Continuous Topological Phase Transition between Two 1D Antiferromagnetic Spin-1 Boson Superfluids with the Same Symmetry.

Spin-1 bosons on a one-dimensional chain, at incommensurate filling with an antiferromagnetic spin interaction between neighboring bosons, may form a spin-1 boson condensed state that contains both a gapless charge and spin excitations. We argue that the spin-1 boson condensed state is unstable, and will become one of two superfluids by opening a spin gap. One superfluid must have a spin-1 ground state on a ring if it contains an odd number of bosons and has no degenerate states at the chain end. The other superfluid has a spin-0 ground state on a ring for any number of bosons and has a spin-1/2 degeneracy at the chain end. The two superfluids have the same symmetry and only differ by a spin-SO(3) symmetry protected topological order. Although Landau theory forbids a continuous phase transition between two phases with the same symmetry, the phase transition between the two superfluids can be generically continuous, which is described by conformal field theory (CFT) su(2)_{2}⊕u(1)_{4}⊕su(2)[over ¯]_{2}⊕u(1)[over ¯]_{4}. Such a CFT has a spin fractionalization: spin-1 excitation can decay into a spin-1/2 right mover and a spin-1/2 left mover. We determine the critical theory by solving the partition function based on emergent symmetries and modular invariance condition of CFTs.