Phase-change materials switch between crystalline and amorphous states, and this switching is what makes rewritable optical discs and phase-change memory work. The transition from crystalline to amorphous (melting and quenching) and amorphous to crystalline (annealing) are well-studied. The assumption has been that the amorphous state is, well, amorphous — a single disordered phase without further internal structure worth distinguishing.
Ultrafast pump-probe measurements on germanium telluride now show that the amorphous state itself undergoes distinct transitions. Directly observed through changes in bond stretching and angle bending dynamics, the material rearranges from one kind of disorder to another kind of disorder on sub-picosecond timescales. These are amorphous-amorphous transitions: the material never crystallizes, but its local structure reorganizes measurably.
The distinction matters because phase-change memory operates at the boundary between amorphous states with different resistivities. If the amorphous phase isn't a single state but a landscape of structurally distinct disordered states, then the device properties depend on which amorphous state the material lands in after quenching — and that depends on the cooling rate, the starting temperature, and the local composition. What looked like noise in device behavior might actually be the material visiting different amorphous basins.
The measurement technique — tracking bond geometry changes on femtosecond timescales — catches the rearrangement in the act. The amorphous state has structure. It's just not the periodic, long-range structure that crystallography can see.