205 research outputs found
Quantitative analysis of electronic transport through weakly-coupled metal/organic interfaces
Using single-crystal transistors, we have performed a systematic experimental
study of electronic transport through oxidized copper/rubrene interfaces as a
function of temperature and bias. We find that the measurements can be
reproduced quantitatively in terms of the thermionic emission theory for
Schottky diodes, if the effect of the bias-induced barrier lowering is
included. Our analysis emphasizes the role of the coupling between metal and
molecules, which in our devices is weak due to the presence of an oxide layer
at the surface of the copper electrodes.Comment: 4 pages, 3 figure
Doping Effect of Nano-Diamond on Superconductivity and Flux Pinning in MgB2
Doping effect of diamond nanoparticles on the superconducting properties of
MgB2 bulk material has been studied. It is found that the superconducting
transition temperature Tc of MgB2 is suppressed by the diamond-doping, however,
the irreversibility field Hirr and the critical current density Jc are
systematically enhanced. Microstructural analysis shows that the diamond-doped
MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50-100 nm)
with highly-dispersed and uniformly-distributed diamond nanoparticles (~10-20
nm) inside the grains. High density of dislocations and diamond nanoparticles
may take the responsibility for the enhanced flux pinning in the diamond-doped
MgB2.Comment: 16 pages, 6 figure
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
Evidence for Insulating Behavior in the Electric Conduction of (NH)KC Systems
Microwave study using cavity perturbation technique revealed that the
conductivity of antiferromagnet (NH)KRbC at 200K is
already 3-4 orders of magnitude smaller than those of superconductors,
KC and (NH)NaRbC, and that the antiferromagnetic
compounds are {\it insulators} below 250K without metal-insulator transitions.
The striking difference in the magnitude of the conductivity between these
materials strongly suggests that the Mott-Hubbard transition in the ammoniated
alkali fullerides is driven by a reduction of lattice symmetry from
face-centered-cubic to face-centered-orthorhombic, rather than by the magnetic
ordering.Comment: accepted for publication in PR
Synthesis and Stoichiometry of MgB2
The system MgxB2 has been investigated to investigate possible
nonstoichiometry in MgB2. When synthesized at 850oC, MgB2 is a line compound
with a possible Mg vacancy content of about 1%. Small changes in lattice
constants as a function of starting composition result from grain interaction
stresses, whose character is different in the Mg-rich, near-stoichiometric, and
Mg-deficient regimes. A small linear decrease of the superconducting transition
temperature, Tc, in the Mg-rich regime results from accidental impurity doping.Comment: Accepted for publication in Physica C. 24 pages, 7 figure
Effect of PVA doping on flux pinning in Bulk MgB2
The synthesis and characterization of PVA (Poly Vinyl Acetate) doped bulk
MgB2 superconductor is reported here. PVA is used as a Carbon source. PVA
doping effects made two distinguishable contributions: first enhancement of Jc
field performance and second an increase in Hc2 value, both because of carbon
incorporation into MgB2 crystal lattice. The susceptibility measurement reveals
that Tc decreased from 37 to 36 K. Lattice parameter a decreased from 3.085 A
to 3.081 A due to the partial substitution of Carbon at Boron site. PVA doped
sample exhibited the Jc values greater than 10^5 A/cm2 at 5 & 10 K at low
fields; which is almost 3 times higher than the pure one, while at high fields
the Jc is increased by an order of magnitude in comparison to pure MgB2. From
R(T)H measurements we found higher Tc values under magnetic field for doped
sample; indicating an increase in Hc2. Also the magnetization measurements
exhibited a significant enhancement in Hirr value. The improved performance of
PVA doped MgB2 can be attributed to the substitution of carbon at boron site in
parent MgB2 and the resulting impact on the carrier density and impurity
scattering. The improved flux pinning behavior could easily be seen from
reduced flux pinning force plots.Comment: 14 Pages of Text + Figs. To appear in Physica
Born Effective Charges and Infrared Response of LiBC
Calculations of the zone center optical mode frequencies (including LO-TO
splitting), Born effective charges Z for each atom,
dielectric constants and , and the dielectric
response in the infrared, using density functional linear response theory, are
reported. Calculated Raman modes are in excellent agreement with experimental
values (170 cm and 1170 cm), while it will require better
experimental data to clarify the infrared active mode frequencies. The Born
effective charges Z (i) have surprisingly different values
for B and C, and (ii) show considerable anisotropy. Relationships between the
effective charges and LO-TO splitting are discussed, and the predicted
reflectivity in the range 0 -- 1400 cm is presented. These results hold
possible implications for Li removal in LiBC, and C substition for B in
MgB.Comment: 6 pages, 3 figure
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.
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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