Far-infrared study of the CuO$_2$ chain layer magnetism in Sr$_{14}$Cu$_{24}$O$_{41}$

Sr$_{14}$Cu$_{24}$O$_{41}$ consists of two-leg ladders and chains of copper ions. Planes of ladders and chains alternate in $b$ axis direction. Oxygen holes, present in stochiometric Sr$_{14}$Cu$_{24}$O$_{41}$, and Cu$^{2+}$ spins in the chain are coupled into Zhang-Rice spin singlet. Chain Cu$^{2+}$ spins that are separated by Zhang-Rice singlets form spin dimers. It is known from inelastic neutron scattering studies that because of inter-chain coupling there are two weakly dispersing triplet modes in the chain layer. We found that these triplet modes are optically active. Light is absorbed at 77.80 and 87.54\,cm$^{-1}$ in zero magnetic field and $T=4$\,K. When magnetic field is applied along the $c$ axis both resonances split. The triplet state g-factor is $2.048\pm0.002$ and $2.063\pm0.006$ for the lower and upper triplet. The results on the absorption line intensities and frequencies as a function of $T$, $B_0$, and light polarization are presented.