Two-component heat diffusion observed in ${\text{LaMnO}}_3$ and ${\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_3$

We investigate the low-temperature electron, lattice, and spin dynamics of ${\text{LaMnO}}_3$ (LMO) and ${\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_3$ (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin $d\text{−}d$ transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the subpicosecond dynamics of colossal magnetoresistance manganites in terms of a phenomenological three-temperature model describing the energy transfer between the electron, lattice, and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long-term relaxation due to film-to-substrate heat diffusion and then develop a modified three-temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the band gap in LMO. In addition, we also model the nonthermal subpicosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.

Figure 1

(Color online) Transient reflectivity in (a) LMO and (b) LCMO for different probe energies when pumping with 1.6 eV. Notice, in particular, the change in the final relaxation time when changing probe energy and the zero crossing ($\sim 100\text{ps}$, LMO and $\sim 550\text{ps}$ LCMO) when probing with 1.47 eV, features that are so far unexplained by the literature.

Figure 2

(Color online) The computed fittings to the experimental (a) LMO and (b) LCMO data plotted on a semilog scale. The dots represent the experimental data while the lines are the corresponding fittings. The inset shows the same fittings using a linear scale. The transients shown are the same as in Fig. 1.

Figure 3

(Color online) The computed fitting parameters of LMO [circles (blue), right axis] and LCMO [triangles (red), left axis] where the error bars indicate 95% confidence intervals.

Figure 4

(Color online) Subpicosecond transients in (a) LMO and (b) LCMO. In both samples we see a roughly Gaussian peak followed by very fast $\left(<1\text{ps}\right)$ decay processes while only LCMO shows signs of coherent optical phonons. The blue dots represent the experimental data while the red lines are the corresponding fittings.

Figure 5

(Color online) The measured coefficient $C_r$ compared to the theoretical curve ${\left(E-E_0\right)}^{-2}$ on a log-log scale.