Ch. Simon

Magnetic order of FeCl3 and CoCl2-graphite intercalation compounds by neutron diffraction

Abstract We have investigated the magnetic properties of first and second stage intercalated FeCl3 and CoCl2 graphite by neutron diffraction at low temperatures. Each case is very different: in FeCl3 we have found that in both stages strong 2-D magnetic correlations develop below 30 K. In the first stage compound we observe a transition from the 2-D phase to a 3-D order at 3.8 K. The in-plane order remains the same and corresponds to an incommensurate magnetic modulation of the propagation vector 0.394 a . By contrast, in both stages of the CoCl2 compounds, we observe a 3-D antiferromagnetism along the c-direction without any 2-D behaviour.

Quantum oscillatory effects and band structure in graphite intercalation compounds

Abstract Results for magnetothermal oscillations and the in situ deHaas-van Alphen effect in acceptor and donor graphite intercalation compounds are presented. The following intercalates have been investigated: C 8 s (NO 3 H) s = 2, 3 and 4, C 7 s (SO 3 HF) s = 2, 3, C 4 s Br s = 2, C 6 s Li s = 2, C 4 s HgK s = 1, where s is the stage. The observed frequencies range between 40 and 1500 Tesla and the effective masses between 0.05 m 0 and 1 m 0 . In acceptor compounds characteristic combinations of frequencies are present which are explained by the magnetic breakdown of the primitive orbits on the FS. A 2D FS model is proposed which includes the electronic structure proposed by Blinowski-Rigaux and the folding due to the periodicity of the intercalate layer. The results for the C 16 (NO 3 H) are analyzed in terms of a detailed FS. The charge transfer coefficient found is ƒ = 0.6 and the Fermi level shift E F = −1.08 eV.

The electronic structure of the graphite acceptor compounds

Abstract Quantum-magnetothermal oscillations in dilute bromine and low stage nitric acid graphite acceptor intercalation compounds are presented. The results are interpreted by using a 2-D electronic structure. For this we assume that localized electrons of the intercalate layer screen out completely each from the other the carbon layers surrounding from each side the interface. These carbon layers are charged with delocalized holes.

Observation of a Critical Behaviour of the Diffusion Constant for Two-Dimensional Melting: HNO3 Intercalated in Graphite

We have studied the behaviour of the diffusion constant by incoherent quasi-elastic neutron scattering at the disorder transition of the second-stage α-HNO3 intercalated in graphite. The transition is continuous. This disordered phase is a lattice liquid phase. It shows simultaneously strong orientational correlations as previously observed by X-ray diffraction and translational diffusion. The diffusion constant follows an exp [- 2b/(T - Tc)1/2] law, as predicted for 2D defect-mediated phase transitions.

Magnetic order of FeCl3 and CoCl2 intercalated graphite

We have investigated by neutron diffraction the magnetic properties of first‐ and second‐stage FeCl3 and CoCl2 graphite intercalation compounds. They can be considered as quasi‐two‐dimensional magnetic systems in which one can vary both the 3D magnetic coupling and the intralayer magnetic interactions. The FeCl3 compounds are Heisenberg systems in which we have found that strong 2D magnetic correlations develop below 40 K; in the first‐stage compound, the 3D coupling is strong enough to allow a transition to a 3D order below 3.8 K. The in‐plane order is always an incommensurate modulation. In the CoCl2 compounds we have observed on the contrary an antiferromagnetism along c axis with the moments in the layer plane. They are 2D X‐Y systems with a Kosterlitz–Thouless transition in which however the 3D coupling plays an important role.

Lattice dynamics of graphite intercalation compounds

We have studied by coherent inelastic neutron scattering the phonon dispersion relation of the second stage bromine graphite intercalation compound. We have observed the almost complete low energy spectrum (<12 THz) with longitudinal and transverse phonons propagating along the direction perpendicular to the layers (c axis) as well as in the layers plane. The modes polarized along the c axis present energy gaps in the c direction branches and the acoustic branch polarized in the layers plane, opposite to the optic ones, is very sensitive to the intercalate nature. The main character of the modes polarized in the layers plane is a dispersionless optic branch along the c direction at 1 THz. A phenomenological Born-von Karman force constant model with non axial symmetry is developed in order to interpret our results.

Experimental determination of the current flow in the quantum Hall regime

Abstract We have developed a new method of determining the interior potential distribution, and hence the current flow, in a quantized two-dimensional electron gas (2DEG) at low temperature and high magnetic fields. Our results show that current carrying states exist in the bulk of the 2DEG and that the distribution of current changes with source-drain current and can vary appreciably over a small range of magnetic field well within the quantum Hall regime. In addition, increasing σxx reduces the variation in the current distribution with magnetic field.