Pressure induced transition from Mott insulator to Q2D metal with ferromagnetic ground states

在强相关电子系统中，特别是在流动铁磁性系统中，在磁序边界上经常发现非常规超导现象。然而，据我们所知，基于具有FM基态的Molt转换系统的SC的报道很少。Ca_2RuO_4是一种具有反铁磁基态的Molt绝缘体，其U/W比Sr_2RuO_4大。在本项目中，我们发现Ca_2RuO_4在P ~ 0.5 GPa时具有金属-绝缘体的转变：在300 K时，从顺磁绝缘体到顺磁准二维金属；在T大于或等于12K时，从AF绝缘子到FM各向异性金属。在P≥1GPa时，沿a、b、c轴的残余磁化率分别为~ 0.4、~ 0.3、~ 0.2μ_B。因此，a轴为易磁化方向，c轴为硬磁化方向。与局域化的Ru^<4+>离子的饱和矩2μ_B相比，残余磁化较小，这表明存在矩流动FM。T_c随P的增加而升高，在~ 5GPa时达到25K，然后逐渐降低。外推Tc和表明在pbbb10gpa处诱导FM量子临界点。因此，我们对p波SC Sr_2RuO_4和金属Ca_2RuO_4的FM之间的联系产生了浓厚的兴趣。此外，将Q2D金属中流动FM的性质与理论预测进行比较也很有趣。受压Ca_2RuO_4中，P_<ab>(T)表现为金属导电，而P_c(T)表现为非金属导电。与我们最近在SRO中对Shubnikov-de Haas效应的研究类比表明，在Ru214系统中，费米表面片在加压下变得更二维。也就是说，P诱导了α和γ片中的双费米表面翘曲的减少。我们推断c轴传导主要由隧穿机制描述。

Unconventional superconductivity (SC) has been frequently found on the border of magnetic order in strongly correlated electron systems, especially itinerant ferromagnetism (FM). However, there is few report on SC based on a Molt transition system with a FM ground state to our knowledge. Ca_2RuO_4, which has a larger U/W than Sr_2RuO_4, is a Molt insulator with an antiferromagnetic (AF) ground state. In this project, we found Ca_2RuO_4 features a metal-insulator transition at P〜0.5 GPa : at 300 K from paramagnetic insulator to paramagnetic quasi-2D metal ; at T【greater than or equal】12K from AF insulator to FM anisotropic metal. We obtain the remnant magnetisations along the a, b, and c axis are〜0.4,〜0.3,〜0.2μ_B at P【greater than or equal】1GPa, respectively. Thus, the a axis is easy direction of magnetisation and the c axis is hard direction. The small remnant magnetisation compared with the saturated moment 2μ_B of localized Ru^<4+> ion implies -moment itinerant FM. The T_c rises with increasing P reaching 25K at〜5GPa, then it decreases gradually. Extrapolation of Tc and suggests the induction of a FM quantum critical point at P>10GPa. We thus have strong interest in the connection between the p-wave SC Sr_2RuO_4, and the FM of metallic Ca_2RuO_4.Moreover, it is also interesting to compare to the properties of the itinerant FM in Q2D metal with theoretical predictions. In pressurised Ca_2RuO_4, P_<ab>(T) shows metallic conduction whereas P_c(T) exhibits a nonmetallic one. This Q2D nature is enhanced by P. Analogy with our recent P study of Shubnikov-de Haas effect in SRO suggests that the Fermi surface sheet becomes more 2D in Ru214 system with pressurising. Namely, P induces a reduction of a double Fermi-surface-warping in both the α and γ sheets. We infer that the c-axis conduction is mainly described by a tunnelling mechanism as in HTC.