Ultrafast electronic photoinduced phase transition in a two-dimensional charge-ordering system

We investigate the ground- and excited-state properties of a two-dimensional charge-ordering system, and theoretically demonstrate that multielectron excitations by one photon occur substantially as a result of frustration effects. These multielectron excitations are naturally regarded as domain excitations, which involve a simultaneous excitation of a part of the system lattice. Furthermore, we show that such domain excitations not only suppress the original charge-ordering phase strongly but also enhance another phase of charge ordering. As a result of such a global change, the overall photoinduced optical conductivity spectra are also modified drastically from the original spectrum, with the modified spectra exhibiting midgap and gapless features.

Figure 1

Schematics of (a) a part of the the checkerboard CO ground state with its considered repulsions and (b) three low-lying excited states in a $6\times 6$ lattice.

Figure 2

(a) Phase diagram reproduced from Ref. [19] along with the corresponding CO patterns in each region. It is noted that phase I is substituted by the $(\pi ,0)$ phase because of compatibility with the $6\times 6$ system size and that the gray circles in the pattern for phase II represent 0.75 electrons per site. (b),(c) Charge correlations for the sets of $(V,V^{\prime })$ specified by the dots in (a). Circles, squares, upward triangles, and downward triangles correspond to the results for $(k_x,k_y)=(\pi ,\pi ),(\pi ,0),(2/3\pi ,2/3\pi )$, and $(2/3\pi ,-2/3\pi )$.

Figure 3

(a)–(d) Optical conductivity spectrum and energy-dependent $N_{\mathrm{ex}}$ values for four sets of $(V,V^{\prime })$ in the weak-coupling case and (e) a spectrum that enlarges a part of (a).

Figure 4

Energy-dependent charge correlations for $(V,V^{\prime })/t_0=(2,1)$. In each panel, the ground-state value is specified by the horizontal line.

Figure 5

(a) Determined eigenenergies measured from the ground-state energy as a function of the input frequency, and (b),(c) photoinduced spectrum ${\sigma }_1\left(\omega \right)$ with the spectrum from the ground state, $\sigma \left(\omega \right)$, for $(V,V^{\prime })/t_0=(2,1)$. (b),(c) The excitation energy is specified by the vertical arrow.

Figure 6

Optical conductivity spectrum and energy-dependent $N_{\mathrm{ex}}$ values for three sets of $(V,V^{\prime })$ in the strong-coupling case.

Figure 7

Same as Fig. 4, but for $(V,V^{\prime })/t_0=(5,4)$.

Figure 8

Photoinduced spectra for $(V,V^{\prime })/t_0=(5,4)$.

Figure 9

Two ground states of the checkerboard CO in the localized picture.

Figure 10

Comparisons between the result based on the full calculation and that with $N_{\mathrm{bond}}=14$. They are obtained for $(V,V^{\prime })/t_0=(2,0)$. Closed circles specify the values of $N_{\mathrm{ex}}$ at those points.

Figure 11

Two patterns that have $6V-6V^{\prime }$ as the excitation energy in the localized picture.