A mysterious form of superconductivity has been found in a twisted crystal, which could help researchers better understand how to make super-efficient electronics.
Superconductivity is a rare property that lets some materials conduct electricity with no resistance. We only know of materials that are superconductors at low temperatures or extreme pressures, but a very high or room-temperature superconductor could transform the world鈥檚 energy systems by allowing us to send limitless energy over vast distances.
Despite extensive efforts, scientists have yet to discover a room-temperature superconductor, and the underlying physical mechanisms for the highest-temperature superconductors we know of, which scientists call unconventional superconductivity, are still hotly debated.
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In 2018, researchers unexpectedly found that when a sheet of graphene, an atom-thick layer of carbon, is stacked on top of another and rotated slightly, it becomes a superconductor. When they repeated this stacking and twisting with other materials, creating what is called a moir茅 pattern, it produced many curious properties, such as exotic magnetism and unusual electrical insulation, but not superconductivity. It was unclear if the original superconductivity was just a quirk of graphene, or whether it might reveal more general principles for how to build a room-temperature superconductor.
Now, at Columbia University in New York and his colleagues have discovered superconductivity in a second atomically thin material, the metal tungsten diselenide. The property manifests when double-layered and twisted crystals of the substance are cooled to 鈭�272.724掳C, around half a degree above absolute zero. 鈥淕raphene isn鈥檛 the only system that does this,鈥� says Dean. 鈥淭hat suggests that this could be a general property of moir茅-patterned materials.鈥�
Dean and his team saw the first hints of superconductivity in tungsten diselenide soon after the graphene experiment, when they cooled the material to extremely low temperatures. However, they couldn鈥檛 properly measure the superconductivity because their electrical contacts stopped working when they tried to cool down the material further. 鈥淲e spent two years to three years trying to figure [it] out. How do we push that temperature window down and [have] our contacts survive?鈥� says Dean. 鈥淓ventually we did that, and lo and behold, the superconductor re-emerged in our new sample.鈥�
Finding superconductivity in a material other than graphene suggests the existence of an entirely new class of superconducting materials, says at Loughborough University, UK. 鈥淥nce you understand the details of these materials and what are the properties which lead to superconductivity, then you can start engineering materials with higher and higher temperatures, and eventually reach the goal [of room-temperature superconductivity],鈥� he says.
It is still unclear exactly how tungsten diselenide is superconducting, says Dean, but there are hints that it is a feature of the material鈥檚 magnetic fields that come from the interactions between the two twisted sheets. Dean and his team only detected the superconductivity next to regions where the magnetic fields are paired in opposite ways, such as a north and south pole lined up next to each other.
鈥淭hat relationship between the onset of superconducting and the onset of magnetic ordering鈥� gives us a good sense that this might be of a similar flavour to some of the unconventional superconductivity that is believed to exist in more conventional materials,鈥� says Dean.
Nature
