Research: Unusual Type of Superconductivity Discovered

2020-04-07

Uppsala researchers have developed a new theory that predicts an unusual type of superconductivity in twisted double layers of graphene.

Uppsala researchers Fabian Schrodi, Alex Aperis and Peter Oppeneer have developed a theory that can explain all superconducting properties that arise for twisted double layers of graphene. The new theory shows that superconducting Cooper pairs are formed with unusual energies that lies outside the Fermi-edge, which is not possible within the Bardeen-Cooper-Schrieffer-theory (BCS), which is the theory that usually explains the phenomenon superconductivity. The theory is based on detailed calculations with the Uppsala Superconductivity code (UppSC), developed by the research group. 

The superconducting gap Δ depicted on the electron band of energy E(k) with vector k in the reciprocal space. The electrons move around within the energy bands close to the Fermi-edge which lies at zero energy. Calculations show that the superconducting gap – the size of the gap is indicated with different colours – is largest on the energy bands outside of the Fermi-edge. Image: Fabian Schrodi.

Graphene is a two-dimensional quantum material consisting of only one layer of carbon atoms, and has a number of unusual properties, but is in itself not superconducting. However, in 2018, the unexpected discovery was made that when two layers of graphene are combined and twisted, as little as one degree in relation to each other, the material becomes superconducting with a critical temperature of about 1.7 Kelvin, i.e. close to the absolute zero. The twist between the two layers makes the electrons strongly correlated and a so called Cooper pair is formed. The movements of the electrons are affected by the twist so that they only have energies that can vary by a very small amount.

Other researchers, who previously have tried to explain superconductivity in twisted double layers of graphene have made use of the BCS-theory, which is the standard theory that is used to explain the phenomenon superconductivity. According to the BCS-theory, two electrons of the same energy at the Fermi-edge, form Cooper pairs through lattice vibrations and thereafter, the Cooper pair may move freely through the material without any energy losses. The BCS-theory though, could not give an explanation of all the superconducting properties that arises in twisted double layers of graphene. For example, according to the BCS-theory, the density of superconducting pairs would be close to zero and the material would not be able to show the Meisner effect, which usually is a characteristic of superconductivity. 

In the new theory though, the Uppsala researchers have managed to expand the BCS-hypothesis that only electrons with energies close to the Fermi-edge can form Cooper pairs.

The Uppsala researchers carried out so called first-principle-calculations, i.e. calculations without freely chosen parameters, with the help of Uppsala Superconductivity code, that accurately can predict if a material becomes superconducting and at what transition temperature. 

The calculations showed that exceptional Cooper pairs are formed in twisted double layers of graphene. And that the electronic energy of the formed Cooper pairs does not lie at the Fermi-edge, but extends over the whole energy band and the superconducting gap opens up mostly away from the Fermi-edge. This is the first time this type of superconductivity has been predicted.

- Our research shows above all that there are new and unexpected forms of superconductivity in addition to what has been known since before, that might have great significance for a deeper and better understanding of the phenomenon superconductivity, says Fabian Schrodi, PhD at the Department of Physics and Astronomy. 

Contact 

Fabian Schrodi, fabian.schrodi@physics.uu.se
Dr. Alex Aperis, 0769-212 312, alex.aperis@physics.uu.se
Prof. Peter Oppeneer, peter.oppeneer@physics.uu.se 

Article reference

F. Schrodi, A. Aperis and Peter M. Oppeneer, Prominent Cooper pairing away from the Fermi level and its spectroscopic signature in twisted bilayer graphene. Physical Review Research – Rapid Communications (2020). Publication Date: March 17, 2020, DOI: https://doi.org/10.1103/PhysRevResearch.2.012066

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Uppsala Superconductivity code (UppSC)

Camilla Thulin
English translation: Johan Wall