched disorder substantially enhances the magnitude of the effect, while magnetic fields drastically reduce the resistivity. A simple picture for the origin of these results is presented. It is also shown that even for the case of just one electron, the resistance curves present metallic and insulating regions by varying the temperature, as it occurs at finite electronic density. Moreover, in the present study these investigations are extended to the more realistic two-orbital model for manganites. The transport results for this model show large peaks in the resistivity vs. temperature curves, located at approximately the Curie temperature, and with associated large magnetoresistance factors. Overall, the magnitude and shape of the effects discussed here closely resemble experiments for materials such as $\rm La_{0.70} Ca_{0.30} Mn O_{3}$, and they are in qualitative agreement with the current predominant theoretical view that competition between a metal and an insulator, enhanced by quenched disorder, is crucial to understand the colossal magnetoresistance (CMR) phenomenon.},
doi = {10.1103/PhysRevB.73.224441},
journal = {Physical Review B},
number = ,
volume = 73,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • The one- and two-orbital double-exchange models for manganites are studied using Monte Carlo computational techniques in the presence of a robust electron-phonon coupling
  • Large-scale Monte Carlo simulation results for the two-orbital model for manganites, including Jahn- Teller lattice distortions, are presented here. At hole density x 1=4 and in the vicinity of the region of competition between the ferromagnetic metallic and spin-charge-orbital ordered insulating phases, the colossal magnetoresistance (CMR) phenomenon is observed with a magnetoresistance ratio 10 000%. Our main result is that this CMR transition is found to be of first order in some portions of the phase diagram, in agreement with early results from neutron scattering, specific heat, and magnetization, thus solving a notorious discrepancy between experiments and previous theoretical studies.more » The first order characteristics of the transition survive, and are actually enhanced, when weak quenched disorder is introduced.« less
  • Considerable progress has been recently made in the theoretical understanding of the colossal magnetoresistance (CMR) effect in manganites. The existence of inhomogeneous states has been shown to be directly related with this phenomenon, both in theoretical studies and experimental investigations. 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 nonuniform clustered states. However, a direct computational study of the CMR effect in realistic models has been difficult since large clusters are needed for this purpose. In this paper, the recently proposed truncatedmore » polynomial expansion method (TPEM) for spin-fermion systems is tested using the double-exchange one-orbital, with finite Hund coupling J{sub H}, and two-orbital, with infinite J{sub H}, models. Two dimensional lattices as large as 48 x 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 analyzed here 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 enough to handle realistic manganite models on large systems. Our results contribute to the next challenge in theoretical studies of manganites, namely a frontal computational attack of the colossal magnetoresistance phenomenon using double-exchange-like models, on large clusters, and including quenched disorder.« less
  • Considerable progress has been recently made in the theoretical understanding of the colossal magnetoresistance
  • The metal-insulator transition, and the associated magnetic transition, in the colossal magnetoresistance CMR regime of the one-orbital model for manganites is studied here using Monte Carlo MC techniques in two-dimensional clusters. Both cooperative oxygen lattice distortions and a finite superexchange coupling among the t2g spins are included in our investigations. Charge and spin correlations are studied. In the CMR regime, a strong competition between the ferromagnetic metallic and the 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 systemmore » 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, identified as a dip in the LDOS at the Fermi energy, is found to exist in the CMR temperature range. The width of the PG in the LDOS is calculated and directly compared to recent scanning-tunneling-spectroscopy STS experimental results. The observed 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. 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.« less