This award supports theoretical research and education aimed to explain the unusual properties of sodium cobaltate materials. These materials are interesting as their properties are the likely result of the combination of strong Coulomb correlation, interplay between correlation and geometrical frustration on the triangular lattice, and the important but subtle role played by the sodium dopants.

The PI's approach builds on recent efforts that treat correlation effects on the electronic structure and Fermi surface topology at a level beyond that of band theory and uses multi-orbital Hubbard and single-band t-U-V models on a triangular lattice. From these Hamiltonians, a description of the cobaltate phase diagram and the observed physical properties will be calculated. Key ideas that will be developed by the research program include: (i) the alleviation of antiferromagnetic frustration through electronic inhomogeneity, charge and spin order in the unconventional insulating phase at x=0.5, (ii) the role of kinetic antiferromagnetism and the nature and the pairing symmetry of the superconducting phase in proximity to charge and spin order (iii) and the strong-coupling Stoner instability and the emergence of in-plane ferromagnetism in the sodium rich part of the phase diagram. As a materials based microscopic approach, the effects of sodium dopants will be studied to understand the physics associated with dopant order at x = 0.5, dopant disorder, and the local moment formation at high sodium concentrations.

The program advances an unrestricted Gutzwiller projection approach through analytical approximations and variational Monte Carlo to address the problem of strongly correlated phases of matter involving orbital carrier transfer, inhomogeneous charge and magnetic order, and unconventional superconductivity.

The research activities provide an ideal training ground for undergraduate and graduate students, including members of under-represented groups. Workshop format courses are developed that integrate the proposed research theme and activities into teaching, education, and include strategies for attracting future generations of materials researchers. Close collaboration with experimental groups will be maintained.


This award supports theoretical research and education aimed to explain the unusual properties of sodium cobaltate materials. These are complex materials that display unusual and interesting properties that lie outside standard textbook descriptions. These properties appear to be the result of strong interactions between electrons acting in combination with the geometry of the lattice. The triangular lattice precludes satisfying the interaction between electrons on neighboring atoms in any simple way.

Understanding materials in which strong interactions between electrons leads to extraordinary correlations in their motions remains a primary challenge in condensed matter physics. This project focuses on one class of these material, sodium cobaltates which consist of alternating layers of cobalt and oxygen atoms. Upon inserting sodium atoms into these materials, electrons are transferred off of sodium atoms and onto the cobalt layers leading to several interesting electronic and magnetic phases which depend upon how much sodium is added. These materials may be superconductors, insulators, magnetic, or thermoelectrics. This research builds new theoretical approaches that address the phase diagram of these materials and contributes to understanding the broad spectrum of experiments on these intriguing materials.

This is fundamental research that contributes to the intellectual foundations of our understanding of complex materials with properties that lie outside our current understanding. A better understanding of such materials may lead to the discovery of new phenomena and to new device technologies. The sodium cobatlates display similarities to another puzzling material, the high superconductors and understanding the cobaltates may reveal important pieces of that puzzle.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Daryl W. Hess
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Boston College
Chestnut Hill
United States
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