Fig. 1.

Normal and superconducting properties of high-quality single crystals of FeSe. (A) Temperature dependence of the in-plane resistivity ρ of FeSe. The structural transition occurs at $T_{\text{s}}\approx$ 90 K. (Upper Inset) Crystal structure. (Lower Inset) $\rho \left(T\right)$ in magnetic field ($\mathit{H}\Vert c$). From bottom to top, ${\mu }_{0}H=$ 0, 2, 4, 6, 8, 10, 12, and 14 T is applied. (B) Temperature dependence of the London penetration depth. (Inset) Superfluid density normalized by the zero temperature value ${\rho }_{\text{s}}\equiv {\lambda }_{\text{L}}^2\left(0\right)/{\lambda }_{\text{L}}^2\left(T\right)$. (C) Tunneling conductance spectrum at $T=0.4$ K. The peaks at $\pm 2.5$ meV (arrows) and shoulder structures at $\pm 3.5$ meV (dashed arrows) indicate the multiple superconducting gaps.