Ionic Liquid/Solid Double Gate Modulation of the LaAlO$_{3}$/SrTiO$_{3}$ Interfacial Electron Gas

An intriguing combination of properties including high electron mobility, superconductivity, ferromagnetism, and strong spin-orbit coupling has been observed at the LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) interfacial quasi-two-dimensional electron gas (q2DEG). To experimentally clarify the electronic band structure of the q2DEG and how these properties evolve with external tuning by the electric field effect is a key challenge. Here we study the transport properties of the q2DEG in a double-gate field-effect transistor geometry utilizing an ionic liquid as the top gate dielectric, and the STO substrate as the back gate. A systematic carrier density and mobility modulation over a previously unobtained parameter range is achieved, providing a clear picture of electrostatic gating in this system. Changes in the carrier density, mobility, and conductivity strongly suggest the filling of heavy- and light- mass subbands in the quantum well as the top gate voltage is increased with respect to the q2DEG. When the heavier-mass electron subbands dominate the conductivity, signatures of negative electronic compressibility were observed, implying the presence of a tunable strong Rashba spin-orbit splitting in the anisotropic heavier-mass bands at this heterointerface.