Ultrafast carrier dynamics in the large-magnetoresistance material ${\mathrm{WTe}}_2$

Ultrafast optical pump-probe spectroscopy is used to track carrier dynamics in the large-magnetoresistance material ${\mathrm{WTe}}_2$. Our experiments reveal a fast relaxation process occurring on a subpicosecond time scale that is caused by electron-phonon thermalization, allowing us to extract the electron-phonon coupling constant. An additional slower relaxation process, occurring on a time scale of $\sim 5$–15 ps, is attributed to phonon-assisted electron-hole recombination. As the temperature decreases from 300 K, the time scale governing this process increases due to the reduction of the phonon population. However, below $\sim 50$ K, an unusual decrease of the recombination time sets in, most likely due to a change in the electronic structure that has been linked to the large magnetoresistance observed in this material.

Figure 1

The transient reflectivity $\mathrm{\Delta }R\left(t\right)/R$ (red line) as a function of time delay in ${\mathrm{WTe}}_2$ at $T=60$ K and $F=3.5\mu \mathrm{J}/{\mathrm{cm}}^2$. The black solid line shows the fit to the measured data, without including the oscillations. The dashed blue line shows the value of the $\mathrm{\Delta }R\left(t\right)/R$ signal at long time delays, $A_0$. The inset shows the $\mathrm{\Delta }R\left(t\right)/R$ signal (red line) and the fitting result (black line) up to 300 ps.

Figure 2

(a)–(e) $\mathrm{\Delta }R\left(t\right)/R$ in ${\mathrm{WTe}}_2$ at several selected temperatures between 5 and 300 K. The pump fluence is $F=3.5\mu \mathrm{J}/{\mathrm{cm}}^2$. (f)–(j) $\mathrm{\Delta }R\left(t\right)/R$ in ${\mathrm{WTe}}_2$ measured with different pump fluences at 5 K. The black solid line through the data in each panel denotes the curve fit to the nonoscillatory response.

Figure 3

(a) and (b) show ${\tau }_f$ and ${\tau }_s$, respectively, as a function of temperature for $F=3.5\mu \mathrm{J}/{\mathrm{cm}}^2$. The red solid curve denotes (a) the TTM fit and (b) the calculated $T$ dependence of the phonon-assisted e-h recombination time, with a fixed density of states in the electron and hole Fermi pockets. The inset of (b) displays the decay rate $1/{\tau }_s$ as a function of the pump fluence, while the red solid line represents a linear fit to the data. (c) Temperature dependence of the MR, normalized by the values at 2 K.

Figure 4

Schematic band structure of ${\mathrm{WTe}}_2$ near the Fermi level ($E_F$) along the $\mathrm{\Gamma }-X$ direction at low temperatures. ${\tau }_f$ describes the e-ph thermalization process and ${\tau }_s$ represents the phonon-assisted interband e-h recombination process.