Nodal gap in Fe-based layered superconductor LaO0.9F0.1-δFeAs probed by specific heat measurements

We report the specific heat measurements on the newly discovered Fe-based layered LaO0.9F0.1-delta FeAs superconductor with the onset transition temperature Tc approximate to 28 K. A nonlinear magnetic field dependence of the electronic specific heat coeffcient gamma(H) has been found in the low temperature limit, which is consistent with the prediction for a nodal superconductor. The maximum gap value Delta(0) approximate to 3.4 +/- 0.5 meV is derived by analysing gamma(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 315.7 K. Our results suggest an unconventional mechanism for this new superconductor.