Dresden 2003 – scientific programme

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PV: Plenarvorträge

PV VII

PV VII: Plenary Talk

Wednesday, March 26, 2003, 12:00–13:00, HSZ/01

Composite Fermions – The Quasi-Particle Notion at its Best — •Jurgen H. Smet — Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart — Träger des Walter-Schottky-Preises 2003

The introduction of suitable fictitious entities or ’quasi-particles’ permits every so often in condensed matter physics to cast an otherwise difficult strongly interacting many-body problem in a single particle form. We can then take the customary physical approach using concepts and representations, which formerly could only be applied to systems with weak interactions, while still capturing the essential physics. A most notable recent example occurs in the conduction properties of a two-dimensional electron system, when it is exposed to a strong perpendicular magnetic field. In this regime, the transport is governed by electron-electron interactions that bring about the celebrated fractional quantum Hall effect. Composite fermions, each assembled from one electron and two elementary flux quanta, were identified as apposite quasi-particles that simplify our understanding of this fundamental correlation phenomenon. We, as well as many others, have devoted considerable effort over the last decade to proof the existence and unravel the properties of these composite fermions. We will review some key experiments which highlight that they behave as legitimate particles. They execute circular cyclotron orbits like electrons, however with a diameter determined by a drastically reduced effective magnetic field rather than the externally applied magnetic field and with a frequency that is no longer controlled by the band mass of the underlying semiconductor crystal, but by an effective mass which is entirely generated from Coulomb interactions. The inclusion of the spin degree of freedom in the composite fermion picture has also reaped rich rewards and has turned out instrumental in accounting for a wealth of spin related phase transition physics in the fractional quantum Hall regime. The insight gained has most recently opened up all new vistas to investigate the often ignored, but surprisingly strong, interactions between electronic spin and the nuclear spin of the crystal lattice in the two-dimensional electron system.