  In the long-wavelength approximation, the effective conductivity tensor is
introduced for graphene ribbons (strips) placed periodically at the interface
between two media. The resulting conducting surface is considered as a coating
for semiconductor nanowire. For the hybrid waves of such nanowire the
dispersion equations are obtained in explicit form. Two types of surface
plasmons are found to exist: (i) the modified surface plasmons, which originate
from the ordinary surface plasmons of a graphene-coated semiconductor nanowire,
and (ii) the spoof plasmons, which arise on the array of graphene ribbons and
may possess forward-wave and backward-wave dispersion. It is revealed that the
spoof surface plasmons are low-loss ones, and their frequencies,
field-confinement and group velocities can be tuned widely by adjusting the
coil angle and width of helical graphene strips.
