Fig. 4: Thickness dependence of the reduced upper critical field Hc2/HP (θ), critical temperature Tc, anisotropy Γ, and coherence lengths ξab, ξc of FeSe thin flakes.
From: Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe
a The reduced superconducting critical field Hc2/HP(0) as a function of reduced transition temperature T/Tc for magnetic field applied either parallel (open symbols) or perpendicular (closed symbols) to the crystallographic c-axis for devices of different thickness. Dashed lines represent fits to the WHH orbital pair-breaking model and the solid line represents a fit to a 2D-Ginzburg Landau model42. b The anisotropy parameter, Γ, defined by the ratio of Hc2 for the two field orientations presented in a at 0.9Tc (left axis) and Tc (right axis) as a function of thickness, t. c The coherence lengths obtained from the slope of Hc2 in the vicinity of Tc as detailed in the Supplementary Material. The horizontal solid line indicates the \(c/\sqrt{2}\)-axis layer spacing of bulk FeSe. The shaded area indicates the crossover towards a two-dimensional highly anisotropic superconducting phase, and the dashed lines in b and c are guides to the eyes. See Methods for definition of error bars.
