Neutron Scattering and Orbital Degeneracy

Stephen E. Nagler,
Oak Ridge National Laboratory


Orbital degeneracy in novel electronic materials can lead to very interesting new
physics. This talk covers some examples of recent neutron scattering work on systems
of particular interest. One of these, MnV2O4, is a cubic spinel system with two magnetic
phase transitions near 56 K and 53 K. Neutron scattering has clarified the nature of these
phase transitions including the magnetic ground state and type of orbital order [1]. The
56 K transition is from a paramagnetic state to a collinear ferrimagnetic phase. The
transition at 53K is second simultaneous structural and magnetic transition to a tetragonal
structure with a non-collinear ferrimagnetic state. The low T magnetic structure has been
definitively resolved, and is seen to be associated with a gap in the magnetic excitation
spectrum. The magnetic structure is compatible with a staggered orbital ordering. A
second system of interest is LiVO2, a layered system with planes containing triangular
lattices of S=1 V3+ ions. A first order phase transition near 500 K leads to an orbitally
ordered trimerized state. The spectrum of excitations contains some interesting features
[2], and we speculate on the possible novel effects of orbital order on magnetic
excitations in an isolated cluster. Other examples may be covered as time permits.


References:
1. V. O. Garlea, S. E. Nagler, D. L. Abernathy, R. Jin, D. Mandrus, B. Roessli, Q.
Huang, M. Miller, and A. J. Schultz, Magnetic and orbital ordering in the spinel
MnV2O4, arXiv:0711.1844, to be published in Phys. Rev. Lett., (2008).
2. W. Tian, M.B. Stone, D. G. Mandrus, B.C. Sales, R. Jin, D.T. Adroja and S. E.
Nagler, Magnetic excitations in the orbitally degenerate triangular lattice LiVO2,
Physica B 385-386, 50 (2006).

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