Orbital Selective Fermi Surface Shifts and Mechanism of High $T_c$ Superconductivity in Correlated $A\mathrm{FeAs}$ ($A=\mathrm{Li}$, Na)

Based on the dynamical mean field theory and angle resolved photoemission spectroscopy, we have investigated the mechanism of high $T_c$ superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the measured angle resolved photoemission spectroscopy. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect within the dynamical mean field theory method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that a fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.

Figure 1

DFT band structures (solid red lines) with (a) $d_{xz,yz}$ and (b) $d_{xy}$ contributions indicated by the size of black circles. (c) (upper panel) Second derivatives of ARPES data along the $\Gamma \mathrm{\text{−}}M$ direction. (lower panel) $\mathrm{DFT}+\mathrm{DMFT}$ spectral functions in red (gray) scale map, and DFT band structures in green (light gray) solid lines. DFT results are rescaled by an average renormalization factor of 2.55.

Figure 2

DMFT (solid lines) and DFT (dashed lines) Fermi surfaces at (a) $k_z=0$, (b) $k_z=\pi$ are compared with the experimental FSs guided by dashed lines. For the experimental data, we applied the 2D curvature method for clarity [38]. The $k_z$ for the measured electron pockets is not exactly at 0 or $\pi$ as the data were taken on a constant total momentum surface (fixed photon energy). (c) $k_z$ dependence of FSs along the $\Gamma \mathrm{\text{−}}M\mathrm{\text{−}}Z$ cut. To check the nesting condition, electron pockets denoted by dash-dot lines are shifted by ($\pi$, $\pi$), and plotted on top of hole pockets (solid lines). DFT and $\mathrm{DFT}+\mathrm{DMFT}$ results are shown in left and right panels, respectively.

Figure 3

(a) Isosurface plot of electron density difference between DFT and $\mathrm{DFT}+\mathrm{DMFT}$ results. The red (brighter) or blue (darker) means increase or decrease of $0.003e/{\AA }^3$ upon the DMFT calculation. The Cartesian $x$ or $y$ axis is chosen along the Fe-Fe bonds, so the increase is related mainly to the $d_{xz}$ or $d_{yz}$ orbital of Fe, while the decrease to $d_{xy}$. (b) Calculated DFT and $\mathrm{DFT}+\mathrm{DMFT}$ total energies against As height relative to the Fe plane.

Figure 4

(a) Bare susceptibilities ${\chi }_0(\mathbf{q})$ calculated by using the DMFT and DFT methods, normalized by ${\chi }_0(\mathbf{q}=0)$. (b) Intraorbital scattering contribution to ${\chi }_0(\mathbf{q})$. (c) and (d) SDW and SC instability plots versus temperature for LiFeAs (Li111) and NaFeAs (Na111), respectively.