{
  "id": "2410.07360",
  "version": "v1",
  "published": "2024-10-09T18:20:40.000Z",
  "updated": "2024-10-09T18:20:40.000Z",
  "title": "Deciphering the origin of spin current in spintronic terahertz emitters and its imprint on their electromagnetic radiation via time-dependent density functional theory",
  "authors": [
    "Ali Kefayati",
    "Yafei Ren",
    "M. Benjamin Jungfleisch",
    "Lars Gundlach",
    "John Q. Xiao",
    "Branislav K. Nikolic"
  ],
  "comment": "7 pages, 3 figures",
  "categories": [
    "cond-mat.mes-hall"
  ],
  "abstract": "Spin current flowing between femtosecond laser pulse (fsLP)-driven ferromagnetic metal and adjacent normal metal (NM) hosting strong spin-orbit coupling is invariably invoked to explain THz radiation believed to be emitted solely by NM layer. Despite being such a central concept, the microscopic origin of such interlayer spin current remains vague. Here, we employ recently developed [A. Kefayati {\\em et al.}, Phys. Rev. Lett. {\\bf 133}, 136704 (2024)] time-dependent density functional theory plus Jefimenko equations approach to extract spin current between Co and NM=Pt or W layer where Co is driven by fsLP responsible for its demagnetization, i.e., shrinking of its magnetization vector, $M^y(t)/M^y(t=0)<1$ [while $M^x(t) \\approx M^z (t) \\approx 0$]. By comparing time dependence of spin current with those of other relevant quantities, we find that: ({\\em i}) spin current is generated by demagnetization dynamics because it {\\em follows} closely $dM^y/dt$, thus it is an example of quantum pumping phenomenon that cannot be captured by phenomenological notions (such as ``spin voltage'') or semiclassical theories; ({\\em ii}) time dependence of pumped spin current {\\em does not follow} closely that of charge current emerging within NM layer via spin-to-charge conversion mechanisms; ({\\em iii}) emitted THz radiation is dominated by charge current within Co layer, rather than usually assumed charge current [from ({\\em ii})] within NM layer, especially in the case of NM=W where spin-to-charge conversion by the inverse spin Hall effect is suppressed, despite large spin Hall angle of W, due to localization of excited electrons onto the outer unfilled $d$-orbitals of W.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2024-10-09T18:20:40.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "time-dependent density functional theory",
      "spintronic terahertz emitters",
      "plus jefimenko equations approach",
      "theory plus jefimenko equations",
      "spin current remains vague"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 7,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}