Flat-band exciton in two-dimensional Kagomé quantum wire systems

Exciton states in the two-dimensional Kagomé lattice, which is fabricated by the semiconductor quantum wires and has the electronic band structure with dispersionless flat bands, are studied theoretically using the tight-binding model. It is found that the binding energy of an exciton in the Kagomé lattice is larger than the exciton binding energies in other two-dimensional lattices and even larger than that in the one-dimensional lattice. It is shown that such large binding energy originates from the macroscopic degree of degeneracy and the localized nature of the flat-band states in the Kagomé lattice. This large binding energy is controllable by applying an external magnetic field. Furthermore, contrary to the exciton state, we also show that both the binding energy of a charged exciton and that of a biexciton in the Kagomé lattice are much smaller than those in other lattices.