FIELD-INDUCED BOSE-EINSTEIN CONDENSATION AND LOCALIZATION OF MAGNONS

我们开发的耦合S=1/2自旋二聚体系统，例如T1CuCl_3和KCuCl_3，经历了新颖的量子相变高磁场。这些系统通常具有带有激发间隙的单线态基态。在强磁场中，能量随磁场减小的三重态之一起着重要作用。三联体可以作为准粒子跳跃到相邻位点，并由于二聚体交换相互作用的横向和纵向分量而相互作用。准粒子具有玻色子的特性，称为磁子。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。当磁振子的跳跃占主导地位时，就会发生场感应反铁磁（AF）排序。这种排序可以描述为磁振子的玻色-爱因斯坦凝聚（BEC）。我们对 TlCuCl 中的场诱导 AF 排序进行了各种测量，包括磁化强度、比热、中子散射……更多散射超声衰减、热导率、热膨胀测量。结合理论分析，我们验证了 TlCuCl 中 1 场诱导的 AF 有序化是磁振子的 BEC。例如，相界的实验临界指数（1152）与理论φ=3/2一致，基于磁振子BEC的理论计算几乎完美地再现了实验相界。通过磁化强度和中子散射测量，我们观察到自旋间隙体系T1CuCl_3和KCuCl_3在静水压力下发生相变到反铁磁态。这是压力诱导的磁量子相变从带隙态到反铁磁态的第一个例子。我们在 Cs2_CuBr_4 的饱和磁化强度的三分之一处观察到磁化平台，该平台被描述为三角形反铁磁体。该平台归因于自旋挫败和量子涨落之间的相互作用。我们开发了许多分子磁体示例，包括 S=1 和 1/2 混合自旋系统 BIPNNBNO、S=1 蜂窝晶格反铁磁体 F_2PNNNO、S=1 自旋梯系统 BIPTENO 和 S=1/2 交替反铁磁链。 F_5PNN。我们在四分之一的饱和磁化强度处观察到了磁化平台，我们观察到了一维反铁磁体的 Tomonaga-Luttinger 液体的磁特性。

Coupled S=1/2 spin dimer systems such as T1CuCl_3 and KCuCl_3 which we developed undergo novel quantum phase transitions 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. Consequently, the system can be described as interaction boson system, where magnetic field corresponds to the chemical potential. 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 perform various measurements on the field-induced AF ordering in TlCuCl, including magnetization, specific heat, neutron sca … More ttering ultrasonic attenuation, heat-conductivity, thermal expansion measurements. Together with theoretical analysis, we verify that 1 field-induced AF ordering in TlCuCl, is the BEC of magnons. Examples include that experimental critical exponent (1152 for the phase boundary agrees with theoretical φ=3/2, and that theoretical calculation based on the magnon BEC reproduces almost perfectly experimental phase boundary. By means of magnetization and neutron scattering measurements, we observed that spin gap systems T1CuCl_3 and KCuCl_3 undergo phase transition to antiferromagnetic state in hydrostatic pressure. This is the first example the pressure-induced magnetic quantum phase transition from the gapped state to the antiferromagnetic state. We observed magnetization plateau at one-third of the saturation magnetization in Cs2_CuBr_4 that is described as triangular antiferromagnet This plateau is attributed to interplay between spin frustration and quantum fluctuation. We developed many molecular magnet Examples include S=1 and 1/2 mixed spin system BIPNNBNO, S=1 honeycomb lattice antiferromagnet F_2PNNNO, S=1 spin ladder system BIPTENO and S=1/2 alternating antiferromagnetic chain F_5PNN. In BIPTENO, we observed magnetization plateau at one-quarter of the saturation magnetization. This plateau is realized by breaking translational symmetry. We observed the magnetic properties characteristic of the Tomonaga-Luttinger liquid for one-dimensional fermion system. We developed the general theory for electron spin resonance for one-dimensional antiferromagnet. Less