Angle-Resolved Photoemission Studies on Ruthenates and Iron-Based Superconductors

Abstract

Angle-resloved photoemission spectroscopy (ARPES) is a powerful technique to study the electronic structure in solids. Its unique ability of resolving the energy and momentum information of electrons inside a solid provides an essential tool in measuring the electronic structure of solids. ARPES has made great contributions in the understanding of correlated system such as high-Tc superconductors and ruthenates. The Metal-insulator transition is a fundamental problem in condensed matter physics. The calcium substituted strontium ruthenate, Ca2-xSrxRuO4, provides a good platform to study the metal-insulator transition in multi-orbital systems. This system has a complex phase diagram that evolves from a p-wave superconductor to a Mott insulator. One of important projects of this thesis focuses on Ca2-xSrxRuO4 The growing evidence for coexistence of itinerant electrons and local moments in transition metals with nearly degenerate d orbitals suggests that one or more electron orbitals undergo a Mott transition while the others remain itinerant. We have observed a novel orbital selective Mott transition (OSMT) in Ca1.8Sr0.2RuO4 by ARPES. While we observed two sets of dispersing bands and Fermi surfaces (FSs) associated with the doubly-degenerate dyz and dzx orbitals, the Fermi surface associated with the dxy orbital which has a wider bandwidth is missing as a consequence of selective Mott localization. Our theoretical calculations have demonstrated that this unusual OSMT is mainly driven by the combined effects of inter-orbital carrier transfer, superlattice potentials and orbital degeneracy, whereas the bandwidth difference plays a less important role. Another important project of this thesis focuses on the recently discovered iron-pnictides superconductors. The idea of inter-FS scattering associated with the near-nesting condition has been proposed to explain the superconductivity in the pnictides. The near-nesting condition varies upon the carrier doping which shifts the chemical potential. We have performed a systematic photoemission study of the chemical potential shift as a function of doping in a pnictide system based on BaFe2As2. The experimentally determined chemical potential shift is consistent with the prediction of a rigid band shift picture by the renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) due to different Fermi velocities for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA from the band structure, which is fully consistent with the experimental phase diagram, strongly supports that inter-FS scattering over the near-nesting Fermi surfaces plays a vital role in the superconductivity of the iron pnictides.