> Abstract: [...] Here we have discerned the quantum critical universality in graphene transport by combining the electrical and thermal conductivities in very high-quality devices close to the Dirac point. We find that they are inversely related, as expected from relativistic hydrodynamics, and the characteristic conductivity converges to a quantized value. We also observe a giant violation of the Wiedemann–Franz law, where the Lorentz number exceeds the semiclassical value by more than 200 times close to the Dirac point at low temperatures. At high temperatures, the effective dynamic viscosity to entropy density ratio close to the Dirac point in the cleanest devices approaches that of a minimally viscous quantum fluid within a factor of four.
Wikipedia lists some limitations of the Wiedemann–Franz law[1], and also some previous violations in other materials.
Reading the Wikipedia page I don't get the sense the law is quite as fundamental as the headline and summary make it sound like.
Here's one of the previous violations:
In 2011, N. Wakeham et al. found that the ratio of the thermal and electrical Hall conductivities in the metallic phase of quasi-one-dimensional lithium molybdenum purple bronze Li0.9Mo6O17 diverges with decreasing temperature, reaching a value five orders of magnitude larger than that found in conventional metals obeying the Wiedemann–Franz law. This due to spin-charge separation and it behaving as a Luttinger liquid.
Still, graphene is cool and seems to be the gift that keeps on giving in terms of surprising results in solid state physics.
There are a few methods to make rhombohedral graphene (which demonstrates superconductivity at room temperature).
Normal carbon stacks into a hexagonal ABAB pattern.
For superconductivity, the layers need to be at least ABC (because twisted bilayer graphene does not demonstrate the effects (superconductivity, quantum hall effect) at room temperature FWIU).
Current process: CVD chemical vapor decomposition and then sort and stack graphene flakes.
Flash heating plastic yields graphene and hydrogen; but you must capture the flue.
There are new plastic recycling methods that intentionally don't produce graphene that maybe could produce more plastic and graphene.
But graphene is hazardous sort of like coal ash; so IIUC if you can make graphene onsite (e.g. from unsorted 'recycled' plastics) and lock it in to glass or another substrate that avoids transport risks.
"Universality in quantum critical flow of charge and heat in ultraclean graphene" (2025) https://www.nature.com/articles/s41567-025-02972-z :
> Abstract: [...] Here we have discerned the quantum critical universality in graphene transport by combining the electrical and thermal conductivities in very high-quality devices close to the Dirac point. We find that they are inversely related, as expected from relativistic hydrodynamics, and the characteristic conductivity converges to a quantized value. We also observe a giant violation of the Wiedemann–Franz law, where the Lorentz number exceeds the semiclassical value by more than 200 times close to the Dirac point at low temperatures. At high temperatures, the effective dynamic viscosity to entropy density ratio close to the Dirac point in the cleanest devices approaches that of a minimally viscous quantum fluid within a factor of four.
Wikipedia lists some limitations of the Wiedemann–Franz law[1], and also some previous violations in other materials.
Reading the Wikipedia page I don't get the sense the law is quite as fundamental as the headline and summary make it sound like.
Here's one of the previous violations:
In 2011, N. Wakeham et al. found that the ratio of the thermal and electrical Hall conductivities in the metallic phase of quasi-one-dimensional lithium molybdenum purple bronze Li0.9Mo6O17 diverges with decreasing temperature, reaching a value five orders of magnitude larger than that found in conventional metals obeying the Wiedemann–Franz law. This due to spin-charge separation and it behaving as a Luttinger liquid.
Still, graphene is cool and seems to be the gift that keeps on giving in terms of surprising results in solid state physics.
[1]: https://en.wikipedia.org/wiki/Wiedemann%E2%80%93Franz_law#Li...
How difficult is it to make clean graphene?
If you're not looking for a perfect sample, pretty simple, the way it was discovered: with sticky tape and graphite.
https://en.wikipedia.org/wiki/Discovery_of_graphene
There are a few methods to make rhombohedral graphene (which demonstrates superconductivity at room temperature).
Normal carbon stacks into a hexagonal ABAB pattern.
For superconductivity, the layers need to be at least ABC (because twisted bilayer graphene does not demonstrate the effects (superconductivity, quantum hall effect) at room temperature FWIU).
Current process: CVD chemical vapor decomposition and then sort and stack graphene flakes.
Flash heating plastic yields graphene and hydrogen; but you must capture the flue.
There are new plastic recycling methods that intentionally don't produce graphene that maybe could produce more plastic and graphene.
But graphene is hazardous sort of like coal ash; so IIUC if you can make graphene onsite (e.g. from unsorted 'recycled' plastics) and lock it in to glass or another substrate that avoids transport risks.