Nature of upper and lower pseudogap phases
Zheng-Yu Weng
Center for Advanced Study, Tsinghua University, Beijing, 100084, P.R. China
The pseudogap phase as a new state of matter will be
explored in the
phase
string model of doped Mott insulators. It is composed of two
distinct
regimes known as the upper and lower pseudogap phases,
respectively.
The former corresponds to the condensate of
resonating-valence-bond
spin pairs and the latter is characterized by
the
formation of the Cooper pair amplitude. Elementary excitation in
the
latter is a spinon-vortex composite, which is charge-neutral
object
carrying spin-1/2 and vortex-like current. Such a lower
pseudogap
phase can be regarded as a vortex liquid state due to the
presence
of free spinon-vortices. Here thermally excited spinon-vortices destroy
the phase coherence and are responsible for nontrivial Nernst effect
and diamagnetism. The superconducting phase is closely related to the
lower pseudogap phase by a topological transition with spinon-vortices
and -antivortices forming bound pairs and the emergence of fermionic
quasiparticles as holon-spinon-vortex bound objects. A quantitative
phase diagram in the parameter space of doping, temperature, and
magnetic field is determined and comparisons with experiments will be
made.
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