Xiaodong Xu

A crystal is like a city: if the streets are crooked, traffic jams. In a crystal, the streets are atomic rows, and the traffic is the flow of electrons. Any tiny imperfection, impurity, or atomic displacement makes electrons scatter. Scientists from several labs have learned to grow crystals so that the "streets" are almost perfectly straight. The method is called horizontal vapor transport.

Layers of a thin material rotated relative to each other resemble stacked grids. Where the lines intersect, a new pattern emerges — a moiré. In this pattern, electrons move differently than in a regular crystal and can form unexpected states. In the new study, researchers took two layers of molybdenum telluride and twisted them by 4.8 degrees.

A sheet of paper with square grid, one with diamond grid, and one with honeycomb — three different universes for an ant. Change the sheet, and the rules of movement change. That's roughly how electrons experience the crystal lattice: geometry dictates whether they become little magnets, superconductors, or something else. In ordinary materials, the lattice is as rigid as if engraved.

Sometimes a crystal behaves like an interpreter who gets distracted by every rustle in the room, and a one-hour meeting stretches into two. In the metal ScV₆Sn₆, atoms are arranged in a kagome pattern (a weave of hexagons and triangles), and upon cooling, a charge density wave emerges — a periodic deformation of the electron cloud.

In a pot of water carefully heated above its boiling point, the liquid remains calm. But the slightest disturbance, and it violently turns to steam. Physicists call this a metastable state: it seems stable, but in reality it's ready to break apart at the slightest nudge.