63 research outputs found
Pressure screening in the interior of primary shells in double-wall carbon nanotubes
The pressure response of double-wall carbon nanotubes has been investigated
by means of Raman spectroscopy up to 10 GPa. The intensity of the radial
breathing modes of the outer tubes decreases rapidly but remain observable up
to 9 GPa, exhibiting a behavior similar (but less pronounced) to that of
single-wall carbon nanotubes, which undergo a shape distortion at higher
pressures. In addition, the tangential band of the external tubes broadens and
decreases in amplitude. The corresponding Raman features of the internal tubes
appear to be considerably less sensitive to pressure. All findings lead to the
conclusion that the outer tubes act as a protection shield for the inner tubes
whereas the latter increase the structural stability of the outer tubes upon
pressure application.Comment: PDF with 15 pages, 3 figures, 1 table; submitted to Physical Review
B NMR study of pure and lightly carbon doped MgB superconductors
We report a B NMR line shape and spin-lattice relaxation rate
() study of pure and lightly carbon doped MgBC for
, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe.
We show that while pure MgB exhibits the magnetic field distribution from
superposition of the normal and the Abrikosov state, slight replacement of
boron with carbon unveils the magnetic field distribution of the pure Abrikosov
state. This indicates a considerable increase of with carbon doping
with respect to pure MgB. The spin-lattice relaxation rate
demonstrates clearly the presence of a coherence peak right below in pure
MgB, followed by a typical BCS decrease on cooling. However, at
temperatures lower than K strong deviation from the BCS behavior is
observed, probably from residual contribution of the vortex dynamics. In the
carbon doped systems both the coherence peak and the BCS temperature dependence
of weaken, an effect attributed to the gradual shrinking of the
hole cylinders of the Fermi surface with electron doping.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
Failure Processes in Embedded Monolayer Graphene under Axial Compression
Exfoliated monolayer graphene flakes were embedded in a polymer matrix and loaded under axial compression. By monitoring the shifts of the 2D Raman phonons of rectangular flakes of various sizes under load, the critical strain to failure was determined. Prior to loading care was taken for the examined area of the flake to be free of residual stresses. The critical strain values for first failure were found to be independent of flake size at a mean value of –0.60% corresponding to a yield stress up to -6 GPa. By combining Euler mechanics with a Winkler approach, we show that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1–2 nm. These results were compared with DFT computations performed on analogue coronene/PMMA oligomers and a reasonable agreement was obtained
muSR study of carbon-doped MgB2 superconductors
The evolution of the superconducting properties of the carbon-doped MgB2
superconductors, MgB(2-x)Cx (x= 0.02, 0.04, 0.06) have been investigated by the
transverse-field muon spin rotation (TF-muSR) technique. The low-temperature
depolarisation rate, sigma(0) at 0.6 T which is proportional to the second
moment of the field distribution of the vortex lattice decreases monotonically
with increasing electron doping and decreasing Tc. In addition, the temperature
dependence of sigma(T) has been analysed in terms of a two-gap model. The size
of the two superconducting gaps decreases linearly as the carbon content
increases, while the doping effect is more pronounced for the smaller gap
related to the 3D pi-sheets of the Fermi surface.Comment: 7 pages, 2 Figures, 1 Table, Europhys. Lett. in pres
Biaxial strain tuning of exciton energy and polarization in monolayer WS2
We perform micro-photoluminescence and Raman experiments to examine the
impact of biaxial tensile strain on the optical properties of WS2 monolayers. A
strong shift on the order of -130 meV per % of strain is observed in the
neutral exciton emission at room temperature. Under near-resonant excitation we
measure a monotonic decrease in the circular polarization degree under applied
strain. We experimentally separate the effect of the strain-induced energy
detuning and evaluate the pure effect coming from biaxial strain. The analysis
shows that the suppression of the circular polarization degree under biaxial
strain is related to an interplay of energy and polarization relaxation
channels as well as to variations in the exciton oscillator strength affecting
the long-range exchange interaction.Comment: 29 pages, 11 figure
Retention of Two-Band Superconductivity in Highly Carbon-Doped MgB2
Tunneling data on MgB_{1.8}C_{0.2} show a reduction in the energy gap of the
pi-bands by a factor of two from undoped MgB2 that is consistent with the Tc
reduction, but inconsistent with the expectations of the dirty limit.
Dirty-limit theory for undoped MgB2 predicts a single gap about three times
larger than measured and a reduced Tc comparable to that measured. Our
heavily-doped samples exhibit a uniform dispersion of C suggestive of
significantly enhanced scattering, and we conclude that the retention of
two-band superconductivity in these samples is caused by a selective
suppression of interband scattering.Comment: 4 pages, 4 figures; added one figure, added one reference, minor
changes to the text, manuscript accepted for publication as a Phys. Rev. B
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