Evidence for two-dimensional nucleation of superconductivity in   MgB$_{2}$
  According to the crystal structure of MgB$_{2}$ and band structure calculations quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical fields and resistance measurements of 30 nm thick MgB$_{2}$ films grown on MgO single crystalline substrate. A linear temperature dependence of the parallel and perpendicular upper critical fields indicate a 3D-like penetration of magnetic field into the sample. Resistivity measurements, in contrast, yield a temperature dependence of fluctuation conductivity above T$_{c}$ which agrees with the Aslamazov-Larkin theory of fluctuations in 2D superconductors. We consider this finding as an experimental evidence of two-dimensional nucleation of superconductivity in MgB$_{2}$. 
A. S. Sidorenko (1,2), L. R. Tagirov (1,3), A. N. Rossolenko (4), V.   V. Ryazanov (4), R. Tidecks (1) ((1) Institut fuer Ph...