Theoretical study on direct observation of electron spin resonance for spin gap

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The electron spin resonance corresponding to the spin gap, which is one of the most interesting quantum phenomena in low-dimensional magnets, cannot be observed because of the spin conservation law. Recently, however, the direct transiti on of the spin gap was observed in some electron spin resonance measurements for the spin Pejeris material CeGeO_3 and Hakdane material NENP. In the present study, the mechanism of the direct transition was revealed and specified for these materials.As the origin of the direct transition, we proposed the effective staggered field induced by the alternating g-tensor and the Dzyaloshinski-Moriya interaction. In order to specify which of them, we introduced a selection rule of the electron spin resonance depending on the relative angle among the external field, crystal axes and rf-field of the incident micro wave : Comparing the selection rule and the experimental results, we concluded that the origin of the direct transition should be the effective staggered field for NENP, while both of the Dzyaloshinski-Moriya interaction and the staggered fieid for CuGeO_3.In addition we investigated some properties of the low-dimensional quantum systems like the spin gap system. For example, the field-induced spin gap due to the frustration of the antiferromagnetic interaction in the spin ladder, the field-induced spin liquid in the spin chain, the pseudogap induced by the spin fluctuation and the charge stripes due to the multispin exchange interaction in the high-Tc cuprate superconductors etc.