Infrared spectroscopic study of ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$

Temperature-dependent infrared spectroscopy has been investigated for ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$ which undergoes a ferromagnetic transition at $T_C\approx 177\mathrm{K}$. It is observed that the spectral weight is transferred from $\sim 4800–14\hspace{0.17em}000{\mathrm{cm}}^{-1}$ to $\sim 0$–$4800{\mathrm{cm}}^{-1}$ as the temperature is lowered around $T_C$. Such a large-range spectral weight transfer is attributed to the Hund's interaction. The phonons in ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$ show minor asymmetric line shapes, implying relatively weak electron-phonon coupling compared with the parent compound ${\mathrm{CaFeO}}_3$. The optical conductivity also reveals a broad peak structure in the range of $\sim 700–1500{\mathrm{cm}}^{-1}$. Fit by the model of single-polaron absorption, the broad peak is interpreted by the excitation of polarons. From the fitting parameters of the polaron peak, we estimate the electron-phonon coupling constant $\alpha \sim 0.4–0.5$, implying that ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$ falls into the weak-coupling regime.

Figure 1

(a) Reflectivity of ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$ at several representative temperatures. (b) Reflectivity of ${\mathrm{CaFeO}}_3$ at several representative temperatures. The insets show the spectra up to 25 $000{\mathrm{cm}}^{-1}$. The arrows in (a) denote the kink structure.

Figure 2

Optical conductivity for (a) ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$ and (b) ${\mathrm{CaFeO}}_3$, respectively. The insets show the spectra up to 25 $000{\mathrm{cm}}^{-1}$. The symbols on the vertical axis are dc values of the conductivity obtained from the transport measurement. The arrow in (a) denotes the polaronlike peak related to the kink structure in Fig. 1.

Figure 3

Temperature dependence of the normalized spectral weight $W\left(T\right)/W\left(300\mathrm{K}\right)$ in different energy regions. The dashed line indicates the FM phase transition. The inset is the temperature dependence of the magnetic susceptibility measured in the field-cooled mode with $H=0.1$ T. The red curves are fittings by the inverse tangent function.

Figure 4

Schematic energy diagrams describing the hopping of an electron between Fe ions. (a) The type-1 excitations with parallel spins. (b),(c) The type-2 excitations with antiparallel spins. The dashed arrow lines from left to right denote the hopping processes. The black dashed arrows in the left side are the initial states of the hopping electrons, while the red arrows in the right side are the final states of the hopping electrons.

Figure 5

Fitting curve of ${\sigma }_1\left(\omega \right)$ at $T=150$ K. The inset shows the spectra up to 15 $000{\mathrm{cm}}^{-1}$.

Figure 6

(a), (b) Line shapes of phonons around 260 and $560{\mathrm{cm}}^{-1}$ in ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$. The Drude response and Lorentz term centered at $\sim 2$ $000{\mathrm{cm}}^{-1}$ have been subtracted from the data. The curves are vertically shifted for clarity. (c), (d) The corresponding phonons in ${\mathrm{CaFeO}}_3$. The red lines are fitting curves by the Fano profiles.

Figure 7

(a), (b) Temperature dependence of the strength parameters ${\mathrm{\Omega }}_1,{\mathrm{\Omega }}_2$, and ${\mathrm{\Omega }}_3$ of the Lorentz oscillators centered at about 2000, 6000, and 15 $000{\mathrm{cm}}^{-1}$, respectively. (c) Temperature dependence of the plasma frequency ${\mathrm{\Omega }}_p$ for the Drude component. (d) Temperature dependence of the scattering rate $1/\tau$ of the Drude component. (e) Polaron intensity $\eta$ obtained from the fitting. (f) Temperature dependence of the electron-phonon coupling constant $\alpha$.