The paper presented in this paper investigates the intricate electronic phenomena observed in twisted bilayer graphene (tBLG), focusing on a twist angle of ~0.93º, which is notably below the established magic angle. This exploration unveils insights into correlated insulating and superconducting states, contributing to the burgeoning field of twistronics.
Observations and Experimental Methodology
The experimental work revealed superconductivity and Mott-like insulating states in tBLG at a sub-magic angle of 0.93º. Previous studies had established that insulating and superconducting phases are observed near the magic angle of ~1.1°, where the electronic bands become extraordinarily flat, facilitating strong electron interactions. These phenomena at a reduced twist angle suggest an expanded range of twist angles where such correlated states emerge, challenging the previously accepted boundary conditions of the magic angle theory.
Transport measurements demonstrated a correlated insulating state at half-filling and the emergence of superconductivity at slightly varied electron densities. Notably, the superconducting phase displayed a two-step transition in resistance as temperature decreased, with critical transitions at approximately 1.5 K and 0.3 K, raising questions about the inhomogeneity and potential competing phases within the device.
Theoretical Implications
Theoretical modeling using the Bistritzer-MacDonald framework, modified for lattice relaxation, was conducted to understand the electronic band structures at this twist angle. The findings indicate that the low-energy moiré Dirac bands are not isolated, diverging from behaviors observed near the magic angle. The presence of high-energy Dirac points explains the observed resistance peaks and their narrowness. Theoretical predictions also highlighted a significant density of states (DOS) in nearly flat bands, further substantiating the emergence of Mott-like behavior beyond the classical magic angle.
Novel Electronic States and Superlattice Symmetry
Intriguingly, the paper reports unidentified resistance peaks at higher electron concentrations of ±5 electrons per moiré unit cell, attributed to correlated insulating states in partially filled high-energy bands. This phenomenon suggests the existence of previously uncharted electronic states in tBLG, possibly pointing towards emergent quasiparticle symmetries beyond the known spin-valley SU(4) symmetry.
Additionally, the manifestation of resistance peaks at ±12 electrons per moiré unit cell indicates Dirac-like points, offering critical insights into the fermionic quasiparticle behavior and supporting the predictions of density of states minima in the studied band structure.
Practical and Theoretical Implications
This paper significantly furthers our understanding of the electronic landscape of tBLG, underscoring that correlated physics can be extended to angles below the classical magic threshold. Its findings necessitate a reevaluation of Dirac point conductance and correlated state behaviors, which may inform the design of future quantum devices exploiting moiré superlattices at lower twist angles.
For theoretical advancements, this work provides a foundation for exploring correlated states in other low-dimensional materials, offering an avenue for examining superconductivity, insulating states, and their hosting lattice structures. Continued exploration could advance both fundamental physics and practical applications in quantum computing and nanoelectronics.
Future investigations may focus on detailed spin-valley interactions and the exploration of superconducting phases under various doping levels. Further theoretical exploration is essential to unravel the complexities of the reported novel states, ensuring these discoveries are ingrained into the broader understanding of quantum materials.
Overall, this research highlights the intricate balance between twist angle, electronic interactions, and external conditions, providing a richer tapestry of phenomena yet to be fully comprehended.