Spin-polarized gap in the magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$

We report a unique type of gap in a magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ where the electrons are spin polarized and preserve the spin-momentum locking feature of Weyl fermions. Such spin-polarized gaps are associated with the Weyl node annihilation, where a pair of Weyl nodes with opposite chirality touch each other at ∼210 meV above the Fermi energy. These are revealed by both time- and spin-resolved ultrafast spectroscopy experiments, combined with first-principles calculations. The spin-polarized gap opening is accompanied by a topological phase transition, and the gap magnitude exhibits an unconventional temperature dependence originated from the Weyl physics. Furthermore, we propose possible circularly polarized terahertz-midinfrared radiation from such a spin-polarized gap. Our results shed light on exploring the topological properties of excited states and endow application potentials for gapped materials based on chirality.

Figure 1

(a) Schematic diagram of the spin-resolved ultrafast pump-probe spectroscopy setup. (b) Time-resolved ultrafast dynamics at various temperatures. The blue and red curves are the dynamics at $T\le T_{\mathrm{WNA}}$ and $T>T_{\mathrm{WNA}}$, respectively. The inset shows the top-view crystal structure of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ for the (001) surface.

Figure 2

(a) The amplitudes $A_{\mathrm{slow}}$ and $A_{\mathrm{slowest}}$. (b) The lifetimes ${\tau }_{\mathrm{slow}}$ and ${\tau }_{\mathrm{slowest}}$. The red curves in (a), (b) at $T>T_{\mathrm{WNA}}$ are fittings according to our modified RT model; the blue curve in (a) is the magnetization of the sample, multiplied by a coefficient (interpreted from Ref. [18]); the red curves at $T<T_{\mathrm{WNA}}$ in (a), (b) and the blue curve in (b) are guides to the eyes. (c) Temperature dependence of the gap. The red curve is the experimental fitted gap according to the RT model and the red dots are the calculated gaps at six given temperatures. $T_{\mathrm{WNA}}$ and $T_{\mathrm{C}}$ are marked by vertical dashed lines.

Figure 3

(a) The $\mathrm{\Delta }R/R$ signal at 100 K, under four circular-circular configurations and one linear-linear polarization configuration, respectively. The red (blue) scanning curves are data taken under right (left) circularly polarized probe beams. (b) The spin-dependent quasiparticle dynamics at 6, 100, 160, 170, and 180 K, respectively.

Figure 4

(a) Temperature dependence of spin-resolved dynamics. (b) Gap evolution with temperature in the topological phase diagram. The light blue and red background regions mark the Weyl and gapped semimetal phases, respectively. The blue dashed diagonal marks the boundary of the Weyl-gapped semimetal topological phase transition. The red circular curve denotes the increasing temperature route. Five typical temperatures are marked by gray points $A$–$E$. Five different schematic electronic structures are illustrated, corresponding to $A$–$E$, where the gold arrows denote spins. $T_{\mathrm{WNA}}$ and $T_{\mathrm{C}}$ are marked in both (a) and (b).

Figure 5

Temperature-dependent band structures by calculations at $T=0$, 41, 131, 160, and 175 K. In the middle row panels we show the three-dimensional perspective views of the band structures in a specific momentum plane parallel to the $k_z$ axis, where a pair of Weyl nodes with opposite chirality exist. The bands with +1/2, 0, and −1/2 spin polarization are colored in red, yellow, and blue, respectively. The top row panels represent the corresponding top views of the middle row panels. A pair of the Weyl nodes are marked as light blue (positive chirality) and green (negative chirality) spots in (a), (b). The white arrows denote the convergence of Weyl nodes with temperature. The bottom row panels illustrate the corresponding band structures of the middle row panels along the momentum path passing through a pair of Weyl nodes with opposite chirality and the Γ point in (a), (b), and only the Γ point in (c)–(e), respectively.