Soft chiral excitations in triangular antiferromagnets.

A. Gozar (1,2), B.S. Shastry (3), G. Blumberg (1), B.S. Dennis (1) and S.-W. Cheong (4).

(1) Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974.
(2) University of Illinois at Urbana-Champaign, Urbana, IL, 61801.
(3) Indian Institute of Science, Bangalore, India, 560012.
(4) Rutgers University, Piscataway, NJ 08854.

Single crystals of LuMnO_3 are studied by Raman spectroscopy. We investigate
the magnetic properties of the spin S = 2 two-dimensional (2D) triangular
antiferromagnetic Mn - O layers above and below the magnetic transition
temperature T_N ~ 90 K. Just below T_N a new mode with a soft character
appears in the Raman spectra and hardens to about 22 cm-1 at T = 10 K.
Symmetry analysis and magnetic field effects show that the low energy feature
is a chiral excitation in the singlet sector transforming like S_i (S_j X S_k),
first discussed as a relevant effective Raman spin operator in the context
of 2D insulating Cu-O layers characteristic of high T_c superconductors [1].
The critical exponents obtained by power law fits to the peak energy and
weight are close to Monte Carlo predictions for the 3D or XY Heisenberg
models in triangular lattices. Surprisingly, if compared to all previous
studies, we find that the soft chiral mode is a sharp and well defined
magnetic feature at low temperatures. The scattering width and strength
demonstrate a distinct relaxation mechanisms and stronger coupling to the
radiation field of what may be a ubiquitous feature of frustrated triangular
antiferromagnets with out-of-plane spin canting. In the symmetric Raman
channel, excitations which could probably be associated with spin pair
scattering are observed as a low energy continuum in the 0 - 200 cm-1 range
in the fluctuation regime around T_N.

[1] B. S. Shastry and B. I. Shraiman, Phys. Rev. Lett. 65, 1068 (1990);
Int. J. Mod. Phys. B 5, 365 (1991).