Ultrafast Electronic Band Gap Control in an Excitonic Insulator

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of $F_C=0.2\mathrm{mJ}{\mathrm{cm}}^{-2}$, the band gap narrows transiently, while it is enhanced above $F_C$. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ with light on the femtosecond time scale.

Figure 1

(a) Schematic band structure of TNS at low temperature (see main text for details). (b) Schematic EI (solid) and semiconductor (dashed) band structures. $E_g$ is the semiconductor band gap and $\mathrm{\Delta }$ the enhancement of the band gap due to the transition to the EI phase. (c) PE spectrum of the TNS occupied electronic structure around $\mathrm{\Gamma }$ at 110 K. The red and yellow bars indicate the momentum intervals of integration for the extracted EDCs at $\mathrm{\Gamma }$ and at $k\ne 0$, respectively. (d) PE intensity at $\mathrm{\Gamma }$ as a function of electron binding energy with respect to $E_F$ (left axis) and pump-probe time delay (bottom axis) at 110 K. Markers indicate the transient energetic position of the two VBs. (e) EDCs at equilibrium (empty gray circles) and 190 fs after photoexcitation (solid red circles). (f) Transient population of the upper VB at $\mathrm{\Gamma }$. (g) Minima from (f) as a function of pump fluence: Above $F_C\approx 0.2\mathrm{mJ}{\mathrm{cm}}^{-2}$, the VB depopulation saturates at $\sim 50\%$. Dashed lines are linear fits.

Figure 2

(a) Shift of the upper VB at $k\ne 0$ as a function of pump-probe time delay for different excitation densities. (b) Same analysis for the lower VB at $\mathrm{\Gamma }$. Black lines in (a) and (b) are single exponential fits to the data convolved with the envelope of the laser pulses.

Figure 3

Shift of the upper VB at $\mathrm{\Gamma }$ as a function of pump-probe time delay for different excitation densities.

Figure 4

(a) The fit amplitude of the shift of the lower VB at $\mathrm{\Gamma }$ (green) and of the upper VB at $k\ne 0$ (yellow) and at $\mathrm{\Gamma }$ (red) as evaluated for a delay of 255 fs [see Fig. 3] as a function of the incident pump fluence. In the inset, the band structure dynamics are schematized for two excitation regimes, below (left) and above (right) $F_C$, respectively. (b) Dispersion relation for the ground state (gray) and a nonthermal distribution function (dark red), obtained from the Hartree-Fock calculations. The dashed lines show the bare semiconducting dispersion (with Hartree shift included). (c) Distribution functions corresponding to the dispersion relations in panel (b) and (d). (d) Comparing the dispersions of the thermal state at elevated temperature (orange) with that of the ground state and the photoexcited nonthermal state.