Ultrahigh Field Cyclotron Resonance in InSb Quantum Wells

We have observed cyclotron resonance (CR) in InSb/Al_0.09In_0.91Sb quantum wells (QWs) at ultrahigh magnetic fields (up to 150 T). At such high fields, the cyclotron energy can exceed the QW barrier height and even become comparable to the band gap, an unusual situation where the standard k.p theory is expected to break down. We studied two modulation-doped samples with densities ~ 2 x 10^11 cm^-2, mobilities ~ 200,000 cm^2 /Vs, and well widths 20 nm and 30 nm. We observed pronounced electron CR in both samples at temperatures from ~20 K to 300 K with 5.5, 9.2 and 10.6 μm radiation from CO and CO₂ lasers. We clearly resolved non- parabolicity-induced spin splitting at high temperatures. In addition to the main CR, we observed a new feature in the 30 nm QW only at high enough temperatures and high enough photon energies. We compare these results with calculations based on the Pidgeon-Brown model