A regulator for unconventional superconductivity

Superconductivity - a phenomenon in which electric current can flow without energy loss - is of great importance for numerous high-tech applications. For example, superconductors can be used to transmit electricity over long distances without loss, whereas in conventional cables energy is lost in the form of heat. They also enable the production of extremely strong electromagnets, such as those used in MRI devices, and are also used in maglev trains, which travel at high speeds with virtually no friction.

A key disadvantage of conventional superconductors is that they only work at very low temperatures and therefore have to be operated with complex and costly cooling systems - often close to absolute zero. This limitation has driven research into unconventional superconductors, which already become superconducting at higher temperatures and therefore require significantly less cooling.

Moiré superconductors are a particularly interesting class. They consist of ultra-thin crystal layers - usually graphene - which are twisted against each other at a precisely defined, small "magic angle". This twisting creates a large-scale moiré pattern, which gives the materials their name. In this pattern, electrons interact strongly with each other and form pairs as they move through the material - a central physical basis for superconductivity.

Recently, a new unconventional superconductor was discovered in twisted layers of tungsten diselenide. However, it was previously unclear exactly how superconductivity arises in this material. In the present study, the researchers show that by specifically varying the angle of twist - i.e. the relative rotation of the two layers - it is possible to investigate how moiré patterns arise and at the same time control how easily electrons couple into pairs and move through the crystal.

Physicists can now use this "controller" to simulate the processes that lead to superconductivity - a control option that is extremely rare for solids. The results could also be transferred to other unconventional superconductors, such as cuprate-based materials that become superconducting at comparatively higher temperatures. Kennes emphasizes the far-reaching significance of the study: "We now have an extremely rare platform that allows us to think specifically about the construction of exotic phases of matter with superconducting properties."