The discovery of the supposedly impossible one-way superconductor

Associate Professor Mazhar Ali and his research group at TU Delft have discovered unidirectional superconductivity without magnetic fields, something that had been thought impossible since the discovery in 1911. The discovery, published yesterday in Nature, uses 2D quantum materials and is a first step towards superconducting computers. Superconductors can make electronics hundreds of times faster, all without losing energy. Ali: “If the 20th century was the century of semiconductors, the 21st century could be the century of superconductors.”

in the 20the century, many scientists – including Nobel laureates – have studied the nature of superconductivity, which was discovered in 1911 by the Dutch physicist Kamerlingh Onnes (read more in the box below† In superconductors, a current flows through a wire without any resistance, which means that it is hardly possible to slow down or even block this current – let alone let the current flow in only one direction and not the other. At Dr. Heng Wu and Dr. Yaojia Wang, the lead researcher in Ali’s group who conducted this study, has succeeded in turning the superconductor into a one-way street – necessary for computers – is remarkable: it’s like inventing a special kind of ice that does not cause friction when you puts one way skating in one direction but invincible friction when skating the other way.

Superconductor: super fast, super green

The benefits of using superconductors in electronics are two-sided. With superconductors, electronics can grow hundreds of times faster, and the use of superconductors in our daily lives would make IT much greener: A superconducting wire from here to the moon would transport energy without loss. For example, the use of superconductors instead of ordinary semiconductors could secure up to ten percent of all Western energy reserves, according to the NWO. An interview with Associate Professor Mazhar Ali on the paper “The field-free Josephson diode in a van der Waals heterostructure”, which establishes a proof-of-concept:

Question: Why has unidirectional superconductivity never worked while normal semiconductors do?
ONE: Mazhar Ali: “Electrical wiring in semiconductors, such as Si, can be one-way due to a fixed internal electrical dipole, so a net built-in potential they can have. The textbook example is the famous” pn junction “where two semiconductors are snapped together: the one has extra electrons (-) and the other has extra holes (+) The separation of the charge creates a built-in net potential that is detected by a flying electron.This breaks the symmetry and can result in “one-way” properties because For example “forward versus backward is not the same anymore. Going in the same direction as the dipole or going against it is different then; similar to whether you are swimming with the river or swimming up the river.”

“Superconductors never had an equivalent to this one-way idea without a magnetic field, as they are more closely related to metals (ie conductors, as the name suggests) than semiconductors, which always conduct in both directions and do not have Josephson Junctions (JJs) built in – which is a sandwich of two superconductors with non-superconducting, classic barrier materials between the superconductors – did not have a symmetry breaking mechanism that resulted in a difference between “forward” and “backward”.

Q: How did you do what seemed impossible at first?
ONE: This result actually came from one of the basic research directions in my group. In what we call “Quantum Material Josephson Junctions” (QMJJs), we replace the classical barrier material in JJs with a quantum material barrier, where the inherent properties of quantum material modulate coupling between the two superconductors in new ways.The Josephson diode was an example of this: we used the quantum material Nb3Br8, a 2D material like the graphene, which is thought to contain a net electric dipole, as the quantum material barrier for our preference and placed it between two superconductors. ”

“We were able to peel a few atomic layers of this Nb3Br8 off and make a very, very thin sandwich – only a few atomic layers thick – that was needed to make the Josephson diode, and that was not possible with normal 3D materials. Nb3Bro8 is part of a group of new quantum materials and was a key element for us in the first realization of the Josephson diode, developed by our collaborators, Professor Tyrel McQueen and his group at Johns Hopkins University in the USA.

Question: What does this discovery mean in terms of efficacy and uses?
ONE: “Many technologies are based on old versions of JJ superconductors, such as MRI technology. Quantum computing is now also based on Josephson Junctions. Technology that was previously only possible with semiconductors can now be made using this building block with superconductors. There are also faster computers, such as computers up to terahertz, 300 to 400 times faster than the computers we use today.This will affect all kinds of social and technological applications.If the 20th century was the century of semiconductors, then it will be 21 . century of the superconductor. “

“The first research direction that we need to address for commercial use is to increase the temperature that we work with. Here we used a very simple superconductor that limited the temperature. Now we want to work with the well-known so-called” High Tc Superconductors “. , and see if we can run Josephson diodes at temperatures above 77 K, as this allows for the cooling of liquid nitrogen.The second point we have to deal with is scaling.Although it’s amazing that we have proven “that this works in nano units, we have only made a handful. The next step is to investigate how we can scale the production to millions of Josephson diodes on one chip.”

Q: How confident are you in your case?
ONE: “All researchers need to take several steps to remain scientifically rigorous. The first is to ensure that their results can be replicated. In this case, we built a lot of units from scratch using different types of materials, and we found the same properties every time. time, even when measured by different people on different machines in different countries, this taught us that the result of the Josephson diode came from our combination of materials and not from a result of dirt, geometry, machine or user error or interpretation. “

We also performed experiments with smoke guns that drastically narrowed the possibilities for interpretation. In this case, to ensure that we had a superconducting diode effect, we actually tried to “switch” the diode; that is, we used the same amount of current “in both forward and reverse directions and showed that we did not actually measure any resistance (superconductivity) in one direction and true resistance (normal conduction) in the other direction.”

“We also measured this effect while applying magnetic fields of different sizes and showed that the effect was clearly present at a zero field and disappeared at an applied field. This is also a smoking gun for our claim that we used a superconducting diode. power at zero field, a very important point for technological applications, as magnetic fields on the nanometer scale are very difficult to control and limit, so for practical applications it is generally desirable to work without local magnetic fields. ”

Question: Is it realistic that ordinary computers (or even supercomputers from KNMI and IBM) can make use of superconductivity?
ONE: Yes, definitely! Not for people at home, but for server farms or for supercomputers it would be smart to implement this. The world today works with centralized calculations. All intensive calculations are performed in centralized facilities, where localization offers enormous advantages in terms of energy management, thermal management, etc. The existing infrastructure could be adapted to work with electronics based on Josephson diodes at no great cost. Chances are, if the challenges discussed in the second issue are overcome, it will revolutionize centralized computing and supercomputing! “

More information

On 18 and 19 May, Associate Professor Mazhar Ali and his collaborators Prof. Valla Fatemi (Cornell University) and Dr. Heng Wu (TU Delft) a “Superconducting Diode Effects Workshop” at the Virtual Science Forum, where 12 international experts will give lectures (published on YouTube) on the current situation in this area as well as on future research and application directions.

Lecturer Mazhar Ali studied at UC Berkeley and Princeton. He took his postdoc at IBM and won the Sofia Kovalevskaya Prize from the Alexander von Humboldt Foundation in Germany, after which he joined the applied faculty in Delft.

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