Atomic-scale design of new superconductors

Source: Shutterstock

Superconductivity is arguably one of the most intriguing and also widely applicable quantum phenomena. When a normal metal (e.g., copper) conducts electricity, part of the electrical energy is always lost due to the resistance of the metal. In a superconductor, on the other hand, this electrical resistance vanishes at lower temperatures, and as such electrical current is conducted in a 100% efficient way. Due to this extraordinary property, superconductors are very promising candidates to be used in super-efficient electronics and sensors, in addition to existing applications such as in powerful electromagnets, widely used in particle accelerators, MRI scanners, etc.

To realize these applications it is crucial to explore new superconductors with optimal properties. Current research is strongly focused on ultra-thin materials, consisting of just one or a few atomically-thin layers. This new materials class has a lot to offer: as the materials are ultra-thin, they are evidently extremely compact, lightweight, and they can be stacked like Lego blocks. Through this atomic-scale Lego a desired functionality can be tailored, to be used in novel electronic components and sensors.

The most famous example of an ultrathin material is graphene. It is one of the thinnest possible materials, being only a single carbon atom thick, but unfortunately not a superconductor. We have investigated whether MXenes, a related family of materials – you could call them cousins of graphene – exhibit superconductivity. MXenes consist of three atomic layers: a layer of carbon or nitrogen (X) sandwiched between two transition metal layers (M), as shown in the figure.

We have considered a total of 20 different MXenes, and have investigated them on the atomic scale, based on a quantum mechanical description. Indeed, quantum mechanics describes the behavior of electrons in materials, including under which circumstances these can exhibit superconductivity. This led to the discovery of 6 new, ultra-thin superconductors, which can now be synthesized in a lab. By identifying the 6 cases with the desired functionality – superconductivity – from a total of 20 candidates, a tremendous amount of trial and error in the lab has been spared, thus also cutting the corresponding research costs.

One of the prime characteristics of a superconductor, which we have calculated, is its critical temperature. This is the temperature below which a material needs to be cooled to adopt the superconducting state, thus the higher this value, the better. The 6 newly discovered superconductors have quite diverse critical temperatures, ranging from a modest 2 kelvin (that means, 2 degrees above absolute zero) to a reasonable 16 kelvin, as shown in the figure.

We also observe in the figure that in one of the novel superconductors, W2N (tungsten nitride), superconductivity coexists with another quantum phenomenon, the charge-density wave (CDW). This is a spatial modulation of the electron’s wave state, characteristic of quantum mechanics. The interplay between the superconducting and charge-density wave states, two competing quantum phenomena, can form the basis of a new generation of electronic devices, which are both ultra-thin and super-efficient.

Therefore, we are currently able to design novel materials with desired functionalities on the atomic scale using highly accurate quantum mechanical descriptions. Subsequently, these materials can be synthesized and characterized in an experimental lab, to ultimately find their way into concrete, everyday applications.

More information to be found in the original publication: Nanoscale, 2020, 12, 17354