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

由于自旋守恒定律，低维磁体中最有趣的量子现象之一--自旋能隙对应的电子自旋共振是无法观测到的。然而，最近在一些自旋PeJeris材料CeGeO_3和Hakdane材料NENP的电子自旋共振测量中观察到了自旋能隙的直接穿越。在本研究中，揭示并明确了这些材料的直接相变机制。作为直接相变的起源，我们提出了由交替的G张量和Dzyaloshinski-Moriya相互作用引起的有效交错场。为了确定它们中的哪一个，我们引入了一个依赖于入射微波的外场、晶轴和射频场之间的相对夹角的电子自旋共振选择规则：将选择规则与实验结果进行比较，我们得出结论：对于NENP，直接跃迁的起始点应该是有效的交错场，而对于CuGeO_3，直接跃迁的起源应该是Dzyaloshinski-Moriya相互作用和交错场。此外，我们还研究了低维量子系统的一些性质，如自旋带隙系统。例如，由于自旋阶梯中的反铁磁相互作用受挫而产生的场致自旋间隙、自旋链中的场致自旋液体、自旋涨落引起的赝隙以及高T_c超导体中由于多自旋交换作用而产生的电荷条纹等。

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.