We calculate the interactions between the Wannier functions of the 8-orbital model for twisted bilayer graphene (TBG). In this model, two orbitals per valley centered at the AA regions, the AA-p ...orbitals, account for the most part of the spectral weight of the flats bands. Exchange and assisted-hopping terms between these orbitals are found to be small. Therefore, the low energy properties of TBG will be determined by the density-density interactions. These interactions decay with the distance much faster than in the 2-orbital model, following a 1 / r law in the absence of gates. The magnitude of the largest interaction in the model, the onsite term between the flat band orbitals, is controlled by the size of the AA regions and is estimated to be ∼ 40 meV. To screen this interaction, the metallic gates have to be placed at a distance ≲ 5 nm. For larger distances only the long-range part of the interaction is substantially screened. The model reproduces the band deformation induced by doping found in other approaches within the Hartree approximation. Such deformation reveals the presence of other orbitals in the flat bands and is sensitive to the inclusion of the interactions involving them.
We analyze the metallic (pi, 0) antiferromagnetic state of a five-orbital model for iron superconductors. We find that with increasing interactions the system does not evolve trivially from the pure ...itinerant to the pure localized regime. Instead we find a region with a strong orbital differentiation between xy and yz, which are halffilled gapped states at the Fermi level, and itinerant zx, 3z super(2) - r super(2), and x super(2) - y super(2). We argue that orbital ordering between the yz and zx orbitals arises as a consequence of the interplay of the exchange energy in the antiferromagnetic x direction and the kinetic energy gained by the itinerant orbitals along the ferromagnetic y direction with an overall dominance of the kinetic energy gain. We indicate that iron superconductors may be close to the boundary between the itinerant and the orbital differentiated regimes and that it could be possible to cross this boundary with doping.
The recently observed superconductivity in twisted bilayer graphene emerges from insulating states believed to arise from electronic correlations. While there have been many proposals to explain the ...insulating behaviour, the commensurability at which these states appear suggests that they are Mott insulators. Here we focus on the insulating states with 2 electrons or holes with respect to the charge neutrality point. We show that the theoretical expectations for the Mott insulating states are not compatible with the experimentally observed dependence on temperature and magnetic field if, as frequently assumed, only the correlations between electrons on the same site are included. We argue that the inclusion of non-local (inter-site) correlations in the treatment of the Hubbard model can bring the predictions for the magnetic and temperature dependencies of the Mott transition to an agreement with experiments and have consequences for the critical interactions, the size of the gap, and possible pseudogap physics. The importance of the inter-site correlations to explain the experimental observations indicates that the observed insulating gap is not the one between the Hubbard bands and that antiferromagnetic-like correlations play a key role in the Mott transition.
Recent experiments on iron pnictides have uncovered a large in-plane resistivity anisotropy with a surprising result: The system conducts better in the antiferromagnetic x direction than in the ...ferromagnetic y direction. We address this problem by calculating the ratio of the Drude weight along the x and y directions, D(x)/D(y), for the mean-field Q=(π,0) magnetic phase diagram of a five-band model for the undoped pnictides. We find that D(x)/D(y) ranges between 0.2<D(x)/D(y)<1.7 for different interaction parameters. Large values of the orbital ordering favor an anisotropy opposite to the one found experimentally. On the other hand, D(x)/D(y) is strongly dependent on the topology and morphology of the reconstructed Fermi surface. Our results point against orbital ordering as the origin of the observed conductivity anisotropy, which may be ascribed to the anisotropy of the Fermi velocity.
We examine the magnetic phase diagram of iron pnictides using a five-band model. For the intermediate values of the interaction expected to hold in the iron pnictides, we find a metallic low moment ...state characterized by antiparallel orbital magnetic moments. The anisotropy of the interorbital hopping amplitudes is the key to understanding this low moment state. This state accounts for the small magnetization measured in undoped iron pnictides and leads to the strong exchange anisotropy found in neutron experiments. Orbital ordering is concomitant with magnetism and produces the large zx orbital weight seen at Γ in photoemission experiments.
The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments call for a prominent role of ...local correlations and place iron superconductors at the entrance of a Hund's metal state, the effect of the electronic correlations on the nematic state has been theoretically poorly investigated. In this work we study the nematic phase of iron superconductors accounting for local correlations, including the effect of the Hund's coupling. We show that Hund's physics strongly affects the nematic properties of the system. It severely constrains the precise nature of the feasible orbital-ordered state and induces a differentiation in the effective masses of the zx/yz orbitals in the nematic phase. The latter effect leads to distinctive signatures in different experimental probes overlooked so far in the interpretation of experiments. As notable examples the splittings between zx and yz bands at Γ and M points are modified, with important consequences for angle-resolved photoemission spectroscopy measurements.
Electronic correlations in Hund metals Fanfarillo, L.; Bascones, E.
Physical review. B, Condensed matter and materials physics,
08/2015, Letnik:
92, Številka:
7
Journal Article
Odprti dostop
To clarify the nature of correlations in Hund metals and its relationship with Mott physics we analyze the electronic correlations in multiorbital systems as a function of intraorbital interaction U, ...Hund's coupling J sub(H) and electronic filling n. We show that the main process behind the enhancement of correlations in Hund metals is the suppression of the double occupancy of a given orbital, as it also happens in the Mott insulator at half-filling. However, contrary to what happens in Mott correlated states the reduction of the quasiparticle weight Z with J sub(H)can happen in spite of increasing charge fluctuations. Therefore, in Hund metals the quasiparticle weight and the mass enhancement are not good measurements of the charge localization. Using simple energetic arguments we explain why the spin polarization induced by Hund's coupling produces orbital decoupling. We also discuss how the behavior at moderate interactions, with correlations controlled by the atomic spin polarization, changes at large U and J sub(H)due to the proximity to a Mott insulating state.
We discuss how to analyze the optical conductivity and Raman spectra of multiorbital systems using the velocity and the Raman vertices in a similar way Raman vertices were used to disentangle nodal ...and antinodal regions in cuprates. We apply this method to iron superconductors in the magnetic and nonmagnetic states, studied at the mean-field level. We find that the anisotropy in the optical conductivity at low frequencies reflects the difference between the magnetic gaps at the X and Y electron pockets. Both gaps are sampled by Raman spectroscopy. We also show that the Drude weight anisotropy in the magnetic state is sensitive to small changes in the lattice structure.
Raman and angle-resolved photoemission spectroscopy experiments have demonstrated that in superconducting underdoped cuprates nodal and antinodal regions are characterized by two energy scales ...instead of the one expected in BCS theory. The nodal scale decreases with underdoping while the antinodal one increases. Contrary to the behavior expected for an increasing energy scale, the antinodal Raman intensity decreases with decreasing doping. Using the Yang-Rice-Zhang model, we show that these features are a consequence of the nonconventional nature of the superconducting state in which superconductivity and pseudogap correlations are both present and compete for the phase space.
Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology ...of the Fermi surface in these materials. Based on this premise, one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. As a matter of fact, they, however, superconduct at very low temperatures or not at all. In this work we establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond angles or bond distances with respect to iron pnictides and chalcogenides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations). Our results indicate that superconductivity in iron-based materials may not always be fully understood based on d or d-p model Hamiltonians only.
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