Magneto-optic far-infrared study of Sr$_{14}$Cu$_{24}$O$_{41}$: 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 Sr$_{14}$Cu$_{24}$O$_{41}$. Two triplet modes T$_1$ and T$_2$ were found in the center of the Brillouin zone at $Delta_1=9.65$ meV and $Delta_2=10.86$ meV in zero magnetic field. The T$_1$ mode was excited when the electric field vector ${bf E}$ of the light was polarized along the b axis (perpendicular to the planes of chains and ladders) and T$_2$ was excited for ${bf E}parallel {bf a}$ (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, $g_{1c}=2.049$ and $g_{2c}=2.044$. Full linewidth at half maximum for both modes was 1 cm$^{-1}$ (0.12 meV) 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 [Matsuda {it et al.}, Phys. Rev. B {bf 59}, 1060 (1999) and Regnault {it et al.}, Phys. Rev. B {bf 59}, 1055 (1999)]. The T$_1$ 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 Sr$_{14}$Cu$_{24}$O$_{41}$ must be modified to include a triplet mode at 9.65 meV in the center of the magnetic Brillouin zone.