A single-layer planar carbon sheet with four- and eight-membered rings, known as T-graphene, could be an intrinsic elemental 2D superconductor with a superconducting transition temperature (Tc) of more than 20 K. This is the new finding from researchers at Nanjing University in China who say that single-layer T-graphene, which could be used to fabricate superconducting nanodevices in the future, might be peeled off from a precursor potassium T-graphene intercalation compound (C4K) and be synthesized under high pressure in the laboratory. The precursor itself could have record values of Tc for layered carbon-based superconductors.
There are single-layer materials, such as iron selenide (FeSe), molybdenum disulphide (MoS2) and niobium selenide (NbSe2) that can be superconductors but some of these need to be doped or strained to exhibit superconductivity. Graphene (a sheet of carbon atoms just one atom thick) is another example. Indeed, researchers recently reported superconductivity in “magic-angle” bilayer graphene, a result that caused quite a stir, but the magic angle can be quite tricky to control in experiments.
The list does not end there: many carbon-allotrope related materials, such as graphene intercalation compounds (GICs), fullerene alkali metal compounds, nanotubes, boron-doped diamond and boron carbide, are also superconducting. GICs, in which metallic atoms intercalate between graphene sheets, are particularly interesting to study, says study team leader Jian Sun who is at the School of Physics and the National Laboratory of Solid State Microstructures at Nanjing University.
T-graphene and its intercalation compound
Alkali metal carbon compounds (such as C8A, where A is K, Rb or Cs) were the first type of GIC superconductors to be studied, he explains. C8K was the most popular compound since it is one of the easiest to fabricate. Indeed, researchers predicted that this material could have Dirac-like fermions and a high Fermi velocity similar to graphene itself and found that the Tc of C8K can be increased to 1.7 K by applying pressures of 1.5 GPa.
Sun and colleagues have now predicted that the T-graphene intercalated material C4K, which has a similar structure to C8K, could have a Tc that is even higher – of up to 30.4 K at 0 GPa. They obtained their result thanks to a machine learning crystal structure search method that they developed last year to predict that a stable phase of C4K can be synthesized in the laboratory at high pressures of around 11.5 GPa and then quickly quenched to ambient pressure.
Direct measurements probe the state of magic-angle twisted graphene
The researchers then employed first-principle calculations based on density functional theory to check the material’s stability and show that a single-layer of T-graphene could, in principle, be peeled off from bulk C4K or from bulk T-graphite (C4), where C4 can be obtained from C4K by evaporating the K atoms. Finally, they used first-principles calculations and electron-phonon coupling theory to study the materials’ electronic structures and superconducting properties. “Most excitingly, we found that the single-layer of T-graphene could be an intrinsic superconductor with a Tc of around 20 K,” Sun tells Physics World.
Fabricating superconducting devices
“Once the single-layer T-graphene is made, it could be stacked with other 2D materials such as graphene or transition metal dichalcogenides (using standard layer-by-layer or so-called vertical techniques) to fabricate superconducting devices,” he says. “What is more, the high-pressure synthesis method we have put forward in this work could be a new strategy to obtain single-layer materials that cannot be easily synthesized at ambient conditions. This provides a road map to obtaining such promising materials and opens a new door in the field.”
The researchers, reporting their work in Chinese Physics Letters 10.1088/0256-307X/36/9/097401, say that they are now using the strategy they have proposed to search for other single-layer materials with superconducting properties.
