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Title: Universal lower bound on spin diffusion in strongly interacting fermions
Plain: When up and down spins are separated, strong collisions slow how fast they mix back. Experiments suggest a floor of about $\hbar/m$, but two kinds of measurement disagree.
Precise: Unitary Fermi gas in 3D (and 2D). Determine the minimum longitudinal spin diffusivity $D_s$ and transverse diffusivity D_perp in units of $\hbar/m$, and whether a universal bound $D \ge c \hbar/m$ holds. Answer: the value of $c$ for each, and a theory explaining the difference.
Settled by: Homogeneous measurements of both diffusivities in the same apparatus near $T_{c}$, with a theory that reproduces both.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status note: Longitudinal (Sommer et al. 2011, about $6.3\,\hbar/\mathrm{m}$) and transverse (Bardon et al. 2014, about $1.3\,\hbar/\mathrm{m}$) minima differ by about a factor of 5, and spin-hydrodynamics work continues in 2026 (arXiv:2607.29647).
Posed since: 2011