At low temperatures and in high magnetic fields, a 2D system’s behavior is no longer governed by the kinetic energies of electrons. Instead, it is driven by electron-electron or hole-hole interactions, leading to exotic quantum phenomena such as fractional quantum Hall states. These states are commonly observed when only a fraction of an energy level or orbital is filled by carriers. Usually, the fraction has an odd denominator, however, although rare, even-denominator states occasionally appear at higher filling levels (for example, 5/2), but their origins remain unclear. Chengyu Wang at Princeton University and his colleagues now report an unexpected even-denominator state in 2D gallium arsenide (GaAs), which could help to identify the origin of these rare states .
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