Emergence of second electronic subband in ultrathin InSe.*

In 2D materials, the energy separation between electronic or hole subbands depends on thickness. We demonstrate the population of the second electronic subband by utilizing high mobility InSe thin enough to host a single layer of two-dimensional electron gas, but at the same time thick enough to allow for electrostatic doping of the second electronic subband. We explore the evolution of Landau levels from both subbands with magnetic and electric fields and investigate the tunability of onset charge density and effective mass of the second subband by perpendicular electric field. At high magnetic fields, the ring-shaped crossings of Landau levels of the two subbands suggest transitions from paramagnetic to ferromagnetic states. We find the electrons in the second subband to manifest tunability unattainable in conventional charge transport.