56 research outputs found
A CASE STUDY OF RANDOM FOREST IN PREDICTIVE DATA MINING
The paper examines the potential of a novel data mining method, the random forest classifier, to support managerial decision making in complex forecasting applications. A modelling paradigm is proposed that embraces a learning curve analysis and grid-search to analyse the model’s sensitivity towards the number of training examples and parameter settings, respectively, and, eventually, produce a final classifier with high predictive accuracy. The effectiveness of the approach is evidenced by experimental evaluation using the data of the 2008 data mining cup competition
SM2RAIN–ASCAT (2007–2018): global daily satellite rainfall data from ASCAT soil moisture observations
Abstract. Long-term gridded precipitation products are crucial for several
applications in hydrology, agriculture and climate sciences. Currently
available precipitation products suffer from space and time inconsistency
due to the non-uniform density of ground networks and the difficulties in
merging multiple satellite sensors. The recent "bottom-up" approach that
exploits satellite soil moisture observations for estimating rainfall
through the SM2RAIN (Soil Moisture to Rain) algorithm is suited to build a consistent rainfall data
record as a single polar orbiting satellite sensor is used. Here we exploit the Advanced SCATterometer (ASCAT) on board three Meteorological Operational (MetOp)
satellites, launched in 2006, 2012, and 2018, as part of the European Organisation for the Exploitation of
Meteorological Satellites (EUMETSAT) Polar
System programme. The continuity of the scatterometer sensor is ensured
until the mid-2040s through the MetOp Second Generation Programme. Therefore, by
applying the SM2RAIN algorithm to ASCAT soil moisture observations, a long-term
rainfall data record will be obtained, starting in 2007 and lasting until the mid-2040s. The
paper describes the recent improvements in data pre-processing, SM2RAIN
algorithm formulation, and data post-processing for obtaining the
SM2RAIN–ASCAT quasi-global (only over land) daily rainfall data record at a
12.5 km spatial sampling from 2007 to 2018. The quality of the SM2RAIN–ASCAT data record
is assessed on a regional scale through comparison with high-quality
ground networks in Europe, the United States, India, and Australia. Moreover, an
assessment on a global scale is provided by using the triple-collocation (TC)
technique allowing us also to compare these data with the latest, fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis
(ERA5), the Early Run version of the Integrated Multi-Satellite Retrievals
for Global Precipitation Measurement (IMERG), and the gauge-based Global
Precipitation Climatology Centre (GPCC) products. Results show that the SM2RAIN–ASCAT rainfall data record performs relatively
well at both a regional and global scale, mainly in terms of root mean square
error (RMSE) when compared to other products. Specifically, the SM2RAIN–ASCAT data
record provides performance better than IMERG and GPCC in data-scarce
regions of the world, such as Africa and South America. In these areas, we
expect larger benefits in using SM2RAIN–ASCAT for hydrological and
agricultural applications. The limitations of the SM2RAIN–ASCAT data record consist
of the underestimation of peak rainfall events and the presence of
spurious rainfall events due to high-frequency soil moisture fluctuations
that might be corrected in the future with more advanced bias correction
techniques. The SM2RAIN–ASCAT data record is freely available at
https://doi.org/10.5281/zenodo.3405563 (Brocca et al., 2019) (recently extended to the end of
August 2019)
Emergent Trion-Phonon Coupling in Atomically-Reconstructed MoSe-WSe Heterobilayers
In low-temperature resonant Raman experiments on MoSe-WSe
heterobilayers, we identify a hybrid interlayer shear mode (HSM) with an
energy, close to the interlayer shear mode (SM) of the heterobilayers, but with
a much broader, asymmetric lineshape. The HSM shows a pronounced resonance with
the intralayer hybrid trions (HX) of the MoSe and WSe layers, only.
No resonance with the neutral intralayer excitons is found. First-principles
calculations reveal a strong coupling of Q-valley states, which are delocalized
over both layers and participate in the HX, with the SM. This emerging
trion-phonon coupling may be relevant for experiments on gate-controlled
heterobilayers.Comment: 6 pages, 3 figure
"What's (the) Matter?", A Show on Elementary Particle Physics with 28 Demonstration Experiments
We present the screenplay of a physics show on particle physics, by the
Physikshow of Bonn University. The show is addressed at non-physicists aged 14+
and communicates basic concepts of elementary particle physics including the
discovery of the Higgs boson in an entertaining fashion. It is also
demonstrates a successful outreach activity heavily relying on the university
physics students. This paper is addressed at anybody interested in particle
physics and/or show physics. This paper is also addressed at fellow physicists
working in outreach, maybe the experiments and our choice of simple
explanations will be helpful. Furthermore, we are very interested in related
activities elsewhere, in particular also demonstration experiments relevant to
particle physics, as often little of this work is published.
Our show involves 28 live demonstration experiments. These are presented in
an extensive appendix, including photos and technical details. The show is set
up as a quest, where 2 students from Bonn with the aid of a caretaker travel
back in time to understand the fundamental nature of matter. They visit
Rutherford and Geiger in Manchester around 1911, who recount their famous
experiment on the nucleus and show how particle detectors work. They travel
forward in time to meet Lawrence at Berkeley around 1950, teaching them about
the how and why of accelerators. Next, they visit Wu at DESY, Hamburg, around
1980, who explains the strong force. They end up in the LHC tunnel at CERN,
Geneva, Switzerland in 2012. Two experimentalists tell them about colliders and
our heroes watch live as the Higgs boson is produced and decays. The show was
presented in English at Oxford University and University College London, as
well as Padua University and ICTP Trieste. It was 1st performed in German at
the Deutsche Museum, Bonn (5/'14). The show has eleven speaking parts and
involves in total 20 people.Comment: 113 pages, 88 figures. An up to date version of the paper with high
resolution pictures can be found at
http://www.th.physik.uni-bonn.de/People/dreiner/Downloads/. In v2 the
acknowledgements and a citation are correcte
SM2RAIN-ASCAT (2007–2018): global daily satellite rainfallfrom ASCAT soil moisture
Abstract. Long-term gridded precipitation products are crucial for several applications in hydrology, agriculture and climate sciences. Currently available precipitation products obtained from rain gauges, remote sensing and meteorological modelling suffer from space and time inconsistency due to non-uniform density of ground networks and the difficulties in merging multiple satellite sensors. The recent bottom up approach that uses satellite soil moisture observations for estimating rainfall through the SM2RAIN algorithm is suited to build long-term and consistent rainfall data record as a single polar orbiting satellite sensor is used. We exploit here the Advanced SCATterometer (ASCAT) on board three Metop satellites, launched in 2006, 2012 and 2018. The continuity of the scatterometer sensor on European operational weather satellites is ensured until mid-2040s through the Metop Second Generation Programme. By applying SM2RAIN algorithm to ASCAT soil moisture observations a long-term rainfall data record can be obtained, also operationally available in near real time. The paper describes the recent improvements in data pre-processing, SM2RAIN algorithm formulation, and data post-processing for obtaining the SM2RAIN-ASCAT global daily rainfall dataset at 12.5 km sampling (2007–2018). The quality of SM2RAIN-ASCAT dataset is assessed on a regional scale through the comparison with high-quality ground networks in Europe, United States, India and Australia. Moreover, an assessment on a global scale is provided by using the Triple Collocation technique allowing us also the comparison with other global products such as the latest European Centre for Medium-Range Weather Forecasts reanalysis (ERA5), the Global Precipitation Measurement (GPM) mission, and the gauge-based Global Precipitation Climatology Centre (GPCC) product. Results show that the SM2RAIN-ASCAT rainfall dataset performs relatively well both at regional and global scale, mainly in terms of root mean square error when compared to other datasets. Specifically, SM2RAIN-ASCAT dataset provides better performance better than GPM and GPCC in the data scarce regions of the world, such as Africa and South America. In these areas we expect the larger benefits in using SM2RAIN-ASCAT for hydrological and agricultural applications.The SM2RAIN-ASCAT dataset is freely available at https://doi.org/10.5281/zenodo.2591215
Absence of a giant spin Hall effect in plasma-hydrogenated graphene
The weak spin-orbit interaction in graphene was predicted to be increased, e.g., by hydrogenation. This should result in a sizable spin Hall effect (SHE). We employ two different methods to examine the spin Hall effect in weakly hydrogenated graphene. For hydrogenation we expose graphene to a hydrogen plasma and use Raman spectroscopy to characterize this method. We then investigate the SHE of hydrogenated graphene in the H-bar method and by direct measurements of the inverse SHE. Although a large nonlocal resistance can be observed in the H-bar structure, comparison with the results of the other method indicate that this nonlocal resistance has a non-spin-related origin
Large-Scale Mapping of Moiré Superlattices by Hyperspectral Raman Imaging.
Moiré superlattices can induce correlated-electronic phases in twisted van der Waals materials: strongly correlated quantum phenomena emerge, such as superconductivity and the Mott-insulating state. However, moiré superlattices produced through artificial stacking can be quite inhomogeneous, which hampers the development of a clear correlation between the moiré period and the emerging electrical and optical properties. Here, it is demonstrated in twisted-bilayer transition-metal dichalcogenides that low-frequency Raman scattering can be utilized not only to detect atomic reconstruction, but also to map out the inhomogeneity of the moiré lattice over large areas. The method is established based on the finding that both the interlayer-breathing mode and moiré phonons are highly susceptible to the moiré period and provide characteristic fingerprints. Hyperspectral Raman imaging visualizes microscopic domains of a 5° twisted-bilayer sample with an effective twist-angle resolution of about 0.1°. This ambient methodology can be conveniently implemented to characterize and preselect high-quality areas of samples for subsequent device fabrication, and for transport and optical experiments
Characterization of highly crystalline lead iodide nanosheets prepared by room-temperature solution processing
Two-dimensional (2D) semiconducting materials are particularly appealing for many applications. Although theory predicts a large number of 2D materials, experimentally only a few of these materials have been identified and characterized comprehensively in the ultrathin limit. Lead iodide, which belongs to the transition metal halides family and has a direct bandgap in the visible spectrum, has been known for a long time and has been well characterized in its bulk form. Nevertheless, studies of this material in the nanometer thickness regime are rather scarce. In this article we demonstrate an easy way to synthesize ultrathin, highly crystalline flakes of PbI2 by precipitation from a solution in water. We thoroughly characterize the produced thin flakes with different techniques ranging from optical and Raman spectroscopy to temperature-dependent photoluminescence and electron microscopy. We compare the results to ab initio calculations of the band structure of the material. Finally, we fabricate photodetectors based on PbI2 and study their optoelectronic properties.We acknowledge financial support from the European Commission under the Graphene Flagship (CNECTICT-604391), and European Research Council (ERC-StG-MINT 307609), the MINECO, the Comunidad de Madrid, the Netherlands Organisation for Scientific Research (NWO), and the German Science Foundation (DFG). JLL and JFR acknowledge financial support by Marie-Curie-ITN 607904-SPINOGRAPH. JFR acknowledges financial support from MEC-Spain (MAT2016-78625-C2)
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