Singlet-triplet transitions of ESR in gapped spin systems

Sakai, Toru; Hijii, Keigo*; Okubo, Susumu*; Ota, Hitoshi*

The spin gap of the low-dimensional quantum spin systems and the strongly correlated electron systems is one of the interesting macroscopic quantum phenomena. Since the spin gap is the energy gap between the singlet ground state and the triplet excitation, the ESR direct transition is prohibited because of the spin conservation law. Thus the properties of the spin gap systems has been observed by some ESR transitions among the triplet excitations. Recently, however, the ESR direct transition corresponding to the spin gap was observed in several gapped spin systems. The mechanism of appearance of these prohibited ESR transition are supposed to be the Dzyaloshinsky-Moriya interaction and/or the staggered g-tensor. In order to identify the origin of the ESR direct transition, we gave some selection rules depending on the relative angle among the crystal axes of each material, the polarization of the microwave and the direction of external magnetic field.