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Scientists who study condensed matter physics (CMP) try to explain macroscopic electrical, magnetic & thermal properties of solids using models of atomic scale interactions of the constituent atoms and electrons. One of our great theoretical successes, the band structure of semiconductors, has literally transformed our world with the invention and evolutionary applications of the common transistor. To this day new observations of physical phenomena continue to challenge condensed matter physicists and have led to much deeper understanding of materials that lead, in turn, to a host of new technologies and products that we all get to enjoy. One example is the discovery of the fantastically-named “giant magneto-resistance” materials that have become the fundamental building blocks for the large-capacity storage discs. These discs give our computers and hand-held devices the ability to store music and video with ease.
In a new paper1 in Nature Physics, Lee and coworkers illustrate how CMP progresses by pushing beyond well-established models to explain newly observed phenomena on real materials. The authors sought to explain the rich behavior seen in KCuF3 crystals as pressure and temperature are varied. The ordering (or spatial arrangement) of copper 3d-electron orbitals in the material is reminiscent of the way electron spins arrange to form long-range magnetic structures. The system is a prototypical example of orbital ordering that was understood through early application of many-electron theory to CMP. As modern measurements have enabled more careful examination of these material, new questions can been asked and a broad set of properties must be reconciled. In particular dynamical properties, characteristic of the lowest energy excited states, are not explained by previous theory.
The authors have extended that theory, postulating two additional interactions that connect electron orbitals and spins to the structural arrangement of fluorine ligands around copper atoms. This so-called “direct orbital-orbital exchange interaction” predicts a new set of ground states, so close in energy the system literally “cannot chose” one over the other above a specific temperature. This nearly degenerate set of states (see figure) could be responsible for observed fluctuations in the crystal structure and the presence of a previously unexplained structural transition observed at 50K.
Nearly degenerate hybrid orbital states. a |HO1> and b, |HO2>. Blue arrows indicate spin direction on the Cu atoms. These two states are thermally occupied in the intermediate-temperature regime (50K<T<800K). The |HO2> state is stabilized by the observed low-temperature, orthorhombic distortion. c Differences between two hybrid orbital states, |HO1> - |HO2>, illustrate the symmetry of fluctuations.
On the experimental side the authors combine synchrotron based soft and hard x-ray diffraction with optical Raman (vibrational) spectroscopy to substantiate the nature and temperature scale of magnetic and structural effects they predict. If this new “direct exchange” interaction can usefully be applied more broadly, it may prove crucial in understanding technically important orbitally active materials including manganates, ruthenates & iron pnictides.
1 “Two-stage Orbital Order and Dynamical Spin Frustration in KCuF3”, James C. T. Lee, Shi Yuan, Siddhartha Lal, Young Joe, Yu Gan, Serban Smadici, Ken Finkelstein, Yejun Feng, Andrivo Rusydi, Paul M. Goldbart, S. Lance Cooper and Peter Abbamonte. Nature Physics – published on-line October 16, 2011
Submitted by: Ken Finkelstein, CHESS, Cornell University