Meroniums in Spin-Orbit Coupled Bose Gases

Abstract

In this work we demonstrate the existence of new types of topological states in a two-component Bose gas with Rashba spin-orbit couplings. We construct a wave function by mapping the degenerate ground states to the circular boundary of the system, and further consider its superposition with the time-reversed counterpart. By employing the imaginary-time evolution method, we identify four types of topological phases. The first one is the well-known half vortex(HV). The second one is called radial wave half vortex(RWHV), which is a combination of HV with a radial wave factor $e^{ikr}$. The spin configurations show that both HV and RWHV are merons. The third and fourth types are of particular interest. The third one is a combination of HV and anti-HV. The spin densities of this phase demonstrates peak structures and the peaks of spin up and spin down densities are mismatched, and hence we call it the double peak (DP) phase. The fourth type is a combination of RWHV and anti-RWHV, due to the superposition of the radial wave factor $e^{ikr}$ and $e^{-ikr}$ the spin densities demonstrate a spiral pattern, then it's called spin spiral (SS) phase. They are both combinations of meron and antimeron. Here, we name them as meroniums. The topological charges of the four phases are calculated and the spin distributions are demonstrated.