In a groundbreaking study, an international team of researchers has unveiled the intricate relationship between temperature-dependent electrical transport and the unique vibrational properties of minimally twisted bilayer graphene. This research, led by Alex Boschi and Héctor Ochoa, alongside collaborators from various institutions, sheds light on how the formation of a moiré superlattice in two-dimensional (2D) materials can dramatically alter their electronic and vibrational properties.
The study focuses on the lowest-energy phonon modes of the moiré superlattice, specifically two acoustic branches known as phasons. These phasons describe the sliding motion of one graphene layer relative to the other. Given their low-energy dispersion and damping, phasons are suspected to play a significant role in scattering electrons within moiré materials. The researchers investigated temperature-dependent electrical transport in minimally twisted bilayer graphene, a system that develops multiple weakly-dispersive electronic bands and a reconstructed lattice structure.
The findings reveal a linear-in-temperature resistivity across the band at temperatures above approximately 10 Kelvin, preceded by a quadratic temperature dependence at lower temperatures. Notably, the linear-in-temperature resistivity observed in minimally twisted bilayer graphene is significantly higher—up to two orders of magnitude—than that in monolayer graphene. However, it is reduced by about a factor of three compared to magic-angle twisted bilayer graphene. Interestingly, this resistivity is modulated by the recursive band filling, with minima occurring near the full filling of each band.
To contextualize these experimental observations, the researchers compared their results with semiclassical transport calculations. Their analysis suggests that the experimental trends are consistent with scattering processes mediated by longitudinal phasons. These phason-mediated processes dominate the resistivity, overshadowing the contributions from conventional acoustic phonons of the monolayer.
This research underscores the profound impact of vibrational modes unique to moiré materials on carrier transport. The findings not only enhance our understanding of the fundamental physics governing these materials but also pave the way for potential applications in electronic and optoelectronic devices. By elucidating the role of phasons in electron scattering, this study opens new avenues for manipulating and optimizing the electronic properties of 2D materials for future technologies.
