Epitaxial growth of MgB$_2$ films at ambient temperature

We grew crystalline MgB$_2$ thin films using molecular beam epitaxy at a low substrate temperature of 110\,$^\circ$C under an ultrahigh vacuum of about 10$^{-6}$\,Pa. MgB$_2$ thin films were deposited on the (001) surface of a 4H-SiC substrate with an epitaxial Mg buffer layer. The epitaxial growth was confirmed by X-ray diffraction measurements. MgB$_2$ thin films show a sharp superconducting transition at 27.2\,K, with a relatively narrow superconducting transition width $\Delta T_{\rm c}$ = 0.9\,K. The growth temperature was lower than any in prior reports on superconducting MgB$_2$ thin films. The presence of the epitaxial Mg buffer layer is crucial for reducing the epitaxial temperature.