Researchers from Ohio State University, Imdea Nanoscience and the National Institute for Materials Science in Tsukuba have demonstrated that superconductivity in twisted bilayer graphene (tBLG) can be tuned - and even completely switched off - by engineering its dielectric environment. Their work reveals that, unlike in conventional phonon-mediated superconductors, the pairing mechanism in this moiré system is strongly controlled by electronic interactions that are highly sensitive to nearby materials.
In the study, the team fabricated twisted bilayer graphene devices and positioned them a few nanometers above a bulk strontium titanate (SrTiO₃) substrate, a synthetic perovskite often referred to as a man‑made “diamond” because of its robustness and very large, tunable dielectric constant. By increasing this dielectric constant in situ, they steadily suppressed both the height and the width of the superconducting dome in magic‑angle devices and, upon further tuning, extinguished superconductivity altogether across the entire dome. At larger twist angles, where devices on standard SiO₂ substrates typically do not superconduct, the SrTiO₃ environment enabled a superconducting “pocket” even in regimes where correlated insulating states were absent, underscoring how delicately the phase diagram depends on dielectric screening.
Microscopically, the results are consistent with a model in which Cooper pairing is driven by Coulomb interactions that are themselves screened by plasmons, electron–hole pairs and longitudinal acoustic phonons in the surrounding structures. In conventional superconductors, reducing repulsive electron-electron interactions generally strengthens phonon-mediated pairing; here, the opposite trend emerges. As the dielectric screening is increased, electron-electron interactions that both mediate and screen the effective attraction weaken the superconducting state instead of enhancing it, revealing the “double‑edged” role of interactions in tBLG and their complex interplay with correlated insulating phases.
To probe this unconventional mechanism, the researchers used tBLG as a platform where interaction strength can be tuned through several knobs: twist angle, carrier density and, crucially, the dielectric environment provided by SrTiO₃. Electron interactions that underlie magnetic order, chemical bonding and correlation-driven insulating behavior inherently come in pairs, and by adjusting the effective pairing “settings” via the substrate, the team could switch superconductivity on and off. As Chun Ning (Jeanie) Lau explains, “Electrons normally repel each other, but in superconductors they form pairs; this pair formation is the key to a superconductor’s ability to conduct electricity without dissipation… Our evidence suggests that electrons themselves, depending on their sensitivity to their nearby environment, are unexpectedly important for material changes.”
The group was particularly struck by the fact that stronger dielectric screening - which suppresses bare Coulomb repulsion - reduced, rather than enhanced, the superconducting signal. This behavior clearly departs from the textbook BCS picture and highlights twisted bilayer graphene as a platform where electronic modes, rather than simple lattice vibrations, likely dominate the pairing glue. In practical terms, the work shows that carefully chosen substrates and gate stacks can be used as an external control knob to stabilize or quench superconductivity, offering a route to designer phase diagrams in graphene-based moiré materials.
Beyond its fundamental implications, the study points toward a pragmatic strategy for future device engineering. If superconductivity can be modulated or even toggled using local dielectric environments, then low‑loss interconnects, adaptive quantum circuits and more robust superconducting elements for quantum technologies could be built without needing to chemically modify the active material itself.
“If you could transmit electricity without energy loss, that would be hugely important for technologies used in our everyday life,” said Lau, noting that environmentally controlled superconductivity offers a new path toward this goal. While the microscopic mechanism in twisted bilayer graphene remains under active investigation, lead author Xueshi Gao emphasizes that their findings provide a concrete framework for testing new interaction channels and for extending these ideas to other strongly correlated and high‑temperature superconductors, where environmental control might similarly boost performance and push operation temperatures closer to the long‑sought room‑temperature regime.
