K. Watanabe

Light in semiconductors is born from pairs of charges, an electron and a hole, that attract each other like a magnet to a fridge. Such a pair is called an exciton. If you add one more charge to the pair, you get a trion — a trio of three particles.

A strange visitor came to light in a fleeting flash. A beam of photons struck a deuterium nucleus (heavy hydrogen), and in the cascade of produced particles, detectors noticed something unusual. At a certain combination of masses and momenta, the signal sharply intensified, as if a new particle had appeared for a brief moment.

Usually, light does not turn an insulator into a conductor: that requires powerful laser pulses that break the material's structure. But in twisted bilayer graphene at the magic angle, things are different.

A light switch clicks, and the light comes on. But imagine a switch that, with the same press, can either turn on the lamp or leave the room dark. It also remembers what it did last time, and sometimes it seems to flip a coin to decide whether to light up. This kind of behavior was found in a crystal of black phosphorus — a semiconductor just a few atomic layers thick.