Specific heat discontinuity, Δc, at T_c in BaFe ₂(As_0.7P_0.3)₂ - Consistent with unconventional superconductivity

We report the specific heat discontinuity, ΔC/T_c, at T_c = 28.2K of a collage of single crystals of BaFe ₂(As_0.7P_0.3)₂ and compare the measured value of 38.5 mJmol^{- 1}K^{- 2} with other iron pnictide and iron chalcogenide (FePn/Ch) superconductors. This value agrees well with the trend established by Bud'ko, Ni and Canfield, who found that for 14 examples of doped Ba_{1 - x}K_xFe₂As₂ and BaFe_{2 - x}TM_xAs₂, where the transition metal TM = Co and Ni. We extend their analysis to include all the FePn/Ch superconductors for which ΔC/T_c is currently known and find and a = 0.083mJmol^{- 1}K^{- 4}. A comparison with the elemental superconductors with T_c > 1K and with A-15 superconductors shows that, contrary to the FePn/Ch superconductors, electron-phonon-coupled conventional superconductors exhibit a significantly different dependence of ΔC on T_c, namely . However ΔC/γT_c appears to be comparable in all three classes (FePn/Ch, elemental and A-15) of superconductors with, for example, ΔC/γT_c = 2.4 for BaFe₂(As_0.7P_0.3)₂. Adiscussion of the possible implications of these phenomenological comparisons for the unconventional superconductivity believed to exist in the FePn/Ch is given.