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

Abstract

© 2019 American Physical Society. 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-12 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)2u(1)4su(2)2u(1)4. Such a CFT has a spin fractionalization: spin-1 excitation can decay into a spin-12 right mover and a spin-12 left mover. We determine the critical theory by solving the partition function based on emergent symmetries and modular invariance condition of CFTs.