Incommensurate magnetic fluctuations and Fermi surface topology in LiFeAs

Using the angle-resolved photoemission spectroscopy data accumulated over the whole Brillouin zone (BZ) in LiFeAs, we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the $(\pi ,\pi )$ point of the BZ, and the large two-dimensional hole pocket, centered around the $\Gamma$ point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign-changing symmetry of the superconducting gap between electron and hole pockets and the sign-preserving gap, depending on the assumptions made for the fermionic damping.

Figure 1

(a) Experimental Fermi surface map with fitting contours superimposed over it. (b) Three general energy-momentum cuts passing through the electron pocket centered at $(-\pi /a;-\pi /b)$ and the hole pockets at $(0;0)$ and $(0;-2\pi /b)$. Photoemission data were collected at the $1^3$ ARPES station (BESSY) using horizontally polarized light with $h\nu =70$ eV at $T\approx 0.9$ K. Further details on experimental geometry and sample preparation can be found elsewhere.[28]

Figure 2

Calculated imaginary part of the bare spin susceptibility in the normal state of LiFeAs as a function of the momentum in the first BZ at $\hslash \Omega =5$ meV.

Figure 3

Calculated imaginary part of the bare (a) and the RPA (b) spin response as a function of the transverse momentum and frequency in the normal state of LiFeAs.

Figure 4

Calculated momentum dependence of the imaginary part of the RPA spin response at $\hslash \Omega =5$ meV in the normal and superconducting $s^{+-}$-wave state, respectively. Symbols refer to the experimental data taken from Ref. 16.

Figure 5

Calculated frequency dependence of the imaginary part of the RPA spin response at the wave vector Q${}_i$ in the normal state and the superconducting state for two different symmetries of the superconducting gap. Difference curves for the $s^{++}$- and $s^{+-}$-wave scenarios refer to the constant fermionic damping of 3 meV and the frequency-dependent damping, as described in the text. Inset: Experimental data, taken from Ref. 16. Observe that the upturn behavior of the experimental curves at energies lower than 1.5 meV is due to the elastic scattering contribution.