Transport properties and electronic ground state of graphite at high magnetic field

Orateur : Benoît FAUQUE

When a magnetic field confines the carriers of a Fermi sea to their lowest Landau level, electron-electron interactions are expected t o play a significant role in determining the electronic ground state. Three decades ago it has been shown that graphite host a field-induced state driven by such interactions. Here we will discuss our magneto-transport and ultra sound measurements at high magnetic field. Studying the magnetoresistance in graphite up to 80 T, we find that the magnetic field induces two successive phase transitions consisting of two distinct ordered states each restricted to a finite field window. In both states, an energy gap opens up in the out-of-plane conductivity and coexists with an unexpected in-plane metallicity for a fully gap bulk system. Such peculiar metallicity may arise as a consequence of edge-state transport expected to develop in the presence of a bulk. The evolution of elastic constant anomalies with temperature and magnetic field across the first order state shows that it evolves in a sequence of 2nd order and first order thermodynamic phase transitions. The complete phas e diagram of graphite at high magnetic field will be discuss in detail. If time allows I will also discuss the observation of magnetoconductance oscillations periodic in magnetic field with an amplitude of the order of e^2/h in macroscopic samples of highly oriented pyrolytic graphite (HOPG) which can be explained by invoking moire superlattices with a discrete distribution in periodicity.

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