Abstract:
Two-dimensional (2D) materials with only few atoms thickness enables us to investigate quantum phases such as superconductivity (SC) and charge density wave (CDW) under 2D limit. Former studies by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) have observed the superconductivity in atomic-thickness layers of Pb and In on Si substrates [1]. Besides, the current discovery of superconductivity in twist bilayer graphene drives the focus on electronic correlations in honeycomb lattices [2]. Similar to Si forming silicene [3], Pb has also been proposed a similar structure as graphene, which is called plumbene. Theoretical predictions indicate that the tuning buckling and spin-orbit couplings in 2D honeycomb lattices may enhance the superconducting critical temperature [4]. In the current study, a plumbene-based superstructure grown by deposition a small amount of Au on Pb(111) surface are found. By STM and temperature-dependent STS, a higher critical temperature (Tc) in plumbene-based superstructure than that of Pb substrate is observed, which is totally beyond the proximity effect [5]. By combining angle-resolved photoemission spectroscopy with density functional theory, the monolayer Au-intercalated low-buckled plumbene sandwiched between the top Au Kagome layer and the bottom Pb(111) substrate is confirmed and the electron–phonon coupling-enhanced superconductivity is revealed.
Another layered material in the family of transition metal dichalcogenides (TMD), NbSe2 shows coexisting behavior of SC and CDW. A 3×3 charge order can be observed in 2D layers below 33.5K [6]. Although CDW can be studied by different techniques, the spatial distribution within a 2D layer has never been systematically visualized. Here, by using STM and density functional theory (DFT) calculation, we monitored the evolution of CDW along the c-axis and realized a tomography scan of CDW of the topmost layer [7]. The results show that the appearance of the CDW varies while tuning the tunneling current and undergoes a transition from the outermost Se level to Nb level. The calculation of orbital charge distributions shows that both CDW intensity modulation and the transition are strongly correlated with the distribution of Se 4p orbitals and Nb 4d orbitals.
Reference
[1] D. Roditchev et al., Nat. Phys. 11 332 (2015). [2] Y. Cao et al., Nature 556 43 (2018). [3] C. L. Lin et al., Appl. Phys. Express 5, 045802 (2012). 556 43 (2018). [4] B. Zhang et al., Physica E 130 114688 (2021). [5] W. H. Chen et al., Adv. Sci. 2023, 2300845 (2023). [6] X. Xi Nat. Nanotech. 10, 765 (2015). [7] J. Y. Wu et al., Phys. Rev. Research 7, 043206 (2025).