Figure 1. Excess conductivity and diamagnetic response of a high-quality single crystal of FeSe.

(a) T dependence of ρ_xx in magnetic fields (H||c). The structural transition occurs at T_s=90 K, which is accompanied by a kink in ρ_xx(T). Inset shows the crystal structure of FeSe. (b) T dependence of ρ_xx (red) and dρ_xx/dT (grey). Below T* shown by arrow, ρ_xx shows a downward curvature. The blue dashed line represents ρ_xx(T)=ρ₀+AT^α with ρ₀=7 μΩ cm A=0.6 μΩ cm K⁻² and α=1.2. (c) Diamagnetic response in magnetization M_dia for H||c. The inset shows the diamagnetic susceptibility χ_dia at 8 T (blue) compared with the estimated χ_AL in the standard Gaussian fluctuations theory (red).