Structural and superconducting properties of ultrathin PbBi3 films fabricated at low temperature

Bismuth (Bi), as the stable heaviest element in the periodic table of elements, has strong spin-orbit coupling, which has attracted a lot attention as the parent material of various known topological insulators. Previous calculations predicted that Bi(111) with a thickness of less than eight bilayers and the ultrathin black-phosphorus-like Bi(110) films were single-element two-dimensional (2D) topological insulators. However, it is generally believed that these crystalline bismuth phases are either non-superconductive or with quite low transition temperature (0.5 mK). Since lead (Pb) is a good superconducting elementary material and there is a relatively small radius difference between the Bi and Pb atoms, they can be mixed to form superconducting alloys with any ratios, according to the Hume-Rothery rule. One may thus expect to form crystalline Bi based superconductors by Pb substitution, which might host intriguing topological superconductivities. While our previous work has demonstrated a low-temperature stable Pb1-xBix (x~0.1) alloy phase in which Pb in the Pb(111) structure is partially replaced by Bi, the Bi crystalline structure-based phases of the superconducting alloys still lack in-depth research. Here, we report a new low-temperature phase of Pb-Bi alloy thin films, namely PbBi3, on the Si(111)-(7×7) substrate, by co-depositing Pb and Bi at a low temperature of about 100 K followed by an annealing treatment of 200 K for 2 h. Using low-temperature scanning tunneling microscopy and spectroscopy (STM/STS), we in situ characterize the surface structure and superconducting properties of the Pb-Bi alloy films with a nominal thickness of about 4.8 nm. Two spatially separated phases with quasi-tetragonal structures are observed in the surface of the Pb-Bi alloy films, which can be identified as the pure Bi(110) phase and the PbBi3 phase, based on their distinct atomic structures, step heights and STS spectra. The PbBi3 film has a base structure similar to Bi(110), where about 25% of the Bi atoms are replaced by Pb, and the surface shows a root2×root2R45° reconstructed structure. The superconducting behavior of the PbBi3 phase is characterized by the STS spectra. The superconducting transition temperature is about 6.13 K, determined by the variable-temperature measurements. The corresponding ratio of 2Delta(0)/kBTc is about 4.62 with the superconducting gap Delta(0)=1.22 meV at 0 K, suggesting that PbBi3 is a strong coupling superconductor. By measuring the magnetic field dependent superconducting coherence lengths, we obtain that the upper critical field is larger than 0.92 T. We further investigate the superconducting proximity effect in the normal metal-superconductor (N-S) heterojunctions consisting of the non-superconducting Bi(110) and the superconducting PbBi3 domains. The N-S heterojunctions with both in-plane and step-like configurations are measured, which suggest that the atomic connection and the area of the quasi-2D Josephson junctions and the external magnetic field can affect the lateral superconducting penetration lengths. We also observe the zero-bias conductance peaks (ZBCPs) in the superconducting gap of the PbBi3 surfaces in some cases at zero magnetic field. By measuring dI/dV spectra at variable temperatures and by adopting a superconducting Nb tip, we identify that the ZBCPs are originated from the superconductor-insulator-superconductor (S-I-S) junction formed between a superconducting tip and the sample, Nevertheless, the Bi(110)-based PbBi3 phase may provide a possible platform to explore the intriguing topological superconducting behaviors by further examining the behaviors at the vortexes under magnetic fields, or in the vicinity of the potentially topological superconducting Bi(110) islands by considering the proximity effect.