Unconventional interactions between 2D excitons and their environment
Excitons and excitonic complexes (EC) are solid-state analogs of atoms and molecules. Simple hydrogen-like states can be studied using a plain Schrodinger equation. However, when an EC is coupled to a medium with a frequency- or momentum-dependent dielectric function, ε(ω, q), the quantum-mechanical description of such system becomes less trivial. The first part of this talk describes a treatment of many-body states of charged particles coupled to a frequencydependent medium. I will show that the problem of dynamically screened ECs can be formally reduced to a simple problem of an EC in an effectively static medium. The environment’s effective dielectric constant ‘perceived’ by ECs depends on the symmetries of EC wavefunctions and their binding energies. This approach is applicable to individual electrons as well as to large many-body complexes. Predictions of environmental effects on EC binding were experimentally verified using neutral, charged and defect-bound excitons in monolayer WS2 screened by liquid, metallic and semiconducting environments. In the second part of the talk I will show that coupling of excitons to some metamaterials with momentum-dependent dielectric functions, ε( ~k), allows creation of ultrarelativistic excitons with high momenta. As a result of the Lorentz contraction, one can observe transient one-dimensional relativistic excitons in two-dimensional graphene.