286 research outputs found
Charge transfer and weak bonding between molecular oxygen and graphene zigzag edges at low temperatures
Electron paramagnetic resonance (EPR) study of air-physisorbed defective
carbon nano-onions evidences in favor of microwave assisted formation of
weakly-bound paramagnetic complexes comprising negatively-charged O2- ions and
edge carbon atoms carrying pi-electronic spins. These complexes being located
on the graphene edges are stable at low temperatures but irreversibly
dissociate at temperatures above 50-60 K. These EPR findings are justified by
density functional theory (DFT) calculations demonstrating transfer of an
electron from the zigzag edge of graphene-like material to oxygen molecule
physisorbed on the graphene sheet edge. This charge transfer causes changing
the spin state of the adsorbed oxygen molecule from S = 1 to S = 1/2 one. DFT
calculations show significant changes of adsorption energy of oxygen molecule
and robustness of the charge transfer to variations of the graphene-like
substrate morphology (flat and corrugated mono- and bi-layered graphene) as
well as edges passivation. The presence of H- and COOH- terminated edge carbon
sites with such corrugated substrate morphology allows formation of ZE-O2-
paramagnetic complexes characterized by small (<50 meV) binding energies and
also explains their irreversible dissociation as revealed by EPR.Comment: 28 pages, 8 figures, 2 tables, accepted in Carbon journa
Spin wave resonances in La_{0.7}Sr_{0.3}MnO_{3} films: measurement of spin wave stiffness and anisotropy field
We studied magnetic field dependent microwave absorption in epitaxial
LaSrMnO films using an X-band Bruker ESR spectrometer. By
analyzing angular and temperature dependence of the ferromagnetic and spin-wave
resonances we determine spin-wave stiffness and anisotropy field. The spin-wave
stiffness as found from the spectrum of the standing spin-wave resonances in
thin films is in fair agreement with the results of inelastic neutron
scattering studies on a single crystal of the same composition [Vasiliu-Doloc
et al., J. Appl. Phys. \textbf{83}, 7343 (1998)].Comment: 15 pages, 7 figures (now figure captions are included
Anomalous spin dynamics in charge ordered, 'two-electron' doped manganite Ca_0.9Ce_0.1MnO_3 : consequence of a spin-liquid phase?
The 'two-electron' doped rare earth mangnites Ca_1-x Ce_x MnO_3 (x = 0.1,0.2)
are probed using resistivity, ac susceptibility and electron paramagnetic
resonance (EPR) measurements across their respective charge ordering (CO)
temperatures T_CO = 173 K and 250 K. The EPR 'g' factor and intensity as well
as the transport and magnetic behaviours of the two compositions are
qualitatively similar and are as expected for CO systems. However, the EPR
linewidth, reflective of the spin dynamics, for x = 0.1, shows a strongly
anomalous temperature dependence, decreasing with decreasing temperature below
T_CO in contrast with the sample with x = 0.2 and other CO systems. Keeping in
view the evidence for magnetic frustration in the system, we propose that the
anomalous temperature dependence of the linewidth is the signature of the
occurrence of a disorder driven spin liquid phase, present along with charge
ordering.Comment: 19 pages including 4 figure
Improved Procedural, Hemodynamic, and Late Clinical Outcomes Using Intravascular Ultrasound Anatomic Guidance During Carotid Artery Stent-Angioplasty
Modification of the ground state in Sm-Sr manganites by oxygen isotope substitution
The effect of O O isotope substitution on electrical
resistivity and magnetic susceptibility of SmSrMnO manganites
is analyzed. It is shown that the oxygen isotope substitution drastically
affects the phase diagram at the crossover region between the ferromagnetic
metal state and that of antiferromagnetic insulator (0.4 0.6), and
induces the metal-insulator transition at for = 0.475 and 0.5. The nature
of antiferromagnetic insulator phase is discussed.Comment: 4 pages, 3 eps figures, RevTeX, submitted to Phys. Rev. Let
High Temperature Treatment of Diamond Particles Toward Enhancement of Their Quantum Properties
Fluorescence of the negatively charged nitrogen-vacancy (NV-) center of diamond is sensitive to external electromagnetic fields, lattice strain, and temperature due to the unique triplet configuration of its spin states. Their use in particulate diamond allows for the possibility of localized sensing and magnetic-contrast-based differential imaging in complex environments with high fluorescent background. However, current methods of NV(-)production in diamond particles are accompanied by the formation of a large number of parasitic defects and lattice distortions resulting in deterioration of the NV(-)performance. Therefore, there are significant efforts to improve the quantum properties of diamond particles to advance the field. Recently it was shown that rapid thermal annealing (RTA) at temperatures much exceeding the standard temperatures used for NV(-)production can efficiently eliminate parasitic paramagnetic impurities and, as a result, by an order of magnitude improve the degree of hyperpolarization of(13)C via polarization transfer from optically polarized NV(-)centers in micron-sized particles. Here, we demonstrate that RTA also improves the maximum achievable magnetic modulation of NV(-)fluorescence in micron-sized diamond by about 4x over conventionally produced diamond particles endowed with NV-. This advancement can continue to bridge the pathway toward developing nano-sized diamond with improved qualities for quantum sensing and imaging
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