Understanding the onset of negative electronic compressibility in single-band and two-band two-dimensional electron gases: Application to LaAlO3 / SrTiO3

We investigate the effects of two electronic bands at the negative electronic compressibility (NEC) in a two-dimensional electron gas (2DEG). We use a simple homogeneous model with Coulombic interactions and first-order multiband coupling to examine the role of effective mass and relative permittivity in relation to the critical carrier density, where compressibility turns negative. We demonstrate that the population of a second band, along with the presence of interband coupling, can dramatically change the crossover carrier density. Given the difficulty in determining and confirming multiband electronic systems, this model provides a potential method for identifying multiband electronic systems using precise bulk electronic properties measurements. To help illustrate this method, we apply our results to the observed NEC in the 2DEG at the LaAlO3/SrTiO3 (LAO/STO) interface and determine that, for the known parameters of LAO/STO, the system is likely a realization of a two-band 2DEG. Furthermore, we provide general limits on the interband coupling with respect to the electronic band population.