FIELD-INDUCED BOSE-EINSTEIN CONDENSATION AND LOCALIZATION OF MAGNONS

我们研制的S=1/2自旋二聚体体系如T1CuCl_3和KCuCl_3在强磁场下发生了新颖的量子相变。这些系统通常具有带激发能隙的单重基态。在强磁场中，能量随磁场减小的三重态之一起着重要作用。由于二聚体间交换相互作用的横向和纵向分量，三重态可以作为准粒子跳跃到相邻位点并相互作用。这种准粒子具有玻色子的特征，称为磁振子。因此，该系统可以被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，场诱导反铁磁（AF）有序化发生。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中的场致AF有序进行了各种测量，包括磁化强度、比热、中子散射、热膨胀系数和热膨胀系数。 关于我们 溅射超声衰减、导热系数、热膨胀测量。结合理论分析，我们验证了TlCuCl中1场诱导的AF有序是磁振子的BEC。例如相界的实验临界指数（1152）与理论值φ=3/2一致，基于磁振子BEC的理论计算几乎完全再现了实验相界。通过磁化强度和中子散射测量，我们观察到自旋能隙体系T1CuCl_3和KCuCl_3在流体静压下发生了向反铁磁态的相变。这是第一个压力诱导的从带隙态到反铁磁态的磁量子相变的例子。在Cs_2_CuBr_4中，在饱和磁化强度的1/3处观察到一个三角形反铁磁体的磁化平台。我们研制了许多分子磁体，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。在BIPTENO中，我们观察到磁化平台在四分之一的饱和磁化强度。这个平台是通过破坏平移对称性来实现的。我们观察了一维费米子系统Tomonaga-Luttinger液体的磁性特征。发展了一维反铁磁体电子自旋共振的一般理论。少

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