Transient electronic structure of the photoinduced phase of ${\text{Pr}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_3$ probed with soft x-ray pulses

We use time-resolved x-ray absorption near-edge structure spectroscopy to investigate the electronic dynamics associated with the photoinduced insulator-to-metal phase transition in the colossal magnetoresistive manganite ${\text{Pr}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_3$. Absorption changes at the $\text{O}K$ and $\text{Mn}L$ edges directly monitor the evolution of the density of unoccupied states in the transient photoinduced phase. We show that the electronic structure of the photoinduced phase is remarkably similar to that of the ferromagnetic metallic phase reached in related manganites upon cooling below the Curie temperature.

Figure 1

(Color online) Electronic structure of ${\text{Pr}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_3$. (a) Sketch of the ${\text{MnO}}_6$ octahedral geometry with relevant $\text{O}2p$ and $\text{Mn}3d$ $e_g$ orbitals. The scheme of the $\text{Mn}d$ electronic structure is shown for ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$ ions. The crystal field splitting (10 Dq) separates $t_{2g}$ from $e_g$ levels and strong exchange interaction aligns electron spins at a given atomic site. Electron hopping occurs between $3d\text{−}{e}_g$ levels of neighboring ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$ species. In the ${\text{Mn}}^{3+}$ ion, Jahn-Teller distortions lift the degeneracy of $t_{2g}$ and $e_g$ levels. $\uparrow$ and $\downarrow$ indicate majority and minority spins, respectively. (b) XANES spectra at the $\text{O}K$ edge (530–550 eV), measured via total electron yield (TEY, solid curve) and total fluorescence yield (TFY, dotted curve), and at the $\text{Mn}{L}_3$ and $L_2$ edges (640–660 eV) measured in TEY mode at room temperature. A schematic of the electronic transitions contributing to $\text{O}K$ and $\text{Mn}L$ XANES spectra is shown.

Figure 2

(Color online) Time-resolved XANES measurements at $\text{O}K$ and $\text{Mn}L$ edges. Static absorption spectra (solid circles) at the (a) $\text{O}K$ pre-edge and (b) $\text{Mn}{L}_3$ edge and spectra measured 300 ps after 800 nm excitation (crosses). Lower panels: corresponding relative change in absorption $\left(\Delta \alpha /\alpha \right)$. (c) and (d) Relative change in absorption $\left(\Delta \alpha /\alpha \right)$ as a function of pulse delay at two representative wavelengths, 529 ($\text{O}K$ edge) and 640.5eV ($\text{Mn}L$ edge). The solid line is a fit of the data using a cross correlation width of 70 ps, corresponding to the temporal resolution of the experiment. Measurements were taken at 80 K via TEY.

Figure 3

Effect of insulator-metal transition on the prestructure of $\text{O-}K$-edge XANES spectra. Spectrally resolved changes in the $\text{O-}K$-edge XANES spectrum for (a) photoinduced phase transition in ${\text{Pr}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_3$, $\Delta \alpha ={\alpha }_{\text{METALLIC}}-{\alpha }_{\text{INSULATOR}}=\alpha \left(300\text{ps}\right)-\alpha \left(-300\text{ps}\right)$. (b)–(d) Temperature-induced phase transition in manganites, $\Delta \alpha ={\alpha }_{\text{METALLIC}}-{\alpha }_{\text{INSULATOR}}=\alpha \left(T<T_{\text{C}}\right)-\alpha \left(T>T_{\text{C}}\right)$. (b) ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3$, $T_{\text{C}}=360\text{K}$ (Ref. 15); (c) ${\text{Pr}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3$, $T_{\text{C}}=250\text{K}$ (Ref. 9); and (d) double-layered ${\text{La}}_{1.3}{\text{Sr}}_{1.7}{\text{Mn}}_2{\text{O}}_7$, $T_{\text{C}}=130\text{K}$ (Ref. 15).

Figure 4

(Color online) Evidence of a nonthermal phase transition. (a) Static absorption spectra at the $\text{O}K$ edge measured via TFY at room temperature (solid circles) and 30 K (crosses). Lower panel: corresponding relative change in absorption $\left(\Delta \alpha /\alpha \right)$. (b) Fluence dependence of photoinduced relative absorption changes at 529 eV (solid squares) from XANES experiments and reflectivity changes at 1.5 eV (open circles), from optical pump-probe experiments (Ref. 3).