Experimental Evidence of Cage Conduction Bands in Superconducting Cement   12CaO$\cdot$7Al$_2$O$_3$
  Natural 12CaO$\cdot$7Al$_2$O$_3$ (C12A7) is a wide bandgap insulator, but conductivity can be realized by introducing oxygen deficiency. Currently, there are two competing models explaining conductivity in oxygen-deficient C12A7, one involving the electron transfer via a "cage conduction band" inside the nominal band gap, the other involving electron hopping along framework lattice sites. To help resolve this debate, we probe insulating and conducting C12A7 with X-ray emission, X-ray absorption, and X-ray photoemission spectroscopy, which provide a full picture of both the valence and conduction band edges in these materials. These measurements suggest the existence of a narrow conduction band between the main conduction and valence bands common in both conducting and insulating C12A7 and support the theory that free electrons in oxygen-deficient C12A7 occupy the low-energy states of this narrow band. Our measurements are corroborated with density functional theory calculations. 
J. A. McLeod and A. Buling and E. Z. Kurmaev and P. V. Sushko and M.   Neumann and L. D. Finkelstein and S.-W. Kim and H. Ho...
cond-mat.supr-con