Electronic state of MgB₂ superconductor

Thanks to the high quality single crystals of MgB₂, we have measured various physical properties to extract important features of the electronic state in this superconductor. The electron density maps calculated from a precise X-ray diffraction analysis and the band dispersions determined by the angle-resolved photoemission spectroscopy demonstrate that the boron orbitals play a central role, just as predicted by the band theory. The strong coupling of the high frequency optical phonon with the boron bands is indicated in resistivity and Raman scattering spectra. Although the multi-band structure significantly affects various normal state properties, no clear multi-gap feature can be observed in the Raman scattering spectra below T_c.