In the crystal La₂₋ₓSrₓCuO₄, at a certain composition, two states coexist: superconductivity, where electrons move without loss, and a charge-density wave — a periodic bunching of electron density. These states compete for the same electron clouds, which resemble cross-shaped petals. To see how they interact, the sample was compressed along one axis, as if gently pressing down on a stack of paper from above.
The compression changed the charge-density wave only within the superconducting state, and differently for different types of electron clouds. The wave associated with the cross-shaped petals weakened, while the wave associated with clouds elongated along the compression axis strengthened. X-ray absorption spectra showed that under compression, electrons more often occupy the elongated clouds. This is like a crowded hall where people move from central seats to side seats to disturb each other less.
This transfer means that superconductivity remains on the cross-shaped clouds, while the charge-density wave moves to the elongated ones. They stop fighting for the same space and can coexist almost without interference. This explains why, in such materials, superconductivity does not disappear even in the presence of a charge-density wave.
Importantly, simple models that describe electron behavior in these materials with only one type of cloud cannot reproduce the observed picture. To understand the full phase diagram, one must account for both types of clouds and their redistribution under pressure.
The uniaxial pressure in this experiment reversibly changed the material's properties: after releasing the load, the electron clouds returned to their original distribution.