Possible unconventional superconductivity in iron-based layered compound LaFePO: Study of heat capacity

Heat capacity measurement was performed on recently discovered, iron-based layered superconductors, namely, nondoped LaFePO and fluorine-doped LaFePO (LaFePO0.94F0.06). A relatively large electronic heat capacity coefficient (gamma) of 10.6 mJ K-2 mol(-1) and a small normalized heat capacity jump (Delta C-p/gamma T-c) of 0.29 at T-c = 3.3 K were observed in LaFePO. LaFePO0.94F0.06 had a smaller gamma of 8.3 mJ K-2 mol(-1) and a larger Delta C-p/gamma T-c of 0.44 at T-c = 5.8 K. These results indicate that these compounds have a strong electron-electron correlation and a large magnetic spin fluctuation, which are the signatures of unconventional superconductivity mediated by spin fluctuation.