Ab initio nonadiabatic molecular dynamics investigation on the dynamics of photogenerated spin hole current in Cu-doped $\mathrm{Mo}{\mathrm{S}}_2$

Fully spin-polarized hole current is theoretically proposed to be generated by photoexcitation in the impurity states of the $\mathrm{Mo}{\mathrm{S}}_2$ monolayer with sulfur partially substituted by copper. To understand the dynamics of the photogenerated spin hole current, we perform a time-domain ab initio nonadiabatic molecular dynamics investigation with different initial excitation and temperature. First, the spin hole relaxes in a band-by-band manner. Therefore a longer lifetime can be achieved if the initial hole is generated at the lower edge of the impurity bands. Second, the phonon excitation is found to affect the spin hole dynamics significantly. When the temperature is decreased from 100 to 50 K, the hole relaxation across the band gap is strongly suppressed by the phonon bottleneck, which is due to the reduction of the phonon occupations. Our results show that the initial hole generation and phonon excitation are two key factors determining the dynamics of the photogenerated spin hole, which provide insights into the design of optimal spintronic devices.

Figure 1

The spin-polarized band structure and the projected density of states (PDOS) (a) and the orbital spatial distribution of the Cu doped $\mathrm{Mo}{\mathrm{S}}_2$ (b). The total density of states (DOS) is projected on the coordinated Mo atoms, the Cu impurity atoms, and the rest of the $\mathrm{Mo}{\mathrm{S}}_2$ host in the supercell. The isosurface value in (b) is set at $0.002e\mathrm{bohr}$. The process of spin hole relaxation within the impurity states is indicated by the arrow in Fig. 1.

Figure 2

Time-dependent evolution of the energy of the impurity states at the Γ point (a, c) and the FT spectra to the autocorrelation function of the energy evolution (b, d) at 100 and 50 K, respectively. The color legend is the same as shown in Fig. 1. (e, f) The spatial localization of each normal phonon mode projected on the Cu impurity and $\mathrm{Mo}{\mathrm{S}}_2$ host, respectively.

Figure 3

The dynamics of a photogenerated hole at 100 K. The averaged energy of the hole and the population on each impurity state are shown in the upper panel, and the AD and NA contributions to the energy relaxation are shown in the lower panel with the initial state specified at the impurity state 1 (a) and 2 (b).

Figure 4

The averaged absolute values of the NAC along the 3-ps trajectory between the impurity states at (a) 100 K and (b) 50 K. The absolute values of the NAC (magenta dotted line) and the oscillation of the gap (black solid line) between the impurity state 2 and 3 along the 3-ps AIMD trajectory at (c) 100 K and (d) 50 K. The blue and red dotted lines indicate the averaged value of the NAC and the gaps, respectively.

Figure 5

The dynamics of a photogenerated hole at 50 K. The averaged energy of the hole and the population on each impurity state are shown in the upper panel, and the AD and NA contributions to the energy relaxation are shown in the lower panel with the initial state specified at the impurity state 1 (a) and 2 (b).