Two-Dimensional Electronic Band Structure of a Pb Monolayer Confined Under Epitaxial Graphene on SiC

Stacking two-dimensional (2D) layers is a promising path towards materials with novel electronic properties beyond conventional bulk systems. Epitaxial graphene on silicon carbide (SiC), also known as epigraphene, is a versatile platform for stabilizing 2D forms of elements at its interface via intercalation, which allows for modifying the properties of graphene [1,2]. Moreover, it is an elegant way of studying the intriguing properties of quantum-confined intercalant elements at the graphene/SiC interface [3,4]. In recent years, lead (Pb), a heavy element superconductor, has gained significant interest as an intercalant for epigraphene [5-9], due to the possible proximity effects, such as superconductivity and spin-orbit coupling [10,11]. In addition, theory calculations for a monolayer Pb on SiC show a non-trivial momentum-space spin texture, making the Pb-intercalated epigraphene a potentially interesting material for spintronics applications [12]. Here we present a detailed electronic band structure study of the interlayer Pb at the graphene/SiC interface using angle-resolved photoelectron spectroscopy (ARPES). The interlayer Pb bands cross the Fermi energy, showing its metallic nature. To a first order approximation, the interlayer bands can be fit with a free-electron-like model. The bands in the repeated Brillouin zone confirm the (1×1) alignment of the Pb layer relative to SiC. The 2D nature of the intercalated Pb is validated using constant initial state mapping, showing no dispersion of interlayer bands with photon energy, hence no bulk origin. Light polarization dependent ARPES measurements additionally shed light on the orbital character associated with various Pb band features along all the high-symmetry directions of its Brillouin zone. Density functional theory calculations for a (1×1) Pb monolayer on SiC show a good qualitative agreement with the experimentally observed interlayer bands, exhibiting a dominant <i>p</i>-orbital character. Furthermore, we demonstrate that the Pb-intercalated quasi-freestanding monolayer graphene on SiC is nearly charge neutral and involves charge transfer from both Pb and SiC. Finally, we discuss how an electron dopant, potassium (K) adsorbed on top of the Pb-intercalated epigraphene influences the band structure of graphene and the interlayer Pb. Apart from that, the Fermi surface illustrates that the K-adlayer orders itself in a (2×2) superstructure relative to the graphene unit cell.<br/><br/>This work is supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through the research unit FOR5242. MAX IV Laboratory (Lund, Sweden) is acknowledged for time at the BLOCH beamline. We are grateful to the beamline scientists at BLOCH for their support.<br/><br/>[1] C. Riedl <i>et al</i>., Phys. Rev. Lett. 103, 246804 (2009)<br/>[2] S. Link<i> et al</i>., Phys. Rev. B 100, 121407(R) (2019)<br/>[3] S. Forti <i>et al</i>., Nat. Commun. 11, 2236 (2020)<br/>[4] Ph. Rosenzweig <i>et al</i>., Phys. Rev. B 101, 201407(R) (2020)<br/>[5] A. Yurtsever <i>et al</i>., Small 12, 3956 (2016)<br/>[6] T. Hu <i>et al</i>., Carbon 179, 151 (2021)<br/>[7] M. Gruschwitz <i>et al</i>., Materials 14, 7706 (2021)<br/>[8] B. Matta <i>et al</i>., Phys. Rev. Res. 4, 023250 (2022)<br/>[9] P. Schädlich <i>et al</i>., Advanced Materials Interfaces 10, 2300471 (2023)<br/>[10] F. Calleja <i>et al</i>., Nat. Phys. 11, 43-47 (2015)<br/>[11] F. Paschke <i>et al</i>., Adv. Quantum Technol. 3, 2000082 (2020)<br/>[12] K. Yang <i>et al</i>., Phys. Rev. Lett. 128, 166601 (2022)