Localized bright luminescence of indirect excitons and trions in MoSe$_2$/WSe$_2$ van der Waals heterostructure
Indirect excitons (IX) in semiconductor heterostructures are bosons, which can cool below the temperature of quantum degeneracy and can be effectively controlled by voltage and light. IX quantum Bose gases and IX devices were explored in GaAs heterostructures where an IX range of existence is limited to low temperatures due to low IX binding energies. IXs in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by large binding energies giving the opportunity for exploring excitonic quantum gases and for creating excitonic devices at high temperatures. TMD heterostructures also offer a new platform for studying single-exciton phenomena and few-particle complexes. In this work, we present studies of IXs in MoSe$_2$/WSe$_2$ heterostructures and report on two IX luminescence lines whose energy splitting and temperature dependence identify them as neutral and charged IXs. The experimentally found binding energy of the indirect charged excitons, i.e. indirect trions, is close to the calculated binding energy of 28 meV for negative indirect trions in TMD heterostructures [Deilmann, Thygesen, Nano Lett. 18, 1460 (2018)]. We also report on the realization of IXs with a luminescence linewidth reaching 4~meV at low temperatures. An enhancement of IX luminescence intensity and the narrow linewidth are observed in localized spots.