Spread complexity and dynamical transition in multimode Bose-Einstein condensates

We study the spread complexity in two-mode Bose-Einstein condensates and unveil that the long-time average of the spread complexity ${\overline{C}}_{K}$ can probe the dynamical transition from self-trapping to Josephson oscillation. When the parameter $\ensuremath{\omega}$ increases over a critical value ${\ensuremath{\omega}}_{c}$, we reveal that the spread complexity exhibits a sharp transition from lower to higher value, with the corresponding phase space trajectory changing from self-trapping to Josephson oscillation. Moreover, we scrutinize the eigenspectrum and uncover the relation between the dynamical transition and the excited state quantum phase transition, which is characterized by the emergence of singularity in the density of states at critical energy ${E}_{c}$. In the thermodynamical limit, the cross point ${E}_{c}(\ensuremath{\omega})$ and the initial energy ${E}_{0}(\ensuremath{\omega})$ determine the dynamical transition point ${\ensuremath{\omega}}_{c}$. Furthermore, we show that the different dynamical behavior for the initial state at a fixed point can be distinguished by the long-time average of the spread complexity, when the fixed point changes from unstable to stable. Finally, we also examine the sensitivity of ${\overline{C}}_{K}$ for the triple-well bosonic model which exhibits the transition from chaotic dynamics to regular dynamics.