Scanning Tunneling Spectroscopy of Superconducting LiFeAs Single Crystals: Evidence for Two Nodeless Energy Gaps and Coupling to a Bosonic Mode

The superconducting compound LiFeAs is studied by scanning tunneling microscopy and spectroscopy. A gap map of the unreconstructed surface indicates a high degree of homogeneity in this system. Spectra at 2 K show two nodeless superconducting gaps with ${\Delta }_1=5.3\pm 0.1\mathrm{meV}$ and ${\Delta }_2=2.5\pm 0.2\mathrm{meV}$. The gaps close as the temperature is increased to the bulk $T_c$, indicating that the surface accurately represents the bulk. A dip-hump structure is observed below $T_c$ with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering.

Figure 1

(a) $80\times 80{\mathrm{nm}}^2$ scanning tunneling topographic image of LiFeAs ($V_B=50\mathrm{mV}$, $I_T=20\mathrm{pA}$) at $T=4.2\mathrm{K}$ and (b) $6.8\times 6.8{\mathrm{nm}}^2$ atomic resolution topographic image of LiFeAs ($V_B=40\mathrm{mV}$, $I_T=100\mathrm{pA}$). (c) Schematic of the crystal structure from the cleaved (001) direction and an edge-on view. (d) $dI/dV$ spectrum in a range from $-550$ to $+550\mathrm{mV}$.

Figure 2

(a) $90\times 90{\mathrm{nm}}^2$ topographic image ($V_B=15\mathrm{mV}$, $I_T=100\mathrm{pA}$) of LiFeAs. (b) 150 individual $dI/dV$ spectra ($T=4.2\mathrm{K}$) from the defect-free 33-nm long line marked in (a). (c) Gap map of the same region in (a). ${\Delta }^{hpp}$ corresponds to half the energy separation between coherence peaks. Both the gap map and the histogram of ${\Delta }^{hpp}$ shown in (d) reveal a high degree in homogeneity of the superconducting properties in LiFeAs.

Figure 3

(a) Two isotropic gaps fit (blue line) on top of measured $dI/dV$ spectra (black line, 2 K). The dashed lines indicate the fitting bias range from $-6.8$ to 6.8 mV. (b) Temperature dependence of the superconducting gap measured by STS between 2 and 20 K. (c) The large gap determined by isotropic $s$-wave fits (red error bars) generally follows the temperature dependence predicted by the BCS weak coupling limit (dashed black line). The development of the smaller gap (blue error bars) is obscured by thermal broadening at elevated temperatures. The bulk $T_C$ of 20 K probed by SQUID magnetometry with a 1 G magnetic field (gray dots and right $y$ axis) agrees with the surface critical temperature, demonstrating agreement between surface and bulk properties.

Figure 4

(a) Normalized STS in a temperature range between 2 and 16.5 K. A dip-hump feature is clearly visible at 2 K but decays with increasing temperature. (b) The normalized second derivative of $I(V)$ at 2 K reveals the position of the main features. The dip in the normalized $dI/dV$ is at $E_D\approx \pm 10\mathrm{mV}$ and the inflection point or kink in $dI/dV$ is at $E_I\approx \pm 12\mathrm{mV}$ (dashed line).