Abstract

Since 1930 it has been known that any system of charged mobile fermions will show ‘quantum oscillation effects’ in all of its thermodynamic and transport properties, as a function of applied field B (and oscillatory in 1/B). These effects (known as dHvA oscillations in the case of the thermodynamic effect) arise from Landau level quantization – as these levels pass through the Fermi energy with increasing field, they dump the fermions to the levels below. Starting with experiments by Shoenberg, and theory by Landau, in 1938, the idea that dHvA effects could be used as ‘calipers’ to measure the Fermi surface began to take hold. By the 1960’s this was a major tool in solid-state physics. However it was never obvious how strong correlations between fermions would affect this. Fundamental work by Luttinger, Bychkov, and Gorkov indicated that in a 3d metal, interactions would not change the fundamental dHvA behaviour, which would now be looking at Fermi surface ‘quasiparticles’ . However in 2D we now know that a fundamental change occurs at very high fields, when the Landau level filling fraction in of order one or less – we than get Fractional Hall fluids, which involve a fundamental reorganization of the wave functions. As I will discuss, the breakdown of the usual ‘quasiparticle’ picture actually persists right down to low fields in 2d – Landau level quantization immediately destroys standard Fermi liquid behaviour in the presence of interactions, and one cannot assume the usual dHvA behaviour, which is radically changed. This requires a reassessment of the recent dHvA results in high-Tc materials as well as that in other quasi-2d systems. In particular, these results do not yet demonstrate Fermi liquid behaviour in these systems.