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Using far-infrared spectroscopy we have studied the magnetic field andtemperature dependence of the spin gap modes in the chains ofSr$_{14}$Cu$_{24}$O$_{41}$. Two triplet modes T$_1$ and T$_2$ were found in thecenter of the Brillouin zone at $\Delta_1=9.65$ meV and $\Delta_2=10.86$ meV inzero 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 theplanes of chains and ladders) and T$_2$ was excited for ${\bf E}\parallel {\bfa}$ (perpendicular to the chains and along the rungs). Up to the maximummagnetic field of 18 T, applied along the chains, the electron $g$ factors ofthese two modes were similar, $g_{1c}=2.049$ and $g_{2c}=2.044$. Full linewidthat half maximum for both modes was 1 cm$^{-1}$ (0.12 meV) at 4K and increasedwith $T$. The temperature dependence of mode energies and line intensities wasin agreement with the inelastic neutron scattering results from two groups[Matsuda {\it et al.}, Phys. Rev. B {\bf 59}, 1060 (1999) and Regnault {\it etal.}, Phys. Rev. B {\bf 59}, 1055 (1999)]. The T$_1$ mode has not been observedby inelastic neutron scattering in the points of the $k$-space equivalent tothe center of the Brillouin zone. Our study indicates that the zone structuremodel of magnetic excitations of Sr$_{14}$Cu$_{24}$O$_{41}$ must be modified toinclude a triplet mode at 9.65 meV in the center of the magnetic Brillouinzone.

Magneto-optic far-infrared study of Sr$_{14}$Cu$_{24}$O$_{41}$: triplet excitations in chains