We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about two, due to electronic correlations. The self-energies for the β and γ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects, but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies, and provides strong evidence for an electronic origin of the observed non-linear frequency dependence of the self-energies, leading to 'kinks' in the quasiparticle dispersion of Sr2RuO4. * present address: ISIS Facility, Rutherford

We show that electron correlations lead to a bad metallic state in chalcogenides FeSe and FeTe despite the intermediate value of the Hubbard repulsion U and Hund's rule coupling J. The evolution of the quasi particle weight Z as a function of the interaction terms reveals a clear crossover at U ≃ 2.5 eV. In the weak coupling limit Z decreases for all correlated d orbitals as a function of U and beyond the crossover coupling they become weakly dependent on U while strongly depend on J. A marked orbital dependence of the Z's emerges even if in general the orbital-selective Mott transition only occurs for relatively large values of U . This two-stage reduction of the quasi particle coherence due to the combined effect of Hubbard U and the Hund's J, suggests that the iron-based superconductors can be referred to as Hund's correlated metals. The role of electron correlations in the iron-based superconductors is still a debated issue, naturally intertwined with the search for the origin of high critical temperatures. We present results that improve the qualitative understanding of how electron correlation influences fundamental electron properties of these compounds, such as the metallicity, which in turn might be important also for the understanding of the pairing mechanism. We choose two candidates of the chalogenides, FeSe and FeTe and employ f irst principles electron structure calculations combined with advanced many-body methods taking into account the local electron correlation. The chalcognides have in contrast to the pnictides a simpler atomic structure, thus easier to synthesize and also to study theoretically. In addition they are non toxic in contrast to the pnictides containing arsenic.In previously known superconductors we can identify either weakly correlated materials, like elemental superconductors or binary alloys, including MgB 2 , or highlycorrelated compound like the copper oxides and heavy fermion materials. In the first set of compounds superconductivity is explained within the Bardeen-CooperSchrieffer framework and its extensions, and it occurs as a pairing instability of a normal metal. In the second set it is widely believed that correlations revolutionize the electronic properties and that both the metallic state and the pairing mechanism deviate from standard paradigms.The iron-based pnictides and chalcognides superconductors do not fit this simple classification. The common labeling "intermediate correlation", referring to properties such as Fermi surface topology or absence of Hubbard bands [1], suggests modest effects of correlations. Conversely, the metallic state appears much less coherent than what these observations imply [2,3]. A magnetic counterpart of this dualism is the localized an itinerant nature of the spin-density-wave state of the parent compound.The characteristic property of the band structure is that several of the five d-bands cross the Fermi level. The multi-orbital nature leads to several exotic electronic properties such as orbital-selectivity [4][5][6][7][8][9] and ...

We present a self-consistent numerical approach to solve the Gutzwiller variational problem for general multi-band models with arbitrary on-site interaction. The proposed method generalizes and improves the procedure derived by Deng et al., Phys. Rev. B. 79 075114 (2009), overcoming the restriction to density-density interaction without increasing the complexity of the computational algorithm. Our approach drastically reduces the problem of the high-dimensional Gutzwiller minimization by mapping it to a minimization only in the variational density matrix, in the spirit of the Levy and Lieb formulation of DFT. For fixed density the Gutzwiller renormalization matrix is determined as a fixpoint of a proper functional, whose evaluation only requires ground-state calculations of matrices defined in the Gutzwiller variational space. Furthermore, the proposed method is able to account for the symmetries of the variational function in a controlled way, reducing the number of variational parameters. After a detailed description of the method we present calculations for multi-band Hubbard models with full (rotationally invariant) Hund's rule on-site interaction. Our analysis shows that the numerical algorithm is very efficient, stable and easy to implement. For these reasons this method is particularly suitable for first principle studies -- e.g., in combination with DFT -- of many complex real materials, where the full intra-atomic interaction is important to obtain correct results.Comment: 19 pages, 7 figure

Strongly correlated systems exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing non-equilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T -TaS2, we identify a spectroscopic signature of double occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on time scales of one electronic hopping cycleh/J ≈ 14 fs. Despite strong electron-phonon coupling the dynamics can be explained by purely electronic effects captured by the single band Hubbard model, where thermalization is fast in the small-gap regime. Qualitative agreement with the experimental results however requires the assumption of an intrinsic hole-doping. The sensitivity of the doublon dynamics on the doping level provides a way to control ultrafast processes in such strongly correlated materials.

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