726 research outputs found
Sample-specific and Ensemble-averaged Magnetoconductance of Individual Single-Wall Carbon Nanotubes
We discuss magnetotransport measurements on individual single-wall carbon
nanotubes with low contact resistance, performed as a function of temperature
and gate voltage. We find that the application of a magnetic field
perpendicular to the tube axis results in a large magnetoconductance of the
order of e^2/h at low temperature. We demonstrate that this magnetoconductance
consists of a sample-specific and of an ensemble-averaged contribution, both of
which decrease with increasing temperature. The observed behavior resembles
very closely the behavior of more conventional multi-channel mesoscopic wires,
exhibiting universal conductance fluctuations and weak localization. A
theoretical analysis of our experiments will enable to reach a deeper
understanding of phase-coherent one-dimensional electronic motion in SWNTs.Comment: Replaced with published version. Minor changes in tex
The CERN Detector Safety System for the LHC Experiments
The Detector Safety System (DSS), currently being developed at CERN under the
auspices of the Joint Controls Project (JCOP), will be responsible for assuring
the protection of equipment for the four LHC experiments. Thus, the DSS will
require a high degree of both availability and reliability. After evaluation of
various possible solutions, a prototype is being built based on a redundant
Siemens PLC front-end, to which the safety-critical part of the DSS task is
delegated. This is then supervised by a PVSS SCADA system via an OPC server.
The PLC front-end is capable of running autonomously and of automatically
taking predefined protective actions whenever required. The supervisory layer
provides the operator with a status display and with limited online
reconfiguration capabilities. Configuration of the code running in the PLCs
will be completely data driven via the contents of a "Configuration Database".
Thus, the DSS can easily adapt to the different and constantly evolving
requirements of the LHC experiments during their construction, commissioning
and exploitation phases.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 5 pages, PDF. PSN THGT00
Correlation between molecular orbitals and doping dependence of the electrical conductivity in electron-doped Metal-Phthalocyanine compounds
We have performed a comparative study of the electronic properties of six
different electron-doped metal phthalocyanine (MPc) compounds (ZnPc, CuPc,
NiPc, CoPc, FePc, and MnPc), in which the electron density is controlled by
means of potassium intercalation. In spite of the complexity of these systems,
we find that the nature of the underlying molecular orbitals produce observable
effects in the doping dependence of the electrical conductivity of the
materials. For all the MPc's in which the added electrons are expected to
occupy orbitals centered on the ligands (ZnPc, CuPc, and NiPc), the doping
dependence of the conductivity has an essentially identical shape. This shape
is different from that observed in MPc materials in which electrons are also
added to orbitals centered on the metal atom (CoPc, FePc, and MnPc). The
observed relation between the macroscopic electronic properties of the MPc
compounds and the properties of the molecular orbitals of the constituent
molecules, clearly indicates the richness of the alkali-doped
metal-phthalocyanines as a model class of compounds for the investigation of
the electronic properties of molecular systems
Double-gated graphene-based devices
We discuss transport through double gated single and few layer graphene
devices. This kind of device configuration has been used to investigate the
modulation of the energy band structure through the application of an external
perpendicular electric field, a unique property of few layer graphene systems.
Here we discuss technological details that are important for the fabrication of
top gated structures, based on electron-gun evaporation of SiO. We perform
a statistical study that demonstrates how --contrary to expectations-- the
breakdown field of electron-gun evaporated thin SiO films is comparable to
that of thermally grown oxide layers. We find that a high breakdown field can
be achieved in evaporated SiO only if the oxide deposition is directly
followed by the metallization of the top electrodes, without exposure to air of
the SiO layer.Comment: Replaced with revised version. To appear on New Journal of Physic
High-performance -type organic field-effect transistors with ionic liquid gates
High-performance -type organic field-effect transistors were developed
with ionic-liquid gates and N,N-bis(n-alkyl)-(1,7 and
1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide)s single-crystals. Transport
measurements show that these devices reproducibly operate in ambient atmosphere
with negligible gate threshold voltage and mobility values as high as 5.0
cm/Vs. These mobility values are essentially identical to those measured in
the same devices without the ionic liquid, using vacuum or air as the gate
dielectric. Our results indicate that the ionic-liquid and -type organic
semiconductor interfaces are suitable to realize high-quality -type organic
transistors operating at small gate voltage, without sacrificing electron
mobility
Possible evidence of extended objects inside the proton
Recent experimental determinations of the Nachtmann moments of the inelastic
structure function of the proton F2p(x, Q**2), obtained at Jefferson Lab, are
analyzed for values of the squared four-momentum transfer Q**2 ranging from ~
0.1 to ~ 2 (GeV/c)**2. It is shown that such inelastic proton data exhibit a
new type of scaling behavior and that the resulting scaling function can be
interpreted as a constituent form factor consistent with the elastic nucleon
data. These findings suggest that at low momentum transfer the inclusive proton
structure function originates mainly from the elastic coupling with extended
objects inside the proton. We obtain a constituent size of ~ 0.2 - 0.3 fm.Comment: 1 reference adde
Bias-dependent Contact Resistance in Rubrene Single-Crystal Field-Effect Transistors
We report a systematic study of the bias-dependent contact resistance in
rubrene single-crystal field-effect transistors with Ni, Co, Cu, Au, and Pt
electrodes. We show that the reproducibility in the values of contact
resistance strongly depends on the metal, ranging from a factor of two for Ni
to more than three orders of magnitude for Au. Surprisingly, FETs with Ni, Co,
and Cu contacts exhibits an unexpected reproducibility of the bias-dependent
differential conductance of the contacts, once this has been normalized to the
value measured at zero bias. This reproducibility may enable the study of
microscopic carrier injection processes into organic semiconductors.Comment: 4 pages, 4 figure
Electron-phonon and electron-electron interactions in organic field effect transistors
Recent experiments have demonstrated that the performances of organic FETs
strongly depend on the dielectric properties of the gate insulator. In
particular, it has been shown that the temperature dependence of the mobility
evolves from a metallic-like to an insulating behavior upon increasing the
dielectric constant of the gate material. This phenomenon can be explained in
terms of the formation of small polarons, due to the polar interaction of the
charge carriers with the phonons at the organic/dielectric interface. Building
on this model, the possible consequences of the Coulomb repulsion between the
carriers at high concentrations are analyzed.Comment: proceedings of the SMEC'07 conference, submitted to the Journal of
Physics and Chemistry of Solid
A negative mass theorem for surfaces of positive genus
We define the "sum of squares of the wavelengths" of a Riemannian surface
(M,g) to be the regularized trace of the inverse of the Laplacian. We normalize
by scaling and adding a constant, to obtain a "mass", which is scale invariant
and vanishes at the round sphere. This is an anlaog for closed surfaces of the
ADM mass from general relativity. We show that if M has positive genus then on
each conformal class, the mass attains a negative minimum. For the minimizing
metric, there is a sharp logarithmic Hardy-Littlewood-Sobolev inequality and a
Moser-Trudinger-Onofri type inequality.Comment: 8 page
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