New polarization effect and collective excitation in $S=1/2$ quasi 1D antiferromagnetic quantum spin chain

Anomalous polarization characteristics of magnetic resonance in $\mathrm{CuGeO_3}$ doped with 2% of Co impurity are reported. For the Faraday geometry this mode is damped for the microwave field $\mathbf B_\omega$ aligned along a certain crystallographic direction showing that the character of magnetic oscillation differs from the standard spin precession. The observed resonance coexists with the ESR on $\mathrm{Cu^{2+}}$ chains and argued not to be caused by “impurity” EPR, as previously claimed, but to correspond to a previously unknown collective mode of magnetic oscillations in an $S=1/2$ AF quantum spin chain.