Could liquid water and liquid phase-change materials (PCMs) have the same underlying physics? The answer could be yes, according to new work by researchers in the US and Germany who have studied a PCM alloy made from germanium (Ge), antimony (Sb) and tellurium (Te) in the ratio of 1:2:4 using quasi-electron neutron scattering. The experiments reveal that the so-called Stokes-Einstein relation, which connects the viscosity of a liquid to the diffusion coefficient of its molecules, appears to break down above the melting point of the material and at very low viscosities – just as in water. The result will be important when making phase-change memories from GeSbTe alloys and related PCMs in the future.

PCMs can be reversibly switched between their glassy and crystalline states by applying a voltage that heats up the material or by directly heating them up with a laser. This ability to switch between two “0” and “1” states – a crystalline state with high electrical conductivity and a meta-stable amorphous state with low electrical conductivity – means that they are one of the new types of non-volatile memory being studied to meet the world’s ever-increasing demand for digital information, the volume of which is doubling every two years. Such memories could work a thousand times faster than current flash computer memory, say the researchers, who are led by Charles Austen Angell of Arizona State University, Tempe. They could also be much more durable with respect to the number of daily read-writes.

The Stokes-Einstein relation and its breakdown

The viscosity and the atomic self-diffusion coefficient play a fundamental role in the ultrafast switching behaviour of PCMs and the connection between the two parameters can be described by the long-established Stokes-Einstein relation (SER), which works surprisingly well for simple liquids at high temperatures near and above their melting temperature. In liquids that can be supercooled, however, this relation beaks down well below the melting point at temperatures where viscosities rapidly increase.

“The situation is different, however, for a small number of unusual liquids (which include water) that can be supercooled,” explains Angell. Here, this breakdown is found to occur above the melting point of the liquid and at very low viscosities. Now, our team has unexpectedly found this behaviour to hold in the high mobility state of PCMs too. Could there be a connection?”

QENS

The researchers obtained their result using quasi-electron neutron scattering (QENS) to study the PCM Ge1Sb2Te4. This technique allows them to directly determine both the so-called structural α-relaxation time (which is proportional to shear viscosity) and the self-diffusion coefficient of a sample.

Their result implies that although the PCM’s viscosity may sharply increase as the material cools, atomic diffusivity can remain high because it favours fast phase switching behaviour when the PCM is very fluid.

“The same behaviour is seen in supercooled silicon and germanium,” Angell tells Physics World. “Can it be that the underlying physics of these liquids has a common basis?” In these materials, the SER breakdown above the melting point and at low viscosities is thought to come from submerged liquid-liquid transitions that kick start crystallization and fragile-strong transitions during ultrafast cooling that preserve the liquid state.

Metal-semiconductor transition in nanoscopic PCM bits

“Above the transition, the liquid is very fluid and crystallization occurs extremely rapidly once nucleated, while below the transition the liquid stiffens up and retains the amorphous, low-conductivity state down to room temperature,” explains Angell. “In nanoscopic ‘bits” made of this material, this amorphous state remains stable indefinitely until instructed by a computer-programmed heat pulse to increase instantly to a temperature at which it flash crystallizes (on a nanosecond timescale) to the conducting, metallic ‘on’ state.

“A second, slightly larger heat pulse can then take the ‘bit’ instantaneously above its melting point, and then with no further heat input and close contact with a cold substrate, for example, it quenches at a rate that is high enough to avoid crystallization. It then freezes into a (semiconducting) ‘off’ state.”

“The amorphous phases of this kind of material can be regarded as ‘semi-metallic glasses’,” explains team member Shuai Wei. “Contrary to the strategy in the research field of ‘metallic glasses’, where people have made efforts for decades to slow down the crystallization in order to obtain the bulk glass, here we want those semi-metallic glasses to crystallize as fast as possible in the liquid, but to stay as stable as possible when in the glass state. I think now we have a promising new understanding of how this is achieved in the PCMs under study.”

The research is detailed in Science Advances 10.1126/sciadv.aat8632.