Superconducting graphene

Materials World magazine
12 Nov 2018

Graphene, while a relatively new kid on the block, is fast becoming a sensation, having a multitude of useful properties and applications.

This two-dimensional net-like arrangement of carbon atoms can conduct electricity very well, but has not been effective as a commercial-scale superconductor – but that could be about to change.

Flat banding graphene

Researchers at Helmholtz-Zentrum Berlin für Materialien und Energie, led by Professor Oliver Rader and Dr Andrei Varykhalov, performed sample analysis at BESSY II lab, where a well-known procedure gave promise of a new application.

The team heated silicon carbide crystal until the surface silicon atoms evaporated, leaving behind a double layer of aligned graphene with an empty mid band.

This semiconductor sample was scanned using angle-resolved photoemission spectroscopy (ARPES), which provides very high-resolution imaging.

The scan revealed that next to the band gap was a flat area, an essential ingredient for superconductivity.   

‘The double layer of graphene has been studied before because it is a semiconductor with a band gap,’ explains Varykhalov, ‘But on the ARPES instrument at BESSY II, the resolution is high enough to recognise the flat area next to this band gap.’

‘It is an overseen property of a well-studied system,’ said lead author Dr Dmitry Marchenko, who believes this oversight was due to using lower resolution tools to study only what was already known about the structure.

Scaling up superconductivity

Unlike past attempts, this effort at reaching graphene superconductivity is stable and can be scaled up to apply to larger areas of material. Rader explains that the interactions between the layers cause the flat band area and says they can ‘predict this behaviour with very few parameters and could use this mechanism to control the band structure’.

However, the challenge remaining is managing Fermi energy.

To be a superconductor, the flat area of the two-layer graphene structure needs to match Fermi energy. At present, the sample’s energy level is 200 milli-electron volts under Fermi range, meaning the team has to raise it by applying external voltage, or by ‘doping’ the sample with additional atoms.

The full paper, Graphene on the way to superconductivity, can be read at Science Advances.