Laser-induced ultrafast insulator-metal transition in $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$

We investigate ultrafast coherent quantum dynamics of undoped $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$ driven by a strong laser pulse. Our calculations demonstrate that in a wide range of radiation frequencies and intensities the system undergoes a transient change from the insulating to the metallic state, where the charge density wave and the corresponding energy spectrum gap vanish. The transition takes place on the ultrafast timescale of tens femtoseconds, comparable to the period of the corresponding lattice vibrations. The dynamics are determined by a complex interplay of the particle-hole excitation over the gap and of the tunneling through it, giving rise to the highly nontrivial time evolution which comprises high harmonics and reveals periodic reappearance of the gap. The time evolution is obtained by solving the dynamical mean-field theory equations with the realistic parameters for the system and radiation. Results are summarized in the phase diagram, helpful for a possible experimental setup to achieve a dynamical control over the conduction state of this and other materials with the similarly strong electron-phonon interaction.

Figure 1

A schematic crystal structure of $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$, with ions Bi (blue), O (red), and Ba (green). (a) The structure before the pulse, distorted by the CDW, has larger and smaller $\mathrm{Bi}{\mathrm{O}}_6$ octahedra, extra electron/hole pairs sit on the upper antibonding ${\text{Bi6s-O2p}}_{{\sigma }^{*}}$ orbitals. (b) The laser excitation breaks the pairs on the antibonding orbitals and removes the lattice distortion.

Figure 2

Double Bethe lattice. Solid and dashed lines correspond to hoppings $t_{\parallel }$ within and $t_{\perp }$ between the lattices, respectively. Each lattice is shown with coordination number 3 for clarity.

Figure 3

(a) Band structure of the tight-binding model (1) for the cubic BBO structure [27]. (b) Single particle DOS calculated for the original TB model with the cubic BBO structure (blue) and for the double Bethe lattice model (red).

Figure 4

Kadanoff-Baym contour with three time domains: real-time forward ${\mathcal{C}}_1$ and backward ${\mathcal{C}}_2$, and imaginary-time ${\mathcal{C}}_3$. The arrows set the contour time ordering; $t_{\text{max}}$ corresponds to the maximum real-time propagation of the solution.

Figure 5

Phase diagram of types of the time evolution. Red color marks the weak excitation domain, blue color denotes the domain of strong excitation with the transient metal-insulator transition. The dashed line is given by $A_0{\omega }_0=\mathrm{\Delta }$ ($=2.5\mathrm{eV}$) is an estimated threshold separating these regimes. The dashed-dotted line is given by $\hslash {\omega }_0=\mathrm{\Delta }$. Dynamics, calculated in points A–E is shown in Fig. 6.

Figure 6

Time evolution of the driven system calculated for parameters at points A–E in the diagram in Fig. 5 is shown, correspondingly, in rows of panels (A)–(E). Panels (A1)–(E1) plot the time evolution of the electronic occupation in sublattices $\alpha =0,1$. The second panel column A2–E2 shows the spectra $I\left(\omega \right)$ of the electron dynamics as follows from the analysis of the Fourier component of the interlattice current flow. Panels (A3)–(E3) plot the time evolution of the dielectric gap. The square root function $\mathrm{\Delta }\left(t\right)=\mathrm{\Delta }\left(0\right){(1-t/t_0)}^{1/2}$ with $\mathrm{\Delta }\left(0\right)=2.5\mathrm{eV}$ is used to fit the time dependence of the gap close to its vanishing point.

Figure 7

Time evolution of the electron DOS, shown as the color density plot, and the phonon-induced deformation of the sublattices, shown by two dashed lines. The blue dashed line shows $X_0\left(t\right)$, and the red $X_1\left(t\right)$. Panels (A)–(E) correspond to points A–E in Fig. 5.

Figure 8

The dynamical response of the system at point C in Fig. 5, the laser field is turned off at the phase transition $t_0=33\mathrm{fs}$. Time evolution of the occupation of sites “0” and “1” is shown by solid lines, lattice displacements are represented by the dashed lines, the dash-dotted line gives the decaying envelope of the phonon displacement.