Manipulating superconductors to create new materials

Physicists from the University of Belgrade believe they’ve found a way to manipulate superthin, wafer-like monolayers of superconductors such as graphene, thus changing the material’s properties to create new artificial materials for future devices. Their work has been published in the Journal of Applied Physics.

Superconductors’ never-ending flow of electrical current could provide new options for energy storage and superefficient electrical transmission and generation — but this lack of electrical resistance is reached only below a certain critical temperature, hundreds of degrees below freezing, and is very expensive to achieve.

With this in mind, the Belgrade researchers examined how conductivity within low-dimensional materials, such as lithium-doped graphene, changed when different types of forces applied a ‘strain’ on the material. Strain engineering has been used to fine-tune the properties of bulkier materials, but the advantage of applying strain to low-dimensional materials, only one atom thick, is that they can sustain large strains without breaking.

“The application of tensile biaxial strain leads to an increase of the critical temperature, implying that achieving high-temperature superconductivity becomes easier under strain,” said first author Vladan Celebonovic, from the university’s LEX Laboratory.

Conductivity depends on the movement of electrons, and although it took seven months of hard work to accurately derive the math to describe this movement in the Hubbard model, the team was finally able to theoretically examine electron vibration and transport. These models, alongside computational methods, revealed how strain introduces critical changes to doped-graphene and magnesium-diboride monolayers.

“Putting a low-dimensional material under strain changes the values of all the material parameters; this means there’s the possibility of designing materials according to our needs for all kind of applications,” said Celebonovic, who explained that combining the manipulation of strain with the chemical adaptability of graphene gives the potential for a large range of potential new materials. Given the high elasticity, strength and optical transparency of graphene, the applicability could be far reaching — think flexible electronics and optoelectric devices.

Celebonovic and colleagues also tested how two different approaches to strain engineering thin monolayers of graphene affected the 2D material’s lattice structure and conductivity. For liquid-phase ‘exfoliated’ graphene sheets, the team found that stretching strains pulled apart individual flakes and so increased the resistance — a property that could be used to make sensors such as touch screens and e-skin.

“In the atomic force microscopy study on micromechanically exfoliated graphene samples, we showed that the produced trenches in graphene could be an excellent platform in order to study local changes in graphene conductivity due to strain,” said co-author Jelena Pesic, from the university’s Graphene Laboratory. “And those results could be related to our theoretical prediction on effects of strain on conductivity in one-dimensional-like systems.”

Although the team foresees many challenges to realising the theoretical calculations from this paper experimentally, they are excited that their work could soon revolutionise the field of nanotechnology.

Image caption: Liquid-phase graphene film deposited on PET substrate. Image credit: Graphene Laboratory, University of Belgrade.

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