We report the first observation of coherent magnetic excitations in a spin ladder system ${\mathrm{NaV}}_2{\mathrm{O}}_5$ by using femtosecond time-domain spectroscopy. A pronounced coherent oscillation is observed at $127{\mathrm{cm}}^{-1\mathrm{}}$ (nearly twice the spin gap energy) and assigned to a two-magnon bound state, based on the temperature dependence of the intensity below the charge ordering phase transition at $T_C=34\mathrm{}\mathrm{K}$. This mode can be observable only when circularly polarized light is used as a pump or a probe beam, suggesting that it corresponds to a spin-flip excitation from the singlet ground state. A phonon mode strongly coupled to the spin state is also found at $303{\mathrm{cm}}^{-1\mathrm{}}$.

• Received 13 June 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.127201