Observation of Bulk and Edge Magnetoplasmons in a Two-Dimensional Electron Fluid

Two-dimensional magnetoplasmons are studied in a classical gas of electrons bound to a liquid-helium surface. A new and unpredicted resonant mode has been observed. The density and magnetic field dependences of this new mode differ strikingly from normal magnetoplasmons. These new resonant modes are interpreted as edge magnetoplasmons of the two-dimensional charge sheet.