Associate Professor Mazhar Ali and his research group at TU Delft have discovered unidirectional superconductivity without magnetic fields, something thought impossible since its discovery in 1911 – until now. The discovery, published in Nature, uses 2D quantum materials and paves the way for superconducting computing. Superconductors can make electronics hundreds of times faster, all without any loss of energy. Ali: “If the 20th century was the century of semiconductors, the 21st can become the century of superconductors.”
During the 20th century, many scientists, including Nobel laureates, wondered about the nature of superconductivity, which was discovered by Dutch physicist Kamerlingh Onnes in 1911. In superconductors, a current flows through a wire without no resistance, which means inhibiting this current or even blocking it is hardly possible – let alone passing current in one direction and not the other. The fact that Ali’s group managed to make unidirectional superconductivity – necessary for computing – is remarkable: you can compare it to inventing a special type of ice that gives you zero friction when skating in a direction, but an insurmountable friction in the other direction.
Superconductor: ultra-fast, ultra-green
The advantages of applying superconductors to electronics are twofold. Superconductors can make electronics hundreds of times faster, and implementing superconductors in our daily lives would make computing much greener: if you were to run a superconducting wire from here to the moon, it would carry energy without any loss. For example, using superconductors instead of regular semiconductors could protect up to 10% of all Western energy reserves according to NWO.
The (im)possibility of applying superconductivity
In the 20th century and beyond, no one could tackle the barrier of making superconducting electrons go in one direction only, which is a fundamental property necessary for computing and other modern electronic devices (consider for example diodes that also go one way). In normal conduction, electrons fly as separate particles; in superconductors, they move in pairs of two, without any loss of electrical energy. In the 1970s, IBM scientists tried the idea of superconducting computing but had to stop their efforts: in their articles on the subject, IBM mentions that without nonreciprocal superconductivity, a computer running on superconductors is impossible. .
Interview with corresponding author Mazhar Ali
Q: Why, when unidirectional direction works with normal semiconductor, has unidirectional superconductivity never worked before?
Electric conduction in semiconductors, like Si, can be unidirectional due to a fixed internal electric dipole, hence net built-in potential they can have. The classic example is the famous pn junction; where we assemble two semiconductors: one has extra electrons (-) and the other has extra holes (+). The separation of charge creates a net integrated potential that an electron flying through the system will feel. This breaks the symmetry and can result in one-way properties because, for example, front and back are no longer the same. There is a difference between going in the same direction as the dipole and going against it; as if you are swimming with the river or going up the river.
Superconductors have never had an analogue of this unidirectional idea without magnetic field; because they are more related to metals (i.e. conductors, as the name suggests) than to semiconductors, which always conduct in both directions and have no built-in potential. Similarly, Josephson (JJ) junctions, which are sandwiches of two superconductors with conventional non-superconducting barrier materials between the superconductors, also did not have any particular symmetry breaking mechanism that resulted in a difference between the front and back.
Q: How did you manage to do what initially seemed impossible?
It was really the result of one of my group’s basic research areas. In what we call Quantum Material Josephson Junctions (QMJJs), we replace the classical barrier material in JJs with a quantum material barrier, where the intrinsic properties of quantum materials can modulate the coupling between the two superconductors in novel ways. . The Diode Josephson was an example: we used the quantum material Nb3BR8which is a 2D material like graphene that has been theorized to host a sharp electric dipole, like our material quantum barrier of choice and placed it between two superconductors.
We were able to peel off only a few atomic layers of this Nb3BR8 and make a very, very thin sandwich – just a few atomic layers thick – that was needed to make the Josephson diode, and that wasn’t possible with normal 3D materials. Number3BR8is part of a group of new quantum materials developed by our collaborators, Professor Tyrel McQueens and his group at Johns Hopkins University in the United States, and played a key role in realizing the Josephson diode for the first time.
Q: What does this discovery mean in terms of impact and applications?
Many technologies are based on older versions of JJ superconductors, for example MRI technology. Moreover, quantum computing today is based on Josephson Junctions. Technology that was previously only possible with semiconductors can now potentially be achieved with superconductors using this building block. This includes faster computers, as in computers with up to terahertz speed, which is 300 to 400 times faster than the computers we use today. This will influence all kinds of societal and technological applications. If the 20th century was the century of semiconductors, the 21st may become the century of the superconductor.
The first research direction we need to address for commercial application is raising the operating temperature. Here we used a very simple superconductor that limited the operating temperature. Now we want to work with the so-called high temperature superconductors and see if we can run Josephson diodes at temperatures above 77 K, as this will allow liquid nitrogen cooling. The second thing to tackle is scaling up production. Although we’ve proven it works in nanodevices, we’ve only made a handful. The next step will be to study how to scale production to millions of Josephson diodes on a chip.
Q: How sure are you of your case?
There are several steps that all scientists must follow to maintain scientific rigor. The first is to ensure that their results are reproducible. In this case, we made many devices, from scratch, with different batches of materials, and found the same properties every time, even when measured on different machines in different countries by different people. This told us that the Josephson diode result was due to our combination of materials and not a false result due to dirt, geometry, machine or user error or interpretation.
We have also conducted smoldering experiments which greatly reduce the possibilities for interpretation. In this case, to be sure to have a superconducting diode effect, we actually tried to switch the diode; as in we applied the same magnitude of current in both forward and reverse directions and showed that we actually measured no resistance (superconductivity) in one direction and real resistance (normal conductivity) in the other direction.
We also measured this effect when applying magnetic fields of different amplitudes and showed that the effect was clearly present at 0 applied field and was killed by an applied field. It is also an irrefutable proof of our claim to have a superconducting diode effect at zero applied field, a very important point for technological applications. Indeed, magnetic fields at the nanometer scale are very difficult to control and limit, so for practical applications, one generally wishes to operate without requiring local magnetic fields.
Q: Is it realistic for ordinary computers (or even supercomputers from KNMI and IBM) to use superconductivity?
Yes it is! Not for people at home, but for server farms or for supercomputers, it would be a good idea to implement this. Centralized computing is really how the world works these days. All of the intensive computing is done in centralized facilities where localization adds huge benefits in terms of power management, heat management, etc. The existing infrastructure could be adapted without too much expense to work with Josephson diode-based electronics. There is a very real chance, if the challenges discussed in the other question are overcome, that it will revolutionize centralized computing and supercomputing.
Mazhar Ali, The fieldless Josephson diode in a van der Waals heterostructure, Nature (2022). DOI: 10.1038/s41586-022-04504-8. www.nature.com/articles/s41586-022-04504-8
Delft University of Technology
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