Linewidth broadening in edge-magnetoplasmon resonance of helium surface state electrons

We employed edge-magnetoplasmon (EMP) resonance technique to study magneto-transport properties of 2DES where edge states play an important role. We measured EMP frequencies and linewidths of 2DES on liquid helium surface under various lateral con nement potentials. The experimental results show that as the lateral con nement potential is reduced measured EMP linewidth takes minimum at a certain strength of con nement potential and broad signal was obtained on further potential reduction. This broadening behavior is absolutely unexpected from the exsisting theories of conventional EMP. We consider that an oscillation mode transition is responsible for the linewidth broadening. The linewidth behavior in the strong con nement region is reasonably explained by conventional EMP, while the broad signals in the weak con nement region is not. We show our experimental data and discuss the origin of the line broadening.