727 research outputs found
Crossover from Coulomb blockade to quantum Hall effect in suspended graphene nanoribbons
Suspended graphene nano-ribbons formed during current annealing of suspended
graphene flakes have been investigated experimentally. Transport measurements
show the opening of a transport gap around charge neutrality due to the
formation of "Coulomb islands", coexisting with quantum Hall conductance
plateaus appearing at moderate values of magnetic field . Upon increasing
, the transport gap is rapidly suppressed, and is taken over by a much
larger energy gap due to electronic correlations. Our observations show that
suspended nano-ribbons allow the investigation of phenomena that could not so
far be accessed in ribbons on SiO substrates.Comment: 5 pages and 5 figures, Accepted in Physical Review Letter
Threshold voltage and space charge in organic transistors
We investigate rubrene single-crystal field-effect transistors, whose
stability and reproducibility are sufficient to measure systematically the
shift in threshold voltage as a function of channel length and source-drain
voltage. The shift is due to space-charge transferred from the contacts, and
can be modeled quantitatively without free fitting parameters, using Poisson's
equation, and by assuming that the density of states in rubrene is that of a
conventional inorganic semiconductor. Our results demonstrate the consistency,
at the quantitative level, of a variety of recent experiments on rubrene
crystals, and show how the use of FET measurements can enable the determination
of microscopic parameters (e.g., the effective mass of charge carriers).Comment: 4 pages, 4 figure
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
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
On-Demand Spin-Orbit Interaction from Which-Layer Tunability in Bilayer Graphene
Spin-orbit interaction (SOI) that is gate-tunable over a broad range is
essential to exploiting novel spin phenomena. Achieving this regime has
remained elusive because of the weakness of the underlying relativistic
coupling and lack of its tunability in solids. Here we outline a general
strategy that enables exceptionally high tunability of SOI through creating a
which-layer spin-orbit field inhomogeneity in graphene multilayers. An external
transverse electric field is applied to shift carriers between the layers with
strong and weak SOI. Because graphene layers are separated by sub-nm scales,
exceptionally high tunability of SOI can be achieved through a minute carrier
displacement. A detailed analysis of the experimentally relevant case of
bilayer graphene on a semiconducting transition metal dichalchogenide substrate
is presented. In this system, a complete tunability of SOI amounting to its
ON/OFF switching can be achieved. New opportunities for spin control are
exemplified with electrically driven spin resonance and topological phases with
different quantized intrinsic valley Hall conductivities.Comment: 8 pages, 3 figure
A ballistic pn junction in suspended graphene with split bottom gates
We have developed a process to fabricate suspended graphene devices with
local bottom gates, and tested it by realizing electrostatically controlled pn
junctions on a suspended graphene mono-layer nearly 2 micrometers long.
Measurements as a function of gate voltage, magnetic field, bias, and
temperature exhibit characteristic Fabry-Perot oscillations in the cavities
formed by the pn junction and each of the contacts, with transport occurring in
the ballistic regime. Our results demonstrate the possibility to achieve a high
degree of control on the local electronic properties of ultra-clean suspended
graphene layers, a key aspect for the realization of new graphene
nanostructures.Comment: 10 pages, 4 figure
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