Abstract
We have measured the plasmon mode frequencies of a disc of electrons above liquid helium as a function of magnetic field. Azimuthal modes of zero field frequency
ω
P split linearly in
ω
c =
eH/
mc for
ω
c ⪡
ω
p, but as the field is increased the frequency of certain modes drops below
ω
c. These are shown to be a new type of 1D perimeter wave propagating at the edge of the disc within a strip of width
C
p
ω
c
and satisfying aa 1D dipersion relation
ω =
νC
p/
R
0 where
C
p is the screened plasmon velocity and
R
0 the radius of the disc. A model including charge accumulation at the disc boundary (Hall effect) gives an excellent quantitative account of the phenomenon.