Electron-hole asymmetry in single-walled carbon nanotubes probed by direct observation of transverse quasidark excitons

Abstract

We studied the asymmetry between valence and conduction bands in single-walled carbon nanotubes through the direct observation of spin-singlet transverse dark excitons using polarized photoluminescence excitation spectroscopy. The intrinsic electron-hole $(e\text{\ensuremath{-}}h)$ asymmetry lifts the degeneracy of the transverse exciton wave functions at two equivalent $K$ and ${K}^{\ensuremath{'}}$ valleys in momentum space, which gives finite oscillator strength to transverse dark exciton states. Chirality-dependent spectral weight transfer to transverse dark states was clearly observed, indicating that the degree of the $e\text{\ensuremath{-}}h$ asymmetry depends on the specific nanotube structure. Based on comparison between theoretical and experimental results, we evaluated the band asymmetry parameters in various carbon nanotube structures and graphene.