Searching for materials with multiple quantum properties

Looking for “spin” in quantum materials. From Unsplash.

What is a MOM? Patient, supportive, loving…no, not quite, in the cold world of materials physics, a MOM is a multi-order material. That means a material with two or more quantum “orders,” where electrons in the material are organized in some way because of quantum mechanics. We call them “orders” because common examples include things like magnetism, an effect caused by electrons in a material “ordering” and all pointing in the same direction. So your refrigerator magnets have quantum order!

MOM: Multi-order material

In this Science Advances paper, we simulated materials on a supercomputer and discovered materials that have two or more interesting quantum properties. Having two quantum effects that coexist in the same material means that we can control one effect by manipulating the other, and vice-versa. That’s like having a soundboard installed directly in the material, with switches and knobs that we can play with to engineer its behavior and make new devices.

Quantum MOMs have knobs and sliders for controlling quantum properties. From Creative Commons.

In our paper, we focused on two specific types of quantum order. One I already mentioned: magnetism. The other is topological order — which is a way of classifying materials that have properties so robust that you can’t destroy them, unless you rip the material apart. Imagine if an elephant dropped on your car and crushed it, but it kept running, and it wouldn’t stop running unless you cut it clean in half. Topological materials are kind of like that.

No more traffic

One of the coolest things that topological order can give is a “surface state.” A topological material with surface states is insulating on the inside (like wood), but conducting on its surface (like copper wire). And the electricity on the surface can only go forward in specific lanes, never backwards. Now imagine that your car is on the highway and every time you inevitably run into construction or an accident, there’s a perfect detour in place, so you never have to slow down and you never hit traffic. Compare that situation to a non-topological material, where electricity is trudging along, bumping into things, always hitting construction and slow-downs. That’s actually what the situation is like for normal metals like copper and iron.

Ok so why would you want something that is both magnetic and topological? Well, say you have a surface state in a material and it has its own magnetic field — the magnetic field aligns all the electrons in the same direction, and the surface state makes sure that electricity only conducts in one direction. Maybe Amazon is trying to get delivery vans from point A to point B, but some of them are going forwards and some are going backwards. It would be much easier if everyone was headed in the same direction — that’s how magnetic topological MOMs conduct electricity.

In materials, we usually only see these kinds of interesting quantum effects at really low temperatures, and it’s expensive to keep things cold all the time. Magnetism and topology can both operate at room temperature though, so there’s hope this kind of effect could be used in new technologies in day-to-day life.

Virtual screening

If this sounds complicated and hard, that’s because it is! We’re hunting for materials with these exotic quantum properties, and not just one, but multiple types. Luckily, the Materials Project database has over 130,000 material structures already available for us to search through. We can narrow down that list to materials that have elements like iron in them, because we expect them to be magnetic. Still, there are over 30,000 materials to consider even if we only pick out ones that are likely to be magnetic and have oxygen in them.

It would be great if there was a laboratory that could grow 30,000 crystals and measure their magnetic and topological properties, but that would take more time and money than anyone has, and most of the materials would be duds or not even possible to make. Fortunately, it’s also unnecessary.

We took a portion of the materials from the database (around 3,000) and simulated all the most likely types of magnetism for them to see what kinds of magnetic materials we could find. Then we took the most interesting ones and calculated their topological properties.

After all that work, we found 18 potential quantum MOMs! Screening the database like this was completely virtual — everything happened with simulations on a supercomputer. Now those candidate materials can be studied further with more detailed calculations, or other scientists may even try to grow them in a lab and measure their properties. 18 is a much more manageable number than 30,000.

Zooming out and zooming in

We also used our calculations and other recently published calculations to train machine learning models to predict quantum properties without requiring huge supercomputers. There are still tens of thousands of other magnetic materials to consider, so hopefully machine learning will provide some guidance on where to look next. We also suspect that there are many more interesting quantum MOMs lurking in our data that we haven’t had a chance to look at yet.

The nature of this kind of virtual screening also means that no one material gets any more attention than the rest. To deeply understand the properties of any particular material, much more work needs to be done.

A new kind of -tronics

The big picture for this kind of work might involve eventual applications in the field of “spintronics.” Roughly speaking, spintronics is the idea that in the future we’ll surpass modern electronics by making devices that use the direction that electrons point as well as their electric charge. Every electronic device around you uses electric charge to do useful things; but electrons have many other properties and it seems obvious that we should be trying to exploit them too. Quantum MOMs are perfect candidates for spintronics, because that’s what they do — move electrons quickly in specific directions, while controlling the direction the electrons point.

Spintronics aren’t the only new kind of -tronics that scientists are pursuing. In the race to beat modern electronics, which have been more or less the same for many decades, there seems to be a new “-tronics” in the news every week. But most of them have one thing in common — they’re built with quantum MOMs.

Getting in touch

If you liked this explainer or have any questions, feel free to reach out over email or connect with me on LinkedIn and Twitter.

If you’re interested in more technical details, you can read the paper here and check out the data and trained models on GitHub and the Materials Project.

You can find out more about my projects and publications on my website or just read a bit more about me.

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