▎ 摘　　要

In a clean Fermi liquid, due to spin up/spin down symmetry, the dc spin current driven by a magnetic field gradient is finite even in the absence of impurities. Hence, the spin conductivity alpha(s) assumes a well-defined collision-dominated value in the disorder-free limit, providing a direct measure of the inverse strength of electron-electron interactions. In neutral graphene, with Fermi energy at the Dirac point, the Coulomb interactions remain unusually strong, such that the inelastic scattering rate comes close to a conjectured upper bound tau(-1)(inel) less than or similar to k(B)T/(h) over bar, similar to the case of strongly coupled quantum critical systems. The strong scattering is reflected by a minimum of spin conductivity at the Dirac point, where it reaches sigma(s) = 0.121/alpha(2) mu(2)(s)/(h) over bar at weak Coulomb coupling alpha, mu(s) approximate to mu(B) being the magnetic moment of the electronic spins. Up to the replacement of quantum units, e(2)/(h) over bar -> mu(2)(s)/(h) over bar, this result equals the collision-dominated electrical conductivity obtained previously. This accidental symmetry is, however, broken to higher orders in the interaction strength. For gated graphene and two-dimensional metals in general, we show that the transport time is parametrically smaller than the collision time. We exploit this fact to compute the collision-limited alpha(s) analytically as sigma(s) = 1/C (mu/T)(2) mu(2)/s/(h) over bar, with C = 4 pi(2)alpha(2) [2/3ln(1/2 alpha) - 1] for weak Coulomb coupling alpha.