Magnetoresistance Of Graphite Research Articles

Extraordinary magnetoresistance in graphite: experimental evidence for the time-reversal symmetry breaking

We report a highly anisotropic in-plane magnetoresistance (MR) in graphite that possesses in-plane parallel line-like structural defects. In a current direction perpendicular to the line defects (LD), MR is negative and linear in low fields with a crossover to a positive MR at higher fields, while in a current direction parallel to LD, we observed a giant super-linear positive MR. These extraordinary MRs are respectively explained by a hopping magnetoresistance via non-zero angular momentum orbitals, and by the magnetoresistance of inhomogeneous media. The linear negative orbital MR is a unique signature of the broken time-reversal symmetry (TRS). We discuss the origin of the disorder-induced TRS-breaking in graphite.

Two Phase Transitions Induced by a Magnetic Field in Graphite

Different instabilities have been speculated for a three-dimensional electron gas confined to its lowest Landau level. The phase transition induced in graphite by a strong magnetic field, and believed to be a charge density wave, is the only experimentally established case of such instabilities. Studying the magnetoresistance in graphite for the first time up to 80T, 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 gap.

Magnetism, magnetoresistance, and Shubnikov-de Haas oscillations in Na-implanted highly oriented pyrolitic graphite

We report on magnetization, magnetoresistance, and Shubnikov-de Haas oscillations experiments in Na-implanted samples of highly oriented pyrolitic graphite (HOPG). Different ion fluences were applied so that samples with Na contents of 0.5, 1.0, 1.5, and 2.0 at. % were obtained in the implanted region. Ferromagnetic-like hysteresis was observed in magnetization experiments where the field was applied parallel to the graphene planes. The observed saturation moment increases systematically as a function of the implanted ion concentration up to Na 1 at. %, where it goes through a maximum before decreasing slightly towards Na 2 at. %. The planar magnetoresistance amplitude at fixed field and temperature closely correlates with the saturation magnetization data. This result suggests that the strong planar magnetoresistance in graphite is at least partially related to a spin dependent mechanism. The magnetoresistance experiments also reveal the occurrence of Shubnikov-de Haas oscillations. The characteristic frequencies and the effective masses could be estimated and do not depend on the Na concentration. The reported experiments show that the expressive enhancement observed in ferromagnetic-like response in Na-implanted HOPG is primarily due to point defects produced by the implantation process.

Sample-Size Effects in the Magnetoresistance of Graphite

Conduction electrons in graphite are expected to have micrometer large de Broglie wavelength as well as mean free path. A direct influence of these lengths in the electric transport properties of finite-size samples was neglected in the past. We provide a direct evidence of this effect through the size dependence of the magnetoresistance, which decreases with the sample size even for samples hundreds of micrometers large. Our findings may explain the absence of magnetoresistance in small few graphene layers samples and ask for a general revision of the experimental and theoretical work on the transport properties of this material.

Integer quantum Hall effect in graphite

We present Hall effect measurements on highly oriented pyrolytic graphite that indicate the occurrence of the integer quantum-Hall-effect. The evidence is given by the observation of regular plateau-like structures in the field dependence of the transverse conductivity obtained in van der Pauw configuration. Measurements with the Corbino-disk configuration support this result and indicate that the quasi-linear and non-saturating longitudinal magnetoresistance in graphite is governed by the Hall effect in agreement with a recent theoretical model for disordered semiconductors.

Point-Contact Spectra of Graphite

The current-voltage characteristics of graphite point contacts were measured in the liquid-helium temperature range. The dynamic resistance (d V /d I ) has a maximum at zero-bias voltage and continuously decreases with increasing bias voltage in contrast to ordinary metal point contacts where d V /d I increases with bias voltage. In the second-derivative signal, we observed structures near 17 mV and 56 mV which agree with the peak energies of phonon density of states as determined by neutron inelastic scattering. In small contact-resistance samples, we observed magnetic field dependence of the spectra which is related to the large magnetoresistance of graphite.

Anomaly in the Magnetoresistance in Graphite under Pulsed High Magnetic Fields

The anomaly of the magnetoresistance in graphite at low temperatures and high magnetic fields has been extensively investigated by d.c. and a.c. measurements using pulsed high magnetic fields up to 45 T. Several new aspects of the anomaly were observed. Especially it was found that the anomaly was suppressed strongly in a.c. measurements. The observed aspects of the anomaly suggest that the anomaly should originate from some many body effect such as a transition into a CDW state.

Magnetoresistance of Graphite: Deviation from the H2 Law and Evaluation of Conduction Parameters

Magnetoresistance of Graphite: Deviation from the H2 Law and Evaluation of Conduction Parameters

The Transverse Magnetoresistance of Graphite in Relation to the Magnetic Field Azimuth

The Transverse Magnetoresistance of Graphite in Relation to the Magnetic Field Azimuth

Analysis of Multicarrier Galvanomagnetic Data for Graphite

The magnetic-field-dependent data of Soule for the Hall effect and magnetoresistance in graphite have been analyzed using a multicarrier model. An improved mode of analysis is used, in which the magnetoconductivity tensor elements are computed as functions of magnetic field strength from experimental data, and then fitted to simple formulas. The formulas represent solutions to the Boltzmann equation in the classical (nonoscillatory) range. The effects of electrons and holes are separated by applying a Kramers-Kronig type relation. The results, which agree with band-model predictions within 20 to 50%, are that there are 2.9\ifmmode\times\else\texttimes\fi{}${10}^{18}$ holes and electrons per ${\mathrm{cm}}^{3}$ in pure graphite at 4.2\ifmmode^\circ\else\textdegree\fi{}K, and 7.0\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ each at 300\ifmmode^\circ\else\textdegree\fi{}K. The mobilities range from about 9\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{2}$/volt sec at 4.2\ifmmode^\circ\else\textdegree\fi{}K to 1.0\ifmmode\times\else\texttimes\fi{}${10}^{4}$ ${\mathrm{cm}}^{2}$/volt sec at 300\ifmmode^\circ\else\textdegree\fi{}K, with the hole-to-electron mobility ratio being 1.2 and 0.9 at the two temperatures. In addition, at room temperatures there are about 6\ifmmode\times\else\texttimes\fi{}${10}^{14}$ minority holes per ${\mathrm{cm}}^{3}$ with a mobility of 15\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{2}$/volt sec and 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ minority electrons per ${\mathrm{cm}}^{3}$ with a mobility 4\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{2}$/volt sec. The relaxation times for the majority carriers are distributed over a range of a factor of four. The average relaxation times are consistent with those deduced from cyclotron resonance experiments.