Laser-induced charge-disproportionated metallic state in ${\mathrm{LaCoO}}_3$

Understanding the origin of the spin transition in ${\mathrm{LaCoO}}_3$ is one of the long-standing aims in condensed matter physics. Aside from its fundamental interest, a detailed description of this crossover will have a direct impact on the interpretation of the semiconductor-to-metal transition (SMT) and the properties of the high-temperature metallic phase in this compound, which has shown to have important applications in environmentally friendly energy production. To date, the spin transition has been investigated mainly as a function of temperature in thermal equilibrium. These results have hinted at dynamical effects. In this paper, we have investigated the SMT by means of pump-probe soft x-ray reflectivity experiments at the O $K$, Co $L$, and La $M$ edges and theoretical calculations within a ${\mathrm{DFT}}^{++}$ formalism. The results point towards a laser-induced metallization in which the optical transitions stabilize a metallic state with high-spin configuration and increased charge disproportionation.

Figure 1

(a) Reflectivity curves taken at the O $K$ edge in the unpumped (solid gray lines) and laser pumped (color marked lines) measurements for different delay times obtained with a maximum laser fluence of 0.43 $\mathrm{mJ}·{\mathrm{cm}}^{-2}$. (b) Difference spectra showing the intensity modifications upon laser pumping for different delays. Three energy regions corresponding to excitation into O $2p$ empty states hybridized with Co $3d$, La $5d$, and Co $4sp$ states are indicated in the figure. (c) Temperature-dependent reflectivity in the range from room temperature to 650 K measured in the same geometrical configuration as the pump-probe data. (d) Thermal reflectivity difference with respect to the 300-K spectra for $s$ and $p$ polarized photons together with the laser-induced reflectivity (black dots).

Figure 2

(a) Reflectivity curves taken at the Co $L$ edge in the unpumped (solid gray line) and laser pumped (color marked lines) experiments for different delay times at the maximum laser fluence of 0.43 $\mathrm{mJ}·{\mathrm{cm}}^{-2}$. Difference signals (b) show important intensity modifications upon laser pumping at the $L_3$ and $L_2$ white lines, as well as at the shoulder of the $L_3$ line. (c) Temperature-dependent reflectivity in the range from room temperature to 650 K measured in the same geometrical configuration as the pump-probe data. (d) Thermal reflectivity difference with respect to the 300 K is displayed together with the laser-induced reflectivity (black dots).

Figure 3

Delay scans taken at the selected photon energies. Two different laser fluence values have been used: 0.20 $\mathrm{mJ}·{\mathrm{cm}}^{-2}$ (a) and 0.09 $\mathrm{mJ}·{\mathrm{cm}}^{-2}$ (b). For comparison, delay scans are plotted for an $\approx 40$-nm-thick film of Ni as a dashed black line, and for a nonresonant energy (777.0 eV, solid gray line) around the Co $L_3$ edge. The latter shows no changes with delay time. Qualitatively similar behavior can be observed at both fluences.

Figure 4

Comparison of the delay scans measured for two different fluences [0.09 $\mathrm{mJ}·{\mathrm{cm}}^{-2}$ (open circles) and 0.2 $\mathrm{mJ}·{\mathrm{cm}}^{-2}$ (filled circles)] for the pseudo-$t{}_{2g}$ O $K$ edge (a) and pseudo-$e{}_g$ (b)], Co $L_3$ edge (c), and La $M$ edge (d).

Figure 5

Fluence dependence of the reflectivity difference at the Co $L_3$ shoulder normalized to the changes at the highest fluence. A linear dependence has been used to fit the data.

Figure 6

Projected DOS on the Co $3d$ and O $2p$ orbitals obtained for two different values of the Coulomb and exchange potentials: (a) $U=6.0$ eV and $J=0.8$ eV as in Ref. [19] and (b) $U=3.0$ eV and $J=0.7$ eV. For each set of potentials are results for two different temperatures: 300 and 650 K have been calculated. The lattice parameters of the real structure at the respective temperatures have been taken from Ref. [9] .

Figure 7

(a) $k$-resolved band dispersion at room temperature for $U=3.0$ eV and $J=0.7$ eV for ${\mathrm{LaCoO}}_3$. The $k$-resolved dispersion for the pure Co $3d$ states of the low- and high-spin configurations is given in (b) and (c), respectively. Vertical optical transitions with 1.55 eV are also displayed in the latter pictures. For the LS configuration, the photon energy is lower than the intra-atomic pseudo-$t{}_{2g}-e{}_g$ energy splitting. On the contrary, the photons can excite transitions to HS pseudo-$t{}_{2g}$ states. For the high spin, both interatomic and intra-atomic transitions from the pseudo-$t{}_{2g}$ could be excited.