Presence and spatial distribution of interfacial electronic states in ${\text{LaMnO}}_3{\text{-SrMnO}}_3$ superlattices

We report direct evidence of interfacial states at the onset of $\text{O}K$ edge confined to a spatial distance of 1 unit-cell full-width at half maximum at the sharp interfaces between epitaxial films of ${\text{LaMnO}}_3$ and ${\text{SrMnO}}_3$ from electron energy-loss spectroscopy (EELS) measurements. The interfacial states are sensitive to interface sharpness; at rough interfaces with interfacial steps of 1–2 unit cells in height, experimental data shows a reduction, or suppression, of the interfacial states. The EELS measurements were performed using a fine electron probe obtained by electron lens aberration correction. By scanning the electron probe across the interface, we are able to map the spatial distribution of the interfacial states across interfaces at high resolution.

Figure 1

STEM image of the superlattice acquired at 80 kV. The dark films are ${\text{SrMnO}}_3$ and the lighter films are ${\text{LaMnO}}_3$. Each ${\text{LaMnO}}_3$ film contains valleys that are filled by ${\text{SrMnO}}_3$. The valleys propagate in the ${\text{LaMnO}}_3$ film as growth proceeds.

Figure 2

(a) High magnification STEM image acquired at 200 kV with line profile markers. (b) Line profile through the films and interfaces. Note that the line profiles go through nearly the entire horizontal of the image. The upper ${\text{LaMnO}}_3{\text{-SrMnO}}_3$ interface is sharp while the lower ${\text{SrMnO}}_3{\text{-LaMnO}}_3$ interface is rough to 1–2 u.c. The line profiles are vertically shifted for clarity.

Figure 3

(Color online) (a) Nanoarea electron-diffraction pattern of the superlattice shows satellite peaks corresponding to a periodic structure. The films in the superlattice are coherently strained to the STO substrate. A survey of high-resolution STEM images in the sample revealed no defects near the substrate-superlattice interface. The weak reflections correspond to orthorhombic ${\text{LaMnO}}_3$. (b) Magnified view of the superlattice reflections shows a high quality, periodic structure.

Figure 4

As-recorded $\text{O}K$ edge and $\text{Mn}{L}_{2,3}$ spectrum images from the line marked in the STEM image. The sampling is 1 point per $2\text{Å}$ and the spectrum images are smoothed to display the major edge features. When the electron beams moves from LMO to SMO, the $\text{O}K$ prepeak rises and the $\text{Mn}{L}_3$ peak maximum shifts toward lower energy loss.

Figure 5

(Color online) (a) HAADF STEM image showing the line where EELS spectra were recorded. (b) EELS spectrum image acquired in $2\text{Å}$ increments. There is a small periodic modulation in EELS intensity as the electron probe moves on and off Mn columns. The $\text{Mn}{L}_3$ edge maximum shifts toward lower energy loss in the ${\text{SrMnO}}_3$ film. (c) Individual $\text{Mn}{L}_3$ spectra through the LMO-SMO and SMO-LMO interfaces from the part of (b) marked by the double white arrows. At the sharp interface, there is an abrupt change in the disappearance of the peak marked by arrow from spectrum 8 to 9. Near the rough interface, there is a smoother shift toward lower energy loss. (d) STEM image where a La EELS profile was acquired. (e) The integrated intensity of the $\text{La}M$ edge from 832 to 854 eV shows the La/Sr intermixing is primarily limited to 2–3 u.c. at the bottom rough interface. The upper interface is sharp to a single unit cell.

Figure 6

A comparison of $\text{Mn}{L}_{2,3}$ edge in XAS and the summed intensity of all spectra in the EELS linescan marked in Fig. 5 shows good agreement between the spectra other than differences in energy resolution of the techniques. The spectra also verify electron-beam damage was avoided as the fine structure and ratios of $L_3$ to $L_2$ edges are the same. The averaged spectrum from the center of the LMO film does not have a shoulder on the $L_3$ edge that bulk LMO samples showed in Ref. 31 Likewise, the averaged spectrum from the center of the SMO film does not have two local minimum on the $L_3$ edge that a bulk SMO film grown on STO shows. The electronic configuration of both LMO and SMO films in the superlattice differs electronically from bulk samples.

Figure 7

(Color online) $\text{O}K$ edge spectra from an average of the center spectrum of the LMO film, SMO film, (a) sharp interface and (b) rough interface. The prepeak shows the hybridization with $\text{Mn}3d$ states is unique in each case. (c) An average of the LMO and SMO spectrum is subtracted from the sharp interface spectrum, showing the difference. There are extra states at the onset of the edge, which are above the Fermi-energy level. (d) The same difference method is applied to the rough interface spectrum, where there are no extra states at the onset. The $\text{O}K$ edge spectrum at the sharp interface is a unique electronic configuration.

Figure 8

(Color online) (a) STEM image with EELS line profile drawn. (b) The difference map of the $\text{O}K$ edge shows extra spectral weight at the sharp interfaces in the energy range of 526 to 531 eV. The extra spectral weight is confined to an area of $4.5\text{Å}$ or approximately 1 u.c. The same positions with the extra spectral weight have a (c) abrupt shift in the maximum of the $\text{Mn}{L}_3$ edge. (d) An integrated intensity profile from 526.7 to 530.3 eV shows the extra $\text{O}K$ spectral weight is above the background noise level.