  We study the spatiotemporal dynamics of ultrafast electron spin transport
across nanometer-thick copper layers using broadband terahertz spectroscopy.
Our analysis of temporal delays, broadening and attenuation of the spin-current
pulse revealed ballistic-like propagation of the pulse peak, approaching the
Fermi velocity, and diffusive features including a significant velocity
dispersion. A comparison to the frequency-dependent Ficks law identified the
diffusion-dominated transport regime for distances larger than 2 nm. The
findings lie the groundwork for designing future broadband spintronic devices.
