Enhanced ultrafast relaxation rate in the Weyl semimetal phase of ${\mathrm{MoTe}}_2$ measured by time- and angle-resolved photoelectron spectroscopy

${\mathrm{MoTe}}_2$ has recently been shown to realize in its low-temperature phase the type-II Weyl semimetal (WSM). We investigated by time- and angle- resolved photoelectron spectroscopy (tr-ARPES) the possible influence of the Weyl points on the electron dynamics above the Fermi level $E_F$, by comparing the ultrafast response of ${\mathrm{MoTe}}_2$ in the trivial and topological phases. In the low-temperature WSM phase, we report an enhanced relaxation rate of electrons optically excited to the conduction band, which we interpret as a fingerprint of the local gap closure when Weyl points form. By contrast, we find that the electron dynamics of the related compound ${\mathrm{WTe}}_2$ is slower and temperature independent, consistent with a topologically trivial nature of this material. Our results shows that tr-ARPES is sensitive to the small modifications of the unoccupied band structure accompanying the structural and topological phase transition of ${\mathrm{MoTe}}_2$.

Figure 1

Crystal structure (a) and three-dimensional Brillouin zone (b) of 1T' ${\mathrm{MoTe}}_2$ [17]. (c), (d) Experimental and calculated band dispersion of 1T' ${\mathrm{MoTe}}_2$ along the $\mathrm{\Gamma }\mathrm{X}$ high-symmetry direction. (e)–(h) Calculated $k$-resolved bulk density of states, projected on the [001] surface BZ, for the low- [(e), (g)] and high- [(f), (h)] temperature phases of ${\mathrm{MoTe}}_2$, at two different binding energies. The presence of two pairs of WPs is revealed in the 1T' phase, at 50 meV (e) and 5 meV (g), respectively. In the centrosymmetric 1T” phase, the WPs vanish and the VB and the CB are well separated in momentum and in energy, as shown in the CEMs at 50 meV (f) and 5 meV (h). (i), (j) The cartoon shows that electron-electron interaction can mediate the scattering between CB and VB along the Weyl cone in the WSM phase (i). Interband scattering requires momentum and energy exchange in the presence of the gap (j).

Figure 2

(a) Band dispersion of ${\mathrm{MoTe}}_2$ along $\mathrm{\Gamma }\mathrm{X}$ at low (left) and high (right) temperatures, measured with 17.5 eV HHG photons 200 fs before optical excitation. (b) Effect of optical excitation on 1T' ${\mathrm{MoTe}}_2$. Difference between ARPES images acquired 80 fs after and 200 fs before the arrival of a $2\mathrm{eV}$ optical excitation with fluence $\sim 0.3\mathrm{mJ}/{\mathrm{cm}}^2$. Red and blue indicate the increase and decrease in the electron population, respectively. (c)–(f) Dynamics in the regions 1–4 of panel (b) near the WPs for the 1T' (blue) and 1T” (red) phases. A fit to the traces (dashed green and black lines) yields quantitative estimates of the characteristic relaxation times, $\tau$.

Figure 3

(a) Band structure of ${\mathrm{WTe}}_2$ along $\mathrm{\Gamma }\mathrm{X}$ calculated for the low-temperature crystal lattice parameters. (b)–(e) Calculated $k$-resolved bulk density of states, projected on the [001] surface BZ, at two different binding energies for the crystal structure at low and high temperatures. Unlike the case of ${\mathrm{MoTe}}_2$ the CB and VB are always well separated in momentum and energy. (f) Band structure along $\mathrm{\Gamma }\mathrm{X}$ for ${\mathrm{WTe}}_2$ measured at $50\mathrm{K}$ (left) and at $300\mathrm{K}$ (right) with $17.5\mathrm{eV}$ HHG photons. (g) The difference between ARPES images acquired 80 fs after and 200 fs before the arrival of a $2\mathrm{eV}$ optical excitation with fluence $\sim 0.3\mathrm{mJ}/{\mathrm{cm}}^2$. (h)–(k) Comparisons of the dynamics in regions 1–4 at low (blue) and high (red) temperature. A fit to the traces (dashed green and black lines) yields quantitative estimations of the characteristic relaxation time, $\tau$, which is three times larger than the gapped 1T” ${\mathrm{MoTe}}_2$. (l) Closeup on the calculated band structures of 1T' ${\mathrm{MoTe}}_2$, 1T” ${\mathrm{MoTe}}_2$, and ${\mathrm{WTe}}_2$ in the region of the WPs. The possible electron-electron and electron-phonon scattering events are shown schematically.