Observation of Negative Terahertz Photoconductivity in Large Area Type-II Dirac Semimetal ${\mathrm{PtTe}}_2$

As a newly emergent type-II Dirac semimetal, platinum telluride (${\mathrm{PtTe}}_2$) stands out from other two dimensional noble-transition-metal dichalcogenides for the unique band structure and novel physical properties, and has been studied extensively. However, the ultrafast response of low energy quasiparticle excitation in terahertz frequency remains nearly unexplored yet. Herein, we employ optical pump-terahertz probe (OPTP) spectroscopy to systematically study the photocarrier dynamics of ${\mathrm{PtTe}}_2$ thin films with varying pump fluence, temperature, and film thickness. Upon photoexcitation the terahertz photoconductivity (PC) of ${\mathrm{PtTe}}_2$ films shows abrupt increase initially, while the terahertz PC changes into negative value in a subpicosecond timescale, followed by a prolonged recovery process that lasted a few nanoseconds. The magnitude of both positive and negative terahertz PC response shows strongly pump fluence dependence. We assign the unusual negative terahertz PC to the formation of small polaron due to the strong electron-phonon ($e\text{−}\text{ph}$) coupling, which is further substantiated by temperature and film thickness dependent measurements. Moreover, our investigations give a subpicosecond timescale of simultaneous carrier cooling and polaron formation. The present study provides deep insights into the underlying dynamics evolution mechanisms of photocarrier in type-II Dirac semimetal upon photoexcitation, which is of crucial importance for designing ${\mathrm{PtTe}}_2$-based optoelectronic devices.

Figure 1

(a) and (b) show the crystal structure of $ab$ and $bc$ side view, respectively. (c) Raman spectra and (d) x-ray diffraction pattern of ${\mathrm{PtTe}}_2$ films.

Figure 2

(a) The transient dynamics of 20 nm ${\mathrm{PtTe}}_2$ film under 780 nm pump at room temperature. The inset plots the transient response in a short time window, the red line is monoexponential fitting. (b) The terahertz transmission, $\mathrm{\Delta }T/T_0$, as a function of delay time under various pump fluences. The inset plots the magnitude of $\mathrm{\Delta }T/T_0$ at $t=0\mathrm{ps}$ (black) and $t=3\mathrm{ps}$ (red) with respect to pump fluence, respectively. (c) The rising time constants obtained from monoexponential fitting with respect to pump fluence, and the solid line is a linear fitting. (d) Terahertz waveform transmission of the sample without pump (brown) and with pump fluence of $542\mu \mathrm{J}/{\mathrm{cm}}^2$ collected at delay time $t=0$ (red) and $t=5\mathrm{ps}$ (blue), respectively. The inset shows the corresponding frequency spectra obtained from Fourier transform.

Figure 3

(a) Temperature dependent $\mathrm{\Delta }T/T_0$ at delay time of $t=0\mathrm{ps}$ (orange) and 3 ps (blue), respectively. Inset shows the transient terahertz transmission at various temperatures at fixed pump fluence of $362\mu \mathrm{J}/{\mathrm{cm}}^2$. (b) The transient terahertz transmission at 5 K under various pump fluences. Inset shows the enlargement of the rising process of $\mathrm{\Delta }T/T_0$ under various pump fluences.

Figure 4

(a) The transient terahertz transmission for 6.8, 20, and 44 nm ${\mathrm{PtTe}}_2$ films under 780 nm pump at room temperature, the optically injected photon density is $3.2\times {10}^{14}{\mathrm{cm}}^{-2}$ in all films. Inset plots the magnitude ratio of $\mathrm{\Delta }{\sigma }_{t=3}/\mathrm{\Delta }{\sigma }_{t=0}$ with respect to the film thickness. (b) The normalized terahertz transmission in long scan window for the three films, inset plots the fitting rise time as a function of the film thickness, the solid line is a guide to the eyes.