Image showing the kink feature of the ARPES data used to extract the electron–photon coupling constant and predict the superconducting transition temperature.
With superconductors and low-dimensional materials being material classes under intense study, the combination of the two is bound to cause a stir. Now, a study by a team from the University of Vienna, along with international collaborators, has uncovered the potential superconducting coupling mechanism in the much-touted wonder material, graphene.
It is only comparatively recently that reports of superconductivity in graphene have appeared, although its close relatives – such as graphite and fullerenes – can be made superconducting through doping. However, this work, reported in Nature Communications [Fedorov, et al., Nat. Commun. (2014), doi:10.1038/ncomms4257], demonstrated the superconducting pairing mechanism in calcium-doped graphene based on the angle-resolved photoemission spectroscopy (ARPES) technique.
In the ARPES method, when a light particle interacts with a material, it can transfer all its energy to an electron inside that material. If the energy of the light is large enough, the electron acquires sufficient energy to escape from the material. The technique helped identify the angle under which the electrons escape from the material, allowing useful information to be gleaned about its electronic properties and interactions.
In this case, ARPES was used to identify an electron donor for monolayer graphene capable of inducing strong electron–phonon coupling and superconductivity, investigating the common electron dopant atoms. It was found that calcium was the most promising candidate for realizing superconductivity in graphene with a critical temperature of about 1.5K.
On examining the strength of the kink in the spectral function in the two crystallographic main directions to estimate the superconducting critical temperature, they found an unexpected low-energy peak for all dopants with an energy and intensity dependent on the dopant atom, demonstrating this peak resulted from dopant-related vibration. As researcher Alexander Grüneis points out, the work does “yield a quantity called the spectral function that is related to the electron energy band structure of a material and the many-body effects. The spectral function allows us to probe the coupling mechanism that in turn allows us to predict superconductivity.”
As the properties of graphene are so easily altered, the study could improve our understanding of superconducting coupling mechanisms, especially that of carbon materials. The team now intend to further explore the stacking of graphene layers to research the transition between 2D and 3D superconductivity, as well as looking at how many layers are needed to achieve superconductivity.
