Quantum Hall effects at Landau level crossings

Interesting ferromagnetic characteristics are found in quantum Hall effects when Landau levels (LLs) cross and carrier interaction effects become important at the crossing points. Such LL crossings are studied in bilayer AlGaAs/GaAs systems and silicon quantum wells having a novel valley splitting. Detailed measurements of bilayer ferromagnetism at the LL crossings reveal easy-plane or axis characteristics depending on spin and pseudospin of the crossing states. In the fractional QH regime, interactions between electron and nuclear spins are observed at the crossings of composite-fermion LLs.