  We propose and demonstrate that relativistic harmonics with a slowly decaying
power law are generated from a femtosecond lase pulse incident parallel to a
micro-scale overdense plasma. It is shown that due to the excitation of a
strong surface wave, dense electron nanobunches are continuously accelerated
forward while oscillating in the transverse laser field. Even around the
stationary phase point, relativistic gamma factors of the nanobunches increase
considerably, leading to a much stronger attosecond burst, compared to the case
with constant gamma. Our two-dimensional particle-in-cell simulations and
analytical theory show that this synergistic function promises a power-law
harmonic spectrum $I_n/I_0 = n^{-1}$. This is much flatter than the other
well-known radiation mechanisms and paves the way to unprecedentedly large
energy attosecond pulses.
