An Electron Spin Resonance Selection Rule for Spin-Gapped Systems

The direct electron spin resonance (ESR) absorption between a singletground state and the triplet excited states of spin gap systems isinvestigated. Such an absorption, which is forbidden by theconservation of the total spin quantum number in isotropicHamiltonians, is allowed by the Dzyaloshinskii-Moriya interaction. Weshow a selection rule in the presence of this interaction, using theexact numerical diagonalization of the finite cluster of thequasi-one-dimensional bond-alternating spin system. The selection ruleis also modified into a suitable form in order to interpret recentexperimentalresults on CuGeO₃ and NaV₂O₅.