Excitonic effects in optical absorption and electron-energy loss spectra of hexagonal boron nitride

Abstract

A new interpretation of the optical and energy-loss spectra of hexagonal boron nitride is provided based on rst-principle calculations. We show that both spectra cannot be explained by independent-particle transitions but are strongly dominated by excitonic e ects. The lowest direct and indirect gaps are much larger than previously reported. The direct gap amounts to 6.8 eV. The rst absorption peak at 6.1 eV is due to an exciton with a binding energy of 0.7 eV. We show that this strongly bound Frenkel exciton is also responsible for the low frequency shoulder of the plasmon in the energy-loss function. Implications for nanotube studies are discussed.