image: When no gate voltage is applied, β-Ag2Te nanoflakes behave as lightly doped topological insulators. After applying a sufficiently large positive gate voltage, the electric field induces a Stark shift of the electronic bands, and the system transitions from a topological insulator to a heavily doped semiconductor, achieving the “on-state.” Conversely, when a sufficiently strong negative gate voltage induces the topological phase transition, the Fermi level moves into the induced band gap, resulting in an insulating “off-state.”
Credit: ©Science China Press
Professor Jinxiong Wu’s team at Nankai University has achieved a significant advance in the field of topological materials by demonstrating the first device-level, reversible topological phase transition (TPT) controlled by an out-of-plane gate voltage. The study, published in National Science Review, overcomes a long-standing hurdle in the practical application of topological physics.
Topological phase transitions, which involve a fundamental change in a material's electronic band structure without altering its crystal lattice, have garnered immense interest for their potential in next-generation, low-power electronics. However, previous methods to induce these transitions—such as structural modification or chemical doping—have proven invasive, irreversible, and incompatible with conventional semiconductor device architectures, severely limiting their practical use.
In this work, the researchers utilized β-Ag2Te nanoflakes and applied a top-gate voltage to achieve a clean, reversible TPT. Through quantum transport measurements, they observed that under a small gate voltage, the material exhibited quantum oscillations with a π Berry phase, a definitive signature of a topologically nontrivial state. When a high gate voltage was applied, the Berry phase reverted to a topologically trivial value. Theoretical calculations revealed that this electrically induced transition originates from the Stark effect, which subtly modifies the material's band structure under an external electric field.
Capitalizing on this gate-tunable TPT, the team successfully fabricated a prototype device: a topological phase transition field-effect transistor (TPT-FET). This transistor demonstrated a high current on-off ratio exceeding 104, showcasing the potential for creating functional electronic components that leverage topological states.