Read an article this past week on how quantum geometry can enable a new form of PCM (phase change memory) that is based on stacks of metallic layers (SciTech Daily article: Berry curvature memory: quantum geometry enables information storage in metallic layers), That article referred to a Nature article (Berry curvature memory through electrically driven stacking transitions) behind a paywall but I found a pre-print of it, Berry curvature memory through electrically driven stacking transitions.
The number one challenge in IT today,is that data just keeps growing. 2+ Exabytes today and much more tomorrow.
All that information takes storage, bandwidth and ultimately some form of computation to take advantage of it. While computation, bandwidth, and storage density all keep going up, at some point the energy required to read, write, transmit and compute over all these Exabytes of data will become a significant burden to the world.
PCM and other forms of NVM such as Intel’s Optane PMEM, have brought a step change in how much data can be stored close to server CPUs today. And as, Optane PMEM doesn’t require refresh, it has also reduced the energy required to store and sustain that data over DRAM. I have no doubt that density, energy consumption and performance will continue to improve for these devices over the coming years, if not decades.
In the mean time, researchers are actively pursuing different classes of material that could replace or improve on PCM with even less power, better performance and higher densities. Berry Curvature Memory is the first I’ve seen that has several significant advantages over PCM today.
Berry Curvature Memory (BCM)
I spent some time trying to gain an understanding of Berry Curvatures.. As much as I can gather it’s a quantum-mechanical geometric effect that quantifies the topological characteristics of the entanglement of electrons in a crystal. Suffice it to say, it’s something that can be measured as a elecro-magnetic field that provides phase transitions (on-off) in a metallic crystal at the topological level.
In the case of BCM, they used three to five atomically thin, mono-layers of WTe2 (Tungsten Ditelluride), a Type II Weyl semi-metal that exhibits super conductivity, high magneto-resistance, and the ability to alter interlayer sliding through the use of terahertz (Thz) radiation.
It appears that by using BCM in a memory,
- To alter a memory cell takes “a few meV/unit cell, two orders of magnitude less than conventional bond rearrangement in phase change materials” (PCM). Which in laymen’s terms says it takes 100X less energy to change a bit than PCM.
- To alter a memory cell it uses terahertz radiation (Thz) this uses pulses of light or other electromagnetic radiation whose wavelength is on the order of picoseconds or less to change a memory cell. This is 1000X faster than other PCM that exist today.
- To construct a BCM memory cell takes between 13 and 16 atoms of W and Te2 constructed of 3 to 5 layers of atomically thin, WTe2 semi-metal.
While it’s hard to see in the figure above, the way this memory works is that the inner layer slides left to right with respect to the picture and it’s this realignment of atoms between the three or five layers that give rise to the changes in the Berry Curvature phase space or provide on-off switching.
To get from the lab to product is a long road but the fact that it has density, energy and speed advantages measured in multiple orders of magnitude certainly bode well for it’s potential to disrupt current PCM technologies.
Potential problems with BCM
Nonetheless, even though it exhibits superior performance characteritics with respect to PCM, there are a number of possible issues that could limit it’s use.
One concern (on my part) is that the inner-layer sliding may induce some sort of fatigue. Although, I’ve heard that mechanical fatigue at the atomic level is not nearly as much of a concern as one sees in (> atomic scale and) larger structures. I must assume this would induce some stress and as such, limit the (Write cycles) endurance of BCM.
Another possible concern is how to shrink size of the Thz radiation required to only write a small area of the material. Yes one memory cell can be measured bi the width of 3 atoms, but the next question is how far away do I need to place the next memory cell. The laser used in BCM focused down to ~1.5 μm. At this size it’s 1,000X bigger than the BCM memory cell width (~1.5 nm).
Yet another potential problem is that current BCM must be embedded in a continuous flow of liquid nitrogen (@80K). Unclear how much of a requirement this temperature is for BCM to function. But there are no computers nowadays that require this level of cooling.
Finally, from my perspective, can such a memory can be stacked vertically, with a higher number of layers. Yes there are three to five layers of the WTe2 used in BCM but can you put another three to five layers on top of that, and then another. Although the researchers used three, four and five layer configurations, it appears that although it changed the amplitude of the Berry Curvature effect, it didn’t seem to add more states to the transition.. If we were to more layers of WTe2 would we be able to discern say 16 different states (like QLC NAND today).
So there’s a ways to go to productize BCM. But, aside from eliminating the low-temperature requirements, everything else looks pretty doable, at least to me.
I think it would open up a whole new dimension of applications, if we had say 60TB of memory to compute with, don’t you think?
[Updated the title from 60TB to PB to 36PB as I understood how much memory PMEM can provide today…, the Eds.]
- Figure 1 from Berry curvature memory through electrically driven stacking transitions paper
- Figure 4 from Berry curvature memory through electrically driven stacking transitions paper
- Figure 3 from Berry curvature memory through electrically driven stacking transitions paper