One method for inducing these ordered phases is by imposing a symmetry-breaking superstructure through the deposition of adatoms on graphene . In 2016, the C atom-centered Kekulé-Y bond flavor (Fig. 1c) was realized in graphene grown on Cu(111), driven by a dilute ensemble of Cu vacancies (“anti-adatoms”) forming a hidden Kekulé lattice [2,3]. More recently, signatures of the originally-proposed hollow-site Kekulé-O bond flavor (Fig. 1b) have been realized through dilute deposition of lithium adatoms on graphene . Curiously, the lithium ad-atom superstructure, which heavily dopes the system, has shown signs of inducing superconductivity below 5.9 K, while the Cu vacancy system has not. In the broader family, calcium-intercalated graphite also exhibits Brillouin Zone folding behavior, and hosts superconductivity with a Tc of 11.5 K .
To date, these ordered phases have been explored using static methods. To better understand how adatom scatterers influence the electron-phonon coupling (EPC) in these various regimes, and its role in stabilizing superconductivity, we will combine Angle Resolved Photoemission Spectroscopy (ARPES) – with its sensitivity to electronic structure – and time-domain pump-probe techniques. Recently, utilizing a unique high-harmonic laser source that combines fine temporal and spectral resolution (~100 fs and 20 meV) with high repetition rate (60 MHz), quantized energy-loss processes involving strongly-coupled optical phonons have been explicitly resolved in time-resolved ARPES studies on graphite, providing a new approach for the direct determination of electron-phonon coupling in the time domain .
The program will include the growth and preparation of ordered phases in graphene samples (Starke-MPI, Damascelli-UBC). The new generation high-harmonic laser-based ARPES in Jones and Damascelli labs at UBC will be used to study the quenching and reformation of Kekulé and CDW phases, as well as the deviation of EPC from pristine graphene, to gain valuable insight into the stability of the ordered phase and their possible interplay/competition with superconductivity.
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