Effect of Li-deficiency impurities on the electron-overdoped LiFeAs superconductor

We use transport, inelastic neutron scattering, and angle-resolved photoemission experiments to demonstrate that the stoichiometric LiFeAs is an intrinsically electron-overdoped superconductor similar to those of the electron-overdoped NaFe${}_{1-x}{T}_x$As and BaFe${}_{2-x}{T}_x$As${}_2$ ($T=\text{Co}$, Ni). Furthermore, we show that although transport properties of the stoichiometric superconducting LiFeAs and Li-deficient nonsuperconducting Li${}_{1-x}$FeAs are different, their electronic and magnetic properties are rather similar. Therefore, the nonsuperconducting Li${}_{1-x}$FeAs is also in the electron overdoped regime, where small Li deficiencies near the FeAs octahedra can dramatically suppress superconductivity through the impurity scattering effect.

Figure 1

(a) Phase diagram of the electron doped NaFe${}_{1-x}$Co${}_x$As from Ref. 27. The inset shows the structure of Na(Li)FeAs and the differences in the angles of the two alkali arsenic planes based on the structural parameters from Ref. 8 for LiFeAs and Ref. 26 for NaFeAs. (b) The temperature dependence of resistivity for the SC LiFeAs (solid line) and NSC Li${}_{0.94}$FeAs (dashed line) up to room temperature. The data are normalized by the size and mass of the single crystals. (c) Expanded view of the temperature dependence of the resistivity for the SC and NSC Li${}_{1-x}$FeAs. The SC LiFeAs has a clear transition to superconductivity at 16 K. (d) Schematic Fermi surfaces of LiFeAs from Ref. 20. The red shadow indicates a flat band in the center of the $\Gamma (0,0)$ point. The incommensurability from the ARPES measurements is defined as ${\delta }_K$, the mismatch of the inner hole Fermi surface and electron Fermi surfaces. (e) The energy dependence of the incommensurability for the incommensurate spin excitations from the SC LiFeAs (the red squared symbols), NSC Li${}_{0.94}$FeAs (the olive diamond symbols), and the APRES measurements (the gray dash line). The violet solid line is the incommensurability value from Ref. 21. The inset shows the locations of the incommensurate peaks near the in-plane AF wave vector $\mathbf{Q}=(1,0)$ in LiFeAs.

Figure 2

Two-dimensional constant-energy plots of the spin excitations in the $[H,K]$ plane at the spin excitation energies indicated (a)–(d) for the SC LiFeAs and (e)–(h) for the NSC Li${}_{0.94}$FeAs at 5 K. The incident neutron energy was $E_i=35$ meV oriented along the $c$ axis. The intensity has been normalized to be in absolute units using a vanadium standard as discussed in Ref. 25.

Figure 3

Constant-energy cuts of spin excitations along the $[1,K]$ direction for the SC LiFeAs and NSC Li${}_{0.94}$FeAs at energy transfers of (a) $E=5\pm 1$ meV, (b) $7\pm 1$ meV, (c) $11\pm 1$ mV, (d) $13\pm 1$ meV, (e) $17\pm 1$ meV, (f) $19\pm 1$ meV; all with $E_i=35$ meV. The solid lines are fits to two Gaussian peaks. The dashed vertical lines in (c) marking peak centers indicate the definition of incommensurability of spin excitations as in previous work.[21] The cuts for the SC and NSC spin excitations spectra were subtracted by the same fitted NSC background at the identical energy. The intensity is in absolute units, and error bars indicate one standard deviation.

Figure 4

(a) and (b) ARPES intensity mappings of the SC and NSC Li${}_{1-x}$FeAs samples recorded with $h\nu =35$ eV photons (corresponding to the $c$ axis momentum transfer $k_Z=\pi$) and integrated within $\pm 20$ meV with respect to the Fermi energy $E_F$. The extracted Fermi surface contours from (a) and (b) are plotted together in (c).