Superconductivity without Nesting in LiFeAs

We have studied the electronic structure of the nonmagnetic LiFeAs ($T_c\sim 18\mathrm{K}$) superconductor using angle-resolved photoemission spectroscopy. We find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a factor of 3, high density of states at the Fermi level caused by a van Hove singularity, and no evidence for either a static or a fluctuating order except superconductivity with in-plane isotropic energy gaps. Our observations suggest that these electronic properties capture the majority of ingredients necessary for the superconductivity in iron pnictides.

Figure 1

Fermi surface shape and topology. (a) Momentum distribution map of the photoemission intensity integrated within 5 meV around the Fermi level. Solid black lines represent $\Gamma$—centered Fermi contours, dashed lines—most pronounced $M$-centered Fermi contours. (b),(c) Momentum-energy cuts along the vertical directions marked by the white dashed lines in panel (a).

Figure 2

Comparison with the LDA calculations. (a)–(c) Fermi surface maps taken at different excitation energies at $T\sim 1\mathrm{K}$. (d) Momentum-energy cut through the $\Gamma$ point measured with the $\sigma$ (left) and $\pi$ (right) polarizations. Photon energy is 67 eV. (e) High-symmetry momentum-energy cuts together with the results of the band structure calculations.

Figure 3

Superconductivity in LiFeAs. (a) Fermi surface crossings at the point $A$ [see panel (d)] taken above and below $T_c(\sim 18\mathrm{K})$. (b) Energy distribution curves from panels (a) corresponding to the $k_F$. (c) Energy distribution curves corresponding to Fermi momenta $A$ and $B$ measured above (20 K) and below (900 mK) $T_c$. (d) Kinetic energies of the leading-edge midpoints of all energy distribution curves from the shown momentum space region in the false color scale. The local maxima of this distribution correspond to $k_F$ points also seen in Fig. 2c and give a good measure of the superconducting gap.

Figure 4

van Hove singularities in Fe pnictides and chalcogenides. Schematic illustration of the main features in the electronic structure of the 122 and 1111 pnictides, LiFeAs, $2H\mathrm{\text{−}}{\mathrm{TaSe}}_2$, and $2H\mathrm{\text{−}}{\mathrm{NbSe}}_2$. Curves represent the band dispersions along the high-symmetry directions. Thick lines represent van Hove sigularities, i.e., the flat extrema of the corresponding bands or the saddle point.