Y. Kopelevich

Ferromagnetism in oriented graphite samples

We have studied the magnetization of various well characterized samples of highly oriented pyrolitic graphite (HOPG), Kish graphite, and natural graphite to investigate the recently reported ferromagneticlike signal and its possible relation to ferromagnetic impurities. The magnetization results obtained for HOPG samples for applied fields parallel to the graphene layers---to minimize the diamagnetic background---show no correlation with the magnetic impurity concentration. Our overall results suggest an intrinsic origin for the ferromagnetism found in graphite. We discuss possible origins of the ferromagnetic signal.

Phase analysis of quantum oscillations in graphite.

The quantum de Haas-van Alphen (dHvA) and Shubnikov-de Haas oscillations measured in graphite were decomposed by pass-band filtering onto contributions from three different groups of carriers. Generalizing the theory of dHvA oscillations for 2D carriers with an arbitrary spectrum and by detecting the oscillation frequencies using a method of two-dimensional phase-frequency analysis which we developed, we identified these carriers as (i) minority holes having a 2D parabolic massive spectrum p(2)(perpendicular)/2m(perpendicular), (ii) massive majority electrons with a 3D spectrum and (iii) majority holes with a 2D Dirac-like spectrum +/-vp(perpendicular) which seems to be responsible for the unusual strongly-correlated electronic phenomena in graphite.

Reentrant Metallic Behavior of Graphite in the Quantum Limit

Magnetotransport measurements performed on several well-characterized highly oriented pyrolitic graphite and single crystalline Kish graphite samples reveal a reentrant metallic behavior in the basal-plane resistance at high magnetic fields, when only the lowest Landau levels are occupied. The results suggest that the quantum Hall effect and Landau-level-quantization-induced superconducting correlations are relevant to understand the metalliclike state(s) in graphite in the quantum limit.

Indication of superconductivity at 35 K in graphite-sulfur composites.

We report magnetization measurements performed on graphite-sulfur composites which demonstrate a clear superconducting behavior below the critical temperature T(c0) = 35 K. The Meissner-Ochsenfeld effect, screening supercurrents, and magnetization hysteresis loops characteristic of type-II superconductors were measured. The results indicate that the superconductivity occurs in a small sample fraction, possibly related to the sample surface.

Signatures of Electron Fractionalization in Ultraquantum Bismuth

Because of the long Fermi wavelength of itinerant electrons, the quantum limit of elemental bismuth (unlike most metals) can be attained with a moderate magnetic field. The quantized orbits of electrons shrink with increasing magnetic field. Beyond the quantum limit, the circumference of these orbits becomes shorter than the Fermi wavelength. We studied transport coefficients of a single crystal of bismuth up to 33 tesla, which is deep in this ultraquantum limit. The Nernst coefficient presents three unexpected maxima that are concomitant with quasi-plateaus in the Hall coefficient. The results suggest that this bulk element may host an exotic quantum fluid reminiscent of the one associated with the fractional quantum Hall effect and raise the issue of electron fractionalization in a three-dimensional metal.

Magnetism in photopolymerized fullerenes

Abstract The phototransformation of bulk C60 and laser- and electron-beam treatment of C60 films in air changes their magnetic properties. Nonlinear magnetization is observed only for samples irradiated in the presence of oxygen, while, in the case of pressure-polymerized C60, oxygen adversely affects the magnetic properties. The contrasting roles of oxygen in these processes are discussed. Magnetic force microscopy shows that laser- and electron-beam irradiation of fullerene films produces magnetic images which are highly correlated with the topographic images.

Field-induced metal-insulator transition in the c-axis resistivity of graphite

We show that the resistivity perpendicular

Dirac and normal fermions in graphite and graphene : Implications of the quantum hall effect

\rho_c

Local ferromagnetism in microporous carbon with the structural regularity of zeolite Y

and parallel

Oscillating Nernst-Ettingshausen effect in bismuth across the quantum limit.

\rho_a