Control of Interaction Parameters and Quantum Phase Transitions under Extreme Conditions in Spin Gap Systems

自旋能隙系统是一个具有激发能隙的单重基态自旋系统。在本项目中，我们研究了带隙自旋系统在外加磁场、流体静压以及杂质和离子替代引起的无序作用下的量子相变。首先，我们对TlCuCl_3进行了低至77 mK和高至7 T磁场下的磁化强度测量。得到了磁振子的临界密度随温度的变化关系和磁相图。估算了磁振子-磁振子相互作用常数。相边界用幂律表示，与基于Hartree-Fock近似的磁振子BEC理论几乎完全一致，得到了真实的色散关系和相互作用常数。随着拟合温度范围的减小，临界指数收敛到由磁振子BEC理论导出的3/2。这些结果有力地支持了BEC描述的Tl中的场致磁有序 ...更多信息 CuCl_3。我们在静水压力下对KCuCl_3进行了磁化测量和中子散射实验。在临界压力P_c= 8.2kbar时，发生了从带隙态到反铁磁态的压致量子相变。在P_c以上的倒易晶格点上观察到了磁布拉格反射，这些点与零压下最低磁激发的点相当。我们还利用新设计的压力池进行了压力下的中子非弹性散射实验，成功地观察到了三重态激发的软化。这些结果证实了所观察到的压力诱导磁有序是由自旋能隙的闭合引起的。我们对S=1/2的一维海森堡反铁磁体KCuGaF 6进行了磁化率、比热和ESR测量。由于DM与交替的D-矢量和/或交错的g-张量的相互作用，当施加外部磁场时，感应交错的磁场。磁场中的有效磁模型可用量子sine-Gordon（SG）模型表示。发现KCuGaF_6中的大多数实验结果都能用量子SG场理论很好地描述。我们发现了一个新的S=1/2Kagome反铁磁体Rb_2Cu_3SnF_<12>，它具有一个单重基态，自旋带隙，与最近的理论预言一致。少

Spin gap system is a spin system that has a singlet ground state with an excitation gap. In the present project, we studied quantum phase transitions in the gapped spin system induced by external magnetic field, hydrostatic pressure and disorder due to nonmagnetic impurities or ion substitution. First, we performed magnetization measurements on TlCuCl_3 down to 77 mK and in magnetic fields up to 7 T. The critical density of magnons as a function of temperature and the magnetic phase diagram were obtained. The magnon-magnon interaction constant was estimated. The phase boundary is expressed by the power law and agrees almost perfectly with the magnon BEC theory based on the Hartree-Fock approximation with realistic dispersion relations and interaction constant obtained. The critical exponent converges to 3/2 that is derived from the magnon BEC theory with decreasing fitting temperature range. These results strongly support the BEC description of the field-induced magnetic ordering in Tl … More CuCl_3. We performed magnetization measurements and neutron scattering experiments on KCuCl_3 under a hydrostatic pressure. A pressure-induced quantum phase transition from a gapped state to an antiferromagnetic state occurs at the critical pressure P_c=8.2 kbar. Above P_c magnetic Bragg reflections were observed at reciprocal lattice points that are equivalent to those for the lowest magnetic excitation at zero pressure. We also performed neutron inelastic scattering under pressure using newly designed pressure cell, and succeeded in observing the softening of triplet excitations. These results confirm that the pressure-induced magnetic ordering observed arises from the closing of the spin gap. We performed magnetic susceptibility, specific heat and ESR measurements on the S=1/2 one-dimensional Heisenberg antiferromagnet KCuGaF6 with the large exchange interaction. Due to the DM interaction with alternating D-vectors and/or the staggered g-tensor, the staggered magnetic field is induced when external magnetic field is applied. The effective magnetic model in magnetic field can be expressed by the quantum sine-Gordon (SG) model. It was found that most experimental results in KCuGaF_6 were beautifully described by the quantum SG field theory. We have found a new S=1/2 Kagome antiferromagnet Rb_2Cu_3SnF_<12>, which has a singlet ground state with the spin gap, as predicted from a recent theory. Less