Metal-insulator transition and novel magnetism driven by Coulomb interactions, spin-orbit coupling and disorder

Strong interactions in transition metal oxides can lead to spectacular phenomena like high Tc superconductivity and colossal magnetoresistance which have dominated materials research in the past decades. The goal of this thesis is to examine the interplay of strong correlations with additional degrees of freedom, like spin orbit coupling (SOC), multiple transition metal ions and disorder. We start with a discussion of Mott insulators, with the transition metal in the d4 configuration, where the competition between superexchange interaction and SOC leads to a novel ferromagnetic insulator. In recent years SOC has been at the center stage of condensed matter research because it can produce band insulators in uncorrelated materials with non-trivial topological properties. Here, we focus on the dual role of SOC and strong interactions, naturally realized in 4d/5d transition metal oxides. We show that in d4 Mott insulators, the local moment can be altered by varying the relative strength of SOC and superexchange, both of which are small parameters compared to the interaction energy scale. In fact, a phase transition from a non-magnetic insulator with J = 0 singlets at every site to an orbitally entangled ferromagnetic insulator occurs with decreasing SOC. Our results challenge the commonly held notion that local moments are robust in a Mott insulator. We identify candidate materials and present predictions for Resonant X-ray Scattering (RXS) signatures of the unusual magnetism in d4 Mott insulators. Next we focus on the double perovskite material Sr2CrOsO6 which is an insulator and has the highest ferromagnetic Tc among all perovskites with a net moment. It presents several puzzles which we address systematically. Its insulating behavior cannot be explained from a band theory point of view or from a naive consideration of Mott physics. Additionii ally, the net moment at low temperature, M(0)=0.75 μB, is unusual for half-filled bands where anti-ferromagnetism is expected, and the non-monotonic temperature dependence of magnetization is quite different from typical magnetic materials. We present a complete theory which resolves all the puzzles by organizing the problem into a hierarchy of energy scales. In the charge sector, where the large onsite interactions operate, we develop a new Mott criterion for double perovskites using slave-rotor mean field theory, and show that Sr2CrOsO6 is indeed a Mott insulator. Then, we discuss an effective low energy magnetic Hamiltonian which naturally explains the unusual magnetic properties of Sr2CrOsO6 in terms of magnetic frustration. Finally, we focus on the nature of the phases and an unexpected disorder-driven Mott insulator to metal transition. In this theory and experiment collaboration, we study a Mott insulator realized in a single crystal of the layered dichalcogenide 1T-TaS2 that is disordered without changing the carrier concentration by Cu intercalation. Angle resolved photoemission spectroscopy (ARPES) measurements reveal that increasing disorder introduces delocalized states within the Mott gap that lead to a finite conductivity, challenging conventional wisdom. The emergent metal also exhibits non-Fermi liquid behavior with a pseudogap in the density of states that persists at finite temperatures. We present detailed theoretical analysis of the two-dimensional disordered Hubbard model which shows that the novel metal is generated by the interplay of strong interaction and disorder.