Excitons in Carbon Nanotubes with Broken Time-Reversal Symmetry

TL;DR

This study uses high magnetic field spectroscopy to demonstrate how magnetic flux breaks time-reversal symmetry in carbon nanotubes, affecting exciton states and valley degeneracy.

Contribution

It provides experimental evidence of magnetic flux-induced symmetry breaking and its impact on exciton optical activity in carbon nanotubes.

Findings

Peak separation correlates with Aharonov-Bohm phase

Magnetic flux lifts valley degeneracy

Dark exciton states become optically active

Abstract

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields ($>$ 55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or ``dark'').