Tunneling observation of Tl quasi-localized impurity states in superconductive semiconductor Pb(Tl) Te

Abstract

The superconductivity of Pb(Tl) Te has shown a possibility of strong correlation with the resonance scattering of carriers with quasi-localized Tl impurity states. In order to detect the quasi-localized states directly, tunneling spectrum observation was carried out with use of several planar and point-contact junctions on well-defined HWE film samples. As a result, two quasi-localized Tl impurity states, QL1 and QL2 at an interval of 13–15 meV, were reproducibly observed on the spectra. Comparing these two states among the spectra observed on the samples with homogeneous Tl distribution, it is found that the upper QL1 state shows a half-width of Γ QL1 = 6±1 meV which is about a half of Γ QL2 for the second QL2 state. This result means the occurrence of stronger resonance scattering with the QL1 state, and shows a degree of consistency with the electrical transport characteristics as had been reported [1]. A larger enhancement in superconducting T c had actually been observed at the upper QL1 state rather than at the QL2 state. The obtained results strongly support the resonance scattering model for the superconductivity in this Pb(Tl) Te system.