Abstract
The direct electron spin resonance absorption between a singlet ground state and the triplet excited states has been observed in some recent experiments on gapped spin systems, although the transition is forbidden because of the spin conservation law. As possible mechanisms to bring about such a transition, we investigate the effective staggered field due to the alternating g-tensor and the Dzyaloshinskii–Moriya interaction, which have some off-diagonal matrix elements between the singlet and triplet states. The selection rules of the direct transition, which depend on the relative angles among the external field, the polarization of the insident submillimeter wave and the crystal axies, were presented for the two mechanisms in our previous works. We review the selection rules and apply them for the recent experiments on the Haldane chain Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ), abreviated NENP, the spin Peierls compound CuGeO 3 and the Shastry–Sutherland system SrCu 2 (BO 3 ) 2 , to specify the origin of the di...
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