Abstract
Using far-infrared spectroscopy, we have studied the magnetic field and temperature dependence of the spin gap modes in the chains of . Two triplet modes and were found in the center of the Brillouin zone at and in zero magnetic field. The mode was excited when the electric field vector of the light was polarized along the axis (perpendicular to the planes of chains and ladders) and was excited for (perpendicular to the chains and along the rungs). Up to the maximum magnetic field of , applied along the chains, the electron factors of these two modes were similar, and . Full linewidth at half maximum for both modes was at and increased with . The temperature dependence of mode energies and line intensities was in agreement with the inelastic neutron scattering results from two groups [M. Matsuda et al., Phys. Rev. B 59, 1060 (1999); L. P. Regnault et al., ibid 59, 1055 (1999)]. The mode has not been observed by inelastic neutron scattering in the points of the space equivalent to the center of the Brillouin zone. Our study indicates that the zone structure model of magnetic excitations of must be modified to include a triplet mode at in the center of the magnetic Brillouin zone.
- Received 28 June 2007
DOI:https://doi.org/10.1103/PhysRevB.76.134418
©2007 American Physical Society