Searching for new 4d, 5d transition metal oxides by high-pressure synthesis and investigation of their physical properties

本课题的主要目标是发现新的4d、5d过渡金属氧化物，并阐明其低自旋态引起的物理性质。此外，还旨在研究相关材料，以加深对低自旋系统物理的认识。高压合成得到了NaRh_2O_4和CaRhO_3。前者被证明是具有强相关电子的n型金属。通过Ca取代Na，金属行为逐渐转变为绝缘行为。固溶体系中未观察到磁跃迁。另一方面，CaRhO_3的物理性质很难因Sr取代Ca而改变，这与CaRuO_3形成鲜明对比。用高压合成方法发现了许多化合物，如Sr_2CoO_4和高压相LiMn_2O_4。Sr_2CoO_4在二维金属中居里温度最高，为255 K，具有中间自旋态。LiMn_2O_4与NaRh_2O_4同构，在10k左右发生自旋玻璃化转变。此外，在高压下制备了超导体eta-Mo_3C_2、gamma-Mo_2N和NbN，测定了超导参数。研究了低自旋体系Nax(H_3O)zCoO_2 yH_2O的超导机理。通过对Co价和Na含量的独立控制，建立了超导相图，并据此确定了活性谱带。此外，在超导相旁边还发现了磁有序相。核四极共振测量有力地表明，超导性是由与磁相相关的量子临界性引起的磁波动引起的。

The target of this research project is mainly on discovering new 4d, 5d transition metal oxides and on elucidation of their physical properties caused by low-spin states. In addition, it is also aimed to study related materials in order to deepen understanding of the physics of low-spin system.NaRh_2O_4 and CaRhO_3 were found by high-pressure synthesis. The former was proved to be an n-type metal with strongly correlated electrons. The metallic behavior is gradually changed into insulating one by substitution of Ca for Na. No magnetic transition was observed in the solid-solution system. On the other hand, physical properties of CaRhO_3 are hard to change by substitution of Sr for Ca, which is in sharp contrast to CaRuO_3.Many compounds, such as Sr_2CoO_4 and high-pressure phase LiMn_2O_4, were discovered by high-pressure synthesis. Sr_2CoO_4 has the highest Curie temperature of 255 K among two-dimensional metals, and was found to have intermediate spin state. LiMn_2O_4, which is isostructural with NaRh_2O_4, has spin-glass transition around 10 K. In addition, superconducting eta-Mo_3C_2, gamma-Mo_2N and NbN were made under high pressures, to determine some superconducting parameters.Superconducting mechanism of low-spin system Nax(H_3O)zCoO_2 yH_2O was investigated. Co valence and Na content were independently controlled to establish the superconducting phase diagram, by which active bands were suggested. Furthermore, magnetically ordered phase was found next to the superconducting phase. Nuclear quadrupole resonance measurements strongly suggest that the superconductivity is induced magnetic fluctuations caused by quantum criticality related to the magnetic phase.