Dichotomy between Large Local and Small Ordered Magnetic Moments in Iron-Based Superconductors

We study a four-band model for iron-based superconductors within the local density approximation combined with dynamical mean-field theory ($\mathrm{LDA}+\mathrm{DMFT}$). This successfully reproduces the results of models which take As $p$ degrees of freedom explicitly into account and has several physical advantages over the standard five $d$-band model. Our findings reveal that the new superconductors are more strongly correlated than their single-particle properties suggest. Two-particle correlation functions unveil the dichotomy between local and ordered magnetic moments in these systems, calling for further experiments to better resolve the short time scale spin dynamics.

Figure 1

Orbital-resolved spectral function of the four-band model for LaFeAsO at $T=460\mathrm{K}$, for $U=1.8\mathrm{eV}$, $J=0.45\mathrm{eV}$ (solid lines) and $U=J=0$ (dashed lines). Including electronic correlations does not change the spectrum drastically.

Figure 2

Spin-spin correlation function for $J=0.45\mathrm{eV}$ and $\beta =25{\mathrm{eV}}^{-1}$. We plot the different intraorbital contributions and the sum of all interorbital contributions. This orbital-resolved presentation clearly shows that the $J$-induced interorbital correlation is particularly large. Inset: Comparison between Ising- and SU(2)-symmetric Hund’s exchange for a related two-band model, showing only quite small differences.

Figure 3

Total spin-spin correlation function for LaFeAsO at two different values of $J$ and $\beta =25{\mathrm{eV}}^{-1}$, compared to the noninteracting $U=J=0$ case. The short-time ($\tau =0$) $\mathrm{LDA}+\mathrm{DMFT}$ local moment for $J=0.45\mathrm{eV}$ is $m_{\mathrm{loc}}=2.16{\mu }_B$, comparably large as in LSDA; whereas the long-time moment is screened to only $m\approx 0.7{\mu }_B$ at $T=50\mathrm{K}$; see text.