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Register a new userWeakness of correlation effects in BaNi$_2$As$_2$: ARPES and LDA+DMFT study
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The BaNi$_2$As$_2$ compound is investigated using both the angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and combined computational scheme of local density approximation together with dynamical mean-field theory (LDA+DMFT). For more realistic comparison of LDA+DMFT spectral functions with ARPES data we take into account several experimental features: the photoemission cross-section, the experimental energy and angular resolutions and the photo-hole lifetime effects. In contrast to isostructural iron arsenides the BaNi$_2$As$_2$ within LDA+DMFT appears to be weakly correlated (effective mass enhancement about $1.2$). This dramatic reduction of the correlation strength comes from the increase of 3d-orbital filling, when going from Fe to Ni, together with rather large bare Ni-3d LDA bandwidth. Nevertheless, even weakened electron correlations cause remarkable reconstruction of the bare BaNi$_2$As$_2$ LDA band structure and corresponding LDA+DMFT calculations provide better agreement with ARPES than just renormalized LDA results.
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Effects of Coulomb correlation on LaOFeAs electronic structure have been investigated by LDA+DMFT(QMC) method. The calculation results show that LaOFeAs is in the regime of intermediate correlation strength with significant part of the spectral density moved from the Fermi energy to Hubbard bands. However the system is not on the edge of metal insulator-transition because increase of the Coulomb interaction parameter value from $U$=4.0 eV to $U$=5.0 eV did not result in insulator state. Correlations affect different d-orbitals not in the same way. $t_{2g}$ states ($xz,yz$ and $x^2-y^2$ orbitals) have higher energy due to crystal filed splitting and are nearly half-filled. Their spectral functions have pseudogap with Fermi energy position on the higher sub-band slope. Lower energy $e_g$ set of d-orbitals ($3z^2-r^2$ and $xy$) have significantly larger occupancy values with typically metallic spectral functions.
The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes and coherence contribute. While the lifted degeneracy of the Fe $d_{xz}$ and $d_{yz}$ dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the nematic response of the spectral weight in BaFe$_2$As$_2$. The symmetry analysis of the ARPES spectra demonstrates that the $d_{xz}$ band gains quasiparticle spectral weight compared to the $d_{yz}$ band for negative antisymmetric strain $Delta epsilon_{yy}$ suggesting the same response inside the nematic phase. Our results are compatible with a different coherence of the $d_{xz}$ and $d_{yz}$ orbital within a Hunds metal picture. We also discuss the influence of orbital mixing.
Pseudogap regime for the prototype high-Tc compounds hole doped Bi2Sr2CaCu2O8-x (Bi2212) and electron doped Nd2-xCexCuO4 (NCCO) is described by means of novel generalized LDA+DMFT+Sk approach. Here conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Sk. In the present case Sk describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material specific model parameters of two neighboring CuO2 layers of Bi2212 and single CuO2 layer of NCCO were obtained within local density approximation (LDA) and constrained LDA method. We show that Fermi surface in presence of the pseudogap fluctuations have perfectly visible hot-spots for NCCO while in Bi2212 there is just rather broad region with strong antiferromagnetic scattering. Results obtained are in good agreement with recent ARPES and optical experiments.
The cobaltates have demonstrated a wide variety complex behavior. The Na rich region of the phase diagram displays various degrees of anomalous behavior, such as Curie-Weiss behavior near a band insulatorcite{Foo:2004}, charge disproportionationcite{Mukhamedshin:2005}, and non-Fermi-liquid behavior in the resistivitycite{Foo:2004}. Alternatively, the Na poor region of the phase diagram appears to be a Fermi-liquid. The magnetic susceptibility displays Pauli behavior, the resistivity is roughly quadratic at low temperaturescite{Foo:2004}, and the system appears to be homogeneouscite{Mukhamedshin:2005}. Therefore, the Na poor region of the phase diagram seems like a natural starting point to attempt to explain the ARPES experiments and heat capacity measurements from a quantitative standpoint.
The interplay of high and low-energy mass renormalizations with band-shifts reflected by the positions of van Hove singularities (VHS) in the normal state spectra of the highest hole-overdoped and strongly correlated AFe$_2$As$_2$ (A122) with A = K, Cs is discussed phenomenologically based on ARPES data and GGA band-structure calculations with full spin-orbit coupling. The big increase of the Sommerfeld coefficient $gamma$ from K122 to Cs122 is ascribed to an enhanced coupling to low-energy bosons in the vicinity of a quantum critical point to an unknown, yet incommensurate phase different from the commensurate Mott one. We find no sizeable increase in correlations for Cs122 in contrast to F. Eilers et al., PRL v. 116, 237003 (2016) [3]. The empirical (ARPES) VHS positions as compared with GGA-predictions point even to slightly weaker correlations in Cs122 in accord with low-$T$ magnetic susceptibility $chi(T)$ data and a decreasing Wilson ratio $propto chi(0)/gamma$.
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