Inelastic Neutron-Scattering Measurements of Incommensurate Magnetic Excitations on Superconducting LiFeAs Single Crystals

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector ($0.5\pm \delta ,0.5\mp \delta ,0$) with $\delta \sim 0.07$ slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.

Figure 1

Distribution of neutron-scattering intensity measured with the flat-cone detector on IN20. The energy transfer is constant in all maps. $(hkl)$ planes with fixed but finite $l$ component are studied by tilting the detector and the sample. Rings of scattering arise from polycrystalline construction material. (a) The scattering of optimally Co-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ crystal in the SC phase ($T=10\mathrm{K}$). One easily identifies the magnetic mode near $Q=(0.5,0.5,0)$ which is of comparable strength to the phonon scattering around the strong nuclear Bragg peaks. (b)–(d) Intensity maps measured on LiFeAs at an energy transfer of 5 meV and $l=0$ ($T=2\mathrm{K}$), of 5 meV and $l=0.5$ ($T=2\mathrm{K}$), and of 10 meV and $l=0$ ($T=2\mathrm{K}$) for (b), (c), and (d), respectively.

Figure 2

Constant-energy scans across the incommensurate positions of magnetic scattering $Q_{\mathrm{inc}}=(0.5+\delta ,0.5-\delta ,0)$ with $\delta \sim \pm 0.07$. The scans in (a) and (b) were recorded on the cold 4F spectrometer with $k_f=1.55{\AA }^{-1}$ with an energy transfer of 1.5 and 2.5 meV, respectively. One clearly recognizes the incommensurate signal even at this low energy in the normal state, but the signal disappears in the SC phase. (c),(d) Scans taken from the flat-cone data at 5 and 10 meV, respectively. Lines are fits with two symmetric Gaussians.

Figure 3

(a) Energy dependence of the incommensurability of magnetic scattering in LiFeAs. (b) The energy dependence of the amplitude of the signal can be well described by a single relaxor function (line), see text, with a characteristic energy of $\Gamma =6.0\pm 0.6\mathrm{meV}$ signaling a near SDW instability.

Figure 4

Energy dependence of the INS intensity at $Q_{\mathrm{inc}}$ at temperatures above and below the SC transition. Panel (a) shows the raw intensity measured on the cold spectrometer (with high-energy resolution, $k_f=1.55\AA -1$), while panel (b) shows data measured on the thermal spectrometer with lower resolution. Vertical bars indicate the energy of the crossover of the shift of spectral weight.

Figure 5

Temperature dependence of scattering intensity at $Q_{\mathrm{inc}}$ for fixed energy transfers, data are measured on the 1T spectrometer (a) and on IN14 (b) with the two large crystals; data in (c) were taken with a smaller crystal on 2T in a $(110)/(001)$ scattering geometry where the incommensurate signal is integrated with the relaxed vertical resolution. Lines are guides to the eye. (d) Temperature dependence of spin-flip scattering at the nuclear Bragg peak (200) reflecting the neutron depolarization due to SC shielding.