Ultrafast Doublon Dynamics in Photoexcited $1T$-${\mathrm{TaS}}_2$

Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide $1T$-$\mathrm{Ta}{\mathrm{S}}_2$, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping $\hslash /J\approx 14\mathrm{fs}$. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale.

Figure 1

(a) False-color representation of the time-dependent photoelectron intensity above $E_F$ at 30 K for $F=60\mu \mathrm{J}/{\mathrm{cm}}^2$ in normal emission. Besides an excitation continuum, the upper Hubbard band is observed at $E-E_F\approx 175\mathrm{meV}$ in the vicinity of $t=0$. (b) and (c) show photoemission spectra after excitation for selected $T$ and $F$, respectively. The spectra were averaged within $\mathrm{\Delta }t=-95\dots -35\mathrm{fs}$. Curves in (b) are offset for better visibility.

Figure 2

(a) Transient photoemission spectra obtained for $T=30\mathrm{K}$ and $F=60\mu \mathrm{J}/{\mathrm{cm}}^2$. The curves are offset for better visibility. (b) Exemplary fit to the transient energy distribution curve obtained at $\mathrm{\Delta }t=+32\mathrm{fs}$. The superposition of the continuum and the Lorentzian line was convoluted with the instrumental energy resolution function. (c) Temporal evolution of the UHB spectral signature in direct comparison to the underlying continuum and electronic gap, as well as the LHB intensity loss that can be described by an error function (solid green line). The temporal derivative $-(dI/dt)$ of the least mean square fit of the LHB dynamics is shown as a dashed line for comparison with the UHB signal. Curves are rescaled for better visibility. (d) Spectral width of the UHB signature as a function of excitation fluence $F$.

Figure 3

(a) Time evolution of the occupation function $G^{<}(\omega ,t)$ for a half-filled band ($n=1$, left panel) and in the hole-doped case ($n=0.95$, right panel). The dashed lines show the equilibrium situation before excitation. (b) Equilibrium occupation function $G^{<}(\omega )$ for $n=0.98$ (solid lines) and $n=0.95$ (dashed lines) at different temperatures $\beta =(k_{B}T{)}^{-1}$, given in units of $W$ ($\beta =1$ corresponds to a temperature of 32 300 K). (c) Spectral width of the UHB signature at $E-E_F\approx 175\mathrm{meV}$ as a function of excitation density $E_0^2$. The dashed line indicates a linear dependence.