Density Functional Dynamical Mean-field Theory Research Articles

Orbital selectivity in the normal state of KFe2Se2 superconductor

Using density functional dynamical mean-field theory, we show how correlation effects lead to pseudogap and Kondo-quasiparticle features in the electronic structure of pure and doped KFe2Se2 superconductor. Therein, correlation- and doping-induced orbital differentiation are linked to the emergence of an incoherent-coherent crossover in the normal state of KFe2Se2 superconductor. This crossover explains the puzzling temperature- and doping-dependent evolution of resistivity and Hall coefficient, seen in experiments of alkali-metal intercalated iron-selenide superconductors. Our microscopic description emphasises the role of incoherent and coherent electronic excitations towards unconventional transport responses of strange, bad metals.

Atomistic electronic structure in the single-chain limit of organic conductors

Using density functional dynamical mean-field theory we show how electronic interactions can tune a dispersive band structure such that it develops atomistic Landau-Fermi liquidness. Thereby, we show how a relaxed single chain with electron densities slightly below half filling can improve orbital selectivity with coexisting metallic and semiconducting states. Our setup provides a well-controlled environment at the atomistic scale to understand the principles behind orbital-selective electronic reconstruction and the interplay between electron interactions and local electron-hole doping in the single-chain limit of organic conductors.

Emergence of orbital-selective marginal Fermi liquidness in compressed RbFe2Te3

Driving a Mott insulator towards an orbital-selective metal with coexisting Mott localized and marginal Fermi liquid electronic states remains central to understanding various correlated emergent phenomena. Here, using density functional dynamical mean-field theory we explore correlation and hole-doping-induced electronic reconstruction in ${\mathrm{RbFe}}_{2}{\mathrm{Te}}_{3}$, a potential two-leg ladder system for hole carrier superconductivity under pressure. We stress the importance of multiorbital Coulomb interactions in concert with band structure calculations for a consistent understanding of intrinsic Mott localization at ambient pressure. We elucidate the nature of pressure-induced Mottness upon tuning the correlation to bandwidth ratio away from the Mott boundary. As a by-product of our analysis, a quantum critical phenomenon with coexisting Mott localized and marginal Fermi liquid electrons is predicted to exist in compressed ${\mathrm{RbFe}}_{2}{\mathrm{Te}}_{3}$ metal.

Theory of two-fluid metallicity in superconducting FeSe at high pressure

We compute the two-fluid resistivity of a compressed FeSe superconductor, treating electronic correlations within density functional dynamical mean-field theory. Results for the emergent strange and bad metal behavior shows good theory-experiment agreement within the mixed tetragonal and pseudohexagonal structural phases at pressures around 8.1 GPa. Our findings call for more studies on unconventional high-temperature (high-${T}_{c}$) superconductors to unearth the consequences of selective Mott localization as the prominent candidate in governing their strange, $T$-linear resistivity.

Electronic structure of BiFeO3 in the presence of strong electronic correlations

Using density-functional dynamical mean-field theory (DFDMFT) we show the importance of multiorbital electronic correlations in determining the insulating state of BiFeO3, a multiferroic material with an electron band gap larger than its bare bandwidth. Within the Fe3+ oxidation state and using realistic values for the on-site Coulomb interaction, we unveil strongly correlated key features probed in x-ray photoelectron and absorption spectra, showing good qualitative theory-experiment agreement. We explore the electronic reconstruction hidden in ferromagnetic BiFeO3, predicting broad orbital- and spin-polarized features at low temperatures. Our proposal for ferromagnetic BiFeO3 is expected to be an important step to understanding the emergent correlated electronic structure of magnetoelectronic and spintronic materials with persisting ordered localized moments coexisting with Coulomb reconstructed electronic states.

Site-selective electronic structure of pure and doped Ca2O3Fe3S2

Using density functional dynamical mean-field theory we investigate the site-selective electronic structure of ${\mathrm{Ca}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{3}{\mathrm{S}}_{2}$. We confirm that the parent compound with two distinct iron sites is a multiorbital Mott insulator similar to ${\mathrm{La}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{2}{\mathrm{S}}_{2}$. Upon electron/hole doping, carrier localization is found to persist in the two active iron channels because the chemical potential lies in a gap structure with anisotropic and almost vanishing states near the Fermi energy. This emergent behavior stems from large electronic reconstruction caused by dynamical spectral weight transfer involving states with distinct $d$-shell occupancies and orbital character at low energies. We detail the implications of our microscopic analysis and discuss the underlying physics which will emerge in future experiments on ${\mathrm{Ca}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{3}{\mathrm{S}}_{2}$.

Kondo-semimetal to Fermi-liquid phase crossover in black phosphorus to pressure-induced orbital-nematic gray phosphorus

We perform a comparative study of the electronic structures and electrical resistivity properties of black $(A17)$ and of pressurized gray $(A7)$ phosphorus, showing band-selective Kondoesque electronic reconstruction in these layered $p$-band phosphorus allotropes. Based on density functional dynamical mean-field theory calculations, we show that gray phosphorus can host a three-dimensional Kondo semimetal to a Fermi liquid phase crossover under anisotropic compression, which lifts in-layer orbital degeneracy. Therein, the $3p$ spectrum is almost unaffected both in the semiconducting and semimetallic Kondo phases, however in the Fermi liquid regime strong electronic reconstruction is predicted to exist in the orbital nematic phase of compressed gray phosphorus. These findings contribute to the microscopic understanding of the role played by dynamical multiorbital electronic interactions in the low energy spectrum of correlated semiconductors and topological semimetals.

Electronic structure and thermoelectric transport of black phosphorus

We investigate anisotropic electronic structure and thermal transport properties of bulk black phosphorus (BP). Using density functional dynamical mean-field theory we first derive a correlation-induced electronic reconstruction, showing band-selective Kondoesque physics in this elemental $p$-band material. The resulting correlated picture is expected to shed light onto the temperature and doping dependent evolution of resistivity, Seebeck coefficient, and thermal conductivity, as seen in experiments on bulk single crystal BP. Therein, large anisotropic particle-hole excitations are key to consistently understand thermoelectric transport responses of pure and doped BP.

Experimental evidence for importance of Hund's exchange interaction for incoherence of charge carriers in iron-based superconductors

Angle-resolved photoemission spectroscopy (ARPES) is used to study the scattering rates of charge carriers from the hole pockets near Gamma in the iron-based high-Tc hole doped superconductors KxBa1-xFe2As2 x=0.4 and KxEu1-xFe2As2 x=0.55$ and the electron doped compound Ba(Fe1-xCox)2As2 x=0.075. The scattering rate for any given band is found to depend linearly on energy, indicating a non-Fermi liquid regime. The scattering rates in the hole-doped compound are considerably larger than those in the electron-doped compounds. In the hole-doped systems the scattering rate of the charge carriers of the inner hole pocket is about three times bigger than the binding energy indicating that the spectral weight is heavily incoherent. The strength of the scattering rates and the difference between electron and hole doped compounds signals the importance of Hund's exchange coupling for correlation effects in these iron-based high-Tc superconductors. The experimental results are in qualitative agreement with theoretical calculations in the framework of combined density functional dynamical mean-field theory.

Selective orbital reconstruction in tetragonal FeS: A density functional dynamical mean-field theory study

Transport properties of tetragonal iron monosulfide, mackinawite, show a range of complex features. Semiconductive behavior and proximity to metallic states with nodal superconductivity mark this d-band system as unconventional quantum material. Here, we use the density functional dynamical mean-field theory (DFDMFT) scheme to comprehensively explain why tetragonal FeS shows both semiconducting and metallic responses in contrast to tetragonal FeSe which is a pseudogaped metal above the superconducting transition temperature. Within local-density-approximation plus dynamical mean-field theory (LDA+DMFT) we characterize its paramagnetic insulating and metallic phases, showing the proximity of mackinawite to selective Mott localization. We report the coexistence of pseudogaped and anisotropic Dirac-like electronic dispersion at the border of the Mott transition. These findings announce a new understanding of many-particle physics in quantum materials with coexisting Dirac-fermions and pseudogaped electronic states at low energies. Based on our results we propose that in electron-doped FeS substantial changes would be seen when the metallic regime was tuned towards an electronic state that hosts unconventional superconductivity.

Electronic reconstruction of hexagonal FeS: a view from density functional dynamical mean-field theory

We present a detailed study of correlation- and pressure-induced electronic reconstruction in hexagonal iron monosulfide, a system which is widely found in meteorites and one of the components of Earth’s core. Based on a perusal of experimental data, we stress the importance of multi-orbital electron-electron interactions in concert with first-principles band structure calculations for a consistent understanding of its intrinsic Mott–Hubbard insulating state. We explain the anomalous nature of pressure-induced insulator-metal-insulator transition seen in experiment, showing that it is driven by dynamical spectral weight transfer in response to changes in the crystal-field splittings under pressure. As a byproduct of this analysis, we confirm that the electronic transitions observed in pristine FeS at moderated pressures are triggered by changes in the spin state which causes orbital-selective Kondo quasiparticle electronic reconstruction at low energies.

Band narrowing and Mott localization in isotropically superstrained graphene

We explore the effect of multiorbital electron-electron interactions in a two-dimensional monolayer made of elemental carbon. Using density functional dynamical mean-field theory (DFDMFT), we show that the interplay between one-particle band narrowing and sizable on-site interactions naturally stabilizes the Mott insulating state in isotropically superstrained graphene. Our theory is expected to be a key step to understanding both the ability of graphene to afford large strain deformations and the changes in electronic degrees of freedom of p -band Coulomb interacting electrons for the next generation of flexible electronics made of semiconductive graphene.

Electronic states in an atomistic carbon quantum dot patterned in graphene

We reveal the emergence of metallic Kondo clouds in an atomistic carbon quantum dot, realized as a single-atom junction in a suitably patterned graphene nanoflake. Using density functional dynamical mean-field theory (DFDMFT) we show how correlation effects lead to striking features in the electronic structure of our device, and how those are enhanced by the electron-electron interactions when graphene is patterned at the atomistic scale. Our setup provides a well-controlled environment to understand the principles behind the orbital-selective Kondo physics and the interplay between orbital and spin degrees of freedom in carbon-based nanomaterials, which indicate new pathways for spintronics in atomically patterned graphene.