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Abstract

It is occasionally possible to interpret strongly interacting
many-body systems within a single-particle framework by
introducing suitable fictitious entities, or 'quasi-particles'.
A notable recent example of the successful application of such
an approach is for a two-dimensional electron system that is
exposed to a strong perpendicular magnetic field. The
conduction properties of the system are governed by electron-
electron interactions, which cause the fractional quantum Hall
effect(1). Composite fermions, electrons that are dressed with
magnetic flux quanta pointing opposite to the applied magnetic
field, were identified as apposite quasi-particles that
simplify our understanding of the fractional quantum Hall
effect(2,3). They precess, like electrons, along circular
cyclotron orbits, but with a diameter determined by a reduced
effective magnetic field(4-10). The frequency of their
cyclotron motion has hitherto remained enigmatic, as the
effective mass is no longer related to the band mass of the
original electrons and is entirely generated from electron-
electron interactions. Here we demonstrate enhanced absorption
of a microwave field in the composite fermion regime, and
interpret it as a resonance with the frequency of their
circular motion. From this inferred cyclotron resonance, we
derive a composite fermion effective mass that varies from 0.7
to 1.2 times that of the electron mass in vacuum as their
density is tuned from 0.6 x 10(11) cm(-2) to 1.2 x 10(11) cm(-
2).