Ultrafast photoinduced conductivity reduction by bonding orbital control in an incommensurate crystal

In this paper, we demonstrate the ultrafast reduction of conductivity in an incommensurate crystal structure within hundreds of femtoseconds. This phenomenon stands in stark contrast to most prior experimental investigations where incident light pulses led to increased conductivity. We achieve this by selectively targeting a specific atomic bond using near-infrared light pulses. Our investigation focuses on misfit layered chalcogenide (${\mathrm{LaS})}_{1.196}{\mathrm{VS}}_2$, known as ${\text{LaVS}}_3$, a semimetal with incommensurability along one crystallographic direction. Our time-resolved electron dynamics investigation reveals that the conductivity decreases as photoexcited electrons are promoted into localized energy states within vanadium clusters due to the incommensurate structure. These findings offer insights into the potential for controlling electronic properties at femtosecond time scales, with implications for the development of ultrafast electronic devices.

Figure 1

(a) Crystal structure of ${\text{LaVS}}_3$, composed by stacking of LaS and ${\text{VS}}_2$ slabs, respectively. (b) Top view of the ${\text{VS}}_2$ layer, showing a periodic change in the interatomic V-V distance. The four closest vanadium neighbors, linked for clarity by a red solid line, form a vanadium cluster.

Figure 2

[(a)–(c)] Transient reflectivity (solid line) and simulated reflectivity (dashed line) of ${\text{LaSV}}_3$ crystal at room temperature as a function of the pump fluence for pump $1.2\mu \mathrm{m}$ (1 eV) and probe $2.4\mu \mathrm{m}$ (0.5 eV) wavelength. (d) and (e) $\mathrm{\Delta }{R}_{\text{max}}$ and $\mathrm{\Delta }{R}_{\text{pl}}$ vs pump fluence. (f) Reflectivity from ellipsometry measurement at equilibrium vs crystal temperature, in the spectral range from 2.3 to $2.5\mu \mathrm{m}$.

Figure 3

Theoretical calculation and bond orbitals in ${\text{LaVS}}_3$. (a) Density of states (DOS) calculation of ${\text{LaVS}}_3$, showing that the $3d$ levels of vanadium are split into $t_{2g}$ and $e_g$ levels and that the main contribution around the Fermi level is given by the $t_{2g}$ orbitals. (b) Partial DOS of vanadium, showing the formation of three subbands resulting from the split of $d_{yz}$ and $d_{xz}$ orbitals, while $d_{xy}$ remains unsplit. (c) Energy splitting in the ${\text{LaVS}}_3$ structure for the vanadium atoms in the cluster and accessible interband transitions with a photon energy of $\sim 1$ eV, shown by the blue arrows.

Figure 4

Occupation level of the $\sigma , d_{xy}$, and ${\sigma }^{*}$ orbitals vs time delay for a pump fluence of 2 ${\text{mJ/cm}}^2$.

Figure 5

$c_{21}$ decay rate from the $d_{xy}$ to the $\sigma$ orbital vs pump fluence.