Magneto-optic far-infrared study of Sr${}_{14}$Cu${}_{24}$O${}_{41}$: 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 Sr${}_{14}$Cu${}_{24}$O${}_{41}$. Two triplet modes T${}_1$ and T${}_2$ were found in the center of the Brillouin zone at $\hat{I}{”}_1=9.65$ meV and $\hat{I}{”}_2=10.86$ 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 $\mathbf{E}\parallel \mathbf{a}$ (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.