Electron Spin Resonance Study of Graphene

Graphene, a honeycomb lattice of a single atom thick plane of carbon atoms has captivated the attention of physicists, materials scientists, and engineers because of its extraordinary physical properties. These include exceedingly high charge carrier mobility, current-carrying capacity, thermal conductivity, and mechanical strength. These properties qualify it as an excellent candidate for new electronic technologies both within and beyond metal oxide semiconductors. The attractive properties of graphene for future applications are observed only in high quality graphene produced by mechanical exfoliation of graphite by using the so-called "scotch tape method". However, the yield is low and it satisfies only the needs of basic research. For applications one has to have material which is produced on large scale but preserves the quality of graphene. There are many production routes proposed in the scientific community and their number is still growing. One of the tasks of this PhD work was to test by Electron Spin Resonance (ESR) the quality of graphene produced by various methods. ESR is a suitable technique for looking for ferromagnetic interactions in graphene predicted by theory, and to determine the intrinsic spin relaxation time? These features are important for integration of graphene in spintronics applications. The main results are obtained on a large assembly of three differently prepared samples: 1. Mechanically exfoliated graphite (MEG), 2. Reduced graphene oxide (RGO) and 3. Liquid phase exfoliated (LPE) graphite. Attempts were done to study graphene synthesized by chemical vapor deposition (CVD) and by epitaxial growth on SiC. The mother compound graphite, as a reference sample was also thoroughly investigated by ESR. The LPG sample, obtained by heavy sonication of graphitic powder in NMP solvent, contains more nanographitic particles than graphene. The overall response resembles that of graphite with the exception of detecting a strong magnetic interaction below 26 K. This unambiguously shows the possibility of creating a strong magnetic response in carbon based material, but the challenge to inducing it graphene still remains. The RGO sample, prepared by chemical methods provides a large quantity of sample, suffers from the harsh oxidative and reduction steps involved in the synthesis. Both steps can be incomplete, leaving behind a large number of defects which Anderson-localize the extended states necessary for electronic applications. The best characteristics are obtained on MEG graphene. The temperature dependence of the magnetic susceptibility follows that predicted by theory, and it is distinctly different from that of graphite. Furthermore, the 10-8 s spin lifetime deduced from the ESR linewidth is by two orders of magnitude longer than that obtained in spin-valve experiments and it is encouraging for spintronics applications

Towards electron spin resonance of mechanically exfoliated graphene

We have attempted to prepare graphene samples by mechanical exfoliation of HOPG (highly oriented pyrolytic graphite) using scotch tape. Random testing of the flakes by AFM has shown in majority single layer graphene. Nevertheless, the presence of ultrathine graphite cannot be excluded in the large assembly of flakes needed for electron spin resonance (ESR) measurements. Graphene flakes sitting on ESR-silent scotch tapes were stacked parallel to form a multilayer sandwich. The ESR measurements performed in the 4-300 K range yielded narrow Lorentzian line. The spin susceptibility was decreasing linearly with decreasing temperature as expected for the conical band dispersion of graphene. Below 70 K the spin susceptibility started to deviate from the linear temperature dependence and a Curie-like behavior was observed. This contribution to the susceptibility is due to the existence of defects or impurities, which are in strong exchange coupling limit with conduction electrons. The temperature dependence of the linewidth suggests Elliott's mechanism for spin relaxation in graphene flakes. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Competitive ion-exchange of manganese and gadolinium in titanate nanotubes

Homogeneous Mn2+ and Gd3+ intercalation of scroll-type trititanate nanotubes using a post-synthesis ion exchange method is reported. Compared to Mn2+, Gd3+ ion-exchange shows larger saturation intercalation levels. Upon co-doping, weak interactions between the dopant ions were found to modify the incorporated concentrations. Electron spin resonance (ESR) measurements, performed at several frequencies, confirmed the homogeneous distribution of Mn2+ and Gd3+. Detailed simulation of ESR spectra identified a large spread of the local structural distortions of the occupied sites as a result of a wide range of curvature radii of the titanate nanotubes. (C) 2016 Elsevier B.V. All rights reserved

Unvailing the spin-phonon coupling in nanocrystalline BiFeO3 by resonant two-phonon Raman active modes

We report on temperature dependence of two-phonon Raman spectra in BiFeO3 nanocrystals, above and below the Neel temperature T-N using a resonant laser excitation line (lambda = 532 nm). Two-phonon modes exhibited anomalous frequency hardening and deviation from the anharmonic decay below T-N. Such behavior strongly supported the existence of spin-two-phonon interaction, because these modes are known to be very sensitive to the antiferromagnetic ordering. Within the mean-field theory for the nearest-neighbor interaction, the linear relationship between spin-spin correlation function and observed two-phonon frequency shift below T-N was obtained. This approach enabled to quantify the spin-phonon interaction by spin-phonon coupling strength for both two-phonon modes and justified the application of mean-field approach. Magnetic measurements revealed the coexistence of antiferromagnetic and weak ferromagnetic phases below T-N, which were found non competitive, additionally supporting the mean-field approach from which we deduced that the two-phonon modes in BiFeO3 are correlated with antiferromagnetic ordering below T-N.LQ

Le Grand écho du Nord de la France

09 janvier 19031903/01/09 (A85,N9).Appartient à l’ensemble documentaire : NordPdeC

Interplay between disorder and electronic correlations in compositionally complex alloys

Owing to their exceptional mechanical, electronic, and phononic transport properties, compositionally complex alloys, including high-entropy alloys, represent an important class of materials. However, the interplay between chemical disorder and electronic correlations, and its influence on electronic structure-derived properties, remains largely unexplored. This is addressed for the archetypal CrMnFeCoNi alloy using resonant and valence band photoemission spectroscopy, electrical resistivity, and optical conductivity measurements, complemented by linear response calculations based on density functional theory. Utilizing dynamical mean-field theory, correlation signatures and damping in the spectra are identified, highlighting the significance of many-body effects, particularly in states distant from the Fermi edge. Electronic transport remains dominated by disorder and potentially short-range order, especially at low temperatures, while visible-spectrum optical conductivity and high-temperature transport are influenced by short quasiparticle lifetimes. These findings improve our understanding of element-specific electronic correlations in compositionally complex alloys and facilitate the development of advanced materials with tailored electronic properties. Compositionally complex alloys have attracted significant attention recently, but the role of electronic correlations in these materials is unknown. Redka et al. study the CrMnFeCoNi alloy using a combination of experimental and theoretical techniques, revealing strong correlation effects far from the Fermi edge

Radiation damage and nuclear heating studies in selected functional materials during the JET DT campaign

A new Deuterium-Tritium campaign (DTE2) is planned at JET in the next years, with a proposed 14MeV neutron budget of 1.7×1021, which is nearly an order of magnitude higher than any previous DT campaigns. The neutron and gamma ray fields inside the JET device during DT plasma operations at specific locations have previously been evaluated. It is estimated that a total neutron fluence on the first wall of JET of up to 1020 n/m2 could be achieved, which is comparable to the fluence occurring in ITER at the end of life in the rear part of the port plug, where several diagnostic components will be located.The purpose of the present work is to evaluate the radiation damage and nuclear heating in selected functional materials to be irradiated at JET during DT plasma operation. These quantities are calculated with the use of the MCNP6 code and the FISPACT II code. In particular the neutron and gamma ray fields at specific locations inside the JET device, dedicated to material damage studies, were characterized. The emphasis is on a potential long term irradiation station located close to the first wall at outboard midplane, offering the opportunity to irradiate samples of functional materials used in ITER diagnostics, to assess the degradation of the physical properties. The radiation damage and the nuclear heating were calculated for selected materials irradiated in these positions and for the neutron flux and fluence expected in DTE2. The studied candidate functional materials include, among others, Sapphire, YAG, ZnS, Spinel, Diamond. In addition the activation of the internal irradiation holder itself was calculated with FISPACT. Damage levels in the range of 10-5 dpa were found

Neutronic analysis of JET external neutron monitor response

The power output of fusion devices is measured in terms of the neutron yield which relates directly to the fusion yield. JET made a transition from Carbon wall to ITER-Like Wall (Beryllium/Tungsten/Carbon) during 2010–11. Absolutely calibrated measurement of the neutron yield by JET neutron monitors was ensured by direct measurements using a calibrated 252Cf neutron source (NS) deployed by the in-vessel remote handling system (RHS) inside the JET vacuum vessel. Neutronic calculations were required in order to understand the neutron transport from the source in the vacuum vessel to the fission chamber detectors mounted outside the vessel on the transformer limbs of the tokamak. We developed a simplified computational model of JET and the JET RHS in Monte Carlo neutron transport code MCNP and analyzed the paths and structures through which neutrons reach the detectors and the effect of the JET RHS on the neutron monitor response. In addition we performed several sensitivity studies of the effect of substantial massive structures blocking the ports on the external neutron monitor response. As the simplified model provided a qualitative picture of the process only, some calculations were repeated using a more detailed full 3D model of the JET tokamak

Activation material selection for multiple foil activation detectors in JET TT campaign

In the preparation for the Deuterium-Tritium campaign, JET will operate with a tritium plasma. The T + T reaction consists of two notable channels: (1) T + T -> He-4 + 2n, (2) T + T -> He-5 + n -> He-4 + 2n. The reaction channel (1) is the reaction with the highest branching ratio and a continuum of neutron energies being produced. Reaction channel (2) produces a spectrum with a peak at 8.8 MeV. A particular problem is the ratio between the individual TT reaction channels, which is highly dependent on the energy of the reacting tritium ions. There are very few measurements on the TT spectrum and the study at JET would be interesting. The work is focused on the determination of the spectral characteristics in the TT plasma discharges, especially on the presence of the 8.8 MeV peak, a consequence of channel (2) of the TT reaction. The possibility to use an optimized set of activation materials in order to target the measurement of the 8.8 MeV peak is studied. The lower limit of detection for the channel (2) ratio within the TT reaction is estimated and the influence of DT source neutrons, which are a consequence of deuterium traces in the plasma, is investigated