Abstract
Emergent phases in the two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides have attracted great attention in the past decade. We present ab-initio electronic structure calculations for the interface between a Mott insulator GdTiO3 (GTO) and a band insulator SrTiO3 (STO) and compare our results with those for the widely studied LaAlO3/SrTiO3 (LAO/STO) interface between two band insulators. Our GTO/STO results are in excellent agreement with experiments, but qualitatively different from LAO/STO. We find an interface carrier density of 0.5 e−/Ti, independent of GTO thickness in both superlattice and thin film geometries, in contrast to LAO/STO. The superlattice geometry in LAO/STO offers qualitatively the same result as in GTO/STO. On the other hand, for a thin film geometry, the interface carrier density builds up only beyond a threshold thickness of LAO. The positive charge at the vacuum surface that compensates the 2DEG at the interface also exhibits distinct behaviors in the two systems. The compensating positive charge at the exposed surface of GTO charge disproportionates due to correlation effect making the surface insulating as opposed to that in LAO which remains metallic within band theory and presumably becomes insulating due to surface disorder or surface reconstruction.
Highlights
Following the pioneering work by Ohtomo and Hwang[1] on La d states for (LAO)/STO, there has been much effort on understanding the interface (IF) between two different insulating ABO3 perovskites
A study of both experimental and theoretical literature[1,2,3,13,14,15,26,27,28] shows that the oxide interfaces have been investigated in two different geometries, (i) superlattice geometry with periodic repetition of alternating layers of STO and, LAO or GTO, and (ii) thin film of LAO or GTO grown on a STO(001) substrate
Most experimental studies on LAO/STO are carried out in a thin film-substrate geometry, while both geometries have been investigated in GTO/STO experiments
Summary
Following the pioneering work by Ohtomo and Hwang[1] on LAO/STO, there has been much effort on understanding the interface (IF) between two different insulating ABO3 perovskites. GTO with Ti ions in 3+ state, is in d1 state as opposed to Al states of LAO which are empty This hints towards the crucial role of Ti d electrons and potential correlation effects in making a difference between GTO/STO and LAO/STO. The 2DEG at the GTO/STO interface shows many other interesting properties[13,14,15], including possible transport signatures for magnetism[16] and strong correlations[17] Motivated by these developments, we present here a detailed electronic structure study of the GTO/STO interface and contrast our results with those obtained for LAO/STO. Intuitive understanding of our detailed DFT results at the end of the paper, summarized in terms of a schematic diagram and the associated discussion
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