Magneto-optic far-infrared study of Sr14Cu24O41 : Triplet excitations in chains

Using far-infrared spectroscopy, we have studied the magnetic field and temperature dependence of the spin gap modes in the chains of Sr₁₄Cu₂₄O₄₁ . Two triplet modes T₁ and T₂ were found in the center of the Brillouin zone at Δ₁=9.65meV and Δ₂=10.86meV in zero magnetic field. The T₁ mode was excited when the electric field vector E of the light was polarized along the b axis (perpendicular to the planes of chains and ladders) and T₂ was excited for E‖a (perpendicular to the chains and along the rungs). Up to the maximum magnetic field of 18T , applied along the chains, the electron g factors of these two modes were similar, g_1c=2.049 and g_2c=2.044 . Full linewidth at half maximum for both modes was 1cm^-1 (0.12meV) at 4K and increased with T . The temperature dependence of mode energies and line intensities was in agreement with the inelastic neutron scattering results from two groups [M. Matsuda , Phys. Rev. B 59, 1060 (1999); L. P. Regnault , ibid 59, 1055 (1999)]. The T₁ mode has not been observed by inelastic neutron scattering in the points of the k space equivalent to the center of the Brillouin zone. Our study indicates that the zone structure model of magnetic excitations of Sr₁₄Cu₂₄O₄₁ must be modified to include a triplet mode at 9.65meV in the center of the magnetic Brillouin zone.