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

自旋间隙对应的电子自旋共振是低维磁体中最有趣的量子现象之一，但由于自旋守恒定律而无法观测到。然而，最近在自旋Pejeris材料CeGeO_3和Hakdane材料NENP的一些电子自旋共振测量中观察到自旋间隙的直接跃迁。在本研究中，揭示并明确了这些材料的直接转变机理。作为直接跃迁的起源，我们提出了由交替g张量和Dzyaloshinski-Moriya相互作用诱导的有效交错场。为了明确是哪一种，我们引入了电子自旋共振的选择规则，该选择规则取决于入射微波的外场、晶体轴和rf场之间的相对角度。将选择规则与实验结果进行比较，我们得出直接跃迁的起源应该是NENP的有效交错场，而Dzyaloshinski-Moriya相互作用和CuGeO_3的交错场都是直接跃迁的起源。此外，我们还研究了低维量子系统如自旋间隙系统的一些性质。如自旋阶梯中反铁磁相互作用受挫引起的场致自旋间隙、自旋链中场致自旋液体、高tc铜超导体中自旋涨落引起的赝间隙和多自旋交换相互作用引起的电荷条纹等。

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.