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Title: Shear viscosity of the graphene Dirac fluid near charge neutrality
Plain: 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: 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.
Settled by: Spatially resolved imaging of the neutrality-point flow profile in channels (e.g. nitrogen-vacancy magnetometry) combined with an independent entropy measurement.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: partially-resolved
Status note: 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.
Posed since: (empty)