arXiv subjects

Alejandro Ramos-Alonso

Publications and source records attributed to Alejandro Ramos-Alonso.

3 recordsLinked to original sources

arXiv2026

Charged moir\'e phonons in twisted bilayer graphene

Moir\'e phonons describe collective vibrations of a moir\'e superlattice produced by long-wavelength relative displacements of the constituent layers. Despite coming from the backfolding of the acoustic phonons of the individual layers, many of these modes become infrared active when the system is doped. We illustrate this effect by a direct calculation of the optical absorption of twisted bilayer graphene (tBG) around different twist angles, including the magic angle. Several modes -- including the acoustic-like phason -- acquire a dipole moment via interband matrix elements of the electron-phonon coupling (EPC) when the flat band is filled or emptied, giving rise to new resonances in the optical conductivity within the single-electron gap that are strongly affected by relaxation. The phason in particular gains a charge that equals the amount of electrons per moir\'e cell added/removed to/from neutrality. Geometrically, this can be understood as the topological quantization of a sliding Chern number. The charged phason yields a Drude-like conductivity with an effective mass that increases with lattice relaxation. Our findings are testable via THz spectroscopy, and provide an experimental knob to characterize EPC strength and disorder in moir\'e materials at small twist angles.

cond-mat.mes-hall↗

arXiv2025

Phason-driven temperature-dependent transport in moir\'e graphene

The electronic and vibrational properties of 2D materials are dramatically altered by the formation of a moir\'e superlattice. The lowest-energy phonon modes of the superlattice are two acoustic branches (called phasons) that describe the sliding motion of one layer with respect to the other. Considering their low-energy dispersion and damping, these modes may act as a significant source of scattering for electrons in moir\'e materials. Here, we investigate temperature-dependent electrical transport in minimally twisted bilayer graphene, a moir\'e system developing multiple weakly-dispersive electronic bands and a reconstructed lattice structure. We measure a linear-in-temperature resistivity across the band manyfold above $T\sim{10}$ K, preceded by a quadratic temperature dependence. While the linear-in-temperature resistivity is up to two orders of magnitude larger than in monolayer graphene, it is reduced (approximately by a factor of three) with respect to magic-angle twisted bilayer graphene. Moreover, it is modulated by the recursive band filling, with minima located close to the full filling of each band. Comparing our results with a semiclassical transport calculation, we show that the experimental trends are compatible with scattering processes mediated by longitudinal phasons, which dominate the resistivity over the contribution from conventional acoustic phonons of the monolayer. Our findings highlight the close relation between vibrational modes unique to moir\'e materials and carrier transport therein.

cond-mat.mes-hall↗

arXiv2024

Flat and tunable moire phonons in twisted transition-metal dichalcogenides

Displacement fields are one of the main tuning knobs employed to engineer flat electronic band dispersions in twisted van der Waals multilayers. Here, we show that electric fields can also be used to tune the phonon dispersion of moir\'e superlattices formed by non-centrosymmetric materials, focusing on twisted transition metal dichalcogenide homobilayers. This effect arises from the intertwining between the local stacking configuration and the formation of polar domains within the moir\'e supercell. For small twist angles, increasing the electric field leads to a universal moir\'e phonon spectrum characterized by a substantially softened longitudinal acoustic phason mode and a flat optical phonon mode, both of which cause a significant enhancement in the vibrational density of states. The phasons also acquire a prominent chiral character, displaying a nonzero angular momentum spread across the Brillouin zone. We discuss how the tunability of the moir\'e phonon spectra may affect electronic properties, focusing on the recently discovered phenomenon of van der Waals ferroelectricity.

cond-mat.mes-hall↗