Inside some materials, electric current flows with zero resistance. These are superconductors, and they only work when it's very cold. Copper oxides are special: they stay superconducting at somewhat higher temperatures, but why has been a puzzle for forty years.
Physicists studied a sample of La$_{2-x}$Sr$_x$CuO$_4$ with an X-ray beam that scatters off electrons and reveals their collective motions. They found that for different compositions and temperatures, all the data collapse onto a single universal curve. It's as if dozens of different roads in a city suddenly merged into one avenue — meaning there's a hidden point in the system where everything converges. That's called a quantum critical point.
The found point is hidden under the 'dome' of superconductivity — the region on the phase diagram where the material becomes superconducting. As long as the dome is there, you can't see the point. But the X-ray beam allowed a peek underneath. The critical exponent turned out to be 0.74—this number describes how quickly fluctuations die out as you move away from the point. It hints that at the point, not only charge waves are involved, but also other types of order, as if several streets meet at the crossroads.
Understanding this point could hint at how to make superconductors that work at room temperature. Then power lines would stop losing energy, and maglev trains would become commonplace.
The quantum critical point in copper oxides hides under the superconducting dome, which is why it took so long to find: it required an X-ray beam with very high resolution.
In Liu Cixin's novel 'The Three-Body Problem,' aliens unfold a proton into a multidimensional structure to manipulate physics at the quantum level — our work also peers into hidden quantum states of matter.