Avoiding the Polarization Catastrophe inLaAlO3Overlayers onSrTiO3(001)through Polar Distortion

Abstract

A pronounced uniform polar distortion extending over several unit cells enables thin LaAlO3 overlayers on SrTiO3(001) to counteract the charge dipole and thereby neutralize the "polarization catastrophe" that is suggested by simple ion counting. This unanticipated mechanism, obtained from density functional theory calculations, allows several unit cells of the LaAlO3 overlayer to remain insulating (hence, fully ionic). The band gap of the system, defined by occupied O 2p states at the surface and unoccupied Ti 3d states at the interface in some cases approximately 20 A distant, decreases with increasing thickness of the LaAlO3 film before an insulator-to-metal transition and a crossover to an electronic reconstruction occurs at around five monolayers of LaAlO3.

Control of electronic conduction at an oxide heterointerface using surface polar adsorbates

“…This simple relationship predicts the same thickness dependence of σ Inter as several more detailed theoretical calculations 20,30 , noting that V Uncom is fixed for an ideal surface. In the work presented here, d LAO is fixed, and the SAP induced change in n 2d is thus associated with a reduction in V Uncom .…”

“…As discussed in several papers 19,20 , and illustrated in Figure 5a, for an idealized LaAlO 3 surface free from surface states, the valence band of LaAlO 3 , E V LAO is the electron source for the interface electrons. Here we assume no conductivity in the LaAlO 3 film itself, and, for simplicity, do not consider other processes that do not affect the electrostatic state of the LaAlO 3 .…”

“…Oxygen vacancies in the SrTiO 3 (ref. 16), atomic interdiffusion 17 , and an electronic reconstruction 4,[18][19][20] have all been suggested to be the origin of the interfacial conductivity in the LaAlO 3 /SrTiO 3 system. In the case of the SAP experiments, we exclude the first two of these as we assume that a SAP at room temperature does not cause significant atomic structure changes at the interface, and we focus on the electronic reconstruction mechanism.…”

“…The intense interest in the LaAlO 3 /SrTiO 3 interface [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] has led to several recent experimental observations that suggest a close relationship between the interface and the LaAlO 3 surface. These include the use of conducting atomic force microscopy probes to toggle a metal insulator transition [10][11][12] through the writing of surface charge 12,13 , and the use of capping layers of SrTiO 3 (ref.…”

“…15) to alter the interface conductivity. The origin of these effects, and indeed the interface conductivity itself, remains in debate 4,[16][17][18][19][20][21][22] . The transfer of electrons from the surface to the interface, to reconcile the polar discontinuity between the neutral {100} layers in SrTiO 3 and the charged layers in {100} LaAlO 3 , is one model that naturally connects the electronic states of the interface and LaAlO 3 surface 4,18-22 .…”