Emergence of a Helical Metal in Rippled Ultrathin Topological Insulator Sb\textsubscript{2}Te\textsubscript{3} on Graphene

TL;DR

This study demonstrates that nanoscale rippling in ultrathin Sb₂Te₃ topological insulator on graphene induces a transition to a helical metallic state with complex spin textures, revealing a new way to engineer spintronic properties via structural modulation.

Contribution

We show that substrate-induced ripples in ultrathin Sb₂Te₃/graphene heterostructures close the hybridization gap and create a dense manifold of helical minibands with unique spin textures, a novel mechanism for spintronic platform design.

Findings

Ripples are driven by substrate strain during cooling.

Ripple-induced structural modulation closes the hybridization gap.

Restores spin polarization and creates a helical metallic state.

Abstract

The integration of topological insulators (TIs) with graphene offers a pathway to engineer hybrid quantum states, yet the impact of strain at the 2D limit remains a critical open question. Here, we investigate the structural properties of ultrathin (1 quintuple layer) Sb$_2$Te$_3$ grown on single-layer graphene and, motivated by the structural modulations observed at the TI surface, explore theoretically how such nanoscale corrugations may influence the electronic behavior of the system. Using low-temperature scanning tunneling microscopy (LT-STM), we observe a periodic rippling of the heterostructure with a wavelength of ~$\sim8.7$ nm. Energetic analysis reveals that these ripples are not intrinsic but are driven by strain from the substrate during cooling. Density functional theory (DFT) calculations show that while the ideal flat heterostructure exhibits a hybridization gap of $\sim40$ meV, the ripple-induced structural modulation closes this gap, restoring a metallic state. This gapless phase is not a trivial metal. By combining an effective moir\'e ladder model with spin-resolved DFT, we find that the proximity-induced spin-orbit coupling is redistributed across a dense manifold of minibands. The resulting ``Helical Metal'' has a complex spin-texture beyond a simple Rashba splitting. Remarkably, while the flat system is effectively spinless in this ultrathin limit due to hybridization, the ripples actively restore the spin polarization. Our findings suggest that rippled TI/graphene heterostructures provide an interesting platform to develop spintronics, where geometric modulation unlocks dense helical states that are inaccessible in the pristine flat limit.