85 research outputs found
Nucleation and growth of single wall carbon nanotubes
Abstract The nucleation and growth of single wall carbon nanotubes from a carbon-saturated catalytic particle surrounded by a single sheet of graphene is described qualitatively by using a very restricted number of elementary processes, namely Stone-Wales defects and carbon bi-interstitials. Energies of the different configurations are estimated by using a Tersoff energy minimization scheme. Such a description is compatible with a broad variety of size or helicity of the tubes. Several mechanisms of growth of the embryos are considered: one of them is made more favourable when the tubes embryos are arranged in a hexagonal network in the graphene plane. All the proposed mechanisms can be indefinitely repeated for the growth of the nanotubes.
Nucleation and growth of single wall carbon nanotubes
The nucleation and growth of single wall carbon nanotubes from a
carbon-saturated catalytic particle surrounded by a single sheet of graphene is
described qualitatively by using a very restricted number of elementary
processes, namely Stone-Wales defects and carbon bi-interstitials. Energies of
the different configurations are estimated by using a Tersoff energy
minimization scheme. Such a description is compatible with a broad variety of
size or helicity of the tubes. Several mechanisms of growth of the embryos are
considered: one of them is made more favourable when the tubes embryos are
arranged in an hexagonal network in the graphene plane. All the proposed
mechanisms can be indefinitely repeated for the growth of the nanotubes.Comment: Solid state communications, in pres
The low-field conductivity of zeolite-encapsulated molecular wires
The first measurements of an upper bound for the low-field conductivity of a molecular wire are
presented here. We were able to encapsulate polypyrrole with chain lengths more than 10 monomers
within the channels of different zeolites. Although the chains are fully oxidized by intrazeolite Fe3 +
ions, and should conduct (when included in a bulk polymer), they do not exhibit, in the zeolite,
significant ac conductivity up to 1 GHz. This suggests that other strategies than low field conductivity
are needed to inject charges and transmit information through isolated molecular wires
Coupled Superconducting Phase and Ferromagnetic Order Parameter Dynamics
Via a direct coupling between the magnetic order parameter and the singlet
Josephson supercurrent, we detect spin-wave resonances, and their dispersion,
in ferromagnetic Josephson junctions in which the usual insulating or metallic
barrier is replaced with a weak ferromagnet. The coupling arises within the
Fraunhofer interferential description of the Josephson effect, because the
magnetic layer acts as a time dependent phase plate. A spin-wave resonance at a
frequency ws implies a dissipation that is reflected as a depression in the
current-voltage curve of the Josephson junction when hbar ws = 2eV. We have
thereby performed a resonance experiment on only 10^7 Ni atoms.Comment: 4 pages, 4 figure
Cubic optical nonlinearity of free electrons in bulk gold
A fast (τresponse <90 fs) free-electron spin-flipping frequency-degenerate nonlinearity with a significant value of |χ(3)xxyy(ω,ω,ω,-ω) χ(3)xyyx(ω,ω,ω,-ω)| ~ 10-8 esu has been observed in bulk gold at 1260 nm by use of a new pump-probe polarization-sensitive technique. <br/
Spin injection and spin accumulation in all-metal mesoscopic spin valves
We study the electrical injection and detection of spin accumulation in
lateral ferromagnetic metal-nonmagnetic metal-ferromagnetic metal (F/N/F) spin
valve devices with transparent interfaces. Different ferromagnetic metals,
permalloy (Py), cobalt (Co) and nickel (Ni), are used as electrical spin
injectors and detectors. For the nonmagnetic metal both aluminium (Al) and
copper (Cu) are used. Our multi-terminal geometry allows us to experimentally
separate the spin valve effect from other magneto resistance signals such as
the anomalous magneto resistance (AMR) and Hall effects. We find that the AMR
contribution of the ferromagnetic contacts can dominate the amplitude of the
spin valve effect, making it impossible to observe the spin valve effect in a
'conventional' measurement geometry. In a 'non local' spin valve measurement we
are able to completely isolate the spin valve signal and observe clear spin
accumulation signals at T=4.2 K as well as at room temperature (RT). For
aluminum we obtain spin relaxation lengths (lambda_{sf}) of 1.2 mu m and 600 nm
at T=4.2 K and RT respectively, whereas for copper we obtain 1.0 mu m and 350
nm. The spin relaxation times tau_{sf} in Al and Cu are compared with theory
and results obtained from giant magneto resistance (GMR), conduction electron
spin resonance (CESR), anti-weak localization and superconducting tunneling
experiments. The spin valve signals generated by the Py electrodes (alpha_F
lambda_F=0.5 [1.2] nm at RT [T=4.2 K]) are larger than the Co electrodes
(alpha_F lambda_F=0.3 [0.7] nm at RT [T=4.2 K]), whereas for Ni (alpha_F
lambda_F<0.3 nm at RT and T=4.2 K) no spin signal is observed. These values are
compared to the results obtained from GMR experiments.Comment: 16 pages, 12 figures, submitted to PR
Phonon-induced spin relaxation of conduction electrons in aluminum
Spin-flip Eliashberg function and temperature-dependent spin
relaxation time are calculated for aluminum using realistic
pseudopotentials. The spin-flip electron-phonon coupling constant
is found to be . The calculations agree with experiments
validating the Elliott-Yafet theory and the spin-hot-spot picture of spin
relaxation for polyvalent metals.Comment: 4 pages; submitted to PR
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