Low temperature specific heat (SH) was measured on the FeAs-based superconducting single crystals Ba$_{ 0.6}$K$_{ 0.4}$Fe$_2$As$_2$ and high pressure synthesized polycrystalline samples SmFeAsO$_{ 0.9}$F$_{ 0.1}$. It is found that the sharp SH anomaly $\Delta C/T|_{ T_c}$ in Ba$_{ 0.6}$K$_{ 0.4}$Fe$_2$As$_2$ reaches an unexpected high value of 98 mJ/mol K$^2$, about one order of magnitude larger than that of SmFeAsO$_{ 0.9}$F$_{ 0.1}$ ($6\sim8$ mJ/mol K$^2$) samples, suggesting very high normal state quasiparticle density of states in FeAs-122 than in FeAs-1111. Furthermore, we found that the electronic SH coefficient $\gamma_e(T)$ of Ba$_{ 0.6}$K$_{ 0.4}$Fe$_2$As$_2$ is weakly temperature dependent and increases almost linearly with the magnetic field in 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 anisotropic or nodal gap. A detailed analysis reveals that the $\gamma_e(T)$ of Ba$_{ 0.6}$K$_{ 0.4}$Fe$_2$As$_2$ cannot be fitted with a single gap of s-wave symmetry probably due to the multigap effect. These results indicate clear difference between the properties of the superconducting state of the holed-doped Ba$_{ 0.6}$K$_{ 0.4}$Fe$_2$As$_2$ and the F-doped LnFeAsO (Ln = rare earth elements) systems, which we believe is originated from the complex Fermi surface structures in different systems.
Categories: cond-mat.supr-con