The superconducting gap of \QTR{em}{in situ} $MgB_{2}$ thin films by   microwave surface impedance measurements
  Precision measurements of the microwave surface resistance $R_{s}$ of in situ $MgB_{2}$ films directly reveal an exponential behavior of $R_{s}$ at low temperature indicating a fully-gapped order parameter. The entire temperature dependence of $R_{s}$ is well described by a Mattis-Bardeen formalism but with a small gap ratio of $\Delta (0)/kT_{c}=0.72$, corresponding to $\Delta (0)=1.9meV$. 
N. Hakim (1), C. Kusko (1), S. Sridhar (1), A. Soukiassian (2), X. H.   Zeng (2), X. X. Xi (2). ((1) Physics Department, Nor...