Specific heat discontinuity, ΔC, at Tc in BaFe2(As0.7P0.3)2-consistent with unconventional superconductivity

We report the specific heat discontinuity, ΔC/Tc, at Tc = 28.2 K of a collage of single crystals of BaFe2(As0.7P0.3)2 and compare the measured value of 38.5 mJ mol-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 ΔC/Tc∝aTc2 for 14 examples of doped Ba1-xKxFe2As2 and BaFe2-xTMxAs2, where the transition metal TM = Co and Ni. We extend their analysis to include all the FePn/Ch superconductors for which ΔC/Tc is currently known and find ΔC/Tc∝aTc1.9 and a = 0.083 mJ mol-1 K-4. A comparison with the elemental superconductors with Tc > 1 K 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 Tc, namely ΔC/Tc∝Tc0.9. However ΔC/γTc appears to be comparable in all three classes (FePn/Ch, elemental and A-15) of superconductors with, for example, ΔC/γTc = 2.4 for BaFe2(As0.7P0.3)2. A discussion of the possible implications of these phenomenological comparisons for the unconventional superconductivity believed to exist in the FePn/Ch is given. (fast track communication)