Figure 4. Concentration x dependence of lower critical field and associated energies for BaFe₂(As_1−xP_x)₂.

(a) Lower critical field H_c1 extrapolated to T=0 and T_c. The location of the QCP is indicated. Error bars on H_c1 represent the combination of uncertainties in extrapolating H_c1(T) to T=0 and in the demagnetizing factor. Error bars on x are s.d. (b) Vortex line energy E_line=E_em+E_core at T=0 from the H_c1(0) data and equations (4) and (3) shown as squares. The electromagnetic energy calculated using equation (4) and different estimates of λ are also shown. The triangles are direct measurements from ref. 9, and the circles are estimates derived by scaling the band-structure value of λ by the effective mass enhancement from specific heat 10. Error bars on E_em (circles) are calculated from the uncertainty in jump size in heat capacity at T_c. (c) Vortex core energy E_core=E_line−E_em along with an alternative estimate derived from the specific heat condensation energy (E_cond) and the effective vortex area (πξ_e²). The uncertainties are calculated from a combination of those in the other panels. The dashed lines in all panels are guides to the eye.