Optical control over topological Chern number in moiré materials
Summary
The authors demonstrate that circularly polarised, narrowband optical excitation can orient the spin–valley degree of freedom in moiré materials and thereby switch the topological Chern number of correlated states. Using photoluminescence and helicity-resolved reflectance measurements, they show optical control of both an integer Chern insulator (ICI) at filling factor ν = −1 and a fractional Chern insulator (FCI) at ν = −2/3. Chern numbers extracted via the Streda relation change sign after optical spin orientation, and the team also reports local optical writing of topological edge modes. The effect is resonant (narrowband) and power-/wavelength-dependent, robust at low temperatures and reversible in situ.
Key Points
- Narrowband circularly polarised pump light optically orients spin–valley polarisation and flips the sign of the Chern number at zero applied magnetic field.
- Demonstrated on moiré devices: ICI at ν = −1 (C ≈ ±1) and FCI at ν = −2/3 (C ≈ ±0.63), with sign changes confirmed by Streda-formula analysis of PL/reflectance cusps.
- Optical writing of topological edge modes is shown — local, reconfigurable control of topological boundaries is achievable with focused resonant light.
- The switching requires resonant, narrowband excitation (pump wavelength and power matter); broadband excitation does not flip the spin–valley state at B = 0.
- The oriented ICI state remains stable up to about 5 K (higher temperatures need more power); related ferromagnetic metal behaviour persists up to ≈1.1 K.
- Measurements use pump–probe sequences and helicity-resolved detection; results are supported by multiple devices and extended-data characterisation.
Content summary
The paper reports experiments on moiré heterostructures where narrowband, circularly polarised optical pumping is used to orient hole spin–valley populations. After optical orientation the team measures photoluminescence and reflectance contrast as a function of carrier density and magnetic field. Characteristic cusps in the optical spectra at filling factors (ν = −1 and ν = −2/3) disperse with magnetic field according to the Streda relation, allowing extraction of Chern numbers. The authors observe that the sign of the extracted Chern number reverses after optical spin orientation, demonstrating direct optical control of topology.
They explore the parameter space (pump wavelength, power, pulse sequences and temperature), showing that narrowband resonant pumping is essential for full switching and that the effect can be locally written to create topological edge modes. Extended data provide details of the experimental setup, pump–probe timing, temperature dependence and comparisons between narrowband and broadband excitation, plus results from a second device.
Context and relevance
This work adds a new, non-invasive knob for controlling topological order in correlated moiré systems: light. It links developments in moiré-engineered correlated phases (integer and fractional Chern insulators) with optical control techniques familiar from exciton and spin physics. For researchers in condensed-matter physics and quantum materials, it suggests routes to reconfigurable topological circuitry, optically addressable edge channels and new hybrid optoelectronic devices. The results build on recent discoveries of fractional Chern insulators in twisted TMDs and demonstrate an optical pathway to manipulate those phases locally and reversibly.
Institutions involved include ETH Zürich, University of Washington and the National Institute for Materials Science (Japan); the work is supported by SNSF, US DOE BES and Japanese funding agencies.
Why should I read this?
Because it’s the sort of neat trick experimentalists dream about: flip a material’s topology with a focused beam of light. If you care about topological electronics, moiré matter, or ways to wire up reconfigurable quantum devices, this paper saves you time — the team shows how to optically write and switch Chern numbers and even draws the limits (wavelength, power, temperature). It’s short on fluff and big on a practical new control tool.