Four members of the NTU team that designed and probed a perovskite Moiré lattice. From left: Dr Simone Zanotti, Asst. Prof. Rui Su, Mr. Feng Jin, and Mr. Jiahao Ren
Over the past decade, researchers have discovered highly unusual physics in twisted bilayer graphene, a two-dimensional composite material of two stacked graphene sheets, one rotated slightly with respect to the other. At certain “magic angles”, some electrons in such a material fall into so-called ” moiré flat bands,” creating unusually strong interactions between electrons and leading to exotic forms of superconductivity or states in which geometry protects the collective behavior of particles, making them unusually robust to disturbances.
To realise a polariton condensate in a synthetic moiré superlattice, researchers at NTU, led by Nanyang Assistant Professor Su Rui and his team at SPMS, carried out an experiment using a semiconductor microcavity etched with moiré patterns.
In graphene, unfortunately, these phenomena only appear at very low temperatures, making them difficult to study or exploit in practical devices. Now Nanyang Assistant Professor Su Rui and and his team at the School of Physical and Mathematical Sciences (SPMS) at the Nanyang Technological University Singapore have demonstrated that closely analogous effects occur at room temperature in a system of so-called exciton-polaritons excited in a synthetic bilayer system realized in a semiconductor chip. They expect the achievement will make it easier to study exotic quantum states of matter, and to put them to practical use.
This research was published in the journal Science Advances in Aug 2025.
“With the synthetic system of exciton polaritons,” says Prof. Su, “we can easily tune the parameters inside the system and directly visualize the formation of moiré flat bands, effectively emulating the behavior already seen in graphene and elsewhere but in an environment where we have more control.”
Exotic quantum effects… but at room temperature?
In twisted bilayer graphene, the periodic lattices of two graphene sheets get stacked with a small mismatch in angle to produce a so-called moiré superlattice, a periodic structure with period larger than the original graphene sheets. At certain special angles, these moiré patterns create the moiré flat bands, decreasing the mobility of electrons and creating fertile ground for correlated behaviors like magnetism and superconductivity.
Inspired by graphene, many researchers have begun building synthetic moiré structures in other systems, including ultracold atoms and photonic crystals. These platforms promise new opportunities for creating analogous physics with other particles playing the role of the electrons in graphene. One such particle, the exciton–polariton, forms when semiconductor excitons (electron–hole pairs bound by Coulomb attraction) strongly couple to photons confined in a microcavity. The result is a quasiparticle that behaves partly like light and partly like matter.
In principle, exciton-polaritons can undergo condensation, with many particles falling into the same quantum state, as happens with photons in a laser. Importantly, these condensates can form even at room temperature. But so far no one had observed exciton polaritons condensation into moiré flat bands in any real system.
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To impose the moiré potential, the team went beyond simply stacking two layers, as in graphene. Instead, they etched two honeycomb patterns into the cavity structure itself, one rotated by a “magic” angle of 21.79° relative to the other. This created the moiré superlattice that gives rise to moiré flat bands. Detailed modelling and spectroscopy confirmed the emergence of such a polaritonic moiré flat band, with polaritons tending to localize in spots where the two etched lattices align most closely and interactions are strongest. When the researchers illuminated the system with pulsed laser light, they observed condensation of polaritons into the moiré flat band at an illumination threshold of just 12.4 μJ/cm², a relatively low energy input, showing how easily the condensate forms. |
 The figure reveals the emergence of a moiré flat band in the team’s set-up – the dark horizontal line (red arrow),in good agreement with the researchers’ theoretical calculations. |
Energy–momentum maps of polariton emission at increasing pump power, obtained by analysing light leaking from the microcavity to reveal polariton energies and momenta. The color scale shows emission intensity, with dark blue corresponding to stronger signals. (A) Below threshold (0.6 Pₜₕ), the emission traces the underlying band structure. (B) Near threshold (1.0 Pₜₕ), polaritons begin to accumulate in the moiré flat band. (C) Above threshold (1.8 Pₜₕ), condensation is complete: the polaritons collapse almost entirely into two wave-like states with fixed momenta, producing the pair of sharp dark-blue peaks. The inset shows the two-dimensional momentum-space distribution. Condensates happen in the moiré flat band, locating at the corners of the twisted hexagons. The red dashed line indicates the slice through momentum space used for the main plots.
Building a home for exciton polaritons
In the new work, Prof. Su and colleagues fabricated a moiré superlattice using CsPbBr₃, a material in a family of semiconductors called perovskites and valued for their strong interaction with light. They placed this material inside an electromagnetic microcavity, formed by sandwiching a thin layer between two mirrors so that photons bounce back and forth. Within the cavity, excitons in the perovskite strongly hybridize with the confined photons to form polaritons — a type of mixed particle part light (photon) and part matter (exciton).
“Moiré exciton–polaritons has been proposed before,” says Feng Jin, who is the first author of the work and a PhD student at SPMS, “but no moiré flat band has been experimentally observed and no collective condensation has been achieved in any polariton system. These are our two achievements over earlier works.”
A new playground for exploring states of quantum matter
“From a fundamental perspective, “says Jiahao Ren, who is the co-first author and PhD student at SPMS, “this work demonstrates that moiré-flat-band physics—the regime of strong correlations that produced superconductivity in twisted graphene—can be recreated in a highly tuneable, optically accessible platform. Unlike graphene, which requires cryogenic temperatures, the polariton system operates at room temperature.”
But from a more practical viewpoint, moiré polariton condensates could also seed the development a new class of polaritonic devices. These might include ultralow threshold lasers, robust quantum light sources, or photonic circuits that exploit strong interactions in flat bands. The combination of light-based control and strong correlations could enable devices with unprecedented efficiency and functionality.
“Future work,” says Prof. Su, “will likely explore whether these condensates can host exotic states analogous to superconductivity or topological order. Researchers may also investigate ways to integrate moiré polariton lattices into on-chip photonic device that could provide a controllable, room-temperature platform for exploring the states of strongly correlated quantum matter.”