Broad-band THz emission by Spin-to-Charge Conversion in Topological Material -- Ferromagnet Heterostructures

TL;DR

This paper demonstrates that topological insulators and Weyl semimetals can efficiently generate broadband terahertz radiation through spin-to-charge conversion, advancing ultrafast spintronic device potential.

Contribution

It introduces the use of time-domain terahertz emission spectroscopy to compare SCC in topological insulators and Weyl semimetals, revealing their broadband emission capabilities.

Findings

Efficient SCC depends on interface quality and composition in TIs.

NbP/W bilayers show THz emission comparable to TIs.

Both TIs and WSMs generate THz pulses up to 8 THz.

Abstract

Terahertz spintronic devices combine ultrafast operation with low power consumption, making them strong candidates for next-generation memory technologies. In this study, we use time-domain terahertz emission spectroscopy to investigate spin-to-charge conversion (SCC) in bilayer heterostructures comprising topological insulators (TIs) or Weyl semimetals (WSMs) with ferromagnetic metals (FMs). SCC is studied in TI materials \ce{Bi2Se3}, Pb-doped \ce{Bi2Se3}, and (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$, and the WSM NbP. Our results reveal that the dependence of SCC on TI thickness varies with interface quality, indicating that thickness dependence alone is not a reliable criterion for distinguishing between inverse spin Hall effect and the inverse Rashba--Edelstein effect mechanisms. We find efficient SCC in TIs depends on both \textit{in-situ} growth to prevent surface oxidation and proper composition. In NbP$\vert$FM bilayers, we observe THz emission with efficiency and bandwidth comparable to that of TIs, highlighting the broader potential of topological materials. Finally, broadband spectral measurements demonstrate that both TIs and WSMs can generate THz pulses with frequencies extending up to 8\,THz. These findings underscore the promise of topological materials as efficient platforms for ultrafast, broadband spintronic applications.