Low temperature specific heat of the hole-doped Ba0.6K0.4Fe2As2 single crystals

Low temperature specific heat (SH) was measured on the FeAs-based superconducting single crystals Ba0.6K0.4Fe2As2. It is found that the sharp SH anomaly Delta C/T parallel to(Tc) in our samples reaches an unexpected high value of 98 mJ/mol K-2, about 1 order of magnitude larger than that of the F-doped system LnFeAsO (Ln=rare-earth elements), suggesting a very high normal-state quasiparticle density of states in the FeAs-122 system than in the FeAs-1111 system. Furthermore, we found that the electronic SH coefficient gamma(e)(T) of Ba0.6K0.4Fe2As2 is weakly temperature dependent and increases almost linearly with the magnetic field in the low temperature region, which may indicate that the hole-doped FeAs-122 system contains a dominant component with a full superconducting gap, although we cannot rule out the possibility of a small component with an anisotropic or nodal gap. A detailed analysis reveals that the gamma(e)(T) of Ba0.6K0.4Fe2As2 cannot be fitted with a single gap of s-wave symmetry, probably due to the multigap effect. These results indicate a clear difference between the properties of the superconducting state of the holed-doped Ba0.6K0.4Fe2As2 and the F-doped LnFeAsO (Ln=rare-earth elements) system, which we believe originated from the complex Fermi-surface structures in different systems.