Magneto-optic far-infrared study ofSr14Cu24O41: Triplet excitations in chains

Abstract

Using far-infrared spectroscopy, we have studied the magnetic field and temperature dependence of the spin gap modes in the chains of ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$. Two triplet modes ${T}_{1}$ and ${T}_{2}$ were found in the center of the Brillouin zone at ${\ensuremath{\Delta}}_{1}=9.65\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and ${\ensuremath{\Delta}}_{2}=10.86\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in zero magnetic field. The ${T}_{1}$ mode was excited when the electric field vector $\mathbf{E}$ of the light was polarized along the $b$ axis (perpendicular to the planes of chains and ladders) and ${T}_{2}$ was excited for $\mathbf{E}\ensuremath{\Vert}\mathbf{a}$ (perpendicular to the chains and along the rungs). Up to the maximum magnetic field of $18\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, 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 $1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ $(0.12\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ at $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 [M. Matsuda et al., Phys. Rev. B 59, 1060 (1999); L. P. Regnault et al., ibid 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 ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ must be modified to include a triplet mode at $9.65\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in the center of the magnetic Brillouin zone.