Experimental Study of Strongly Correlated Fermion Systems under Extreme Conditions: Two-Dimensional 3He at Ultra-Low Temperatures and Graphite in the Magnetic Ultra-Quantum Limit

摘要

This work explores both the effect of strong correlations in two-dimensional fermion systems, using 3He films on graphite at ultra-low temperatures as a model system, and the correlation induced phases of graphite in the magnetic ultra-quantum limit.Two-dimensional 3He is a strongly correlated Fermi system that undergoes a Mott transition from an isotropic neutral Landau Fermi liquid at low densities into a magnetically frustrated solid on a triangular lattice. The magnetisation, frequency shift and spin dynamics of a series of 3He samples across the phase diagram were measured using pulsed low-frequency SQUID-NMR down to 200 µK. On increasing the density approaching the Mott transition the effective mass of the 3He quasiparticles diverges. At the lowest temperatures the Mott insulator is preceded by an intervening phase, the nature of which is discussed. The Mott insulator is a highly frustrated quantum magnet, which shows a Fermi liquid-like magnetisation down to lowest temperatures, and other features, which support the formation of a gapless quantum-spin liquid.In the second part of this thesis it will be shown how magnetic fields appliedalong the crystallographic c-axis of graphite influence its underlying band structure, incorporating electronic correlations. The quasi-one dimensional dispersion of the Landau levels gives rise to the formation of a series of charge-density waves below 10 K and above 30 T. Pulsed magnetic field magneto-transport experiments have been performed up to 60 T. These show the onset, commensuration and collapse of these charge-density wave states. The observations, in conjunction with theoretical calculations, conclusively identify these states.