Researchers succeed in developing a unidirectional superconductor

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A team from the Delft University of Technology, in the Netherlands, has just accomplished what was considered impossible until now: they have succeeded in designing a superconductor that only allows current to flow in one direction. This discovery could open the way to a new generation of computers and electronic devices for which this property is fundamental.

The phenomenon of superconductivity was discovered in 1911 by physicist Heike Kamerlingh Onnes. By definition, a superconductor is able to conduct electric current without any resistance, therefore without loss of energy. In other words, with a superconductor, the current can theoretically flow almost infinitely, because the energy is not dissipated. It also has the property of totally expelling the magnetic field that surrounds it. The phenomenon occurs at very low temperatures, close to absolute zero, and is based on the formation of pairs of electrons (called “Cooper pairs”).

In contrast, in a standard electrical or electronic circuit, when current is flowing, the electrons encounter some resistance as they move (due to interactions with surrounding atoms); therefore, part of the electrical energy is lost in the form of heat. This is also the reason why electrical appliances become hot to the touch after a few minutes of operation. If these devices operated on the basis of superconductors, they would not only be more efficient, but also much more economical in terms of electricity.

Two superconductors separated by a quantum material

Superconductors can make electronics hundreds of times faster, and their implementation would make computing much “greener”. According to the Dutch Research Council (NWO), using superconductors instead of ordinary semiconductors could save up to 10% of all Western energy reserves. For this to be possible one day, however, superconducting electrons must only move in one direction in circuits, because this is how computing and electronics work – a seemingly impossible challenge. to be noted, given the very high conductance of superconductors…

Professor Mazhar Ali and his research group at the Technical University of Delft have nevertheless achieved this feat, which is absolutely remarkable: it is like inventing a type of ice on which it is only possible to skate one way! ” If the 20th century was the century of semiconductors, the 21st can become the century of the superconductor “said the scientist in a press release.

As the physicist points out, with semiconductors, the problem does not arise: their conductivity can be controlled by doping — which consists of integrating a small quantity of impurities into the material in order to produce an excess or a deficit of electrons. Differently doped semiconductors can then be brought into contact to create junctions: “ The classic example is the famous “pn junction”, where two semiconductors are joined together: one has extra electrons (-) and the other has extra holes (+). The charge separation creates an embedded net potential that an electron passing through the system will feel. This breaks symmetry and can give rise to ‘one way’ properties “says Ali.

It has never been possible to obtain an analogous behavior without a magnetic field with superconductors, which always conduct current in both directions and have no integrated potential. But Ali and his team came up with the idea of ​​using “quantum material Josephson junctions”. Josephson junctions are assemblies of two superconductors, separated by a non-superconducting insulating or metallic material; this time they opted for a two-dimensional quantum material (like graphene), with the formula Nb3Br8 – which is part of a group of new quantum materials developed by a team from Johns Hopkins University, in the United States .

Other challenges to overcome before a commercial application

The theory showed that Nb3Br8 hosted a sharp electric dipole. Sandwiched between two layers of niobium diselenide (NbSe2), it made it possible to create a junction which can be superconductive with a positive current, while being resistive with a negative current.

To confirm their results, the researchers tried to “switch” the diode, applying the same magnitude of current in both directions. They thus showed that they measured no resistance (superconductivity) in one direction, but real resistance (normal conductivity) in the other. They also ensured that the effect only occurred in the complete absence of a magnetic field — a particularly important point, as nanoscale magnetic fields are very difficult to control and limit, the scientist points out.

Thus, a technology that was previously only possible with semiconductors can now be achieved with superconductors. This new approach could make it possible to develop computers 300 to 400 times faster than current computers. However, there remains another challenge to be met before considering a commercial application: raising the operating temperature of the junction (the superconductor used in this study requires temperatures below -266°C).

« We now want to work with the known so-called “high critical temperature” superconductors and see if we can run Josephson diodes at temperatures above 77 K (-196°C), as this will allow liquid nitrogen cooling “, specifies Ali in the press release. This done, it will still be necessary to find a way to produce these components on a large scale, the objective being to be able to obtain chips equipped with millions of Josephson diodes.

Source : H. Wu et al., Nature

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