Stadler, Katharina Maria (2019): A model study of strong correlations in Hund metals: the Numerical Renormalization Group as efficient multiband impurity solver for Dynamical MeanField Theory. Dissertation, LMU München: Faculty of Physics 

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Abstract
For a long time strong electronic correlations in metals have mainly been associated with Mottness, the proximity to a Mott metalinsulator transition (MIT), where large Coulomb interactions induce the localization of charges. However, triggered by the discovery of the ironbased superconductors about ten years ago, it was realized that multiorbital materials with only moderate Coulomb but sizeable Hund’s rule interactions – socalled Hund metals – allow for a distinct screening mechanism towards strong correlations: Hundness. Here, Hund’s rule constrains the spin rather than the charge dynamics. This discovery led to a vividly debated fundamental issue in the field of strongly correlated condensed matter systems, which is the main topic of the present thesis: what is the origin of strong correlations in the normal phase of Hund metals, Mottness or Hundness? And what are their decisive fingerprints? The goal of this dissertation is twofold. First, we present and advance our method: the numerical renormalization group (NRG) as viable realfrequency multiband impurity solver for dynamical meanfield theory (DMFT), a common approach to tackle strongly correlated systems. Second, we apply DMFT+NRG to shed light on the Hundmetal problem raised above. In the first part of this thesis we present our stateoftheart NRG solver, which offers direct access to data with unprecedented realfrequency spectral resolution at arbitrarily low energies and temperatures in contrast to commonly used Quantum Monte Carlo solvers. It is based on matrix product states and exploits nonabelian symmetries to reduce numerical costs. In the case of orbital symmetry, this allows us to treat multiband models with more than two bands, and thus to tackle the Hundmetal problem for the first time with NRG. For multiband models without orbital symmetry, an “interleaved” scheme of NRG (iNRG) was recently developed, dramatically increasing the numerical efficiency. Remarkably, the accuracy of iNRG is comparable to standard NRG, as we reveal in a detailed study. This finding establishes iNRG as a promising DMFT solver for materialspecific model simulations. In the second part of this thesis we study a minimal toy model for Hund metals with DMFT+NRG, the orbitalsymmetric threeband HubbardHund model (3HHM) close to a lattice filling of 1/3. Our major insight is “spinorbital separation” (SOS), a Hund’sruleinduced twostage Kondotype screening process, in which orbital screening occurs at much higher energies than spin screening. In Hund metals, i.e. far from a MIT phase boundary, SOS thus causes large electron masses by strongly reducing the coherence scale below which a Fermi liquid is formed. Further, it opens up a broad incoherent and strongly particlehole asymmetric intermediate energy regime that reaches up to bare excitation scales. This SOS regime shows fractional powerlaw behavior and is characterized by resilient “Hund quasiparticles” with itinerant orbital degrees of freedom coupled nontrivially to quasifree large spins. At zero temperature, the local density of states exhibits a twotier quasiparticle peak on top of a broad incoherent background. In contrast, in Mottcorrelated metals, i.e. close to the MIT phase boundary, the SOS regime becomes negligibly small and the Hubbard bands are well separated. These findings lead to distinct signatures of Hundness and Mottness in the temperature dependence of ARPES spectra, static local susceptibilities, resistivity, thermopower and entropy, many of which were also found in realistic simulations of the archetypal Hund and Mottcorrelated materials, Sr2RuO4 and V2O3. In summary, we provide evidence that and elucidate how Hundness evokes strong correlation effects in Hund metals. This might help to better interpret experimental results and guide superconducting theories.
Item Type:  Thesis (Dissertation, LMU Munich) 

Keywords:  Strong electronic correlations, Hund metals, Hund's rule coupling, Numerical Renormalization Group, Dynamical MeanField Theory, spinorbital separation, 3band HubbardHund model 
Subjects:  500 Natural sciences and mathematics 500 Natural sciences and mathematics > 530 Physics 
Faculties:  Faculty of Physics 
Language:  English 
Date of oral examination:  28. February 2019 
1. Referee:  Delft, Jan von 
MD5 Checksum of the PDFfile:  0592c9390b1f2c5d6b7491426970ba75 
Signature of the printed copy:  0001/UMC 26197 
ID Code:  23843 
Deposited On:  26. Mar 2019 15:34 
Last Modified:  26. Mar 2019 15:34 