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. In the quantum world, such situations are everywhere — from tiny magnets to, possibly, the very fabric of spacetime. Watching a metastable state decay in real time is incredibly difficult: usually everything happens too fast and unpredictably.
In the new work, scientists found a way not only to observe this process but also to control it. They used an intricate layered material made of semiconductors — a so-called moiré quantum material (in which layers of atoms are rotated relative to each other, creating a large pattern reminiscent of silk moiré). In such structures, electrons behave very unusually: they can spontaneously align their magnetic moments in one direction, turning the material into a kind of flat magnet — a ferromagnet. The main advantage of this system is that a laser can prepare a state that is not the most energetically favorable — the very metastable "superheated ice."
The authors illuminated the sample with a laser beam twisted into a spiral (a Laguerre-Gauss beam). This beam, like a tiny mixer, flipped the magnetic moments of electrons in a small region, creating a "bubble" with opposite magnetization inside the original magnet. This bubble is the embodiment of a metastable state: its walls tend to collapse, while its interior tends to expand. The outcome of this struggle depends on the initial size of the bubble, which is set by the laser power.
The most striking result appeared when the scientists turned on an external magnetic field directed against the bubble's magnetization. Under certain conditions, when the number of electrons in the material exactly matched fractional or integer numbers per moiré pattern, the bubble's lifetime increased a hundredfold. At these moments, the material entered a special quantum state — a Chern topological insulator, where electrons flow along the edges without resistance while the bulk remains insulating. It was this rigid topological structure, like rebar in concrete, that prevented the magnetic bubble from collapsing quickly.
Now let's return to our superheated water metaphor. Instead of guessing when it will finally boil, we can not only precisely create a steam bubble in the right place but also stretch out the boiling moment over a long time, simply by choosing the right "composition" of the liquid. These experiments open the way to controlling unstable states in a wide variety of quantum materials and may help better understand how our universe could have survived the false vacuum epoch in the first moments after the Big Bang.
Moiré patterns, which gave their name to a whole class of materials, get their name from the silk-processing technique of moiré: if you fold two layers of thin fabric with a slight twist, a large wavy pattern emerges. Similarly, two rotated layers of atoms create a new, much larger periodic pattern for electrons.
The idea of a false vacuum that could suddenly decay underlies many science fiction plots — from H.G. Wells's "The Time Machine" to Isaac Asimov's "The End of Eternity," where heroes fear a cataclysm that could reshape the entire universe.