Monopoles of orbital angular momentum (OAM) are a tantalising prospect for orbitronics because OAM is uniform in all directions. This would mean that information flows could be generated in any direction. Image © Paul Scherrer Institute/ Monika Bletry.
Orbitronics is an emerging field that could rival and improve upon the electronics we have grown used to over the decades. Professor Dr Michael Schüler, of the Paul Scherrer Institute’s Center for Computing, Theory and Data, explains the concepts behind the field and his recent discovery of orbital angular momentum monopoles that mark a major advance for the technology.
Orbital angular momentum monopoles have been the subject of great theoretical interest as they offer major practical advantages for the emerging field of orbitronics, a potential energy- efficient alternative to traditional electronics. Now, through a combination of robust theory and experiments at the Swiss Light Source at Paul Scherrer Institute, their existence has been demonstrated.
ScienceVega spoke to Professor Dr Michael Schüler of the PSI’s Center for Computing, Theory and Data, to find out more about the field of orbitronics and its potential applications.
ScienceVega: Can you explain the concept of orbitronics?
MS: Orbitronics is about orbital angular momentum. If you think of me sitting on a frictionless chair and spinning around, because it is frictionless I would actually be spinning forever because all the angular momentum is conserved. It’s a fundamental conservation law of nature that I have all the angular momentum.
So then we move onto orbitronics, which is the idea of making use of orbital angular momentum of electrons in solids. This angular momentum can be transferred across the solid, generating a current of rotating electrons through a material.
Coming back to the analogy of me sitting on the chair, that would mean that somebody is moving the chair so that my angular momentum is moving in space. The point is that this stores information through the rotation directionality. If I move this angular momentum from place A to place B then I have transferred information. That’s the overall idea.
Electronics is noisy: devices heat up and there are fundamental limits. This is one of the reasons why we cannot scale down chips any further. Image © DustyDingo, Public domain, via Wikimedia Commons.
ScienceVega: How can you use orbitronics to provide an energy efficient alternative to electronics?
MS: With electronics if you apply an electric field, some current flows and this current does something. The question is how much current do you need for what you want to do? If you’re thinking of computing for example, in principle you want to use lower and lower currents, because then you can make smaller devices. But currents fluctuate, devices heat up and there are fundamental limits. This is one of the reasons why we cannot scale down computer chips any further. And a computer also needs a lot of energy to run, to operate and to do its computing.
With orbitronics, the angular momentum is in principle conserved and that means the orbital current is more stable.
So the robustness helps and in principle allows you to run smaller currents because they don’t dissipate so easily. Therefore we can run this with smaller currents and still transmit the same amount of information.
One other thing to say, specifically about the materials that we’ve looked at so far, is that we can launch these currents of orbital angular momentum with relatively small fields. You just need a small electric field and you still get sizeable currents. That’s what I find interesting: which materials are such that they just need a small external application to generate sizeable effects?
ScienceVega: Orbitronics and spintronics are related but different branches of science, with spintronics perhaps the more established. What are the pros and cons of each?
MS: Spintronics is more established in terms of fundamental research, but maybe not so much in terms of applications. While both of these fields are still at a fairly fundamental level of research at this point, it’s now time to also explore the potential for applications.
Orbital angular momentum is probably more robust than spin because there are more mechanisms to flip the spins of electrons. If you want to launch these spin currents with spin-polarised electrons, then there are different ways of doing that, but a lot of the materials that you need rely on spin orbit coupling and for that you need heavy atoms. Heavy atoms are much rarer in nature. If we think about some of the most promising materials for spintronics, they contain bismuth, which is a very heavy element and it’s pretty rare on Earth.
Whereas last year there was a nice demonstration of orbital currents using a layer of titanium. Titanium is very abundant on Earth. The materials we’ve been researching also contain relatively abundant elements.
So the comparison of orbitronics and spintronics is about these two things: firstly the robustness of the current and the second point is, if you want to make any of this reality, you need to find materials that can be produced without too much of a cost.
ScienceVega: So do you know what materials you could use for orbitronics, or are there others out there that you need to trial to see how they react?
MS: Excellent question! What we discussed in our paper is one class of materials that we think is exciting, known as chiral materials. They have a crystal structure that looks a bit like DNA, a Helix-type structure. If you have this chirality, then the electrons in the solid have angular momentum already. There is no flow, but the electrons that ‘live’ in the solid already have angular momentum. It sums up to net zero, so it’s not that the whole thing is spinning, but you can break this balance of angular momentum of electrons circling one way and circling the other. You don’t need spin orbit coupling for this, so you can do this with any material that is chiral.
They also have this property in that they already have some kind of circulating electrons, so if you apply an electric field, then you would generate a net current of angular momentum; you can basically launch this current relatively easily. How many chiral materials are there? It seems there are quite a few. That is why we are so excited about them.
ScienceVega: Where do you see the main areas where orbitronics can be applied in everyday usage?
MS: It’s hard to say at the moment, but we’ve been thinking mostly about whenever you need to transmit some information and you need to trigger some electronic device. We are looking at making it consume less power. Think of something like a conventional computer, but it runs using less power. Or whenever you need to transmit information.
Supercomputers, like the IBM Blue Gene/ P supercomputer in Argonne National Laboratory, USA (pictured) require ever more amounts of energy to run. Orbitronics may make it easier in future. Image © Argonne National Laboratory’s Flickr page, CC BY-SA 2.0 < https://creativecommons.org/licenses/by-sa/2.0 >, via Wikimedia Commons.
ScienceVega: So it could be for things where low battery usage or long-term battery usage would be advantageous?
MS: Exactly. That’s the whole plan that you just need less power and you can run the electronics that we have with less power.
ScienceVega: What do you want to research next?
MS: What we did specifically in the paper was come up with the recipe of identifying ‘hotspots’ of the electronic properties that we know that these materials need to have in order to launch large orbital currents. We came up with a recipe of a relatively simple measuring technique to investigate whatever material and then you can be sure it has this property.
What I mostly do is develop simulations for materials. The good thing is we can actually do this for many materials. We can run our procedure, simulating an experiment for candidate materials and then go to the experimentalists and tell them which materials to inspect.
That’s how simulations also really help to save money. We can sample many materials beforehand and select the most promising candidates and then directly confirm this with experimentalists. That’s definitely one research direction.
The other thing is a little bit funky, shall we say! The background is that electronics are still relatively slow. If you think about computing, and of the frequencies at which electrons go back and forth, they are relatively slow.
We have reached the limit of how fast CPUs can really run in terms of clock speed. But there’s this relatively new idea of light wave electronics, where you use lasers to drive electronic currents back and forth, and that would be orders of magnitude faster than conventional electronics.
That is a very modern line of research, but basically we’ve been asking ourselves whether we could launch these orbital currents in a faster way. We are doing quite a bit of research on these ultra-fast phenomena, and we also want to combine these fields of ultra-fast current generation and orbitronics. It’s fantasy at the moment, but it’s not that unrealistic.
