Electronic structure of a conventional superconductor, ZrB¬12 -a high resolution photoemission study

I will discuss the electronic structure of a BCS type superconductor, ZrB12 using high resolution photoemission spectroscopy and single crystalline samples. X-ray photoemission spectroscopy in the hard x-ray regime reveals boron deficiency at the surface relative to the bulk leading to multiple valence states of  Zr. Intense satellite features are observed in the bulk Zr core level spectra suggesting importance of electron correlation among the conduction electrons while the surface appears to be uncorrelated. These results reveal complexity in the electronic structure even in a conventional superconductor. Comparisons of valence band spectra at different photon energies reveal that Zr 4d states are the main contributing states near the Fermi level. High resolution spectra reveal a dip in spectral density of states at Fermi level, gradually increases with the decrease in temperature down to 10 K. We observe a quasiparticle peak in the density of states at 10 K; the value of gap obtained is 8 meV. Large ratio of 2Δ/kBTc ~7.4 is observed at the surface, indicating strong coupling in the system. The spectral line shape shows a (є- єf)1.1 dependence at all temperature studied, emphasizing that appearance of pseudogap is not related to any disorder in the system.