A Stradivarius violin can only be made from wood that grew under special conditions. For a long time, it was believed that graphene for delicate quantum effects had to be exfoliated from graphite with tape, and that a crystal grown in a test tube was incapable of such feats. This work shows that graphene grown from a gas mixture can sound like a Stradivarius.

Two graphene crystals were grown on a single copper grain so that their lattices were aligned, then transferred onto a substrate and stacked with a twist of about 1.2 degrees. This angle is called the magic angle: at this twist, electrons in the two layers start moving in concert, giving rise to flat bands where their velocity drops to nearly zero. It is in these bands that superconductivity and other exotic states emerge. Previously, they were seen only in manually exfoliated samples, while grown films were considered too dirty.

The sample surface was cleaned by "sweeping" it with an atomic force microscope tip, and then the electronic structure was measured using photoemission with a spot size of about one micrometer. The spectra revealed flat bands near the Fermi level, with a width of about 24 millielectronvolts, comparable to the best exfoliated samples. Analysis revealed a domain with a near-magic angle measuring 15 by 10 micrometers, and a more precise fit gave an area of 84 ± 41 square micrometers. This is like finding a patch in a planted forest where the trees are aligned to within a centimeter.

Before this work, it was unknown whether grown graphene could preserve the fragile flat bands. Now it is clear that it can, and the quality matches the benchmark. One problem remains: the magic-angle domain gradually relaxes to the normal stacking, and the length of this transition is 4.1 micrometers. But even such an area is enough to place contacts and gates and test whether the exotic states survive at low temperatures.

So the test-tube violin has sounded. Now we need to tune it so that every specimen produces a clear note, not just one lucky one.