Metal-insulator transition at a surface of a ferromagnetic-metal La$_{0.75}$Ca$_{0.25}$MnO$_3$/SrTiO$_3$(100) thin film

We have performed low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on a ferromagnetic-metal La$_{0.75}$Ca$_{0.25}$MnO$_3$/SrTiO$_3$(100) thin film surface. Our topographic images show two-domain zigzag patterns with ($\sqrt{2}\times\sqrt{2}$) periodicities in the perovskite structure. In addition, we measured an energy gap at the Fermi level in our STS spectra, in contrast to the ferromagnetic-metal properties obtained by ensemble measurements. First-principle calculations suggest that the topmost zigzag structure is caused by the structural relaxation based on the orthorhombic nature in bulk, suppressing the carrier itinerancy.