Direct manifestation of the Fermi pressure in a two-dimensional electron system

We have measured the dispersion of magnetoplasmons in high-density InAs/GaSb quantum wells. At zero magnetic field we find an offset 3/4${\mathit{v}}_{\mathit{F}}^2$${\mathit{q}}^2$ (q is the plasmon wave vector, ${\mathit{v}}_{\mathit{F}}$ is the Fermi velocity) with respect to the local plasmon frequency which is a direct manifestation of the Fermi pressure in a two-dimensional electron system. © 1996 The American Physical Society.