1,026 research outputs found
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
Photon-assisted electron transport in graphene
Photon-assisted electron transport in ballistic graphene is analyzed using
scattering theory. We show that the presence of an ac signal (applied to a gate
electrode in a region of the system) has interesting consequences on electron
transport in graphene, where the low energy dynamics is described by the Dirac
equation. In particular, such a setup describes a feasible way to probe energy
dependent transmission in graphene. This is of substantial interest because the
energy dependence of transmission in mesoscopic graphene is the basis of many
peculiar transport phenomena proposed in the recent literature. Furthermore, we
discuss the relevance of our analysis of ac transport in graphene to the
observability of zitterbewegung of electrons that behave as relativistic
particles (but with a lower effective speed of light).Comment: 5 pages, 2 figure
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
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
Competition between Spin-Orbit Interaction and Zeeman Coupling in Rashba 2DEGs
We investigate systematically how the interplay between Rashba spin-orbit
interaction and Zeeman coupling affects the electron transport and the spin
dynamics in InGaAs-based 2D electron gases. From the quantitative analysis of
the magnetoconductance, measured in the presence of an in-plane magnetic field,
we conclude that this interplay results in a spin-induced breaking of time
reversal symmetry and in an enhancement of the spin relaxation time. Both
effects, due to a partial alignment of the electron spin along the applied
magnetic field, are found to be in excellent agreement with recent theoretical
predictions.Comment: 4 figures and 4 page
Organic Single-Crystal Field-Effect Transistors
We present an overview of recent studies of the charge transport in the field
effect transistors on the surface of single crystals of organic
low-molecular-weight materials. We first discuss in detail the technological
progress that has made these investigations possible. Particular attention is
devoted to the growth and characterization of single crystals of organic
materials and to different techniques that have been developed for device
fabrication. We then concentrate on the measurements of the electrical
characteristics. In most cases, these characteristics are highly reproducible
and demonstrate the quality of the single crystal transistors. Particularly
noticeable are the small sub-threshold slope, the non-monotonic temperature
dependence of the mobility, and its weak dependence on the gate voltage. In the
best rubrene transistors, room-temperature values of as high as 15
cm/Vs have been observed. This represents an order-of-magnitude increase
with respect to the highest mobility previously reported for organic thin film
transistors. In addition, the highest-quality single-crystal devices exhibit a
significant anisotropy of the conduction properties with respect to the
crystallographic direction. These observations indicate that the field effect
transistors fabricated on single crystals are suitable for the study of the
\textit{intrinsic} electronic properties of organic molecular semiconductors.
We conclude by indicating some directions in which near-future work should
focus to progress further in this rapidly evolving area of research.Comment: Review article, to appear in special issue of Phys. Stat. Sol. on
organic semiconductor
Electrostatic confinement of electrons in graphene nano-ribbons
Coulomb blockade is observed in a graphene nanoribbon device with a top gate.
When two pn junctions are formed via the back gate and the local top gate,
electrons are confined between the pn junctions which act as the barriers. When
no pn junctions are induced by the gate voltages, electrons are still confined,
as a result of strong disorder, but in a larger area. Measurements on five
other devices with different dimensions yield consistent results.Comment: 4 figures, 1 table, 4.4page
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