FIELD-INDUCED NEW QUANTUM PHENOMENA IN MAGNETIC SYSTEMS

我们已经开发了许多具有不同晶体结构的量子磁体，并研究了它们在强磁场中的磁性。以下是代表性结果。耦合S=1/2自旋二聚体系统在强磁场中经历了新的量子相变。这些系统通常具有具有激励间隙的单线态基态。在强磁场中，能量随磁场而减小的三重子之一起着重要的作用。由于中间二聚体交换相互作用的横向和纵向分量，三重态可以以准粒子的形式跳跃到邻近的位点并相互作用。这种准粒子具有玻色子的特性，称为磁振子或三介子。因此，该系统可以描述为相互作用玻色子系统，其中磁场对应于化学势。我们发现在SrCu_2(BO_3)_2和其他体系中存在磁化平台。当磁振子的跳变由于自旋受挫或宇称而受到抑制，磁振子（维格纳晶体）的周期排列由于斥力相互作用而稳定时，就会出现磁化平台。这是一种典型的量子多体效应。利用核磁共振在SrCu_2(B0_3)_2上的强磁场，验证了磁振子维格纳晶体的实现。另一方面，磁场诱导的反铁磁（AF）有序发生在以磁振子的跳变为主导的情况下，这种有序可以描述为磁振子的玻色-爱因斯坦凝聚（BEC）。我们对TlCuCl_3的场致AF有序进行了各种测量。结合理论分析，验证了TlCuCl_3中场致AF有序是磁振子的BEC。本研究表明，在某些情况下，量子磁体表现出量子粒子系综的性质特征。这提供了一个新的磁学概念。为了鼓励我们的研究项目，我们组织了三次邀请国外研究人员的公开研讨会和量子自旋系统QSS04国际研讨会。我们还支持了三次国际会议。少

We have developed many quantum magnets, which have various crystal structures, and have investigated their magnetic properties in high magnetic fields. Followings are representative results. Coupled S=1/2 spin dimer systems undergo novel quantum phase transitions in high magnetic fields. These systems often have singlet ground states with excitation gaps. In high magnetic field, one of the triplets whose energy decreases with magnetic field plays an important role. The triplet can hop to neighboring site as a quasi-particle and interact due to the transverse and longitudinal components of the interdimer exchange interaction. The quasi-particle has characteristics of boson and is called magnon or triplon. Consequently, the system can be described as interaction boson system, where magnetic field corresponds to the chemical potential. We found that the magnetization plateaus in SrCu_2(BO_3)_2 and other systems. The magnetization plateau can occur when the hopping of magnons are strongly … More suppressed due to spin frustration or parity and the periodic arrangement of magnons (Wigner crystal) are stabilized due to the repulsive interaction. This is a typical quantum many-body effect By means of NMR in high magnetic fields on SrCu_2(B0_3)_2, we verify the realization of Wigner crystal of magnons. On the other hand, the field-induced antiferromagnetic (AF) ordering occurs when the hopping of magnons is dominant The ordering can be described as the Bose-Einstein condensation (BEC) of magnons. We have performed various measurements on the field-induced AF ordering in TlCuCl_3. Together with theoretical analysis, we verify that the field-induced AF ordering in TlCuCl_3 is the BEC of magnons. The present study demonstrated that there are cases where the quantum magnets exhibit the nature characteristic of ensemble of quantum particles. This provides a new concept of magnetism.To encourage our research project, we organized three open symposiums inviting researchers from abroad and International Symposium on Quantum Spin Systems QSS04. We also supported three international meetings. Less