31 research outputs found
2D layered transport properties from topological insulator BiSe single crystals and micro flakes
Low-field magnetotransport measurements of topological insulators such as
BiSe are important for revealing the nature of topological surface
states by quantum corrections to the conductivity, such as
weak-antilocalization. Recently, a rich variety of high-field magnetotransport
properties in the regime of high electron densities ( cm)
were reported, which can be related to additional two-dimensional layered
conductivity, hampering the identification of the topological surface states.
Here, we report that quantum corrections to the electronic conduction are
dominated by the surface states for a semiconducting case, which can be
analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the
case of strong spin-orbit interaction. However, in the metallic-like case this
analysis fails and additional two-dimensional contributions need to be
accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance
prove as strong indications for the two-dimensional layered metallic behavior.
Temperature-dependent magnetotransport properties of high-quality BiSe
single crystalline exfoliated macro and micro flakes are combined with high
resolution transmission electron microscopy and energy-dispersive x-ray
spectroscopy, confirming the structure and stoichiometry. Angle-resolved
photoemission spectroscopy proves a single-Dirac-cone surface state and a
well-defined bulk band gap in topological insulating state. Spatially resolved
core-level photoelectron microscopy demonstrates the surface stability.Comment: Sci. Rep. (2016
Electrical Transport Properties of Vanadium‐Doped Bi2Te2.4Se0.6
Vanadium‐doped Bi2–xTe2.4Se0.6 single crystals, with x = 0.015 and 0.03, are grown by the Bridgman method. Bandstructure characterization by angle‐resolved photoemission spectroscopy (ARPES) measurements shows gapless topological surface states for both vanadium concentrations. The Van‐der‐Pauw resistivity, the Hall charge carrier density, and the mobility in the temperature range from 0.3 to 300 K are strongly dependent on vanadium concentration, with carrier densities as low as 1.5 × 1016 cm−3 and mobilities as high as 570 cm2 V−1s−1. As expected for transport in gapless topological surface states, the resistivity, carrier density, and mobility are constant below 10 K. The magnetoresistance shows weak antilocalization for both vanadium concentrations in the same temperature range. The weak antilocalization is analyzed with the Hikami–Larkin–Nagaoka model, which yields phase‐coherence lengths of up to 250 nm for x = 0.015.Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Helmholtz-Gemeinschaft
http://dx.doi.org/10.13039/501100001656Peer Reviewe
Polymorphisms in Plasmodium falciparum chloroquine resistance transporter and multidrug resistance 1 genes: parasite risk factors that affect treatment outcomes for P. falciparum malaria after artemether-lumefantrine and artesunate-amodiaquine.
Adequate clinical and parasitologic cure by artemisinin combination therapies relies on the artemisinin component and the partner drug. Polymorphisms in the Plasmodium falciparum chloroquine resistance transporter (pfcrt) and P. falciparum multidrug resistance 1 (pfmdr1) genes are associated with decreased sensitivity to amodiaquine and lumefantrine, but effects of these polymorphisms on therapeutic responses to artesunate-amodiaquine (ASAQ) and artemether-lumefantrine (AL) have not been clearly defined. Individual patient data from 31 clinical trials were harmonized and pooled by using standardized methods from the WorldWide Antimalarial Resistance Network. Data for more than 7,000 patients were analyzed to assess relationships between parasite polymorphisms in pfcrt and pfmdr1 and clinically relevant outcomes after treatment with AL or ASAQ. Presence of the pfmdr1 gene N86 (adjusted hazards ratio = 4.74, 95% confidence interval = 2.29 - 9.78, P < 0.001) and increased pfmdr1 copy number (adjusted hazards ratio = 6.52, 95% confidence interval = 2.36-17.97, P < 0.001 : were significant independent risk factors for recrudescence in patients treated with AL. AL and ASAQ exerted opposing selective effects on single-nucleotide polymorphisms in pfcrt and pfmdr1. Monitoring selection and responding to emerging signs of drug resistance are critical tools for preserving efficacy of artemisinin combination therapies; determination of the prevalence of at least pfcrt K76T and pfmdr1 N86Y should now be routine
Neuroimaging-based classification of PTSD using data-driven computational approaches: A multisite big data study from the ENIGMA-PGC PTSD consortium
Background: Recent advances in data-driven computational approaches have been helpful in devising tools to objectively diagnose psychiatric disorders. However, current machine learning studies limited to small homogeneous samples, different methodologies, and different imaging collection protocols, limit the ability to directly compare and generalize their results. Here we aimed to classify individuals with PTSD versus controls and assess the generalizability using a large heterogeneous brain datasets from the ENIGMA-PGC PTSD Working group. Methods: We analyzed brain MRI data from 3,477 structural-MRI; 2,495 resting state-fMRI; and 1,952 diffusion-MRI. First, we identified the brain features that best distinguish individuals with PTSD from controls using traditional machine learning methods. Second, we assessed the utility of the denoising variational autoencoder (DVAE) and evaluated its classification performance. Third, we assessed the generalizability and reproducibility of both models using leave-one-site-out cross-validation procedure for each modality. Results: We found lower performance in classifying PTSD vs. controls with data from over 20 sites (60 % test AUC for s-MRI, 59 % for rs-fMRI and 56 % for D-MRI), as compared to other studies run on single-site data. The performance increased when classifying PTSD from HC without trauma history in each modality (75 % AUC). The classification performance remained intact when applying the DVAE framework, which reduced the number of features. Finally, we found that the DVAE framework achieved better generalization to unseen datasets compared with the traditional machine learning frameworks, albeit performance was slightly above chance. Conclusion: These results have the potential to provide a baseline classification performance for PTSD when using large scale neuroimaging datasets. Our findings show that the control group used can heavily affect classification performance. The DVAE framework provided better generalizability for the multi-site data. This may be more significant in clinical practice since the neuroimaging-based diagnostic DVAE classification models are much less site-specific, rendering them more generalizable
Quantum transport investigations of low-dimensional electron gases in AlxGa1-xAs/GaAs- and Bi2Se3-based materials
Die Transporteigenschaften eines Elektronengases mit reduzierter Dimensionalität
werden von den Welleneigenschaften der Elektronen bestimmt. Dies ermöglicht es,
verschiedene Quanteneffekte, wie Quanteninterferenz, zu beobachten.
Im ersten Teil dieser Arbeit werden geätzte Quantenringe und eindimensionale
(1D) Verengungen, basierend auf AlxGa1-xAs/GaAs-Heterostrukturen, hinsichtlich
ihrer Transporteigenschaften untersucht. Messungen des thermischen Rauschens
im Gleichgewichtszustand zeigen, dass der Erwartungswert mit den Rauschspektren
aller 1D Verengungen übereinstimmt, jedoch um bis zu 60 % bei allen Quantenringen
überschritten wird.
Rauschmessungen im thermischen Nichtgleichgewicht ergeben, dass der Wärmefluss
in Quantenringen mithilfe einer globalen Steuerelektrode (Topgate) an- und
ausgeschaltet werden kann. Die magnetische Widerstandsänderung der Quantenringe
zeigt Oszillationen, die dem Aharonov-Bohm-Effekt zugeordnet werden. Die
Beobachtbarkeit dieser Oszillationen hängt stark von dem Abkühlvorgang der Probe
ab und die Oszillationen zeigen Hinweise auf ein Schwebungsmuster sowie auf Phasenstarre.
Im zweiten Teil der Arbeit werden die Oberflächenzustände von exfolierten Bi2Se3
Mikroflocken untersucht. Für Mikroflocken mit metallischen Temperaturabhängigkeiten
des Widerstandes wurde schwache Anti-Lokalisierung beobachtet. Diese
Beobachtung deutet darauf hin, dass sich die magnetische Widerstandsänderung
weniger ausschließlich aus den 2D Oberflächenkanälen als vielmehr aus einem geschichtetem
Transport von 2D Kanälen im Volumenkörper zusammensetzt. Eine
Mikroflocke mit halbleitenden Eigenschaften zeigt keine Hinweise auf solch einen
geschichteten 2D Transport und es wird angenommen, dass ihre magnetische Widerstandsänderung
ausschließlich von den 2D Oberflächenzuständen verursacht wird.The transport properties of an electron gas with reduced dimensionality are dominated
by the electron’s wave nature. This allows to observe various quantum effects,
such as quantum interference.
In the first part of this thesis etched quantum rings and one-dimensional (1D)
constrictions, based on AlxGa1-xAs/GaAs heterostructures, are investigated with
respect to their transport properties. Thermal noise measurements in equilibrium
show that the expectation value agrees with the noise spectra of all 1D constrictions
but is exceeded by up to 60 % for the noise spectra of all quantum rings.
Noise measurements in thermal non-equilibrium reveal that the heat flow can
be switched on and off for a quantum ring by a global top-gate. The measured
magnetoresistance of the quantum rings shows oscillations that are attributed to
the Aharonov-Bohm effect. The observability of these oscillations strongly depends
on the cooling process of the sample and the oscillations show indications of a beating
as well as phase rigidity.
In the second part of the thesis the surface states of exfoliated Bi2Se3 microflakes
are studied. For microflakes that show a metallic temperature dependence of the
resistance weak anti-localization is observed. This observation suggests that the
magnetoresistance is a result of layered transport of 2D channels in the bulk rather
than just the surface 2D channels. A microflake with semiconducting characteristics
does not show indications of such a 2D layered transport and its magnetoresistance
is considered to be carried by the 2D surface states only
Electrical Transport Properties of Vanadium‐Doped Bi2Te2.4Se0.6
Vanadium‐doped Bi2–xTe2.4Se0.6 single crystals, with x = 0.015 and 0.03, are grown by the Bridgman method. Bandstructure characterization by angle‐resolved photoemission spectroscopy (ARPES) measurements shows gapless topological surface states for both vanadium concentrations. The Van‐der‐Pauw resistivity, the Hall charge carrier density, and the mobility in the temperature range from 0.3 to 300 K are strongly dependent on vanadium concentration, with carrier densities as low as 1.5 × 1016 cm−3 and mobilities as high as 570 cm2 V−1s−1. As expected for transport in gapless topological surface states, the resistivity, carrier density, and mobility are constant below 10 K. The magnetoresistance shows weak antilocalization for both vanadium concentrations in the same temperature range. The weak antilocalization is analyzed with the Hikami–Larkin–Nagaoka model, which yields phase‐coherence lengths of up to 250 nm for x = 0.015.Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Helmholtz-Gemeinschaft
http://dx.doi.org/10.13039/501100001656Peer Reviewe
Critical factors characterizing consumers’ intentions to use drones for last-mile delivery : does delivery risk matter?
Uncrewed Aerial Vehicle (UAV), commonly known as a drone, has become popular in military and recreational circles. Although their usage in commerce is relatively low, a continuous rise in commercial use, especially for last-mile delivery, in the future is anticipated. Consequently, there is a necessity for a greater understanding of consumers' readiness to accept the latest technological application to increase knowledge, business, and managerial practice. Specifically, this study aims to investigate consumers’ intentions to deploy drones for last-mile delivery. The study applies the social cognitive theory and the model of goal-directed behaviour. It investigates the effect of outcome expectancy, lifestyle compatibility, perceived self-efficacy, consumer attitude, and the desire of usage for delivery drones among European millennial consumers. Additionally, it examines whether delivery risk moderates the influence of attitude and desire to use delivery drones. The authors discovered that the aforementioned are positively related to consumer attitude. Consumer attitude as such is positively associated with the desire and intention to use this method of delivery. Furthermore, the intention to use drone delivery is positively influenced by the desire for this delivery, outcome expectancy, and lifestyle compatibility. These findings indicate the importance of desire and lifestyle compatibility as predictors. At the theoretical level, the results support the perspective that social cognitive theory, together with the model of goal-directed behaviour, is an adequate framework to account for consumer intentions.http://www.elsevier.com/locate/jretconser2023-07-09hj2022Gordon Institute of Business Science (GIBS