Abstract
The rich interlayer couplings in low-dimensional graphitic materials, such as one-dimensional (1D) carbon nanotubes and two-dimensional (2D) graphene multilayers, and versatile controlling knobs to tune their physical properties have led to significant advancements in condensed matter physics in recent years. In this talk, I will first show the spectroscopic discovery of uncharted phonon modes in one commensurate and three incommensurate DWNT crystals, three of which concurrently exhibit strongly reconstructed electronic band structures [1]. Our density functional theory (DFT) calculations for the experimentally observed commensurate DWNT (7,7) @ (12,12) reveal that this new phonon mode originates from a (nearly) degenerate coupling between two transverse acoustic modes (ZA modes) of constituent inner and outer nanotubes having approximately trigonal and pentagonal rotational symmetry along the nanotube circumferences. Such coupling strongly hybridizes the two phonon modes in different nanotubes and leads to the formation of a unique lattice motion featuring evenly distributed vibrational amplitudes over inner and outer nanotubes, distinct from any known phonon modes in 1D systems.
The second topic is the nanoscale optical imaging of purely stacking-induced electric polarization and polarization switching in the thinnest natural graphene polytypes hosting broken inversion and mirror symmetries [2]. To date, the studies of electric polarization and ferroelectricity in 2D van der Waals materials have predominantly reported semiconducting and/or insulating compounds, and spontaneous electric polarization and polarization switching in 2D elemental metals remain scarce and unexplored. We discover ABAC and ABCB stacking orders in tetralayer graphene featuring opposite out-of-plane electric polarizations from the distinct optical response upon electron and hole doping, which also suggests the coexistence of electric polarization and appreciable free carrier density. We also provide the first microscopic evidence of polarization switching via domain wall (DW) sliding in gated tetralayer graphene, a 2D elemental metal. We further achieve controlled local switching between two adjacent polar stackings with opposite polarizations by both mechanical manipulation and local electric field application.
References
- S. Sun et al., Strongly hybridized phonons in one-dimensional van der Waals crystals. Physical Review Letters 134, 176101 (2025) (Preprint arXiv:2408.08596)
- Z. Zhou, X. Peng et al., Optical imaging of spontaneous electric polarization and polarization switching in tetralayer graphene. Preprint arXiv:2504.06874 (2025)
Anyone interested is welcome to attend.