Magneto-optical determination of the T-point energy gap in bismuth

We have carried out infrared magnetotransmission measurements on a 5000-thick bismuth film grown by molecular-beam epitaxy onto a CdTe substrate. The data for photon energies below 300 meV display at least ten orders of minima due to interband transitions at the L points. In contrast to previous results from magnetoreflectivity experiments on bulk Bi, two concurrent series of resonances separated by a nearly constant energy of 5 meV are observed. An additional series of strong oscillations emerges at somewhat higher photon energies, due to interband transitions at the T point. The energy dispersion of these resonances are fit quite well by a simple nonparabolic model, allowing us to directly determine the T-point energy gap (407 meV) and the electron-hole reduced mass (0.027m0, which implies an electron mass of 0.048m0).