Intrinsic Instability of Electronic Interfaces with Strong Rashba Coupling

We consider a model for the two-dimensional electron gas formed at the interface of oxide hetero-structures, which includes a Rashba spin-orbit coupling proportional to the electric field perpendicular to the interface. Based on the standard mechanism of polarity catastrophe, we assume that the electric field has a contribution proportional to the electron density. Under these simple and general assumptions, we show that a phase separation instability (signaled by a negative compressibility) occurs for realistic values of the spin-orbit coupling and of the electronic band-structure parameters. This provides an intrinsic mechanism for the inhomogeneous phases observed at the LaAlO3/SrTiO3 or LaTiO3/SrTiO3 interfaces.