Correlated electronic structure of LaOFeAs

Abstract: We compute the electronic structure, momentum resolved spectral function and optical conductivity of the new superconductor LaO$_{1-x}$F$_x$FeAs within the combination of the Density functional theory and the Dynamical Mean Field Theory. We find that the compound in the normal state is a strongly correlated metal and the parent compound is a bad metal at the verge of the metal insulator transition. We argue that the superconductivity is not phonon mediated.

• The paper finds that strong correlations in Fe 3d states drive LaOFeAs close to a Mott transition with quasiparticle renormalization factors around 0.2–0.3.
• The paper reveals that the itinerant As 4p states significantly disperse electronic charge, overlapping with the Fe states and altering band character.
• The paper demonstrates via DMFT that electron doping creates distinct electron pockets and anisotropic optical conductivity, underscoring non-phonon mechanisms for superconductivity.

Correlated Electronic Structure of LaO$_{1-x}$F$_x$FeAs

The paper by Haule, Shim, and Kotliar presents an in-depth analysis of the electronic structure and optical properties of the iron-based superconductor LaO$_{1-x}$F$_x$FeAs, utilizing a combination of Density Functional Theory (DFT) and Dynamical Mean Field Theory (DMFT). The research provides an evaluation of the normal state of LaO$_{1-x}$F$_x$FeAs, describing it as a strongly correlated metal while identifying the parent compound as a bad metal close to a metal-insulator transition. A noteworthy conclusion from their study is that superconductivity in this compound is not phonon-mediated, which challenges conventional superconductivity mechanisms.

Key Findings

1. Electronic Structure and Correlations:
   • The study emphasizes the importance of Fe $3d$ states at the Fermi level, with DFT predicting a density of states (DOS) characterized by a steep slope at this energy. The authors observe strong correlations, estimating a Coulomb repulsion of approximately $4\,$eV between these Fe $3d$ states.
   • The bandwidth of these $3d$ states is around $3\,$eV, suggesting that, despite the high degeneracy of the $d$ bands, the LaOFeAs compound approaches a Mott transition, highlighted by quasiparticle renormalization factors in the range of $Z \sim 0.2-0.3$.
2. Contribution of As and Band Character:
   • The As $4p$ states demonstrate notable itinerant character, significantly influencing the dispersion and spreading of the electronic charge beyond atomic boundaries. This results in an intricate overlapping of As states with Fe states, contrary to the case for a typical oxygen-heavy metal mixing seen in transition metal oxides.
3. Application of DMFT:
   • The DMFT computations reveal a narrow width for the low-energy renormalized band and significant weight transfer to the Hubbard bands. The electron-doped variant of the compound demonstrates more definitive electron pockets at the Fermi level, pointing towards improved conduction compared to the parent compound.
4. Optical Conductivity Analysis:
   • The study provides insights into the anisotropic nature of the optical conductivity within the compound, displaying the absence of a Drude peak and reinforcing the bad metal characterization. The presence of peaks corresponding to electron transitions between Fe $3d$ states and As $4p$ states substantiates the electronic intricacies hinted at by the DFT-DMFT studies.

Implications and Future Developments

The implications of these findings span both theoretical perspectives and practical applications. The identification of strong correlations near a Mott transition raises interesting avenues for theoretical exploration in multiorbital correlated systems. The study challenges existing paradigms for understanding superconductivity beyond phonon-mediated mechanisms, suggesting a potential role for spin and orbital fluctuations.

The results highlight the necessity of non-local correlations potentially accessible by extensions such as cluster DMFT, which could elucidate the low-energy phenomena observed in this and similar materials. Future studies could focus on more complex models that incorporate these non-local effects to gain further understanding of unconventional superconductivity mechanisms in iron-based superconductors.

Overall, the paper by Haule et al. advances our understanding of the electronic complexities in LaO$_{1-x}$F$_x$FeAs, providing a robust platform for future research aimed at elucidating the underpinnings of nascent superconductivity phenomena.