Role of interlayer shear phonons on lattice symmetry switching in the transition metal dichalcogenide WTe$_{2}$

Abstract

Coherent phonon control using ultrashort pulse trains is the key to realizing structural phase transitions in solids by non-thermal pathways. By combining double-pulse excitation and time-resolved second harmonic generation techniques under high-density electronic excitation in a 2D layered material, WTe$_{2}$, we demonstrate that the lattice symmetry switching from the Weyl semimetallic T$_{d}$ to the semimetallic 1T$^{\prime}$ phases is independent of the amplitude of the coherent interlayer shear phonons after the arrival of the second pump pulse. This finding provides new insights into the mechanisms for symmetry switching that electronic excitation-driven shear sliding plays a dominant role.

Figures (4)

• Figure 1: (a) Schematic of the time-resolved SHG measurements with a reflection geometry. The time interval between the 1st and 2nd pump pulse is expressed as $\Delta t$. (b) Measured spectra for the incoming probe light (1230 nm) and induced SHG (615 nm) from $T_{d}$-WTe$_{2}$ at the probe fluence of 3.2 mJ/cm$^{2}$. (c) Polarization angle dependence of static SHG intensity.
• Figure 2: Time-domain signal of normalized change of $\Delta I_{\mathrm{SHG}}/I_{\mathrm{SHG}}$ obtained for the double pulse excitation at a total fluence of 4.6 mJ/cm$^{2}$. The blue and red lines represent the case for constructive and destructive excitation, respectively. The Gaussian-shape train indicates the arrival time of the double pulses. The inset shows the time-domain signal of $\Delta I_{\mathrm{SHG}}/I_{\mathrm{SHG}}$ under the single-pulse excitation at the fluence of 1.5 mJ/cm$^{2}$.
• Figure 3: (a) Time-domain SHG intensity for constructive excitation of the shear phonon using double-pulse with $\Delta t$=2$T$ at the total fluence from 1.6 mJ/cm$^{2}$ to 10.6 mJ/cm$^{2}$. (b) Time-domain SHG intensity for destructive excitation of the shear phonon with double-pulse with $\Delta t$=1.5$T$ at the total fluence from 1.8 mJ/cm$^{2}$ to 10.6 mJ/cm$^{2}$. The dotted lines are the fit using exponential decay functions described in the main text.
• Figure 4: The pump fluence dependence of the normalized SHG intensity change. The green closed circles show single-pulse excitation, while the closed blue triangles and red squares show the case for the constructive ($\Delta t$=2$T$) and destructive ($\Delta t$=1.5$T$) excitation of the shear phonon by the double-pulse, respectively. The gray thick lines represent the two different slopes obtained by the linear fits. The arrows represent the critical fluence for double-pulse excitation ($\sim$6 mJ/cm$^{2}$) and single-pulse excitation ($\sim$4 mJ/cm$^{2}$). The error bars represent fluctuations of the laser output from the amplifier ($\pm$ 4%).