316 research outputs found
Phosphatogenèse en atolls polynésiens : la filiation mattes cyanobactériennes-phosphorites
Des mattes cyanobactériennes épaisses (kopara) sont présentes dans certaines mares et lagons saumâtres des atolls polynésiens. La matière organique ainsi accumulée est préservée grâce à l'anoxie du milieu. La productivité élevée de ces mattes microbiennes repose sur la richesse en seuls nutritifs du milieu interstitiel des carbonates sous-jacents, et procède donc du fonctionnement global des atolls par endo-upwelling géothermique (Rougerie et Wauthy, 1986, 1993). Des analyses chimiques ainsi que des observations en microscopie électronique d'échantillons de phosphates insulaires ont été effectuées, parallèlement à une analyse comparative kopara-phosphate menée sur la fraction hydrocarbures de la matière organique. Les résultats indiquent une relation causale entre l'évolution diagénétique des mattes algaires (kopara) et la précipitation in situ des apatites ... (D'après résumé d'auteur
A tunable, dual mode field-effect or single electron transistor
A dual mode device behaving either as a field-effect transistor or a single
electron transistor (SET) has been fabricated using silicon-on-insulator metal
oxide semiconductor technology. Depending on the back gate polarisation, an
electron island is accumulated under the front gate of the device (SET regime),
or a field-effect transistor is obtained by pinching off a bottom channel with
a negative front gate voltage. The gradual transition between these two cases
is observed. This dual function uses both vertical and horizontal tunable
potential gradients in non-overlapped silicon-on-insulator channel
Thermionic charge transport in CMOS nano-transistors
We report on DC and microwave electrical transport measurements in
silicon-on-insulator CMOS nano-transistors at low and room temperature. At low
source-drain voltage, the DC current and RF response show signs of conductance
quantization. We attribute this to Coulomb blockade resulting from barriers
formed at the spacer-gate interfaces. We show that at high bias transport
occurs thermionically over the highest barrier: Transconductance traces
obtained from microwave scattering-parameter measurements at liquid helium and
room temperature is accurately fitted by a thermionic model. From the fits we
deduce the ratio of gate capacitance and quantum capacitance, as well as the
electron temperature
Full Current Statistics in Diffusive Normal-Superconductor Structures
We study the current statistics in normal diffusive conductors in contact
with a superconductor. Using an extension of the Keldysh Green's function
method we are able to find the full distribution of charge transfers for all
temperatures and voltages. For the non-Gaussian regime, we show that the
equilibrium current fluctuations are enhanced by the presence of the
superconductor. We predict an enhancement of the nonequilibrium current noise
for temperatures below and voltages of the order of the Thouless energy
E_Th=D/L^2. Our calculation fully accounts for the proximity effect in the
normal metal and agrees with experimental data. We demonstrate that the
calculation of the full current statistics is in fact simpler than a concrete
calculation of the noise.Comment: 4 pages, 2 figures (included
Peroxisome proliferator-activated receptor β activation promotes myonuclear accretion in skeletal muscle of adult and aged mice
We reported recently that peroxisome proliferator-activated receptor β (PPARβ) activation promotes a calcineurin-dependent exercise-like remodelling characterised by increased numbers of oxidative fibres and capillaries. As physical exercise also induces myonuclear accretion, we investigated whether PPARβ activation alters myonuclear density. Transgenic muscle-specific PPARβ over-expression induced 14% increase of myonuclear density. Pharmacological PPARβ activation promoted rapid and massive myonuclear accretion (20% increase after 48 h), which is dependent upon calcineurin/nuclear factor of activated T cells signalling pathway. In vivo bromodeoxyuridine labelling and proliferating cell nuclear antigen immunodetection revealed that PPARβ activation did not promote cell proliferation, suggesting that the PPARβ-promoted myonuclear accretion involves fusion of pre-existing muscle precursor cells to myofibres rather than cell division. Finally, we showed that in skeletal muscle, ageing led to a down-regulation of PPARβ accompanied by decrease of both oxidative fibre number and myonuclear density. PPARβ pharmacological activation counteracts, at least in part, the ageing-driven muscle remodelling
Nonlinear shot noise in mesoscopic diffusive normal-superconducting systems
We study differential shot noise in mesoscopic diffusive
normal-superconducting (NS) heterostructures at finite voltages where nonlinear
effects due to the superconducting proximity effect arise. A numerical
scattering-matrix approach is adopted. Through an NS contact, we observe that
the shot noise shows a reentrant dependence on voltage due to the
superconducting proximity effect but the differential Fano factor stays
approximately constant. Furthermore, we consider differential shot noise in the
structures where an insulating barrier is formed between normal and
superconducting regions and calculate the differential Fano factor as a
function of barrier height.Comment: 4 pages, 6 figure
Energy dependent counting statistics in diffusive superconducting tunnel junctions
We present an investigation of the energy dependence of the full charge
counting statistics in diffusive
normal-insulating-normal-insulating-superconducting junctions. It is found that
the current in general is transported via a correlated transfer of pairs of
electrons. Only in the case of strongly asymmetric tunnel barriers or energies
much larger than the Thouless energy is the pair transfer uncorrelated. The
second cumulant, the noise, is found to depend strongly on the applied voltage
and temperature. For a junction resistance dominated by the tunnel barrier to
the normal reservoir, the differential shot noise shows a double peak feature
at voltages of the order of the Thouless energy, a signature of an ensemble
averaged electron-hole resonance.Comment: 8 pages, 5 figure
Two-dimensional array of diffusive SNS junctions with high-transparent interfaces
We report the first comparative study of the properties of two-dimensional
arrays and single superconducting film - normal wire - superconducting film
(SNS) junctions. The NS interfaces of our SNS junctions are really high
transparent, for superconducting and normal metal parts are made from the same
material (superconducting polycrystalline PtSi film). We have found that the
two-dimensional arrays reveal some novel features: (i) the significant
narrowing of the zero bias anomaly (ZBA) in comparison with single SNS
junctions, (ii) the appearance of subharmonic energy gap structure (SGS), with
up to n=16 (eV=\pm 2\Delta/n), with some numbers being lost, (iii) the
transition from 2D logarithmic weak localization behavior to metallic one. Our
experiments show that coherent phenomena governed by the Andreev reflection are
not only maintained over the macroscopic scale but manifest novel pronounced
effects as well. The behavior of the ZBA and SGS in 2D array of SNS junctions
strongly suggests that the development of a novel theoretical approach is
needed which would self-consistently take into account the distribution of the
currents, the potentials, and the superconducting order parameter.Comment: RevTex, 5 pages, 5 figure
Shot Noise through a Quantum Dot in the Kondo Regime
The shot noise in the current through a quantum dot is calculated as a
function of voltage from the high-voltage, Coulomb blockaded regime to the
low-voltage, Kondo regime. Using several complementary approaches, it is shown
that the zero-frequency shot noise (scaled by the voltage) exhibits a
non-monotonic dependence on voltage, with a peak around the Kondo temperature.
Beyond giving a good estimate of the Kondo temperature, it is shown that the
shot noise yields additional information on the effects of electronic
correlations on the local density of states in the Kondo regime, unaccessible
in traditional transport measurements.Comment: 4 pages, 1 figur
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