Edge magnetoplasma-like resonance in two-dimensional electrons on the surface of ³He-⁴He liquid mixtures

We have observed unexpected resonance phenomena in two-dimensional electron system formed on the free surface of dilute ³He-⁴He liquid mixtures (with ³He concentrations up to 21%), in a Corbino conductivity measurement. The resonances are observed only on the mixture surfaces, and were not observed both on pure ⁴He and pure ³He. The resonance frequency is inversely proportional to the magnetic field applied perpendicular to the electron sheet. This property is quite similar to the characteristics of edge magnetoplasmons (EMPs) or inter-edge magnetoplasmons (IEMPs), although the resonance frequency is much smaller than the ordinary EMPs frequencies. The resonances are altered by tilting the electrodes. These results suggest that the inhomogeneity in the electron density on a macroscopic scale is induced by adding a small amount of ³He into liquid ⁴He (possibly due to the growth of ³He surface layers), and that the IEMPs propagate at the boundary between two regions with different electron densities.