Polar catastrophe and electronic reconstructions at theLaAlO3/SrTiO3interface: Evidence from optical second harmonic generation

Abstract

The so-called "polar catastrophe," a sudden electronic reconstruction taking place to compensate for the interfacial ionic polar discontinuity, is currently considered as a likely factor to explain the surprising conductivity of the interface between the insulators LaAlO 3 and SrTiO 3 . We applied optical second harmonic generation, a technique that a priori can detect both mobile and localized interfacial electrons, to investigating the electronic polar reconstructions taking place at the interface. As the LaAlO 3 film thickness is increased, we identify two abrupt electronic rearrangements: the first takes place at a thickness of 3 unit cells, in the insulating state; the second occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for which the samples become conducting. Two possible physical scenarios behind these observations are proposed. The first is based on an electronic transfer into localized electronic states at the interface that acts as a precursor of the conductivity onset. In the second scenario, the signal variations are attributed to the strong ionic relaxations taking place in the LaAlO 3 layer.

Electronic Reconstruction at the Interface Between Band Insulating Oxides: The LaAlO3/SrTiO3 System

“…However, structural and XAS spectroscopic data [37], core-level photoemission spectroscopy [50], second har-monic generation [83] suggest an orbital reconstruction below the critical thickness.…”

LaAlO3 stoichiometry is key to electron liquid formation at LaAlO3/SrTiO3 interfaces

“…The origin of this 2-DEL has been attributed to a ''polar catastrophe'', an intrinsic electronic reconstruction 1,[14][15][16][17][18] , which acts to remove the diverging electric potential caused by the atomic layer arrangement at the interface-(SrO) 0 -(TiO 2 ) 0 -(LaO) þ 1 -(AlO 2 ) À 1 . This hypothesis for the origin of the observed interfacial conductivity has been questioned and there has been much debate as to the role of defects on the conductivity of this system [19][20][21][22][23] .…”

Perovskite oxide heteroepitaxy

“…In figure 2.8a, the SHG intensity for the single LAO/STO interfaces is presented [68]. There is large scatter in the signal for the three unit cell layer samples and for thicker samples significant SHG intensity is measured.…”

“…Two papers describe measurements of the SHG intensity of LAO/STO interfaces. Savoia et al measured the SHG signal of single n-type LAO/STO interfaces with different LAO thicknesses [68]. Ogawa et al measured the SHG intensity of LAO/STO superlattices (both n-and p-type interfaces) for different sublayer thicknesses [69].…”