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A comparative study of the low temperature conductivity of an ensemble of multiwall
carbon nanotubes and semiconductor nanowires is presented. The
quasi one-dimensional samples are made in nanoporous
templates by electrodeposition and CVD growth. Three different
structures are studied in parallel: multiwall carbon nanotubes,
nanowires, and silicon nanowires. It is shown that the Coulomb blockade
regime dominates the electronic transport below 50 K, together with weak and
strong localization effects. In the Coulomb blockade regime, a
scaling law of the conductance measured as a function of the
temperature and the voltage is systematically observed. This allows
a single scaling parameter α to be defined. This parameter
accounts for the specific realization of the “disorder”, and plays
the role of a fingerprint for each sample. Correlations between
α and the conductance measured as a function of temperature
and voltage, as a function of the perpendicular magnetic field, and
as a function of the temperature and voltage in the localized regime
below 1 K have been performed. Three universal laws are reported.
They relate the coefficient α (1) to the normalized Coulomb
, (2) to the phase coherence length
, and (3) to the activation energy
. These observations suggest a description of the wires
and tubes in terms of a chain of quantum dots; the wires and tubes
break into a series of islands. The quantum dots are defined by
conducting islands with a typical length on the order of the phase
coherence length separated by poorly conducting regions (low
density of carriers or potential barriers due to defects). A
corresponding model is developed in order to put the three
universal laws in a common frame.
In Danes we replicated the 3′APOB–VNTR gene/longevity association study previously carried
out in Italians, by which the Small alleles (less than 35 repeats) had been identified as frailty alleles
for longevity. In Danes, neither genotype nor allele frequencies differed between centenarians and
20–64-year-old subjects. However, when Danish and Italian data were compared, a significant
difference (p = 0.0004) was found between the frequencies of Small alleles in youths, which
disappeared in centenarians (p = 0.290). Furthermore, the demographic-genetic approach revealed
in Danes a significant gene–sex interaction relevant to Long alleles (more than 37 repeats). The
different findings in Denmark and Italy suggest that gene/longevity associations are population-specific,
and heavily affected by the population-specific genetic and environmental history.
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