Superconducting at higher temperatures: a material passes laboratory test

ONE OF the constraints we face in transportation of electricity is the resistance of materials, such as the wires, that carry the current. Most materials offer some kind of resistance because of which transmission losses in electricity take place, the energy getting dissipated in the form of heat. This resistance is quite useful in certain circumstances, especially in situations where the flow of electrical current needs to be regulated and controlled.

However, in certain situations we like this resistance to be as low as possible. It is possible to have very low resistance, even zero resistance, in some materials in certain special conditions. These materials are called superconductors, but they exhibit this property only at very low temperatures, typically below -200°C. Coils made of superconducting wires can withstand very high current and produce high magnetic fields that are used in MRI imaging. One of the objectives in superconductivity research has been to induce superconductivity in materials at higher temperatures, preferably at room temperature, so that they can be used for everyday applications such as transporting electricity through overhead wires without any transmission losses and more energysaving electronic devices can be realised.

Generally, elementary particles, depending on their quantum behaviour, are distinguished in two broad classes — the bosons named after Indian physicist Satyendra Nath Bose, and fermions named after Italian scientist Enrico Fermi. For example, recently discovered “Higg’s particles” are bosons while electrons are fermions.

Electrons are described by a theory developed by English scientist Paul Diarc, who combined quantum theory with Einstein's special theory of relativity, and consequently the electrons can be further classified as Dirac fermions. An extension of this theory predicts the existence of other special classes of fermions, such as the Weyl fermions named after the German mathematician and physicist Hermann Weyl who proposed their existence in 1929.

The Weyl fermions are mass-less particles but they are expected to be real. Weyl fermions were initially expected to be observed in cosmic radiations but that has never happened. Instead, a couple of years ago, they were observed to exist as quasi-particles, collective excitations of electrons, in a semi-metal tantalum arsenide (a compound of tantalum and arsenic) which is now also referred to as a Weyl semi-metal.

Our earlier work had shown that in a different kind of very complex materials, socalled topological Dirac semi-metals, we were able to induce superconductivity in special situations. After the discovery of Weyl semi-metals, we were interested in studying whether the Weyl fermions also have any bearing on superconductivity.

Our recent research at IISER has shown that this indeed is a case. Weyl fermions in tantalum arsenide can not only take part in superconductivity but also do so in a more conventional manner and at relatively high temperatures under certain controllable conditions. So Weyl semi-metals offer a much better possibility of realising superconductivity at higher temperatures. This result can have important consequences for research aimed at obtaining superconductivity at normal temperatures and used for everyday purposes such as electricity transmission without appreciable losses.

But there are more immediate exciting implications. The superconducting phases realised on Weyl semi-metals, in presence of a magnetic field, might also host another type of elusive particles called the Majorana fermions, initially predicted by Italian scientist Ettore Majorana in 1937. One of the major obstacles in quantum computing, the new-age computing that involves quantum data bits (called “qubits”) for processing and storing information, are fragile and easily perturbed by disorders or impurities in a material. The Majorana fermions are known to be “fault tolerant” — they are almost insensitive to disorder. Thus, it is possible to use them in fault-tolerant quantum computing.

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