Electronic Properties and Atomic Composition of Single-Atomic-Layer (Pb,Bi)/Ge Compound Structures

抄録

Recently, the appearance of spin-triplet Cooper pairs in single atomic-layer (SAL) superconductors due to the combination with Rashba-type spin-splitting has become a topic of great interest since its first discovery in the SAL (Tl,Pb) structure on semiconductor substrates. In the search for other structures exhibiting these phenomena, several (Pb,Bi) SAL structures have been found to be promising candidates due to their strong spin-orbit coupling. In theoretical studies, the (Pb,Bi)/Ge-√3×√3 has been predicted to exhibit chiral topological superconductivity in addition to the emergence of a van Hove singularity [1-3]. Experimentally, the (Pb,Bi)/Ge-2×2, -2√3×2√3 mixed structure was successfully fabricated and its surface structure confirmed by scanning tunneling microscopy [4]. As for the electronic properties of these structures, no experimental results have been reported.

For this study, we measured the electronic structure of various (Pb,Bi) SAL structures using angle-resolved photoemission spectroscopy (ARPES). We successfully fabricated the (Pb,Bi)/Ge-1×1, -√3×√3 and -2×2 structures, for which the corresponding low-energy electron diffraction (LEED) patterns are shown in Figures 1 (a)-(c). Figure 1 (d) shows the core-level photoemission spectra of the Pb and Bi 5d peaks. These spectra were taken at the beamline BL-13 of the Saga Light Source with a beam energy of hν = 70eV. To determine the Pb to Bi composition ratio of each structure, we compared the intensities of these photoelectron spectra. We found that the (Pb,Bi)/Ge-2×2 (-√3×√3) structures has a ratio of 0.5:1 (1.85:1), while previous reports state a 1:1 (3:1) ratio [1-4]. In addition, we found that the (Pb,Bi)/Ge-1×1 structure has a similar Pb to Bi ratio with 2.04:1 as the -√3×√3 structure. Moreover, we observed that all of our structures have a metallic band dispersion that did not match the reported ones from previous studies.

In this presentation, we will show the electronic states of these structures and compare them to calculated band structures and discuss the relationship between the electronic structures and composition ratios.

[1] W. Qin et al., Nat. Phys. 15, 796 (2019).

[2] L. Li et al., Phys. Rev. B 102, 035150 (2020).

[3] S. Sun et al., Phys. Rev. B 103, 235149 (2021).

[4] A. Mihalyuk et al., Front. Mater. 9, 882008 (2022).