  The spin Coulomb drag effect, arising from the exchange of momentum between
electrons of opposite spins, plays a crucial role in the spin transport of
interacting electron systems. This effect leads to the emergence of a spin mass
and the finite lifetime of spin currents, posing challenges for the accurate
description of spin dynamics. Using the state-of-the-art Variational
Diagrammatic Monte Carlo approach, we investigate the spin-resolved
exchange-correlation (XC) kernel in the three-dimensional uniform electron gas.
Our analysis reveals a distinct nature in the spin response, characterized by a
$1/q^2$ divergence in the spin XC kernel at finite frequencies. This so-called
ultranonlocal behavior, stemming from the spin Coulomb drag effect, is absent
in the charge channel. By extracting precise values for the spin mass
enhancement factor, we observe a trend of increasing enhancement with
decreasing electron density. Furthermore, we find compelling evidence for the
suppression of spin diffusion at low temperatures, characterized by vanishing
inverse relaxation times. These findings deepen our understanding of the
intricate relation between the Coulomb interaction and the spin transport,
providing valuable insights for the development of accurate density functional
approximations and the advancement of spintronics and quantum technologies.
