Detecting charge and lattice dynamics in photoinduced charge-order melting in perovskite-type manganites using a 30-femtosecond time resolution

We investigated the ultrafast dynamics of photoinduced melting of charge and orbital order in a perovskite-type manganite, ${\text{Nd}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$, by means of pump-probe reflection spectroscopy. By irradiation with a 20-fs laser pulse, only the charge sectors become melted within $\sim 30\text{fs}$ through strong electron correlations. During this process the oxygen displacements associated with the ordered phase persist. These displacements are subsequently released accompanied by several coherent oscillations. The coherent oscillation corresponding to the Jahn-Teller mode has relatively large amplitude demonstrating that the orbital order plays a significant role on the stabilization of the charge order.

Figure 1

(Color online) (a) Schematic illustrations of charge and orbital order in ${\text{Nd}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$. (b) Upper panel: reflectivity spectra for $E\parallel \mathbf{c}$ and $E\perp \mathbf{c}$ at $T=10\text{K}$ (solid lines) and $T=230\text{K}$ (broken lines) (Ref. 20). Lower panel: spectra of photoinduced reflectivity change $\left(\Delta R/R\right)$ (open circles) for $E\perp \mathbf{c}$ at $T=10\text{K}$ with the pump energy of 1.55 eV and the excitation photon density of 0.01 ph/Mn, and differential reflectivity spectra $\left[\left(R_{230\text{K}}-R_{10\text{K}}\right)/R_{10\text{K}}\right]$ (Ref. 16).

Figure 2

Time evolutions of $\Delta R/R$ (open circles) for a probe energy of 1.72 eV for $E\parallel \mathbf{c}$ (the upper panel) and $E\perp \mathbf{c}$ (the lower panel) at 10 K. The pump energy is $1.94\text{eV}\left(E\perp \mathbf{c}\right)$ and the excitation photon density is $6.5\times {10}^{-3}\text{ph}/\text{Mn}$. The solid line represents the result of fitting. The broken and the dotted line denote the fast and slow component, respectively (see text).

Figure 3

(Color online) (a) Time profile of $\Delta R/R$ observed at the probe energy of 1.72 eV for $E\perp \mathbf{c}$ at 10 K for the three excitation photon densities $x_{\text{ph}}$ marked by the solid arrows in (b) (lower panel) and the cross-correlation profile between pump (1.94 eV) and probe (1.72 eV) pulses (upper panel). (b) $x_{\text{ph}}$-dependence of $\left|\Delta R/R\right|$ at $t_d=40\text{fs}$.

Figure 4

(Color online) [(a) and (b)] Oscillatory component (open circles) extracted from the time profile of $\Delta R/R$ for $E\perp \mathbf{c}$ at 10 K (lower panel of Fig. 2). The solid line is a fitting curve, which is the sum of the four damped oscillators shown in the lower parts.

Figure 5

(Color online) (a) Temperature dependence of the Raman-scattering spectra in ${\text{Nd}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$. The bands $\mathbf{d}$ and $\mathbf{e}$ are the JT and breathing mode, respectively. (b) Temperature dependences of the amplitudes for the coherent oscillations associated with oxygen atoms [$A_i$ ($i=2$, 3, and 4)] and the intensities of the corresponding Raman bands ($\mathbf{b}$, $\mathbf{c}$, and $\mathbf{d}$).

Figure 6

(Color online) Schematic views of Raman modes: [i] and [ii] the rotational modes of ${\text{MnO}}_6$ octahedra, [iii] the JT mode, and [iv] the breathing mode. The bands $\mathbf{b}\text{−}\mathbf{e}$ are assigned to modes [i]–[iv], respectively.

Figure 7

The Fourier transform of the Gaussian response function $\left[\text{exp}\left(-t^2/{\tau }^2\right)\right]$ in the measurement system $\left[\text{exp}\left(-{\pi }^2{c}^2{\tau }^2{k}^2\right),\tau =20\text{fs}\right]$, which corresponds to the wave-number dependence of the detection sensitivity for coherent oscillations.

Figure 8

(Color online) Schematics for the overall dynamics of photoinduced melting of charge and orbital order in ${\text{Nd}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$. (a) The charge- and orbital-order state. (b) The melting of the charge order by a photoirradiation. The JT distortions remain just after the photoirradiation. (c) The release of the JT distortion. In this process, the coherent oscillations of the JT mode are generated (not shown). (d) The nonordered state. Recovery to the charge- and orbital-order state occurs with the time constant of 0.6 ps and simultaneously, the temperature of the system is increased (not shown).