Photoemission evidence of electronic stabilization of polar surfaces inK3C60

We present a detailed investigation of the surface electronic structure of solid ${\mathrm{K}}_3{\mathrm{C}}_{60}$ in connection with its electronic transport properties. We find that the conductivity is extremely sensitive to the K concentration of the surface layer, and that the best-conducting samples with the highest superconducting transition temperatures have surfaces with the highest density of states at the Fermi level as measured by photoemission. The ${\mathrm{C}}_{60}$ ions at the surface have, however, a valence that deviates appreciably from the $3-$ bulk value, namely $2.5-$ or even $1.5-,$ depending on the preparation procedure. We attribute this as being the result of an electronic rather than an atomic surface reconstruction to avoid the divergence of the electrostatic potential associated with the polar (111) surface termination of ${\mathrm{K}}_3{\mathrm{C}}_{60}.$ We argue that such a mixed-valence surface should always be a metal, irrespective of whether the bulk is a metal or a Mott-Hubbard insulator.