Plasmons at the LaAlO_3/SrTiO_3 interface and in the graphene-LaAlO_3/SrTiO_3 double layer

摘要

We study plasmon modes of the two-dimensional electron gas residing at the interface of band insulators LaAlO_3 and SrTiO_3 (LAO/STO) and the plasmon excitations of graphene-LAO/STO double layer as well. Considering the electron-electron interaction within random phase approximation, we calculate the plasmon dispersions of both systems numerically and in the long-wavelength limit analytical expressions for collective modes are found. One optical mode and two (three) acoustic modes are predicted for the LAO/STO (graphene- LAO/STO) system where only the uppermost acoustic mode of both systems can emerge above the electron-hole continuum depending on the characteristics of each system. In the case of LAO/STO interface, thanks to the spatial separation between t_(2g) orbitals, the upper acoustic mode might be undamped at the long-wavelength limit depending on the exact value of the dielectric constant of SrTi03. Same as other double layer systems, the interlayer distance for the graphene-LAO/STO system plays a crucial role in damping the upper acoustic mode. Faster damping of all plasmon modes of the present double layer system in comparison with the ones with conventional two-dimensional electron gas instead of t2g electron gas is also found due to heavier effective masses of the gas and also stronger interlayer Coulomb interaction.