Photoinduced metallization of excitonic insulators

Utilizing the time-dependent density-matrix renormalization-group technique, we numerically prove photoinduced pairing states in the extended Falicov-Kimball model (EFKM) at half-filling, both with and without internal SU(2) symmetry. In the time-dependent photoemission spectra, an extra band appears above the Fermi energy after pulse irradiation, indicating an insulator-to-metal transition. Even in the absence of the SU(2) structure, the pair correlations are enhanced during the pump, and afterwards they decrease over time. This implies the possible metallization of ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$, a strong candidate for an excitonic insulator material, for which the EFKM is considered to be the minimal theoretical model. Simulating the photoemission with optimized pulse parameters, we demonstrate a photoinduced quantum phase transition, in accord with recent findings in time- and angle-resolved photoemission spectroscopy experiments on ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$.

Figure 1

Contour plots of $\tilde{P}(q=0,t)$ (a) and ${\tilde{N}}_{\mathrm{ex}}(q=0,t)$ (b) at $t\times t_c=15$ in the ${\omega }_{\mathrm{p}}-A_0$ plane for an infinite half-filled EFKM chain with $t_f=-t_c, U/t_c=8, \langle {\widehat{n}}_c\rangle =0.25$, and $D/t_c=0.75$. (c) Time evolution of $\tilde{P}(0,t)$ and ${\tilde{N}}_{\mathrm{ex}}(0,t)$ near the peak position of panel (a), $A_0=0.4$ and ${\omega }_{\mathrm{p}}/t_c=6.6$ marked by the “$+$” symbol in panels (a) and (b). The pump pulse has width ${\sigma }_{\mathrm{p}}\times t_c=2$ and is centered at time $t_0\times t_c=10$.

Figure 2

(a)–(i) Snapshots of the photoemission spectra $A_{\alpha =c,f}^{-}(k,\omega ;t)$ and $A^{-}(k,\omega ;t)={\sum }_{\alpha }{A}_{\alpha }^{-}(k,\omega ;t)$ for the $\mathrm{\Delta }$-pairing dominant state. Here, $A_0=0.4$ and ${\omega }_{\mathrm{p}}/t_c=6.6$, denoted as “$+$” in Fig. 1. Results are displayed during the pump at $t\times t_c=5$ [(a)–(c)], 10 [(d)–(f)], and 15 [(g)–(i)]. (j)–(l) Transient integrated density of states $A_{\left(\alpha \right)}^{-}(\omega ;t)$ from Eq. (12) obtained from the results in (a)–(i). All data were obtained using the (i)TEBD technique with IBCs, with the pump pulse parametrized as in Fig. 1. The Fermi energy $E_{\mathrm{F}}$ is set to $\omega =0$.

Figure 3

Contour plots of $\tilde{P}(0,t)$ (a) and ${\tilde{N}}_{\mathrm{ex}}(0,t)$ (b) in the ${\omega }_{\mathrm{p}}\text{−}{A}_0$ plane for an infinite EFKM chain after pulse irradiation ($t\times t_c=15$). The EFKM model parameters are $t_f/t_c=-0.5, U/t_c=1, D/t_c=0.03$, and $\langle {\widehat{n}}_j^c\rangle =0.1$. (c) Time evolution of $\tilde{P}(0,t)$ and ${\tilde{N}}_{\mathrm{ex}}(0,t)$ near the peak position of panel (a), i.e., $A_0=0.8$ and ${\omega }_{\mathrm{p}}/t_c=0.8$ marked by the “$+$” symbol in panels (a) and (b). The pump pulse has width ${\sigma }_{\mathrm{p}}\times t_c=2$ and is centered at time $t_0\times t_c=10$.

Figure 4

(a)–(i) Snapshots of the photoemission spectra $A_{\alpha =c,f}^{-}(k,\omega ;t)$ and $A^{-}(k,\omega ;t)={\sum }_{\alpha }{A}_{\alpha }^{-}(k,\omega ;t)$ of the 1D EFKM with $U/t_c=1$ and $\langle {\widehat{n}}_c\rangle =0.1$. Results are given, for the $\mathrm{\Delta }$-pairing dominant state, during the pump at $t\times t_c=5$ [(a)–(c)], 10 [(d)–(f)], and 15 [(g)–(i)], where $A_0=0.8$ and ${\omega }_{\mathrm{p}}/t_c=0.8$ (marked by the “$+$” in Fig. 3). (j)–(l) The transient integrated density of states $A_{\left(\alpha \right)}^{-}(\omega ;t)$ obtained from panels (a)–(i). All data were obtained using the (i)TEBD technique with IBCs.