Fermi polaron fine structure in strained van der Waals heterostructures

The fine structure of attractive Fermi polarons in van der Waals heterostructures based on monolayer transition metal dichalcogenides in the presence of elastic strain is studied theoretically. The charged excitons (trions), three particle bound states of two electrons and a hole or two holes and an electron, do not show any strain-induced fine structure splitting compared to neutral excitons whose radiative doublet is split by the strain into linearly polarized components. The correlation of the trions with Fermi sea holes gives rise to attractive Fermi polarons. We show that this results in a fine structure splitting of the polaron into states polarized along the main axes of the strain tensor. This effect is related to the bosonic statistics of Fermi polarons. We develop a microscopic theory of the effect and calculate the strain-induced splitting of Fermi polarons for both tungsten- and molybdenum-based monolayers, identifying the role of inter- and intravalley exciton-electron interactions. The fine structure splitting of the attractive Fermi polaron is proportional to both the excitonic splitting and the Fermi energy. The Fermi polaron fine structure in bilayers is briefly analyzed, and the role of electron and trion localization in moire potentials is discussed.