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Spin liquids on a honeycomb lattice: Projective Symmetry Group study of Schwinger fermion mean-field theory
Spin liquids are novel states of matter with fractionalized excitations. A
recent numerical study of Hubbard model on a honeycomb lattice\cite{Meng2010}
indicates that a gapped spin liquid phase exists close to the Mott transition.
Using Projective Symmetry Group, we classify all the possible spin liquid
states by Schwinger fermion mean-field approach. We find there is only one
fully gapped spin liquid candidate state: "Sublattice Pairing State" that can
be realized up to the 3rd neighbor mean-field amplitudes, and is in the
neighborhood of the Mott transition. We propose this state as the spin liquid
phase discovered in the numerical work. To understand whether SPS can be
realized in the Hubbard model, we study the mean-field phase diagram in the
spin-1/2 model and find an s-wave pairing state. We argue that s-wave
pairing state is not a stable phase and the true ground state may be SPS. A
scenario of a continuous phase transition from SPS to the semimetal phase is
proposed. This work also provides guideline for future variational studies of
Gutzwiller projected wavefunctions.Comment: 13 pages, 4 figures, Revtex
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