# Metal-to-Insulator Transition in Ultrathin Manganite Heterostructures
## 4 Mechanism of the MIT
### Valence Change  
According to the phase diagram of bulk La\({}_{1-x}\)Sr\({}_{x}\)MnO\({}_{3}\) (see Figure 1b), the ground state strongly depends on the Sr doping level. The La\({}_{1-x}\)Sr\({}_{x}\)MnO\({}_{3}\) is metallic when the doping is in the range \(0.2<x<0.5\). There is a narrow range \(0.15<x<0.2\) where the La\({}_{1-x}\)Sr\({}_{x}\)MnO\({}_{3}\) is a ferromagnetic insulator, making the doping range (\(0.15<x<0.5\)) for the ferromagnetic ground state a little larger than that for the metallic phase. The very explicit consequence of the hole doping by Sr is the increasing valence from Mn\({}^{3+}\) to Mn\({}^{3+x}\). This fact may suggest that another mechanism is responsible for the origin of the dead layer: a change of Mn valence in LSMO. A change of Mn valence can be caused by different reasons: (I) polar catastrophe-induced charge transfer or oxygen vacancy formation [38; 55; 56]; (II) Sr-segregation [46; 47; 48; 49]; (III) interface diffusion [44; 49; 50]. The polar catastrophe model proposed by [PERSON] et al. [56] is shown in Figure 10. The LSMO is a polar material with an alternate stacking of charge planes of (La,Sr)O\({}^{0.67}\) and MnO\({}^{-0.67}\). No matter grown on SrO-termination STO or TiO\({}_{2}\) termination STO, a potential is built in from interface to surface due to polar discontinuity across LSMO/STO interface (see Figure 10a,b). This built-in potential results in charge instability and certain amount of electrons have to be transferred from the surface to the interface, to remove the built-in potential. Such mechanism will electron-dope the interface, and therefore deteriorate the properties. If this is the case, inserting another polar material (La\({}_{0.33}\)Sr\({}_{0.67}\)TiO\({}_{3}\)) at the interface (Figure 10c,d); cancelling this effect should have significant consequences on the transport properties. Indeed, [PERSON] et al. have demonstrated enhanced conductivity and magnetization by such interface charge engineering (see Figure 10e,f). For example, non-interface-engineered (nIE) 6 u.c. LSMO is insulating, but interface-engineered (IE) 6 u.c. LSMO is metallic. Microscopic investigation of the polar catastrophe model has been done by using atomic resolved electron energy loss spectra (EELS) performed in a STEM [55]. The layer-res  
Figure 10: Polar catastrophe model of LSMO film on STO. The termination of the STO substrate is TiO\({}_{2}\) in (**a**,**c**), and is SrO in (**b**,**d**). The black line and green dotted line indicate the electrostatic potential profile before and after charge transfer, respectively. Comparison of (**e**) magnetization and (**f**) transport properties between non-interface-engineered (nIE) and interface-engineered (IE) LSMO films. Reproduced with permission from [PERSON] et al. [56]. Copyright 2012 John Wiley and Sons. obtained for different doping La\({}_{1-x}\)Sr\({}_{x}\)MnO\({}_{3}\) (\(x\) = 0-0.5) grown on STO. From the valence profile, [PERSON] et al. concluded that the enrichment of electrons in LSMO near the interface, and the extra electrons at the interface, quantitatively agree with the predicted charge (q = (\(x/2-1/2\)) e\({}^{-}\)) that is needed to cancel the polar catastrophe for insulating La\({}_{1-x}\)Sr\({}_{x}\)MnO\({}_{3}\). However, the transfer charge dramatically drops when La\({}_{1-x}\)Sr\({}_{x}\)MnO\({}_{3}\) is metallic, due to the onset of screening effects. For \(x\) = 0.3, the total transfer charge is around 0.1 e\({}^{-}\). This fact suggests the overestimation of the charge transfer due to polar discontinuity, and its effect on LSMO magnetization and transport properties, since the extra 0.1 e\({}^{-}\) that which varies the Mn valence to 3.23\({}^{+}\) at upper limit is not large enough to move the LSMO into antiferromagnetic ground state (see Figure 1b). The spreading of the charge in space would reduce the change of the Mn valence.