Possible Z₂ phase and spin-charge separation in electron-doped cuprate superconductors

The SU(2) slave-boson mean-field theory for the tt^′J model is analyzed. The role of next-nearest-neighbor hopping t^′ on the phase diagram is studied. We find a pseudogap phase in hole-doped (HD) materials (where t^′<0). The pseudogap phase is a U(1) spin liquid (the staggered-flux phase) with a U(1) gauge interaction and no fractionalization. This agrees with experiments on HD samples. The same calculation also indicates that a positive t^′ favors a Z₂ state with true spin-charge separation. The Z₂ state that exists when t^′≳0.5J can be a candidate for the pseudogap phase of electron-doped (ED) cuprates (if such a phase exists). The experimental situation in ED materials is also addressed.