Oxygen isotope effect and phase separation in the optical conductivity of ${\left({\mathrm{La}}_{0.5}{\mathrm{Pr}}_{0.5}\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ thin films

The optical conductivities of films of ${\left({\mathrm{La}}_{0.5}{\mathrm{Pr}}_{0.5}\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ with different oxygen isotopes (${}^{16}\mathrm{O}$ and ${}^{18}\mathrm{O}$) have been determined in the spectral range from 0.3 to 4.3 eV using a combination of transmission in the midinfrared and ellipsometry from the near-infrared to ultraviolet regions. We have found that the isotope exchange strongly affects the optical response in the ferromagnetic phase in a broad frequency range, in contrast to the almost isotope-independent optical conductivity above $T_C$. The substitution by ${}^{18}\mathrm{O}$ strongly suppresses the Drude response and a midinfrared peak while enhancing the conductivity peak at 1.5 eV. A qualitative explanation can be given in terms of the phase separation present in these materials. Moreover, the optical response is similar to the one extracted from measurements in polished samples and other thin films, which signals to the importance of internal strain.

Figure 1

(Color online) Phase diagram of (a) ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ (from Ref. 1), and (b) ${\left({\mathrm{La}}_{1-y}{\mathrm{Pr}}_y\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ (from Ref. 5 for ${}^{16}\mathrm{O}$ and as suggested in Ref. 10 for ${}^{18}\mathrm{O}$). The richness of these phase diagrams is evident. They comprise the following states: canted antiferromagnet (CAF), charge order (CO), ferromagnetic insulator (FI), ferromagnet (FM), and antiferromagnet (AF). The arrow in (b) indicates the composition used in this work. However, it has to be taken into account that due to internal strains, the film actually lies closer to the region of phase separation (see Ref. 11).

Figure 2

(Color online) Transmission of two ${\left({\mathrm{La}}_{0.5}{\mathrm{Pr}}_{0.5}\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ films with ${}^{16}\mathrm{O}$ (S16) and ${}^{18}\mathrm{O}$ (S18) (thickness $\sim 60\mathrm{nm}$) grown on STO substrates. The spectra presented in the main panels correspond to temperatures between 10 and 296 K every 10 K . Insets: Temperature dependence of the transmission at $2000{\mathrm{cm}}^{-1}$ and the dc conductivity, both measured in the heating mode with no magnetic field. The vertical dotted line indicates the minimum in ${\sigma }_{dc}$ which is associated with the entrance to the ferromagnetic state (we will refer to it as $T_C$). The inset of the upper panel also shows the temperature dependence of the transmission (shifted down by 30%) of a STO substrate. It is clear that the temperature effects are solely due to the changes in the films.

Figure 3

(Color online) (a) Measured complex dielectric function of STO at 296 and 7 K. (b) Optical conductivity of the S18 film obtained from the raw data at 5 K and using the STO dielectric function at 296 (blue) and 7 K (red). (c) Relative difference, expressed in percentage, between the two obtained conductivities. This result indicates the negligible effect of the temperature variation of the substrate.

Figure 4

(Color online) Parameters $\Psi$ and $\Delta$ for S16 as obtained directly from ellipsometry at an angle of incidence of 80° between 10 and 296 K every 10 K.

Figure 5

(Color online) Optical conductivity of the S16 and S18 samples between 10 and 296 K every 10 K. Insets: Temperature dependencies of the optical conductivity at 3000 and $12000{\mathrm{cm}}^{-1}$.

Figure 6

(Color online) Isotope substitution effect on the optical conductivity in the paramagnetic (200 K) and ferromagnetic (10 K) states.

Figure 7

(Color online) Results of the Drude-Lorentz fit. Optical conductivity (gray circles), fit (thin solid line) and individual oscillators (dashed lines) at (a) 10 K and (b) room temperature.

Figure 8

(Color online) Energy diagram of the manganites (adapted from Refs. 1, 20). (a) The Mn d levels, due to the crystal field, are split in the so-called $t_{2g}$ and $e_g$ levels. If the $e_g$ level is occupied, this state is further split by $E_{JT}$, due to the Jahn-Teller distortions of the surrounding O atoms. If the $e_g$ level is empty, its original energy can be shifted by $E_B$ due to a “breathing” distortion which couples to changes in the $e_g$ occupation density. Furthermore, the $e_g$ level can correspond to spin states either parallel or antiparallel to the total spin of the $t_{2g}$ electrons. In the latter case, the energy of $e_g$ is higher by an energy given by the Hund’s coupling, $J_H$. We have plotted this situation for the case of an occupied $e_g$ level. (b) Possible transitions occurring in the manganites. The initial states (a single-occupied $e_g$ level and a filled O 2p band) are depicted in the center. To determine the final energies, one has to take into account the different situations noted before, the origin of the excited electron (either coming from an $e_g$ or an O $2p$ state), and the fact that putting an extra electron in an already occupied $e_g$ level costs the on-site Coulomb repulsion energy $U$. When an excited electron comes from an O site, the charge transfer energy $\Delta$ has to be added.

Figure 9

(Color online) Difference between optical conductivities, $\Delta {\sigma }_1(\omega ,T)\equiv {\sigma }_1(\omega ,T)-{\sigma }_1(\omega ,10\mathrm{K})$ for the two samples. There are two clear regions where ${\sigma }_1\left(\omega \right)$ decreases below $T_C$. The insets show the temperature dependencies at 12 000 and $22000{\mathrm{cm}}^{-1}$.

Figure 10

(Color online) Temperature dependence of the (a) position, ${\omega }_o$, and (b) plasma frequency, ${\omega }_p^2$, of the oscillators number 2 and 3 (see Fig. 7). Notice also that in sample S18 above 110 K, peak 2 almost merges with peak 3. Therefore, above this temperature, we have kept ${\omega }_{p,2}$ constant.

Figure 11

(Color online) Integrated spectral weight, $N_{eff}$, in the whole measured range. The insets show the temperature dependence of $\Delta N_{eff}\equiv N_{eff}\left(T\right)-N_{eff}\left(10\mathrm{K}\right)$ at two different frequencies.