865 research outputs found
Charge Transport Properties of a Metal-free Phthalocyanine Discotic Liquid Crystal
Discotic liquid crystals can self-align to form one-dimensional
semiconducting wires, many tens of microns long. In this letter, we describe
the preparation of semiconducting films where the stacking direction of the
disc-like molecules is perpendicular to the substrate surface. We present
measurements of the charge carrier mobility, applying temperature-dependent
time-of-flight transient photoconductivity, space-charge limited current
measurements, and field-effect mobility measurements. We provide experimental
verification of the highly anisotropic nature of semiconducting films of
discotic liquid crystals, with charge carrier mobilities of up to
2.8x10cm/Vs. These properties make discotics an interesting choice
for applications such as organic photovoltaics.Comment: 5 pages, 5 figure
Electron-electron interaction in multiwall carbon nanotubes
Magnetic susceptibility of pristine and brominated arc-produced sample
of multiwall carbon nanotubes was measured from 4.2 to 400 K. An additional
contribution to diamagnetic susceptibility of carbon
nanotubes was found at T 50 K for both samples. It is shown that are dominated by quantum correction to for interaction
electrons (interaction effects-IE). The IE shows a crossover from
two-dimensional to three-dimensional at = 5.5 T. The effective interaction
between electrons for interior layers of nanotubes are repulsion and the
electron-electron interaction was estimated to be 0.26.Comment: 10 pages, 7 figure
In-plane magnetic field-induced spin polarization and transition to insulating behavior in two-dimensional hole systems
Using a novel technique, we make quantitative measurements of the spin
polarization of dilute (3.4 to 6.8*10^{10} cm^{-2}) GaAs (311)A two-dimensional
holes as a function of an in-plane magnetic field. As the field is increased
the system gradually becomes spin polarized, with the degree of spin
polarization depending on the orientation of the field relative to the crystal
axes. Moreover, the behavior of the system turns from metallic to insulating
\textit{before} it is fully spin polarized. The minority-spin population at the
transition is ~8*10^{9} cm^{-2}, close to the density below which the system
makes a transition to an insulating state in the absence of a magnetic field.Comment: 4 pages with figure
Magnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit
We report on the magnetic field (0TT) dependence of the
longitudinal thermal conductivity of highly oriented pyrolytic
graphite in the temperature range 5 K 20 K for fields parallel to
the axis. We show that shows large oscillations in the
high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are
observed in the Hall resistance. With the measured longitudinal electrical
resistivity we show that the Wiedemann-Franz law is violated in the high-field
regime.Comment: 4 Figures, to be published in Physical Review B (2003
Non-monotonic magnetic field and density dependence of in-plane magnetoresistance in dilute two-dimensional holes in GaAs/AlGaAs
We studied low temperature (T=50mK) in-plane magnetoresistance of a dilute
two-dimensional hole system in GaAs/AlGaAs heterostructure that exhibits an
apparent metal-insulator transition. We found an anisotropic magnetoresistance,
which changes dramatically at high in-plane fields (B_{\parallel}\agt5T) as
the hole density is varied. At high densities where the system behaves metallic
at , the transverse magnetoresistance is larger than the
longitudinal magnetoresistance. With decreasing the hole density the difference
becomes progressively smaller, and at densities near the "critical" density and
lower, the longitudinal magnetoresistance becomes larger than the transverse
magnetoresistance
Thermoelectric properties of lead chalcogenide core-shell nanostructures
We present the full thermoelectric characterization of nanostructured bulk
PbTe and PbTe-PbSe samples fabricated from colloidal core-shell nanoparticles
followed by spark plasma sintering. An unusually large thermopower is found in
both materials, and the possibility of energy filtering as opposed to grain
boundary scattering as an explanation is discussed. A decreased Debye
temperature and an increased molar specific heat are in accordance with recent
predictions for nanostructured materials. On the basis of these results we
propose suitable core-shell material combinations for future thermoelectric
materials of large electric conductivities in combination with an increased
thermopower by energy filtering.Comment: 12 pages, 8 figure
L-SeqSleepNet: Whole-cycle Long Sequence Modelling for Automatic Sleep Staging
Human sleep is cyclical with a period of approximately 90 minutes, implying
long temporal dependency in the sleep data. Yet, exploring this long-term
dependency when developing sleep staging models has remained untouched. In this
work, we show that while encoding the logic of a whole sleep cycle is crucial
to improve sleep staging performance, the sequential modelling approach in
existing state-of-the-art deep learning models are inefficient for that
purpose. We thus introduce a method for efficient long sequence modelling and
propose a new deep learning model, L-SeqSleepNet, which takes into account
whole-cycle sleep information for sleep staging. Evaluating L-SeqSleepNet on
four distinct databases of various sizes, we demonstrate state-of-the-art
performance obtained by the model over three different EEG setups, including
scalp EEG in conventional Polysomnography (PSG), in-ear EEG, and around-the-ear
EEG (cEEGrid), even with a single EEG channel input. Our analyses also show
that L-SeqSleepNet is able to alleviate the predominance of N2 sleep (the major
class in terms of classification) to bring down errors in other sleep stages.
Moreover the network becomes much more robust, meaning that for all subjects
where the baseline method had exceptionally poor performance, their performance
are improved significantly. Finally, the computation time only grows at a
sub-linear rate when the sequence length increases.Comment: 9 pages, 4 figures, updated affiliation
Low-field magnetoresistance in GaAs 2D holes
We report low-field magnetotransport data in two-dimensional hole systems in
GaAs/AlGaAs heterostructures and quantum wells, in a large density range, cm, with primary focus on
samples grown on (311)A GaAs substrates. At high densities, cm, we observe a remarkably strong positive magnetoresistance.
It appears in samples with an anisotropic in-plane mobility and predominantly
along the low-mobility direction, and is strongly dependent on the
perpendicular electric field and the resulting spin-orbit interaction induced
spin-subband population difference. A careful examination of the data reveals
that the magnetoresistance must result from a combination of factors including
the presence of two spin-subbands, a corrugated quantum well interface which
leads to the mobility anisotropy, and possibly weak anti-localization. None of
these factors can alone account for the observed positive magnetoresistance. We
also present the evolution of the data with density: the magnitude of the
positive magnetoresistance decreases with decreasing density until, at the
lowest density studied ( cm), it vanishes and is
replaced by a weak negative magnetoresistance.Comment: 8 pages, 8 figure
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