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

Abstract

We report the specific heat discontinuity,ΔC/Tc, atTc = 28.2 K of a collage ofsingle 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 valueagrees well with the trend established by Bud’ko, Ni and Canfield, who found that for 14 examples of doped Ba1 − xKxFe2As2 and BaFe2 − xTMxAs2, where thetransition metal TM = Co and Ni. We extend their analysis to include all the FePn/Ch superconductors for whichΔC/Tc is currently known and find and a = 0.083 mJ mol − 1 K − 4. A comparison with the elemental superconductors withTc > 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 ofΔC onTc, namely . However ΔC/γTc appears to be comparable in all three classes (FePn/Ch, elemental and A-15) of superconductors with, forexample, ΔC/γTc = 2.4 for BaFe2(As0.7P0.3)2. A discussion of the possible implications of these phenomenological comparisons forthe unconventional superconductivity believed to exist in the FePn/Ch is given.