Fragmentation of a trapped multiple-species bosonic mixture

Abstract

We consider a multiple-species mixture of interacting bosons, $N_1$ bosons of mass $m_1$, $N_2$ bosons of mass $m_2$, and $N_3$ bosons of mass $m_3$ in a harmonic trap of frequency $\omega$. The corresponding intraspecies interaction strengths are $\lambda_{11}$, $\lambda_{22}$, and $\lambda_{33}$, and the interspecies interaction strengths are $\lambda_{12}$, $\lambda_{13}$, and $\lambda_{23}$. When the shape of all interactions are harmonic, this is the generic multiple-species harmonic-interaction model which is exactly solvable. We start by solving the many-particle Hamiltonian and concisely discussing the ground-state wavefunction and energy in explicit forms as functions of all parameters, the masses, numbers of particles, and the intraspecies and interspecies interaction strengths. We then move to compute explicitly the reduced one-particle density matrices for all the species and diagonalize them, thus generalizing the treatment in [J. Chem. Phys. {\bf 161}, 184307 (2024)]. The respective eigenvalues determine the degree of fragmentation of each species. As applications, we focus on aspects that do not appear for the respective single-species and two-species systems. For instance, placing a mixture of two kinds of bosons in a bath made by a third kind, and controlling the fragmentation of the former by coupling to the latter. Another example exploits the possibility of different connectivities (i.e., which species interacts with which species) in the mixture, and demonstrates how the fragmentation of species $3$ can be manipulated by the interaction between species $1$ and species $2$, when species $3$ and $1$ do not interact with each other. We thereby highlight properties of fragmentation that only appear in the multiple-species mixture. Further applications are briefly discussed.