Shear viscosity of the graphene Dirac fluid near charge neutrality
In plain words
When graphene has equal numbers of electrons and holes, its carriers form a nearly ideal relativistic fluid. Its viscosity has been estimated only indirectly, so how close it comes to a conjectured quantum lower limit is not settled.
Precise statement
For monolayer graphene at charge neutrality with $50\,\mathrm{K} < T < 300\,\mathrm{K}$, determine the shear viscosity $\eta$, the entropy density s, and $\eta/s$ in units of $\hbar/k_B$, to compare with the conjectured lower bound $1/(4\pi)$ (Kovtun, Son, Starinets 2005) and with kinetic theory at the measured effective coupling. Answer: $\eta/s(T)$ with uncertainty.
What would settle it
Spatially resolved imaging of the neutrality-point flow profile in channels (e.g. nitrogen-vacancy magnetometry) combined with an independent entropy measurement.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Majumdar et al. (Nature Physics 2025, doi:10.1038/s41567-025-02972-z) inferred $\eta/s$ within a factor of four of $1/(4 \pi)$ from combined electrical and thermal transport; a direct flow-profile measurement of $\eta$ with independent $s$ is lacking.
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