# Metal-to-Insulator Transition in Ultrathin Manganite Heterostructures
## 4 Mechanism of the MIT
### Orbital Hybridization  
While the orbital reconstruction and valence change are the two focused models explaining the origin of the dead layer, another mechanism of reduced bandwidth-driven metal-to-insulator transition in ultrathin LSMO has been proposed recently [40]. The role of bandwidth in determining the electronic ground state in LSMO has been demonstrated long time ago in bulk LSMO [64]. The relation between the bandwidth and the bond angle can be described by an empirical formula, \(w\propto\frac{Cos(x-\theta)/2}{d^{35}}\)[65]. Here, \(d\) is the Mn-O bond length, and \(\theta\) is the Mn-O-Mn bond angle. By isovalent substitution of the A site with different rare earth elements for obtaining different structure distortions while maintaining the same charge doping level, [PERSON] et al. demonstrated that the \(T_{\rm C}\) decreases with a reducing Mn-O-Mn bond angle [64]. For the LSMO thin film, more and more experiments indicate that the octahedra has different rotation patterns near the interface and away from the interface, due to interfacial oxygen octahedral coupling (OOC) [63; 66; 67; 40]. The interfacial OOC rearranges the octahedral tilting, to retain oxygen octahedral connectivity at the interface between two dissimilar perovskite oxides. X-ray diffraction of LSMO on STO revealed a gradual evolution of the octahedral tilt pattern from the interface, to a distant interface region [66]. At the LSMO/STO interface, the tilting disappears, and LSMO becomes a\({}^{0}\)a\({}^{0}\)a\({}^{0}\) in Glazer notation [68]. Away from STO, the tilt pattern becomes a\({}^{+}\)a\({}^{-}\)c\({}^{0}\), and the tilt angle gradually relaxes. Thanks to the advanced annular bright field (ABF) technique, oxygen atoms can be spatially resolved in STEM images, allowing us to visualize the octahedral tilt at the atomic scale. Using the ABF technique, the change of the octahedral tilt near the interface has been confirmed microscopically [63; 67; 40]. One example of the visualization of the octahedra tilt across the LSMO/NGO interface is shown in Figure 11. A clearly enhanced octahedral tilt of LSMO is observed at the LSMO/NGO interface. The profile of the tilt angle \(\beta\), as shown in Figure 11b, further illustrates the large degree of tilting near the interface, and the tilting angle gradually increases when further away from the interface region. With inserting an STO buffer layer to locally engineer the OOC, the tilt in the LSMO near the interface then can be dramatically reduced (see Figure 11a,b). A direct link between the bond angle and the ferromagnetic ordering is shown in Figure 11c. For non-STO-buffered LSMO, \(T_{\rm C}\) is 150 K, which is promoted to 180 K and 250 K by 1 u.c. and 9 u.c. STO buffer layers, respectively. This strongly suggests the impact of octahedral tilting, and therefore the orbital hybridization on the ferromagnetic ordering. Experimentally, the effect of the octahedral tilt on the strength of p-d hybridization is directly measured by atomically resolved fine-structure changes at the O-K edge in EELS. Figure 12 shows layer resolved spectra of the O-K edge spectrum taken from the individual layers across a LSMO film grown on NGO substrate. The pre-edge peak located near 530 eV arises from the mixture of O-2p and Mn-3d character [69; 70]. Getting closer to the LSMO/NGO interface, the pre-peak intensity is found to be smaller, suggesting weaker O-2p and Mn-3d hybridization at the interface. This smaller Mn-O orbital hybridization is consistent with the smaller Mn-O-Mn bond angle near the interface. The change of the orbital hybridization from the bulk region towards the interface agrees well with the schematic structure profile shown in Figure 12a. Figure 11: **(a)** Inversed annular bright-field STEM images of (**left** panel) LSMO/NdGaO\({}_{3}\)(LSMO/NGO), (**middle** panel) LSMO/STO (9 u.c. )/NGO and (**right** panel) LSMO/STO (1 u.c. )/NGO heterostructures. The connectivity of oxygen octahedra across the interfaces is indicated.