  We have proposed an exactly solvable quantum spin-3/2 model on a square
lattice. Its ground state is a quantum spin liquid with a half integer spin per
unit cell. The fermionic excitations are gapless with a linear dispersion,
while the topological "vison" excitations are gapped. Moreover, the massless
Dirac fermions are stable. Thus, this model is, to the best of our knowledge,
the first exactly solvable model of half-integer spins whose ground state is an
"algebraic spin liquid."
