Nodal Gap in Fe-Based Layered Superconductor LaO _0.9 F _0.1-δ FeAs Probed by Specific Heat Measurements

Abstract

We report the specific heat measurements on the newly discovered Fe-based layered LaO0.9F0.1-δFeAs superconductor with the onset transition temperature Tc ≈ 28K. A nonlinear magnetic field dependence of the electronic specific heat coefficient γ(H) has been found in the low temperature limit, which is consistent with the prediction for a nodal superconductor. The maximum gap value Δ0 ≈ 3.4 ± 0.5 meV is derived by analysing γ(H) based on the d-wave model. We also detected the electronic specific heat difference between 9 T and 0 T in a wide temperature range, a specific heat anomaly can be clearly observed near Tc. The Debye temperature of our sample is determined to be about 315.7K. Our results suggest an unconventional mechanism for this new superconductor.