Emergence of unstable avoided crossing in the collective excitations of spin-1 spin-orbit coupled Bose-Einstein condensates

Abstract: We present the analytical and numerical results on the collective excitation spectrum of quasi-one-dimensional spin-orbit (SO) coupled spin-1 Bose-Einstein condensates. The collective excitation spectrum, using Bogoliubov-de-Gennes theory, reveals the existence of a diverse range of phases in the SO and Rabi ($k_L-\Omega$) coupling plane. Based on eigenvalue of the excitation spectrum, we categorize the $k_L-\Omega$ plane into three distinct regions. In region I, a stable mode with phonon-like excitations is observed. In region IIa, single and multi-band instabilities are noted with a gapped mode, while multi-band instability accompanied by a gapless mode between low-lying and first excited states is realized in region IIb, which also provides evidence of unstable avoided crossing between low-lying and first excited modes The gap between low-lying and first-excited states increases upon increasing the Rabi coupling while decreases upon increase of SO coupling. Using eigenvector analysis, we confirm the presence of the spin-dipole mode in the spin-like modes in Region II. We corroborate the nature of the collective excitation through real-time dynamical evolution of the ground state perturbed with the quench of the trap using the mean-field Gross-Pitaevskii model. This analysis suggests the presence of dynamical instability leading to the disappearance of the $0$-th component of the condensate. In Region III, mainly encompassing $\Omega \sim 0$ and finite $k_L$, we observe phonon-like excitations in both the first excited and the low-lying state. The eigenvectors in this region reveal alternative in- and out-of-phase behaviours of the spin components. Numerical analysis reveals the presence of a super stripe phase for small Rabi coupling in this region, wherein the eigenvector indicates the presence of more complicated spin-like-density mixed modes.