ESR selection rules for direct transition of spin gap

Abstract

The spin gap in low-dimensional systems, such as the bond alternating chain, the spin 1-chain or spin ladders, has attracted much interest, both experimental and theoretical. Electron spin resonance (ESR) experiments at very low temperatures provide very high-energy resolution measurements of the spin gap. Tuning an external magnetic field, the field-dependent energy of the excited triplet state is adjusted to the frequency of the electromagnetic propagating wave. A direct transition from the singlet ground state to the gapped triplet state may be observed. Such a direct transition would be, however, forbidden by the conservation law of the total spin quantum number. The ESR direct transition from the ground state to the first excited state has been observed in some recent experiments on gapped spin systems, although such a transition is forbidden because of the conservation law of the spin quantum number. The two possible mechanisms of the direct transition between the singlet and triplet states are considered; the Dzyaloshinsky–Moriya interaction and the staggered field because of the alternating g-tensor. The angle-dependent selection rules are presented for the two mechanisms, respectively. The chapter concludes with a discussion on the mechanism of the singlet-triplet transition observed in the recent experiment on CuGeO3.