Evidence for internal field in graphite: A conduction electron spin resonance study

We report conduction electron spin resonance measurements performed on highly oriented pyrolitic graphite samples between 10 K and 300 K using S (f = 4 GHz), X (f = 9.4 GHz), and Q (f = 34.4 GHz) microwave bands for the external dc-magnetic field applied parallel (H || c) and perpendicular (H perp c) to the sample hexagonal c-axis. The results obtained in the H || c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field Heff-int(T,H) that increases as the temperature decreases and the applied dc-magnetic field increases. We associate the occurrence of the Heff-int(T,H) with the field-induced metal-insulator transition in graphite and discuss its origin in the light of relevant theoretical models.Comment: 10 pages (tex), 5 figures (ps

EEG spike source localization before and after surgery for temporal lobe epilepsy: a BOLD EEG-fMRI and independent component analysis study

Simultaneous measurements of EEG-functional magnetic resonance imaging (fMRI) combine the high temporal resolution of EEG with the distinctive spatial resolution of fMRI. The purpose of this EEG-fMRI study was to search for hemodynamic responses (blood oxygen level-dependent - BOLD responses) associated with interictal activity in a case of right mesial temporal lobe epilepsy before and after a successful selective amygdalohippocampectomy. Therefore, the study found the epileptogenic source by this noninvasive imaging technique and compared the results after removing the atrophied hippocampus. Additionally, the present study investigated the effectiveness of two different ways of localizing epileptiform spike sources, i.e., BOLD contrast and independent component analysis dipole model, by comparing their respective outcomes to the resected epileptogenic region. Our findings suggested a right hippocampus induction of the large interictal activity in the left hemisphere. Although almost a quarter of the dipoles were found near the right hippocampus region, dipole modeling resulted in a widespread distribution, making EEG analysis too weak to precisely determine by itself the source localization even by a sophisticated method of analysis such as independent component analysis. On the other hand, the combined EEG-fMRI technique made it possible to highlight the epileptogenic foci quite efficiently.58258

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 ferromagnetic-like 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.Comment: 11 figure

Magnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit

We report on the magnetic field (0T$\le B \le 9$T) dependence of the longitudinal thermal conductivity $\kappa(T,B)$ of highly oriented pyrolytic graphite in the temperature range 5 K $\le T\le$ 20 K for fields parallel to the $c-$axis. We show that $\kappa(T,B)$ shows large oscillations in the high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are observed in the Hall resistance. With the measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violated in the high-field regime.Comment: 4 Figures, to be published in Physical Review B (2003

Holographic description of strongly correlated electrons in external magnetic fields

We study the Fermi level structure of (2+1)-dimensional strongly interacting electron systems in external magnetic field using the AdS/CFT correspondence. The gravity dual of a finite density fermion system is a Dirac field in the background of the dyonic AdS-Reissner-Nordstrom black hole. In the probe limit the magnetic system can be reduced to the non-magnetic one, with Landau-quantized momenta and rescaled thermodynamical variables. We find that at strong enough magnetic fields, the Fermi surface vanishes and the quasiparticle is lost either through a crossover to conformal regime or through a phase transition to an unstable Fermi surface. In the latter case, the vanishing Fermi velocity at the critical magnetic field triggers the non-Fermi liquid regime with unstable quasiparticles and a change in transport properties of the system. We associate it with a metal-"strange metal" phase transition. We compute the DC Hall and longitudinal conductivities using the gravity-dressed fermion propagators. As expected, the Hall conductivity is quantized according to integer Quantum Hall Effect (QHE) at weak magnetic fields. At strong magnetic fields, new plateaus typical for the fractional QHE appear. Our pattern closely resembles the experimental results on graphite which are described using the fractional filling factor proposed by Halperin.Comment: 39 pages, 11 figures, Ch. 21, Lect. Notes Phys. "Strongly interacting matter in magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A. Schmitt, H.-U. Yee. arXiv admin note: substantial text overlap with arXiv:1011.405

Conduction Electron Spin Resonance Evidence For Internal Field In Graphite

We report conduction electron spin resonance measurements performed on highly oriented pyrolitic graphite samples between 10 and 300K using S (ν = 4GHz), X (ν = 9.4GHz), and Q (ν = 34.4GHz) microwave bands for the external DC-magnetic field applied parallel (H ∥ c) and perpendicular (H ⊥c ) to the sample hexagonal c-axis. The results obtained in the H ∥ c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field, H int eff(T, H), that increases as the temperature decreases and the applied DC-magnetic field increases. We associate the occurrence of the H int eff(T, H) with the field-induced metal-insulator transition in graphite and discuss its origin in the light of relevant theoretical models. © 2002 Elsevier Science B.V. All rights reserved.3201-4413415Xu, S., (1996) Phys. Rev. Lett., 76, p. 483Kopelevich, Y., (1999) Fiz. Tverd. Tela (St. Petersburg), 41, p. 2135Kopelevich, Y., (1999) Phys. Solid State, 41, p. 1959Kopelevich, Y., (2000) J. Low Temp. Phys., 119, p. 691Kempa, H., (2000) Solid State Commun., 115, p. 539Khveshchenko, D.V., (2001) Phys. Rev. Lett., 8720, p. 6401Dresselhaus, M.S., Dresselhaus, G., Sugihara, K., Spain, I.L., Graphite fibers and filaments Springer Series in Material Science, 5, pp. 179-188. , references therein, Springer, BerlinDresselhaus, M.S., (1981) Adv. Phys., 30, p. 139Sercheli, M.S., (2002) Solid State Commun., 121, p. 579Laughlin, R.B., (1998) Phys. Rev. Lett., 80, p. 5188Ferrer, E.J., cond-mat/010130

EEG spike source localization before and after surgery for temporal lobe epilepsy: a BOLD EEG-fMRI and independent component analysis study

Simultaneous measurements of EEG-functional magnetic resonance imaging (fMRI) combine the high temporal resolution of EEG with the distinctive spatial resolution of fMRI. The purpose of this EEG-fMRI study was to search for hemodynamic responses (blood oxygen level-dependent - BOLD responses) associated with interictal activity in a case of right mesial temporal lobe epilepsy before and after a successful selective amygdalohippocampectomy. Therefore, the study found the epileptogenic source by this noninvasive imaging technique and compared the results after removing the atrophied hippocampus. Additionally, the present study investigated the effectiveness of two different ways of localizing epileptiform spike sources, i.e., BOLD contrast and independent component analysis dipole model, by comparing their respective outcomes to the resected epileptogenic region. Our findings suggested a right hippocampus induction of the large interictal activity in the left hemisphere. Although almost a quarter of the dipoles were found near the right hippocampus region, dipole modeling resulted in a widespread distribution, making EEG analysis too weak to precisely determine by itself the source localization even by a sophisticated method of analysis such as independent component analysis. On the other hand, the combined EEG-fMRI technique made it possible to highlight the epileptogenic foci quite efficiently

Eeg Spike Source Localization Before And After Surgery For Temporal Lobe Epilepsy: A Bold Eeg-fmri And Independentcomponent Analysis Study

Simultaneous measurements of EEG-functional magnetic resonance imaging (fMRI) combine the high temporal resolution of EEG with the distinctive spatial resolution of fMRI. The purpose of this EEG-fMRI study was to search for hemodynamic responses (blood oxygen level-dependent - BOLD responses) associated with interictal activity in a case of right mesial temporal lobe epilepsy before and after a successful selective amygdalohippocampectomy. Therefore, the study found the epileptogenic source by this noninvasive imaging technique and compared the results after removing the atrophied hippocampus. Additionally, the present study investigated the effectiveness of two different ways of localizing epileptiform spike sources, i.e., BOLD contrast and independent component analysis dipole model, by comparing their respective outcomes to the resected epileptogenic region. Our findings suggested a right hippocampus induction of the large interictal activity in the left hemisphere. Although almost a quarter of the dipoles were found near the right hippocampus region, dipole modeling resulted in a widespread distribution, making EEG analysis too weak to precisely determine by itself the source localization even by a sophisticated method of analysis such as independent component analysis. On the other hand, the combined EEG-fMRI technique made it possible to highlight the epileptogenic foci quite efficiently.426582587Engel Jr., J., Introduction to temporal lobe epilepsy (1996) Epilepsy Res, 26, pp. 141-150Ives, J.R., Warach, S., Schmitt, F., Edelman, R.R., Schomer, D.L., Monitoring the patient's EEG during echo planar MRI (1993) Electroencephalogr Clin Neurophysiol, 87, pp. 417-420Allen, P.J., Josephs, O., Turner, R., A method for removing imaging artifact from continuous EEG recorded during functional MRI (2000) Neuroimage, 12, pp. 230-239Ritter, P., Villringer, A., Simultaneous EEG-fMRI (2006) Neurosci Biobehav Rev, 30, pp. 823-838Zijlmans, M., Huiskamp, G., Hersevoort, M., Seppenwoolde, J.H., van Huffelen, A.C., Leijten, F.S., EEG-fMRI in the preoperative work-up for epilepsy surgery (2007) Brain, 130, pp. 2343-2353Lemieux, L., Krakow, K., Fish, D.R., Comparison of spike-triggered functional MRI BOLD activation and EEG dipole model localization (2001) Neuroimage, 14, pp. 1097-1104Bagshaw, A.P., Kobayashi, E., Dubeau, F., Pike, G.B., Gotman, J., Correspondence between EEG-fMRI and EEG dipole localisation of interictal discharges in focal epilepsy (2006) Neuroimage, 30, pp. 417-425Belliveau, J.W., Rosen, B.R., Kantor, H.L., Rzedzian, R.R., Kennedy, D.N., McKinstry, R.C., Functional cerebral imaging by susceptibility-contrast NMR (1990) Magn Reson Med, 14, pp. 538-546Ogawa, S., Lee, T.M., Kay, A.R., Tank, D.W., Brain magnetic resonance imaging with contrast dependent on blood oxygenation (1990) Proc Natl Acad Sci U S A, 87, pp. 9868-9872Delorme, A., Makeig, S., EEGLAB: An open source toolbox for analysis of single-trial EEG dynamics including independent component analysis (2004) J Neurosci Methods, 134, pp. 9-21Plummer, C., Harvey, A.S., Cook, M., EEG source localization in focal epilepsy: Where are we now? (2008) Epilepsia, 49, pp. 201-218Leal, A.J., Dias, A.I., Vieira, J.P., Analysis of the EEG dynamics of epileptic activity in gelastic seizures using decomposition in independent components (2006) Clin Neurophysiol, 117, pp. 1595-1601Kobayashi, E., Bagshaw, A.P., Benar, C.G., Aghakhani, Y., Andermann, F., Dubeau, F., Temporal and extratemporal BOLD responses to temporal lobe interictal spikes (2006) Epilepsia, 47, pp. 343-354Mintzer, S., Cendes, F., Soss, J., Andermann, F., Engel Jr, J., Dubeau, F., Unilateral hippocampal sclerosis with contralateral temporal scalp ictal onset (2004) Epilepsia, 45, pp. 792-80