Large anisotropic normal-state magnetoresistance in clean MgB$_2$ thin films

MgB$_2$ is arguably the first material which shows clear multiband superconductivity and two energy gaps. The interplay between the interband and intraband scattering as well as electron-phonon coupling has manifested in many physical properties. In this talk, I will review the magnetoresistance measurements of the normal state of MgB$_2$. We have shown large normal-state magnetoresistance with temperature-dependent anisotropy in very clean epitaxial MgB$_2$ thin films (residual resistivity much smaller than 1 $\mu \Omega$cm) grown by hybrid physical-chemical vapor deposition.[1] The magnetoresistance shows a complex dependence on the orientation of the applied magnetic field, with a maximum magnetoresistance (MR~136\%) observed at the field $H$ parallel to $ab$ plane at low temperature. However, the angular dependence changes dramatically as the temperature is increased, and at high temperatures ($T >$ 100 K), the magnetoresistance maximum changes to the $H$ perpendicular to $ab$ direction. We attribute the large magnetoresistance and the evolution of its angular dependence with temperature to the multiple bands with different Fermi surface topology in MgB$_2$ and the relative scattering rates of the $\sigma$ and $\pi$ bands, which vary with temperature due to stronger electron-phonon coupling for the sigma bands. The change of anisotropy with disorder has also been reported in neutron irradiated MgB$_2$ thin films. \newline Work done in collaboration with B. T. Liu, Y. F. Hu, J. Chen, H. Gao, L. Shan, H. H. Wen, A.V. Pogrebnyakov, J. M. Redwing, and X. X. Xi. \newline \newline [1] Qi Li et al., Phys. Rev. Lett., 96, 167003 (2006).