Scientists have found that a superconducting current flows in only one direction through a chiral nanotube, marking the first observation of the effects of chirality on superconductivity. Until now, superconductivity has only been demonstrated in achiral materials, in which the current flows in both directions equally.
Chiral superconductivity combines two typically unrelated concepts in a single material: Chiral materials have mirror images that are not identical, similar to how left and right hands are not identical because they cannot be superimposed one on top of the other. And superconducting materials can conduct an electric current with zero resistance at very low temperatures.
Observing chiral superconductivity has been experimentally challenging due to the material requirements. Although carbon nanotubes are superconducting, chiral, and commonly available, so far researchers have only successfully demonstrated superconducting electron transport in nanotube assemblies and not in individual nanotubes, which are required for this purpose.
The achievement is only possible with a new two-dimensional superconducting material called tungsten disulfide, a type of transition metal dichalcogenide, which is a new class of materials that have potential applications in electronics, photonics, and other areas. The tungsten disulfide nanotubes are superconducting at low temperatures using a method called ionic liquid gating and also have a chiral structure. In addition, it’s possible to run a superconducting current through an individual tungsten disulfide nanotube.
When the researchers ran a current through one of these nanotubes and cooled the device down to 5.8 K, the current became superconducting—in this case, meaning its normal resistance dropped by half. When the researchers applied a magnetic field parallel to the nanotube, they observed small antisymmetric signals that travel in one direction only. These signals are negligibly small in nonchiral superconducting materials, and the researchers explain that the chiral structure is responsible for strongly enhancing these signals
Chirality of materials are known to affect optical, magnetic and electric properties, causing a variety of nontrivial phenomena such as circular dichiroism for chiral molecules, magnetic Skyrmions in chiral magnets and nonreciprocal carrier transport in chiral conductors. On the other hand, effect of chirality on superconducting transport has not been known. Here we report the nonreciprocity of superconductivity—unambiguous evidence of superconductivity reflecting chiral structure in which the forward and backward supercurrent flows are not equivalent because of inversion symmetry breaking. Such superconductivity is realized via ionic gating in individual chiral nanotubes of tungsten disulfide. The nonreciprocal signal is significantly enhanced in the superconducting state, being associated with unprecedented quantum Little-Parks oscillations originating from the interference of supercurrent along the circumference of the nanotube. The present results indicate that the nonreciprocity is a viable approach toward the superconductors with chiral or noncentrosymmetric structures.
WS2 chiral NT and asymmetric superconducting transport in chiral NTs.
SOURCES- Phys.org, Nature Communications