  We present a new mechanism of attosecond extreme-ultraviolet (XUV) pulses
generation from a relativistic laser-driven overdense plasma surfaces in the
wavebreaking regime. Through particle-in-cell simulations and analysis, we
demonstrate that the observed ultrashort XUV emission for the parameters we
considered is predominantly due to a strong plasma-density oscillation
subsequent to wavebreaking. The coupling of the strong density variation and
the transverse fields in the front surface layer gives rise to the transmitted
emission with frequencies mainly around the local plasma frequency. This
mechanism provides new insights into the scenarios of XUV generation from solid
surfaces and the dynamics of laser-plasma interactions.
