Nodal Gap in Fe-Based Layered Superconductor LaO_0.9F_0.1-δFeAs Probed by Specific Heat Measurements

We report the specific heat measurements on the newly discovered Fe-based layered LaO_0.9F _0.1-δFeAs superconductor with the onset transition temperature T_c≈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 ∆₀≈3.4 ±0.5meV is derived by analysing γ(H) based on the d-wave model. We also detected the electronic specific heat difference between 9T and 0T in a wide temperature range, a specific heat anomaly can be clearly observed near T_c. The Debye temperature of our sample is determined to be about 315.7K. Our results suggest an unconventional mechanism for this new superconductor.