5. ON THE EFFECTIVE MASS OF COMPOSITE FERMIONS
Fermions (electrons, protons, neutrons) are particles that obey the Pauli exclusion principle: i.e., no two fermions of the same kind can occupy the same quantum state.
The term "quasiparticle" refers to a propagated perturbation in a medium (or field) that behaves as a particle, with energy (mass) and momentum, and that can be treated as such theoretically.
In classical physics, the Hall effect is the development of a transverse voltage across a current-carrying conductor in a magnetic field, the voltage being perpendicular to both the direction of the current and the direction of the magnetic field. In quantum physics, there are two other Hall effects, an integer charge quantum Hall effect, and a fractional charge quantum Hall effect, these quantum Hall effects being observed at extremely low temperatures (a few degrees Kelvin) and extremely intense magnetic fields (at least several tesla). Both quantum Hall effects were first noted in the 1980s, and the fractional quantum Hall effect, although experimentally observed, has not been theoretically resolved.
L.V. Kukushkin et al (Max Planck Institute for Solid-State Physics Stuttgart, DE) discuss composite fermions, the authors making the following points:
1) 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. They precess, like electrons, along circular cyclotron orbits, but with a diameter determined by a reduced effective magnetic field.(5) 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.
2) The authors experimentally demonstrate enhanced absorption of a microwave field in the composite fermion regime, and interpret it as a resonance with the frequency of composite fermion circular motion. From this inferred cyclotron resonance, the authors 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).