In extreme cold — minus 268 degrees — a crystalline perovskite film starts transmitting light differently depending on its polarization. The beam seems to split in two: one part travels slightly faster, the other slightly slower. The energy difference between them is just 45 thousandths of an electron-volt, but that's enough to clearly separate two optical channels.
The authors grew nanowalls of MAPbI3 at a steep angle to the substrate, rigidly setting the direction of crystalline folds — a ferroelastic texture (a pattern of strain that persists in the crystal). When the temperature rises to minus 113 degrees, the old and new structures coexist, and a third peak appears in the spectrum — 58 meV below the center. This indicates that emission begins to strongly mix with lattice vibrations.
Upon further heating, the anisotropy smooths out, and the light loses its polarization tint. The crystal seems to 'relax,' and its optical response becomes the same in all directions. A simple symmetry-based mathematical model nicely describes this transition from split excitons (bound electron-hole pairs) to mixed states.
Ferroelasticity — a crystal's ability to retain residual deformation after the load is removed, much like a bent spoon stays bent.
In Lem's novel 'Solaris,' the planet's ocean changed its physical properties in response to external influence — isn't that a giant ferroelastic?