We study the spatiotemporal dynamics of ultrafast electron spin transport across nanometer-thick copper layers using ultrabroadband terahertz emission spectroscopy. Our analysis of temporal delays, broadening, and attenuation of the spin-current pulse reveals ballisticlike propagation of the pulse peak, approaching the Fermi velocity, and diffusive features including a significant velocity dispersion. A comparison to the frequency-dependent Fick's law identifies the diffusion-dominated transport regime for distances >2 nm. These findings lay the groundwork for designing future broadband spintronic devices.

Department of Physical Chemistry Fritz Haber Institute of the Max Planck Society 14195 Berlin Germany

Department of Physics Freie Universität Berlin 14195 Berlin Germany

Faculty of Mathematics and Physics Charles University 121 16 Prague Czech Republic

Institut für Physik Martin Luther Universität Halle 06120 Halle Germany

Institut für Physik Universität Greifswald 17489 Greifswald Germany

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Terahertz Spin-Conductance Spectroscopy: Probing Coherent and Incoherent Ultrafast Spin Tunneling