Host: The Japan Society of Vacuum and Surface Science
Name : Annual Meeting of the Japan Society of Vacuum and Surface Science 2024
Location : [in Japanese]
Date : October 20, 2024 - October 24, 2024
Graphene-based superconductors are promising for various applications due to their optical transparency, mechanical strength, and flexibility, with twisted bilayer graphene being a prominent example [1]. This system exhibits unconventional superconductivity characterized by strong correlation effects and a van Hove singularity (VHS) in the electronic structure.
An alternative approach to achieving graphene-based superconductors is the synthesis of interlayer compounds, similar to graphite intercalation compounds. These compounds are typically understood to exhibit conventional superconductivity via electron-phonon interactions, but the presence of VHS in the electronic structure remains a topic of interest. VHS has often been observed using angle-resolved photoemission spectroscopy (ARPES) following the intercalation of various metals into the interface of epitaxial graphene and its substrate. Despite the potential for VHS-driven unconventional superconductivity in intercalated systems, experimental confirmation has been limited by the absence of integrated vacuum systems combining sample growth, ARPES, and cryogenic measurements.
In this study, we investigated the correlation between VHS and superconductivity in calcium-intercalated bilayer graphene [2-4] using an all-in-one multi-probe system that includes molecular beam epitaxy, photoelectron spectroscopy, and electrical transport measurements at cryogenic temperatures. Our findings indicate that in the dense calcium phase, an epitaxial layer of calcium forms at the graphene-SiC interface, enhancing the superconducting transition temperature (Tc). Both theoretical and experimental analyses revealed a metallic state at the interface and its hybridization with one of the Dirac cones. This leads to a distinctive VHS state that increases the density of states near the Fermi level. Two types of attractive interactions are proposed to contribute to the enhanced Tc through VHS: increased electron-phonon coupling via low-energy phonon modes and direct Coulomb interactions between electrons and holes.
Furthermore, in lithium-intercalated bilayer graphene, we demonstrated that the VHS of the Dirac band is also situated at the Fermi level. Interestingly, the Fermi level slightly shifts from the VHS as the number of graphene layers increases. This phenomenon suggests potential for superconductivity and exploring various electronic ground states associated with the VHS.
References:[1] Y. Cao et al., Nature 556, 43 (2018).[2] S. Ichinokura et al., ACS Nano 10, 2761 (2016).[3] H. Toyama et al., ACS Nano 16, 3582 (2022).[4] S. Ichinokura et al., ACS Nano, 18, 13738 (2024).[5] S. Ichinokura, et al., Physical Review B 105, 235307 (2022).
