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Chirality is known to affect a number of properties including those of an optical, electrical or magnetic nature. The effect chirality has on these properties causes a variety of nontrivial phenomena such as circular dichroism for chiral molecules, magnetic Skyrmions in chiral magnets and nonreciprocal carrier transport in chiral conductors.

But one phenomenon until now has never been studied, and it is the effect that chirality has on superconducting transport.

An international research team has found conclusive evidence that the chirality of a nanotube lattice causes nonreciprocal superconductivity. The superconductivity has been found to reflect the chiral structure, where the forward and backwards supercurrent flows are not equivalent due to the breaking up of the inversion symmetry. This superconductivity can be realised through ionic gating in certain individual nanotubes, namely those of tungsten disulphide.

Principles behind the research

Chirality within structures is recognised as a powerful source of unique optical and electrical properties with novel functionalities. It is also a well-known fact that the polarisation of light and the spin of electrons are sensitive to chirality.

Chirality is an asymmetrical effect that arises when two molecules are the same but have a different atomic arrangement in space. Many molecules can appear in two forms where they are mirror opposites of each other, i.e. you cannot superimpose one of the molecules on top of the other. The two mirror images are commonly referred to as enantiomers. Normally, unless isolated, the two enantiomers coexist in a sample and is referred to as a racemic mixture.

Nanotubes come in many forms, with the most common ones that people know being carbon-based. However, many people don’t realise that there are also inorganic nanotubes. The researchers used tungsten disulphide, which was the first material ever to form an inorganic nanotube.

Superconductivity in carbon nanotubes was discovered over 15 years ago, but it has taken its inorganic rival many years to catch up, despite being discovered over 20 years ago.

Tungsten disulphide, WS2, is a transition metal dichalcogenide (TMD) that can form many inorganic-based structures that we see with carbon, including 2D sheets and tubular structures. Tungsten disulphide is classed as a semi-conducting material, without the need for any doping. Tungsten disulphide can also form nanotubes with noncentrosymmetric chiral structures.

What did the research achieve?

One area present in chiral molecules that can be exploited is the unidirectional resistance in electron transfer mechanisms. Under a magnetic field, the two current injections are non-identical due to the chiral nature of the nanotubes- when the current applied is parallel to the nanotube, the nanotube shows a nonreciprocity, which could yield nontrivial quantum transport in the superconducting states.

The research used the above principles as a basis for the research and studied the transport properties of WS2 nanotubes using an ionic liquid gating technique and a resistance measurement, on the first and second harmonic signals in alternating-current (AC) mode. The researchers focused primarily on tubular structures- i.e. a multi-walled nanotube.

The researchers observed an ambipolar transfer curve in regions of electrostatic doping; and superconductivity in regions of electrochemical doping. Individual, non-tubular, nanotubes were also investigated, where the observed anisotropy of the superconductivity and Little-Parks oscillations were consistent to those found in the tubular structures.

However, the most significant finding was the experimentally produced nonreciprocal superconducting transport via the second harmonic signal, showing that chirality influences the superconductivity. This nonreciprocal signal was found to enhance the superconducting state, affect the magnetic flux quantum and show periodic oscillations.

The current discovery now paves the way to study the interplay between superconductivity, chirality and noncentrosymmetric. The noncentrosymmetric cylinder structure also has the potential to make these tungsten disulphide nanotubes a great candidate to explore exotic quantum phenomena and nontrivial Cooper pairing.

Sources and Further Reading

Qin F., Shi W., Ideue T., Yoshida M., Zak A., Tenne R., Kikitsu T., Inoue D., Hashizume D., Iwasa Y., Superconductivity in a chiral nanotube, Nature Communications, 2017, 8, 14465

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