Magnetic Study of Metal-Complex-Based Molecular Conductors and Magnets under Pressure

本研究项目的主要目的是开发在压力下测量静态磁化强度的技术，适用于研究基于金属络合物的分子导体和磁体的电子状态。尽管在分子材料中存在各种显著的压力诱导现象，但迄今为止，压力下的磁性研究尚未受到重视，因为此类化合物显示出约10^<-4>至10^<-3>emu/mol的小磁化率。在这项研究中已经证明，仔细选择用于夹式细长压力室的材料，以最大限度地减少背景贡献的不良影响，可以明显提高压力下测量的磁化强度的精度。这使我们能够测量顺磁磁化率的温度依赖性约为10^<-4>emu/mol，这是令人满意的检测一个典型的分子导体的磁相变。在0.5GPa压力下，对[Pd（dmit）_2]系列盐的磁性进行了研究。对[Pd（dmit）_2]盐的磁性研究，包括常压下的磁性研究，揭示了与三角晶格上量子自旋的阻挫行为有关的各种新的相变，如轨道-二聚体畸变-电荷转移相互作用引起的价态不稳定性，空间各向异性引起的反铁磁相变以及价键有序化。因此，压力的应用有望使人们更好地理解这种行为的物理起源。在EtMe_3P盐（P2_1/m相）中发现了压力诱导的体超导电性：在0.2GPa下T_c = 5 K。通过绝缘体到金属的Mott转变，没有发现磁化率的显著变化，这表明即使在金属状态下电子也是强关联的。

The main purpose of this research project is to develop techniques for measurements of static magnetization under pressure, as being applicable to study of electronic states of molecular conductors and magnets based on metal-complexes. In spite of various remarkable pressure-induced phenomena in molecular materials, magnetic study under pressure has not been appreciated so far, since such compounds exhibit small susceptibility of about 10^<-4> to 10^<-3> emu/mol. It has been demonstrated in this study that careful choice of materials used for the clamp-type slender pressure cell to minimize undesirable effect of the background contributions, can appreciably improves the precision of the magnetization measured under pressure. This enables us to measure the temperature dependence of paramagnetic susceptibility of about 10^<-4> emu/mol, which is satisfactory for the detection of magnetic phase transitions of a typical molecular conductor. This technical development has been applied to the magnetic study of a series of [Pd(dmit)_2] salts under pressure up to about 0.5 GPa. Magnetic studies of the [Pd(dmit)_2] salts, including those under ambient pressure, has revealed variety of novel phase transitions associated with the frustrated behavior of quantum spins on the triangular lattice, such as the valence instability due to orbital-dimer distortion-charge transfer interplay, the antiferromagnetic transitions induced by spatial anisotropy, and the valence bond ordering. Therefore, application of pressure is expected to give more improved understanding of the physical origin of such behavior. Pressure-induced bulk superconductivity has been found in the EtMe_3P salt (P2_1/m phase) : T_c = 5 K at 0.2 GPa. Through the insulator-to-metal Mott transition, no remarkable change in susceptibility has been found, which indicates that the electrons are strongly correlating even in the metallic state.