Nodal Gap in Fe-Based Layered Superconductor LaO_0.9F_{0.1-delta}FeAs Probed by Specific Heat Measurements

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We report the specific heat measurements on the newly discovered Fe-based layered superconductor LaO_0.9F_{0.1-delta}FeAs with the onset transition temperature T_c \approx 28 K.

• A nonlinear magnetic field dependence of the electronic specific heat coefficient 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 \approx 3.4$\pm$0.5 meV was derived by analyzing gamma(H) based on the d-wave model.
• We also detected the electronic specific heat difference between 9 T and 0 T in wide temperature region, a specific heat anomaly can be clearly observed near T_c.