Friday 31 July 2015- Emilio Artacho - On the origin of the two-dimensional electron gas at the interface between insulating perovskites

Coming from the field of electronic structure and DFT, Emilio Artacho discusses “On the origin of

the 2D electron gas at the interface between insulating perovskites”. He introduces the “polar catastrophe” argument for LaAlO3/SrTiO3 – for an idealized heterostructure, the charged layers in LaAlO3 up against the neutral layers in SrTiO3 creates electrostatic boundary conditions which would be resolved with 0.5 electrons/2D unit cell for one interface, and 0.5 holes for the other.  Assuming idealized structures, and bulk stoichiometry, the electric field building up, as the LaAlO3 layers are stacked, drives charge transfer from the surface to the interface. DFT calculations (on superlattices) show this and correspond quite well with a simple parallel plate capacitor model.

Emilio wants to convey 2 key points in his talk:

1) The 0.5 interface charge is not only what you find in the simple ionic limit – rather it is robust to the realistic generalization (including covalency, etc.). To show this, he points out that the dipole moment/unit cell is an ill-defined concept, in that it is dependent on the choice of the origin (pointed out by Richard Martin in 70’s). A “dipole-free” unit cell can be chosen and thus projecting all the charge issues to the surface. David Vanderbilt used these ideas to make the “Berry’s phase connection” (in his case for ferroelectrics) in the 90’s, such that the boundaries can be treated much as is the case for topological insulators nowadays. Another analogy is the 1D Haldane chain, in that the relevant spin degrees of freedom are at the ends of the chain. Ultimately, the 0.5 charge is the robust consequence of being between two materials with different topological index.

2) The second point he emphasizes is that rather than purely dealing with bulk idealized stoichiometry (discussed above), another relevant degree of freedom is the stoichiometry – i.e., the 2DEG can be triggered by redox processes. The overall point is that the electrostatic boundary conditions can be resolved not just by “mobile electrons”, but also by defect chemistry. Ultimately, both processes are at play and may interplay.

Final points he makes include: not all carriers at the interface are mobile; disorder is expected; and that depending on origin, we may not necessarily have Mott-Anderson behavior, free carriers moving in a smoothly, weakly disordered potential.

Discussions include possible connections to phase separation scenarios previously discussed in the conference (Grilli et al.).

Blogged by Harold Hwang