Magneto-optic far-infrared study of Sr14 Cu24 O41: 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 Sr14 Cu24 O41. Two triplet modes T1 and T2 were found in the center of the Brillouin zone at Δ1 =9.65 meV and Δ2 =10.86 meV 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 18 T, applied along the chains, the electron g factors of these two modes were similar, g1c =2.049 and g2c =2.044. Full linewidth at half maximum for both modes was 1 cm-1 (0.12 meV) at 4 K 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. 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 Sr14 Cu24 O41 must be modified to include a triplet mode at 9.65 meV in the center of the magnetic Brillouin zone.