Studying electron-phonon coupling in Kekule and Charge Density Wave phases in graphene

The project consists of pump-probe studies of Kekulé distorted phases in graphene under the supervision of A. Damascelli and D. J. Jones (QMI-UBC, Vancouver), and U. Starke (MPI-FKF, Stuttgart), experts in the fields of photoemission, ultrafast spectroscopy, and graphene.

Graphene, a single two-dimensional sheet of graphite, is composed of carbon atoms arranged in a honeycomb lattice. In addition to hosting quasiparticles that mimic massless Dirac fermions, graphene has been shown to support many interesting emergent exotic phases, such as superconductivity [1] and charge density wave (CDW) order [2,3]. The Kekulé distortion (KD) is one such emergent phase, consisting of a commensurate CDW and a periodic lattice distortion whereby the C-C bond symmetry breaks into a super-lattice of short – and long – atomic bonds (Fig. 1a, blue/black bonds).

Fig 1. Kekulé distortions in graphene

a) lattice of graphene, red arrows denote lattice vectors. b), c) The Kekulé-O (lithium adatom deposition) and Kekulé-Y (Cu-vacancy) flavor of the distorted phase, showing the modification of particular bonds of the hexagonal lattice. The new lattice vectors are shown in yellow.

One method for inducing these ordered phases is by imposing a symmetry-breaking superstructure through the deposition of adatoms on graphene [1]. 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 [4]. 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 [5]. 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)[6], 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 [7]. 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. [1] Ludbrook et al., PNAS 112, 11795 (2015). [2] Cheianov et al., Solid State Communications 149, 1499, (2009). [3] Gutiérrez et al., Nature Physics 12, 950, (2016). [4] Qu, et al., in preparation. [5] Yang et al., Nature Communications 5, 3493, (2014). [6] Mills, et al., arXiv:1902.05997 (2019). [7] Na*, Mills*, et al., arXiv:1902.05572 (2019).