Our colleague Iva Šrut Rakić together with her collaborators from the University of Illinois, Urbana Champaign (USA) have published a paper in a pristine journal Nano Letters where they demonstrate the ability to manipulate graphene’s electronic properties using a precisely engineered strain superlattice. This study reveals the formation of both integer and fractional pseudo-Landau levels (pLLs) in graphene, a feat that could pave the way for discovering new quantum phases of matter and designing advanced electronic devices.
Interaction Effects and Non-Integer Pseudo-Landau Levels in Engineered Periodically Strained Graphene
Iva Šrut Rakić, Matthew J. Gilbert, Preetha Sarkar, Anuva Aishwarya, Marco Polini, Vidya Madhavan, and Nadya Mason, Nano Lett. 2025, 25, 1, 41–47.
DOI: https://doi.org/10.1021/acs.nanolett.4c03542
Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, has long been hailed for its exceptional electronic and mechanical properties. This study of our colleague and her collaborators leverages graphene’s remarkable flexibility to impose periodic strain, creating a superlattice by draping the material over an array of silica nanospheres. The strain induces pseudomagnetic fields (PMFs) reaching up to 55T causing electrons to form flat energy bands and quantized density of states known as pseudo-Landau levels. The use of silica nanospheres as a substrate introduces a globally tunable quasi-periodic strain pattern, unlike previous methods reliant on uncontrolled processes.
One of the most remarkable findings is the observation of fractional pLLs, a novel quantum state predicted by theory but experimentally elusive until now (Figure 1.). Fractional pLLs arise due to enhanced electron-electron interactions in the flat energy bands generated by the strain. They also detected a splitting of the zeroth pLL—a signature of sublattice symmetry breaking and band flattening—offering deeper insights into the role of Coulomb interactions in graphene’s electronic behavior. Using scanning tunneling microscopy and spectroscopy, the team mapped these features at nanometer precision. Theoretical calculations confirmed the emergence of these phenomena and revealed how increasing strain can further flatten the bands, enhancing many-body interactions that give rise to exotic quantum phases.
Figure 1. Scanning tunneling spectroscopy spectra taken across graphene strained over SiO2 nanosphere. Figures (B) and (D) clearly show presence of fractional pseudo Landau levels along with integer ones. These states propagate over several nanometers.
This strain-engineered graphene system not only demonstrates fundamental quantum phenomena but also presents a scalable platform for exploring interaction-driven phases, such as fractional quantum Hall effects, ferromagnetism, and superconductivity. The ability to tune electronic states through substrate design could be transformative for quantum computing, valleytronics, and nanoscale electronics. Moreover, this approach could extend beyond graphene to other 2D materials and van der Waals heterostructures, enabling a broader exploration of strain-engineered quantum systems being, at the same time, compatible with CMOS (complementary metal-oxide-semiconductor) technology.