[Paper Review] Open Questions in CMR Manganites, Relevance of Clustered States, and Analogies with other Compounds

Elbio Dagotto|arXiv (Cornell University)|Feb 26, 2003

High-pressure geophysics and materials3 citations

TL;DR

This paper argues that inhomogeneous 'clustered' states—such as phase-separated or mixed-phase regions—are a fundamental and widespread phenomenon in correlated electron systems, particularly in colossal magnetoresistance (CMR) manganites. It proposes that the CMR effect arises from competition between metallic ferromagnetic and insulating (charge/orbital-ordered or antiferromagnetic) phases, with a universal temperature scale $T^*$ marking the onset of anomalous behavior, a feature shared across manganites, cuprates, organic superconductors, and heavy fermions.

ABSTRACT

This is an informal paper that contains a list of ``things we know'' and ``things we do not know'' in manganites. It is adapted from the conclusions chapter of a recent book by the author, {\it Nanoscale Phase Separation and Colossal Magnetoresistance. The Physics of Manganites and Related Compounds}, Springer-Verlag, Berlin, November 2002. The main new result of recent manganite investigations is the discovery of tendencies toward inhomogeneous states, both in experiments and in simulations of models. The colossal magnetoresistance effect appears to be closely linked to these mixed-phase tendencies, although considerably more work is needed to fully confirm these ideas. The paper also includes information on cuprates, diluted magnetic semiconductors, relaxor ferroelectrics, cobaltites, and organic and heavy fermion superconductors. These materials potentially share some common phenomenology with the manganites, such as a temperature scale $T^*$ above the ordering temperature where anomalous behavior starts. Many of these materials also present low-temperature phase competition. The possibility of colossal-like effects in compounds that do not involve ferromagnets is briefly discussed. Overall, it is concluded that inhomogeneous ``clustered'' states should be considered a new paradigm in condensed matter physics, since their presence appears to be far more common than previously anticipated.

Motivation & Objective

• To identify and clarify the open questions in colossal magnetoresistance (CMR) manganites, particularly regarding the role of inhomogeneous, clustered states.
• To investigate the relevance of phase competition—especially between ferromagnetic metal and charge/orbital-ordered insulating states—for the origin of CMR.
• To explore formal and phenomenological analogies between manganites and other correlated electron systems, including cuprates, diluted magnetic semiconductors, relaxor ferroelectrics, cobaltites, and heavy fermion compounds.
• To argue that the emergence of nanoscale phase separation and clustered states is not an anomaly but a new paradigm in condensed matter physics.
• To highlight the presence of a universal $T^*$ scale above which anomalous electronic behavior begins, observed across diverse materials with competing ground states.

Proposed method

• Analysis of experimental and simulation data on manganites, focusing on inhomogeneous states such as phase-separated clusters and mixed-phase behavior.
• Comparison of phase diagrams across multiple correlated electron systems, including manganites, cuprates, organic superconductors, and heavy fermions, to identify common features.
• Use of phenomenological models to describe competing phases (e.g., FM metal, CO, AF, SC, SDW), with emphasis on first-order transitions and phase coexistence.
• Identification of a universal $T^*$ scale in various materials, marking the onset of pre-transitional anomalies such as pseudogaps and charge fluctuations.
• Application of concepts from double-exchange and Jahn-Teller physics to explain orbital and spin ordering in manganites at $x=0$ and $x=1$.
• Use of mean-field and Monte Carlo simulations to support the existence of complex, competing ground states in manganites, particularly at half-doping ($x=0.5$).

Experimental results

Research questions

• RQ1 What is the role of nanoscale phase separation and clustered states in the colossal magnetoresistance (CMR) effect in manganites?
• RQ2 How do competing phases—such as ferromagnetic metal, charge-ordered insulator, and antiferromagnetic states—interact to produce CMR?
• RQ3 To what extent is the $T^*$ scale, marking the onset of anomalous behavior, a universal feature across diverse correlated electron systems?
• RQ4 What are the formal and phenomenological analogies between manganites and other complex oxides, including cuprates, organic superconductors, and heavy fermions?
• RQ5 Can the presence of inhomogeneous, clustered states be considered a new paradigm in condensed matter physics, given their prevalence across multiple material classes?

Key findings

• Nanoscale phase separation and clustered states are prevalent in manganites, both in experiments and simulations, and are strongly linked to the CMR effect.
• The CMR effect is not explained by the ferromagnetic metallic state alone but arises from competition between metallic and insulating phases, particularly at phase boundaries.
• At $x=0$, LaMnO₃ exhibits A-type antiferromagnetic order with staggered orbital ordering, driven by strong electron-Jahn-Teller coupling.
• At $x=1$, the fully hole-doped system forms a G-type antiferromagnetic state, indicating a reversal of magnetic order with doping.
• At intermediate doping ($x=0.5$), charge-ordered and orbital-ordered states coexist with the ferromagnetic metal, and their competition is central to understanding CMR.
• A universal $T^*$ scale is observed across manganites, cuprates, organic superconductors, and heavy fermions, marking the onset of pre-transitional anomalies such as pseudogaps and charge fluctuations.