# Metal-to-Insulator Transition in Ultrathin Manganite Heterostructures
## 4 Mechanism of the MIT
### Orbital Reconstruction
Though several reports show 3\({}^{2}\)-r\({}^{2}\) orbital reconstruction, linear-dichroism (LD) measurements by [PERSON] et al. revealed different results that the x\({}^{2}\)-y\({}^{2}\) orbital is preferentially occupied without any orbital reconstruction at the interface [18]. Their results show very similar LD signals between thick films (e.g., 70 u.c.) and ultrathin films (e.g., 5 u.c.). This fact indicates that the orbital plays a minor role in inducing the dead layer. The proposed interface that enhanced 3\({}^{2}\)-r\({}^{2}\) orbital reconstruction would also suggest a larger 3\({}^{2}\)-r\({}^{2}\) orbital population and reduced magnetism for STO capped LSMO, which do not agree with the experimentally observed reduced dead layer thickness by STO capping [39]. Since the observed orbital polarization in these LSMO films is very small, another open question then arises for whether this small orbital polarization  
Figure 8: Orbital reconstruction and induced antiferromagnetic (AFM) ordering, due to tetragonal distortion. is significant enough to unbalance the competition between double exchange and superexchange, and practically cause an AFM ground state. A systematical investigation of the correlation of orbital polarization with magnetic ordering has been performed on LSMO films grown on NGO (110) substrates [40], as shown in Figure 9. To quantify the orbital polarization, the Mn x\({}^{2}\)-y\({}^{2}\) and 3\({}^{2}\)-r\({}^{2}\) orbitals were preferentially probed by in-plane (\(\sigma\)) and out-of-plane (\(\pi\)) polarized XAS, respectively, at the Mn L\({}_{2,3}\) edge (see Figure 9a). The measured XAS is shown in the top panel of Figure 9b. For different films, their two spectra I(\(\sigma\)) and I(\(\pi\)) all show slight differences, indicating a small linear dichroism. The X-ray linear dichroism (XLD) is calculated as the intensity difference between the signals with different polarization (I(\(\sigma\)) \(-\) I(\(\pi\))). As shown in the bottom panel of Figure 9b, all films exhibited quite similar XLD characteristics, e.g., 6 u.c. LSMO (noted as L\({}_{6}\))- and 9 u.c. STO-buffered 6 u.c. LSMO (noted as L\({}_{6}\)/S\({}_{9}\)) on the one hand; L\({}_{30}\) and L\({}_{30}\)/S\({}_{6}\) on the other hand. By integrating the spectra in the range 649-659 eV (around the Mn L\({}_{2}\) edge), the area under the XLD peak (ALD) is obtained and plotted as a function of the Curie temperature (\(T_{\rm C}\)) for different films (see Figure 9c). A positive (or negative) ALD indicates a preferential occupation of the 3\({}^{2}\)-r\({}^{2}\) (or x\({}^{2}\)-y\({}^{2}\)) orbital. The ALDs of all different films are within \(\sim\)1%, indicating a very small orbital polarization with slightly preferential occupation of the 3\({}^{2}\)-r\({}^{2}\) orbital. Since the XLDs were taken at 300 K, which were well above the \(T_{\rm C}\) of LSMO films that were thinner than 30 u.c. (Figure 9c), therefore, the contribution of magnetization to XLD can be ruled out. A change of orbital polarization is not accompanied by any change of the Curie temperature. Therefore, one can infer that the small degree of orbital polarization present plays a minor role in magnetic ordering when the lattice mismatch is relatively small. It is noteworthy that the 6 u.c. LSMO had a smaller orbital polarization than thicker films. This indicates a competitive action of compressive strain and octahedral tilt on the tetragonal distortions of the octahedra. Since the Mn-Mn distances in the (110) plane are locked-in by the NGO substrate, the tilting of the octahedra will increase the Mn-O bond length in the (110) plane, compensating the effect from compressive strain which tends to elongate the Mn-O bond in the direction normal to the interface. This effect subsides with increasing thickness, and hence orbital polarization will increase in  
Figure 9: **(a)**, Sketch of the X-ray absorption measurement of 3\({}^{2}\)-r\({}^{2}\) and x\({}^{2}\)-y\({}^{2}\) orbital occupations by \(\pi\)- and \(\sigma\)-polarized X-rays, respectively.