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Register a new userObservation of Kondo hybridization with an orbital-selective Mott phase in 4d Ca2-xSrxRuO4
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The heavy fermion state with Kondo-hybridization (KH), usually manifested in f-electron systems with lanthanide or actinide elements, was recently discovered in several 3d transition metal compounds without f-electrons. However, KH has not yet been observed in 4d/5d transition metal compounds, since more extended 4d/5d orbitals do not usually form flat bands that supply localized electrons appropriate for Kondo pairing. Here, we report a doping- and temperature-dependent angle-resolved photoemission study on 4d Ca2-xSrxRuO4, which shows the signature of KH. We observed a spectral weight transfer in the {gamma}-band, reminiscent of an orbital-selective Mott phase (OSMP). The Mott localized {gamma}-band induces KH with the itinerant b{eta}-band, resulting in spectral weight suppression around the Fermi level. Our work is the first to demonstrate the evolution of the OSMP with possible KH among 4d electrons, and thereby expands the material boundary of Kondo physics to 4d multi-orbital systems.
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Iron-based superconductors display a variety of magnetic phases originating in the competition between electronic, orbital, and spin degrees of freedom. Previous theoretical investigations of the multi-orbital Hubbard model in one dimension revealed the existence of an orbital-selective Mott phase (OSMP) with block spin order. Recent inelastic neutron scattering (INS) experiments on the BaFe$_2$Se$_3$ ladder compound confirmed the relevance of the block-OSMP. Moreover, the powder INS spectrum reveled an unexpected structure, containing both low-energy acoustic and high-energy optical modes. Here we present the theoretical prediction for the dynamical spin structure factor within a block-OSMP regime using the density-matrix renormalization group method. In agreement with experiments we find two dominant features: low-energy dispersive and high-energy dispersionless modes. We argue that the former represents the spin-wave-like dynamics of the block ferromagnetic islands, while the latter is attributed to a novel type of local on-site spin excitations controlled by the Hund coupling.
We study the phase transition in Cu-substituted iron-based superconductors with a new developed real-space Greens function method. We find that Cu substitution has strong effect on the orbital-selective Mott transition introduced by the Hunds rule coupling. The redistribution of the orbital occupancy which is caused by the increase of the Hunds rule coupling, gives rise to the Mott-Hubbard metal-insulator transition in the half-filled $d_{xy}$ orbital. We also find that more and more electronic states appear inside that Mott gap of the $d_{xy}$ orbital with the increase of Cu substitution, and the in-gap states around the Fermi level are strongly localized at some specific lattice sites. Further, a distinctive phase diagram, obtained for the Cu-substituted Fe-based superconductors, displays an orbital-selective insulating phase, as a result of the cooperative effect of the Hunds rule coupling and the impurity-induced disorder.
We report a quantum phase transition between orbital-selective Mott states, with different localized orbitals, in a Hunds metals model. Using the density matrix renormalization group, the phase diagram is constructed varying the electronic density and Hubbard $U$, at robust Hunds coupling. We demonstrate that this transition is preempted by charge fluctuations and the emergence of free spinless fermions, as opposed to the magnetically-driven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strong-coupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediate-coupling phase.
Electrons in a simple correlated system behave either as itinerant charge carriers or as localized moments. However, there is growing evidence for the coexistence of itinerant electrons and local moments in transition metals with nearly degenerate $d$-orbitals. It demands one or more selective electron orbitals undergo the Mott transition while the others remain itinerant. Here we report the first observation of such an orbital selective Mott transition (OSMT) in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ by angle-resolved photoemission spectroscopy (ARPES). While we observed two sets of dispersing bands and Fermi surface associated with the doubly-degenerate $d_{yz}$ and $d_{zx}$ orbitals, the Fermi surface associated with the wider $d_{xy}$ band is missing, a consequence of selective Mott localization. Our theoretical calculations demonstrate that this novel OSMT is mainly driven by the combined effects of interorbital carrier transfer, superlattice potential, and orbital degeneracy, whereas the bandwidth difference plays a less important role.
We study non-local correlations in a three-orbital Hubbard model defined on an extended one-dimensional chain using determinant quantum Monte Carlo and density matrix renormalization group methods. We focus on a parameter with robust Hunds coupling, which produces an orbital selective Mott phase (OSMP) at intermediate values of the Hubbard U, as well as an orbitally ordered ferromagnetic insulating state at stronger coupling. An examination of the orbital and spin-correlation functions indicates that the orbital ordering occurs before the onset of magnetic correlations in this parameter regime as a function of temperature. In the OSMP, we find that the self-energy for the itinerant electrons is momentum dependent, indicating a degree of non-local correlations while the localized electrons have largely momentum independent self-energies. These non-local correlations also produce relative shifts of the hole-like and electron-like bands within our model. The overall momentum dependence of these quantities is strongly suppressed in the orbitally-ordered insulating phase.
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