Pressure induced Novel quantum phenomena in Q2D Mott transition system Ca_2RuO_4

探索非常规超导、巡游磁、大磁电阻效应等新的量子现象是凝聚态物理中最重要和最有趣的课题之一。Mott转变体系在单层碳烯酸盐氧化物中引起了广泛的关注。这是因为这些材料显示出多功能的量子现象，从反铁磁（AF）莫特绝缘体到铁磁（FM）金属以及超导体。特别是，压力相图是相当独特的，不同于掺杂系统。在这个项目中，我们有两个有趣的现象如下：1。Mott相变附近的各向异性巨磁电阻效应我们观察到4d电子系统Ca_2RuO_4在压致Mott相变附近的各向异性巨磁电阻效应。在10 T磁电阻效应下，纵向磁电阻效应在10 T时达到10 T-80%，横向磁电阻效应在低温时达到10 T +80%。负MR被解释为在T_c处FM自旋涨落的减小。相比之下，观察到的大的正效应是罕见的，可能是由于通过沿c轴沿着施加场诱导的金属-非金属转变。准二维金属态的巡回铁磁性我们揭示了加压到Ca_2RuO_4使其从AF基态的Mott绝缘体转变为FM基态的准二维金属。这种铁磁性已被观察到的金属相，然后包含一个巡回FM的功能。首先，M_<rem>（35）μ_B比Ru^（4+）离子的2μ_B饱和磁矩小得多.第二，局域自旋系统的p_<eff>/ M_<rem>∞_5比1大得多。最后，M不容易被H饱和。特别是在低温下的磁化过程在Arrott图（M^2<rem>对H/M图）中表现出良好的线性。这些FM性质与MnSi的典型巡回铁磁体相当。

To explore novel quantum phenomena such as unconventional superconductivity, itinerant magnetism, and large magnetoresistance effect is one of the most important and interesting subjects in condensed matter physics. Much attention has been paid for a Mott transition system in single-layered ruthenate oxides. This is because the materials display versatile quantum phenomena, ranging from an antiferromagnetic (AF) Mott insulator to a ferromagnetic (FM) metal, as well as the superconductor. In particular, the pressure phase diagram is quite unique and different from that of the doped system. In this project, we feature two interesting phenomena as follows :1. Anisotropic giant magnetoresistance effect in the vicinity of Mott transitionWe have observed an anisotropic and giant magnetoresistance (MR) of a 4d-electron system Ca_2RuO_4 in the vicinity of a pressure induced Mott transition. At〜2GPa, the MR effect at 10T reaches〜-80% at〜T_c for longitudinal and 〜+80% at low temperatures for the transverse effects. The negative MR is interpreted as a reduction of a FM spin fluctuation at T_c. In contrast, the observed large positive effect is rare and likely due to metal-nonmetal transition induced by applying fields along the c axis.2. Itinerant ferromagnetism in the quasi-2D metallic stateWe reveal that pressurisation to Ca_2RuO_4 transforms it from a Mott insulator with an AF ground state to a quasi-2D metal with a FM ground state. This ferromagnetism has been observed in the metallic phase, and then contains features of an itinerant FM. Firstly, M_<rem>〜35μ_B is quite smaller than the saturated moment of 2μ_B of localised Ru^<4+> ion. Secondly, a ratio of p_<eff> / M_<rem>〜5 is much larger than 1 of localized spins systems. Lastly, M is not easily saturated with H. In particular, the magnetisation process at low temperatures exhibits good linearity in an Arrott plot (M_<rem>^2 versus H/M plot). These FM natures are comparable to typical itinerant ferromagnet of MnSi.