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Register a new userNature of the Doping-Driven Metal-Insulator Transition in Correlated Electron Systems with strong Hunds Exchange Coupling
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We study the doping-driven Mott metal-insulator transition for multi-orbital Hubbard models with Hunds exchange coupling at finite temperatures. As in the single-orbital Hubbard model, the transition is of first-order within dynamical mean field theory, with a coexistence region where two solutions can be stabilized. We find, that in the presence of finite Hunds coupling, the insulating phase is connected to a badly metallic phase, which extends to surprisingly large dopings. While fractional power-law behavior of the self-energies on the Matsubara axis is found on both sides of the transition, a regime with frozen local moments develops only on the branch connected to the insulating phase.
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Characterizing non-local magnetic fluctuations in materials with strong electronic Coulomb interactions remains one of the major outstanding challenges of modern condensed matter theory. In this work we address the spatial symmetry and orbital structure of magnetic fluctuations in perovskite materials. To this aim, we develop a consistent multi-orbital diagrammatic extension of dynamical mean field theory, which we apply to an anisotropic three orbital model of cubic $t_{2g}$ symmetry. We find that the form of spatial spin fluctuations is governed by the local Hunds coupling. For small values of the coupling, magnetic fluctuations are anisotropic in orbital space, which reflects the symmetry of the considered $t_{2g}$ model. Large Hunds coupling enhances collective spin excitations, which mixes orbital and spatial degrees of freedom, and magnetic fluctuations become orbitally isotropic. Remarkably, this effect can be seen only in two-particle quantities; single-particle observables remain anisotropic for any value of the Hunds coupling. Importantly, we find that the orbital isotropy can be induced both, at half-filling and for the case of 4/3 electrons per orbital, where the magnetic instability is associated with different, antiferromagnetic and ferromagnetic modes, respectively.
We point out the generic competition between the Hunds coupling and the spin-orbit coupling in correlated materials, and this competition leads to an electronic dilemma between the Hunds metal and the relativistic insulators. Hunds metals refer to the fate of the would-be insulators where the Hunds coupling suppresses the correlation and drives the systems into correlated metals. Relativistic Mott insulators refer to the fate of the would-be metals where the relativistic spin-orbit coupling enhances the correlation and drives the systems into Mott insulators. These contradictory trends are naturally present in many correlated materials. We study the competition between Hunds coupling and spin-orbit coupling in correlated materials and explore the interplay and the balance from these two contradictory trends. The system can become a spin-orbit-coupled Hunds metal or a Hunds assisted relativistic Mott insulator. Our observation could find a broad application and relevance to many correlated materials with multiple orbitals.
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