Polyexcitons and charged polyexcitons in two-dimensional multivalley semiconductors

We perform a diffusion Monte Carlo study on the excitonic complexes in the electron-hole (e-h) bilayer systems realized in multivalley semiconductors. Our focus is on polyexcitons with $n$ e-h pairs and charged polyexcitons consisting of $n$ e-h pairs plus one excess electron. By introducing the spin and two-valley degrees of freedom for each carrier, one can relax Pauli-blocking effects. Consequently, we find stable polyexcitons up to $n=4$ and charged polyexcitons up to $n=3$ for small interlayer distances and for any e-h mass ratio. It turns out that the separation energies required to remove an exciton from these excitonic complexes increase linearly with $n$, suggesting that each pair of excitons is bound by an identical ``chemical bond,'' irrespective of the number $n$ or the type of excitonic complex. In charged polyexcitons, each exciton exhibits an ``affinity'' with the excess electron. We posit excitons and excess electron as the fundamental building blocks of such complexes by providing a compelling picture of their internal structures.