  It is generally accepted that doped Mott insulators can be well characterized
by the t-J model. In the t-J model, the electron fractionalization is dictated
by the phase string effect. We found that in the underdoped regime, the
antiferromagnetic and superconducting phases are dual: in the former, holons
are confined while spinons are deconfined, and {\it vice versa} in the latter.
These two phases are separated by a novel phase, the so-called Bose-insulating
phase, where both holons and spinons are deconfined. A pair of Wilson loops was
found to constitute a complete set of order parameters determining this
zero-temperature phase diagram. The quantum phase transitions between these
phases are suggested to be of non-Landau-Ginzburg-Wilson type.
