Pressure induced Novel quantum phenomena in Q2D Mott transition system Ca_2RuO_4

探索非常规超导、巡游磁性和大磁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。单层Ruthenate氧化物中的Mott转变系统引起了人们的广泛关注。这是因为这种材料表现出各种各样的量子现象，从反铁磁(AF)Mott绝缘体到铁磁(FM)金属，以及超导体。特别是，压力相图是非常独特的，不同于掺杂体系的相图。在这个项目中，我们发现了两个有趣的现象：1.莫特相变附近的各向异性巨磁电阻效应我们在压致莫特相变附近观察到了4d电子系统Ca_2RuO_4的各向异性巨磁电阻(MR)。在~2 GPa10T时，纵向磁流变效应达到-80%，低温磁流变效应达到+80%。负MR被解释为在T_c处FM自旋涨落的减小。相反，观察到的大的正效应是罕见的，可能是由于沿c轴施加磁场而引起的金属-非金属相变。准2D金属态的巡游铁磁性我们揭示了对Ca_2RuO_4的加压使其从具有AF基态的Mott绝缘体转变为具有FM基态的准2D金属。这种铁磁性是在金属相中观察到的，然后包含巡回调频的特征。首先，~35μ_B比定域Ru^&lt；4+&gt；离子的2μ_B的饱和力矩小得多。第二，局域自旋系统的p&lt；ef&gt；/M&lt；rem&gt；~5比1大得多。最后，M不容易与H饱和，特别是低温下的磁化过程在Arrott图(M_&lt；rem&gt；^2与H/M图)中表现出良好的线性。这些调频特性可与典型的MNSI巡回铁磁体相媲美。

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.