204 research outputs found
Pavlov's dog associative learning demonstrated on synaptic-like organic transistors
In this letter, we present an original demonstration of an associative
learning neural network inspired by the famous Pavlov's dogs experiment. A
single nanoparticle organic memory field effect transistor (NOMFET) is used to
implement each synapse. We show how the physical properties of this dynamic
memristive device can be used to perform low power write operations for the
learning and implement short-term association using temporal coding and spike
timing dependent plasticity based learning. An electronic circuit was built to
validate the proposed learning scheme with packaged devices, with good
reproducibility despite the complex synaptic-like dynamic of the NOMFET in
pulse regime
OvA-INN: Continual Learning with Invertible Neural Networks
In the field of Continual Learning, the objective is to learn several tasks
one after the other without access to the data from previous tasks. Several
solutions have been proposed to tackle this problem but they usually assume
that the user knows which of the tasks to perform at test time on a particular
sample, or rely on small samples from previous data and most of them suffer of
a substantial drop in accuracy when updated with batches of only one class at a
time. In this article, we propose a new method, OvA-INN, which is able to learn
one class at a time and without storing any of the previous data. To achieve
this, for each class, we train a specific Invertible Neural Network to extract
the relevant features to compute the likelihood on this class. At test time, we
can predict the class of a sample by identifying the network which predicted
the highest likelihood. With this method, we show that we can take advantage of
pretrained models by stacking an Invertible Network on top of a feature
extractor. This way, we are able to outperform state-of-the-art approaches that
rely on features learning for the Continual Learning of MNIST and CIFAR-100
datasets. In our experiments, we reach 72% accuracy on CIFAR-100 after training
our model one class at a time.Comment: to be published in IJCNN 202
Four-point measurements of n- and p-type two-dimensional systems fabricated with cleaved-edge overgrowth
We demonstrate a contact design that allows four-terminal magnetotransport
measurements of cleaved-edge overgrown two-dimensional electron and hole
systems. By lithographically patterning and etching a bulk-doped surface layer,
finger-shaped leads are fabricated, which contact the two-dimensional systems
on the cleave facet. Both n- and p-type two-dimensional systems are
demonstrated at the cleaved edge, using Si as either donor or acceptor,
dependent on the growth conditions. Four-point measurements of both gated and
modulation-doped samples yield fractional quantum Hall features for both n- and
p-type, with several higher-order fractions evident in n-type modulation-doped
samples.Comment: 3 pages, 3 figure
Classical-to-stochastic Coulomb blockade cross-over in aluminum arsenide wires
We report low-temperature differential conductance measurements in aluminum
arsenide cleaved-edge overgrown quantum wires in the pinch-off regime. At zero
source-drain bias we observe Coulomb blockade conductance resonances that
become vanishingly small as the temperature is lowered below . We
show that this behavior can be interpreted as a classical-to-stochastic Coulomb
blockade cross-over in a series of asymmetric quantum dots, and offer a
quantitative analysis of the temperature-dependence of the resonances
lineshape. The conductance behavior at large source-drain bias is suggestive of
the charge density wave conduction expected for a chain of quantum dots.Comment: version 2: new figure 4, refined discussio
Stellar and thermal neutron capture cross section of ⁹Be
The neutron capture cross section of ⁹Be for stellar energies was measured via the activation technique using the Karlsruhe Van de Graaff accelerator in combination with accelerator mass spectrometry at the Vienna Environmental Research Accelerator. To characterize the energy region of interest for astrophysical
applications, activations were performed in a quasistellar neutron spectrum of kT = 25 keV and for a spectrum
at En = 473 ± 53 keV. Despite the very small cross section, the method used provided the required sensitivity for
obtaining fairly accurate results of 10.4 ± 0.6 and 8.4 ± 1.0 μb, respectively. With these data it was possible to
constrain the cross section shape up to the first resonances at 622 and 812 keV, thus allowing for the determination
of Maxwellian-averaged cross sections at thermal energies between kT = 5 and 100 keV. In addition, we report
a new experimental cross section value at thermal energy of σth = 8.31 ± 0.52 mb.This work was partly funded by the Austrian Science
Fund (FWF), Projects No. P20434 and No. I428, and by
the Australian Research Council, Projects No. DP140100136
and No. DP180100496
Optical excitations of a self assembled artificial ion
By use of magneto-photoluminescence spectroscopy we demonstrate bias
controlled single-electron charging of a single quantum dot. Neutral, single,
and double charged excitons are identified in the optical spectra. At high
magnetic fields one Zeeman component of the single charged exciton is found to
be quenched, which is attributed to the competing effects of tunneling and
spin-flip processes. Our experimental data are in good agreement with
theoretical model calculations for situations where the spatial extent of the
hole wave functions is smaller as compared to the electron wave functions.Comment: to be published in Physical Review B (rapid communication
High fidelity optical preparation and coherent Larmor precession of a single hole in an InGaAs quantum dot molecule
We employ ultrafast pump-probe spectroscopy with photocurrent readout to
directly probe the dynamics of a single hole spin in a single, electrically
tunable self-assembled quantum dot molecule formed by vertically stacking
InGaAs quantum dots. Excitons with defined spin configurations are initialized
in one of the two dots using circularly polarized picosecond pulses. The
time-dependent spin configuration is probed by the spin selective optical
absorption of the resulting few Fermion complex. Taking advantage of sub-5 ps
electron tunneling to an orbitally excited state of the other dot, we
initialize a single hole spin with a purity of >96%, i.e., much higher than
demonstrated in previous single dot experiments. Measurements in a lateral
magnetic field monitor the coherent Larmor precession of the single hole spin
with no observable loss of spin coherence within the ~300 ps hole lifetime.
Thereby, the purity of the hole spin initialization remains unchanged for all
investigated magnetic fields
Signatures of spin pairing in a quantum dot in the Coulomb blockade regime
Coulomb blockade resonances are measured in a GaAs quantum dot in which both
shape deformations and interactions are small. The parametric evolution of the
Coulomb blockade peaks shows a pronounced pair correlation in both position and
amplitude, which is interpreted as spin pairing. As a consequence, the
nearest-neighbor distribution of peak spacings can be well approximated by a
smeared bimodal Wigner surmise, provided that interactions which go beyond the
constant interaction model are taken into account.Comment: 5 pages, 3 figure
The visibility of IQHE at sharp edges: Experimental proposals based on interactions and edge electrostatics
The influence of the incompressible strips on the integer quantized Hall
effect (IQHE) is investigated, considering a cleaved-edge overgrown (CEO)
sample as an experimentally realizable sharp edge system. We propose a set of
experiments to clarify the distinction between the large-sample limit when bulk
disorder defines the IQHE plateau width and the small-sample limit smaller than
the disorder correlation length, when self-consistent edge electrostatics
define the IQHE plateau width. The large-sample or bulk QH regime is described
by the usual localization picture, whereas the small-sample or edge regime is
discussed within the compressible/incompressible strips picture, known as the
screening theory of QH edges. Utilizing the unusually sharp edge profiles of
the CEO samples, a Hall bar design is proposed to manipulate the edge potential
profile from smooth to extremely sharp. By making use of a side-gate
perpendicular to the two dimensional electron system, it is shown that the
plateau widths can be changed or even eliminated altogether. Hence, the
visibility of IQHE is strongly influenced when adjusting the edge potential
profile and/or changing the dc current direction under high currents in the
non-linear transport regime. As a second investigation, we consider two
different types of ohmic contacts, namely highly transmitting (ideal) and
highly reflecting (non-ideal) contacts. We show that if the injection contacts
are non-ideal, however still ohmic, it is possible to measure directly the
non-quantized transport taking place at the bulk of the CEO samples. The
results of the experiments we propose will clarify the influence of the edge
potential profile and the quality of the contacts, under quantized Hall
conditions.Comment: Substantially revised version of manuscript arXiv:0906.3796v1,
including new figures et
Nuclear data from AMS & nuclear data for AMS - some examples
We summarize some recent cross-section measurements using accelerator mass spectrometry (AMS). AMS represents an ultra-sensitive technique for measuring a limited, but steadily increasing number of longer-lived radionuclides. This method implies a two-step procedure with sample activation and subsequent AMS measurement. Applications include nuclear astrophysics, nuclear technology (nuclear fusion, nuclear fission and advanced reactor concepts and radiation dose estimations). A series of additional applications involves cosmogenic radionuclides in environmental, geological and extraterrestrial studies. Lack of information exists for a list of nuclides as pointed out by nuclear data requests. An overview of some recent measurements is given and the method is exemplified for some specific neutron-induced reactions.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard
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