Spin-charge separation: From one hole to finite doping

In the presence of nonlocal phase shift effects, a quasiparticle can remain topologically stable even in a spin-charge separation state due to the con finement effect introduced by the phase shifts at finite doping. True decon finement only happens in the zero-doping limit where a bare hole can lose i ts integrity and decay into holon and spinon elementary excitations. The Fe rmi surface structure is completely different in these two cases, from a la rge band-structure-like one to four Fermi points in one-hole case, and we a rgue that the so-called underdoped regime actually corresponds to a situati on in between.