Observation of the Two-Dimensional Plasmon in Silicon Inversion Layers

The two-dimensional plasmon of an $n$ inversion layer of (100) $p$-type Si is observed at a fixed wave vector as a function of electron density. The position, width, and strength of the resonance agree with existing theory at electron densities ≳${10}^{12}$/${\mathrm{cm}}^2$. At lower densities, the resonance position is below the predicted value, implying a substantial increase in the electron mass.