ADSORBED METALLIC CHAINS AS APPROXIMATIONS TO QUASI-ONE-DIMENSIONAL SYSTEMS

A study of quasi-one-dimensional Pb, Tl and mixed Pb-Tl overlayers on noble metal substrates using LEED, Auger electron spectroscopy and photoelectron spectroscopy is presented. A [66/22BAR] [or p(6 square-root 2 x 2 square-root 2)R45-degrees] Tl chain structure on Cu(100) shows a temperature dependent Fermi level bandgap and a structure consistent with a simple dimerization along the chains indicating the formation of a Peierls distortion or charge-density wave (CDW). In contrast a lower density [22/22BAR] [or c(4 x 4)] chain structure on Cu(100) and a higher density [10/02] [or p(2 x 1)] chain structure on Ag(100) show no evidence for the formation of superstructures and no detectable bandgap. In mixed Pb-Tl layers on Cu(100) LEED indicates the formation of interpenetrating chains and the formation of a Tl chain phase with the same density as the [66/22BAR] structure but with the Peierls distortion suppressed. Preliminary conductivity measurements indicate that Tl chains on Cu(100) with a slightly lower density than the [66/22BAR] CDW structure may have entered the superconducting region of the 1D phase diagram with a transition temperature in the region of 220 K.