Unusual Hall effect in superconductingMgB2films

We have investigated the temperature and magnetic field dependence of the Hall coefficient of two well-characterized superconducting ${\mathrm{MgB}}_{2}$ films ${(T}_{c0}=38.0\mathrm{K})$ in both the normal and superconducting states. Our results show that the normal-state Hall coefficient ${R}_{H}$ is positive and increases with decreasing temperature, independent of the applied magnetic field (to 8 T). We find that ${R}_{H}^{\ensuremath{-}1}\ensuremath{\propto}T(40--300\mathrm{K})$ and $\mathrm{cot}{\ensuremath{\theta}}_{H}\ensuremath{\propto}{T}^{2}(100--300\mathrm{K}).$ As the sample is cooled below ${T}_{c}(H),$ ${R}_{H}$ decreases rapidly with temperature and changes sign before it reaches zero. The position and magnitude at which ${R}_{H}$ shows a minimum depends on the applied field.