Novel electronic states in new layered pnictides, Sr2VO3FeAs and SrMnBi2

Jun Sung Kim

 Since the discovery of high-Tc superconductivity in Fe-pnictides, layered pnictides have attracted a lot of attention as a new platform for inducing various novel electronic states. Here we report on two layered pnictides showing intriguing electronic states: (1) Sr2VO3FeAs, a hybrid system of Mott-insulating oxides with Fe-pnictides, and (2) SrMnBi2, a new Dirac material with Bi square nets. First, we discuss Sr2VO3FeAs consisting of a SrVO3 layer and a FeAs layer, which exhibits superconductivity at Tc of 25 K. By measuring transport properties of single crystals, we found a significant anisotropy in the superconducting properties, suggesting insulating nature of the inserted SrVO3 layer. In the normal state, anomalous negative magnetoresistance is observed with out-of-plane magnetic fields, which is reduced until T ~ 150 K, indicating magnetic ordering in the SrVO3 layer. These findings suggest that the inserted SrVO3 layers are, in fact, Mott-insulating, in contrast to the LDA predictions, due to the electron correlation of V-states. Secondly, we report a discovery of another novel electronic state with relativistic Dirac fermions in a layered Mn-pnictide SrMnBi2, consisting of a Bi square net stacked with MnBi layers. Based on first principle calculations, angle resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals, we show that the electronic properties of SrMnBi2 are determined by Dirac fermions residing in a double-sized Bi square net, while the antiferromagnetic order is developed in the MnBi layer. In contrast to the commonly observed isotropic Dirac cone, as found in e.g. graphene and topological insulators, the Dirac cone in SrMnBi2 is highly anisotropic with a large momentum-dependent disparity of Fermi velocities. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provide a new platform that hosts highly anisotropic Dirac fermions.