Photoemission study of (Cu,Tl)-1223 and Tl-1223 with T/sub c/ above 130 K

Abstract

A change of electronic structure and chemical bond nature of high pressure synthesized (Cu,Tl)Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub 9-/spl delta// [(Cu,Tl)-1223] and TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub 9-/spl delta// [Tl-1223] with reduction-annealing, which leads to a significant rise of their superconducting transition temperature T/sub c/ above 130 K, has been investigated by in-situ photoemission spectroscopy. Their intrinsic surface nature has been achieved by fracturing in vacuum. In (Cu,Tl)-1223, a Fermi edge is observed for the first time. Trivalent Tl and heavily over-doped states of Cu-O clusters are dominant in as-grown specimens of both systems. The annealing resulted in remarkable shifts of photoemission spectra of Ba and Tl core levels towards higher binding energy with an increase of the annealing temperature, which means a selective oxygen-depletion around their charge-reservoirs and a decrease of valency of Tl. In contrast, there are quite little shape-changes and movements in valence band spectra and core signals of Cu and O. These results reveal that, in the high temperature-treated specimens, major portions of the electrons left should be absorbed into the cations in their charge reservoir, which consequently should cause a much more moderate change of the hole-concentration of the CuO/sub 2/ planes. It suggests that the utilization of this charge-redistribution mechanism in over-doped specimens should be an advantageous way to optimize the electronic structure of the 1223 phases for high T/sub c/.