  The Fermi surface as a contour of the gapless quasiparticle excitation in
momentum space is studied based on a mean-field theory of the doped Mott
insulator, where the underlying pseudogap phase is characterized by a
two-component resonating-valence-bond (RVB) order that vanishes in the
overdoping at $\delta>\delta^*$. Here the quasiparticle emerges as a
``collective'' mode and a Fermi arc is naturally present in the pseudogap
regime, while a full Fermi surface is recovered at $\delta>\delta^*$. The area
enclosed by the gapless quasiparticle contour still satisfies the Luttinger
volume in both cases, and the ``Fermi arc'' at $\delta<\delta^*$ is actually
due to a significant reduction of the spectral weight caused by a quasiparticle
fractionalization in the antinodal region. The endpoints of the Fermi arcs
exhibit enhanced density of states or ``hotspots'', which can further give rise
to a charge-density-wave-like quasiparticle interference pattern. At the
critical doping $\delta^*$, the fractionalized spin excitations become gapless
and incoherent which is signaled by a divergent specific heat. At
$\delta>\delta^*$, the quasiparticle excitation restores the coherence over the
full Fermi surface, but the fractionalization still persists at a higher
energy/temperature which may be responsible for a strange metal behavior.
Different mechanisms for the Fermi arc and experimental comparisons are briefly
discussed.
