# Metal-to-Insulator Transition in Ultrathin Manganite Heterostructures
## 3 The Dead Layer in LSMO
The STO buffer layer inserted between LSMO and NGO plays a role to reduce octahedra tilting via the octahedral coupling effect [40]. The capping layer alleviates/eliminates surface distortion, and restores the surface magnetization, as a result further reducing the dead layer thickness. These optimization leads to a record of the thinnest dead layer of only 3 u.c. (1.2 nm) [39]. As shown in Figure 5, the 4 u.c. LSMO film still exhibits very good metallic behavior, with resistivity of only 1 m\(\Omega\)-cm at 2 K. For the 3 u.c. film, we can still observe a MIT at 80 K, though it quickly reenters into insulating phase at 42 K. The MIT at 80 K should be due to the existence of paramagnetic to ferromagnetic transition, while the low temperature reentrant insulator phase should be related to the electron localized effect, due to unavoidable disorder. This fact suggests that the magnetic dead layer should be no more than 2 u.c. Of other thickness-dependent magnetization measurements [18; 40], layer-resolved magnetization [40; 43; 44] also suggests that the magnetic dead layer can be as small as a single unit cell. All of these results indicate that the dimensionality effect plays a minor role for the formation of the dead layer. Figure 4: **(a)** Temperature-dependent resistivity of different thickness of LSMO thin films grown on STO (001) substrate. Figure is adapted from Ref. [39]. **(b)** Reported thicknesses of the dead layer of LSMO thin films grown on different substrates. \(\epsilon\equiv(\text{a}_{\text{Sub}}-\text{a}_{\text{LSMO}})/\text{a}_{ \text{LSMO}}\). The magnetic dead layer thickness can be resolved by determining the magnetic profile along the surface normal direction. The magnetic depth profile can be obtained by RXR or PNR [40, 42, 43, 44, 45]. Figure 6 shows an example of the magnetic profile obtained by RXR [40]. The chemical profile is shown as well, for comparison. The 6 u.c. LSMO films showed a magnetic dead layer near the top surface of the LSMO layers, and suppressed magnetization near the interface (see Figure 6a). With the STO buffer layer, the interfacial magnetization was restored (see Figure 6b), which should be due to the reduced structure distortion [40]. However, the surface dead layer still existed. This suggests that the surface dead layer should be related to surface broken symmetry. However, the surface dead layer was absent when the sample was capped with STO, both for the buffered and non-buffered LSMO films (see Figure 6c,d). The stoichiometry profiles extracted from RXR for both buffered and non-buffered LSMO films verified their similarities and their sharp chemical contrasts across interfaces. RXR also revealed a stoichiometric profile from the interface to the bulk region except the top surface. The variation of the La/Sr stoichiometry near the top surface is probably important for explaining the role of the capping layer, since the capped samples have a more stoichiometric surface than the non-capped samples. The enhanced surface structural distortion, as indicated by DFT calculation (see Supplementary information in Ref. [40]) and experimentally confirmed by surface X-ray diffraction [51], should play a strong role in inducing a surface dead layer. The STO capping layer can enhance the surface magnetism by reducing the surface distortion and minimizing the surface off-stoichiometry. Figure 5: Temperature dependent resistivity of different thicknesses of STO-sandwiched LSMO layers grown on NdGaO\({}_{3}\) (110) substrate. Figure is adapted from Ref. [39]. Figure 6: Stoichiometry and magnetic profiles for different LSMO films with/without STO buffer/capping layers. The depth profiles of the Ga, Ti, Mn, Nd, La, Sr, O atomic concentration, and Mn magnetization were measured at 20 K. Reproduced with permission from [PERSON] et al. [40]. Copyright 2017 John Wiley and Sons.