  The collective excitation of surface plasmons in a massless Dirac plasma
(e.g., graphene) half-space (bounded by air) is investigated using a
relativistic quantum fluid model. The unique features of such surface waves are
discussed and compared with those in a Fermi plasma. It is found that in
contrast to Fermi plasmas, the long-wavelength surface plasmon frequency
$(\omega)$ in massless Dirac plasmas is explicitly nonclassical, i.e.,
$\omega\propto1/\sqrt{\hbar}$, where $h=2\pi\hbar$ is the Planck's constant.
Besides some apparent similarities between the surface plasmon frequencies in
massless Dirac plasmas and Fermi plasmas, several notable differences are also
found and discussed. Our findings elucidate the properties of surface plasmons
that may propagate in degenerate plasmas where the relativistic and quantum
effects play a vital role.
