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Ultrahigh magnetic field spectroscopy of graphite

The continuous drive towards higher magnetic fields as a research tool in condensed matter science is motivated by the general rule, that the higher the field, the more detailed the information one can obtain about the system under study. The 100 Tesla threshold was recently passed with a non-destructive magnet at the Los Alamos pulsed field facility, but much higher fields do not currently seem feasible by this method, as no known engineering material can resist the Lorentz forces generated inside such magnets. To go to higher fields, one has therefore to resort to semi-destructive methods, which sacrifice the magnet during the field shot, but leave the sample and its direct environment intact. Only very few installations of this type exist in the world, and one of them is located at the LNCMI in Toulouse. This so-called MegaGauss installation was originally developed at the Humboldt University in Berlin, where it has generated up to 350 T, with pulse durations of 5 µs. In Toulouse it is now being integrated into the user program, to start with for optical and spectroscopic experiment in the UV-VIS-NIR range, at temperatures between 300 and 4 K. Developments of other experimental techniques adapted to the special conditions of this installation are under way. Below we describe the first results obtained with the MegaGauss installation by one of our users, Robin Nicholas (Oxford University), on graphite.

The recent activities on two-dimensional electronic systems formed from monolayer and bilayer graphene have rekindled the interest in the properties of bulk graphite. In particular, the many exciting properties of bilayer graphene are crucially dependent on understanding the interlayer coupling that is also operative in bulk graphite. Most of the properties of graphite can be described quite simply at the high symmetry points of the Brillouin zone by a combination of a single layer graphene model for massless Dirac fermions at the H point of its bandstructure and a bilayer graphene model for massive particles at the K point. However, there is very little quantitative information on the bandstructure that results from the interlayer coupling. By using the ultrahigh magnetic fields generated by the MegaGauss installation, we can now extend the magnetospectroscopy of graphene and graphite into the regime of much higher energies. This has allowed us to perform for the first time the magnetospectroscopy of the interlayer split-off bands. Our results show that their behaviour can be modelled very well by the analogue of the relativistic behaviour predicted by the bi-layer model.

(a) Schematic view of the experimental system, (b) graphite band structure, (c) typical time dependence of the magnetic field and sample transmission for 1220 nm radiation.

Reference: R. Nicholas, P.Y. Solane and O. Portugall, Phys. Rev. Lett. 111, 096802 (2013)