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

本研究计画的主要目标为发现新的4d、5d过渡金属氧化物，并阐明其低自旋态所造成的物理性质。此外，为了加深对低自旋体系物理性质的理解，还对相关材料进行了研究。通过高压合成，发现了NaRh_2O_4和CaRhO_3。前者被证明是具有强关联电子的n型金属。Ca取代Na后，材料的金属性逐渐转变为绝缘性。在固溶体系中没有观察到磁性转变。与CaRuO_3相比，Sr替代Ca对CaRhO_3的物理性质没有明显的影响。高压合成法发现了Sr_2CoO_4和高压相LiMn_2O_4等化合物。Sr_2CoO_4具有二维金属中最高的居里温度（255 K），并具有中间自旋态。LiMn_2O_4与NaRh_2O_4同构，在10 K左右发生自旋玻璃转变。此外，还在高压下制备了超导的eta-Mo_3C_2，γ-Mo_2N和NbN，测定了一些超导参数，研究了低自旋系统Nax（H_3O）zCoO_2yH_2O的超导机制。通过控制Co价态和Na含量，建立了超导相图，并据此提出了活性带。此外，磁有序相被发现旁边的超导相。核四极共振测量强烈地表明，超导性是由与磁相有关的量子临界引起的感应磁涨落。

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.