347 research outputs found
Computer-supported Exploration of a Categorical Axiomatization of Modeloids
A modeloid, a certain set of partial bijections, emerges from the idea to
abstract from a structure to the set of its partial automorphisms. It comes
with an operation, called the derivative, which is inspired by
Ehrenfeucht-Fra\"iss\'e games. In this paper we develop a generalization of a
modeloid first to an inverse semigroup and then to an inverse category using an
axiomatic approach to category theory. We then show that this formulation
enables a purely algebraic view on Ehrenfeucht-Fra\"iss\'e games.Comment: 24 pages; accepted for conference: Relational and Algebraic Methods
in Computer Science (RAMICS 2020
Electrically driven photon emission from individual atomic defects in monolayer WS2.
Quantum dot-like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources
Resonant and bound states of charged defects in two-dimensional semiconductors
A detailed understanding of charged defects in two-dimensional semiconductors is needed for the development of ultrathin electronic devices. Here, we study negatively charged acceptor impurities in monolayer WS2 using a combination of scanning tunneling spectroscopy and large-scale atomistic electronic structure calculations. We observe several localized defect states of hydrogenic wave function character in the vicinity of the valence band edge. Some of these defect states are bound, while others are resonant. The resonant states result from the multivalley valence band structure of WS2, whereby localized states originating from the secondary valence band maximum at Γ hybridize with continuum states from the primary valence band maximum at K/K′. Resonant states have important consequences for electron transport as they can trap mobile carriers for several tens of picoseconds
Lithographically defined synthesis of transition metal dichalcogenides
Transition metal dichalcogenides (TMDs) promise to revolutionize optoelectronic applications. While monolayer exfoliation and vapor phase growth produce extremely high quality 2D materials, direct fabrication at wafer scale remains a significant challenge. Here, we present a method that we call ‘lateral conversion’, which enables the synthesis of patterned TMD structures, with control over the thickness down to a few layers, at lithographically predefined locations. In this method, chemical conversion of a metal-oxide film to TMD layers proceeds by diffusion of precursor propagating laterally between silica layers, resulting in structures where delicate chalcogenide films are protected from contamination or oxidation. Lithographically patterned WS2 structures were synthesized by lateral conversion and analyzed in detail by hyperspectral Raman imaging, scanning electron microscopy and transmission electron microscopy. The rate of conversion was investigated as a function of time, temperature, and thickness of the converted film. In addition, the process was extended to grow patterned MoS2, WSe2, MoSe2 structures, and to demonstrate unique WS2/SiO2 multilayer structures. We believe this method will be applicable to a variety of additional chalcogenide materials, and enable their incorporation into novel architectures and devices
Enlarged magnetic focusing radius of photoinduced ballistic currents
We exploit GaAs-based quantum point contacts as mesoscopic detectors to
analyze the ballistic flow of photogenerated electrons in a two-dimensional
electron gas at a perpendicular magnetic field. Whereas charge transport
experiments always measure the classical cyclotron radius, we show that this
changes dramatically when detecting the photoinduced non-equilibrium current in
magnetic fields. The experimentally determined radius of the trajectories
surprisingly exceeds the classical cyclotron value by far. Monte Carlo
simulations suggest electron-electron scattering as the underlying reason.Comment: pdf-file includes both main article and supplementary informatio
How Substitutional Point Defects in Two-Dimensional WS Induce Charge Localization, Spin-Orbit Splitting, and Strain
Control of impurity concentrations in semiconducting materials is essential
to device technology. Because of their intrinsic confinement, the properties of
two-dimensional semiconductors such as transition metal dichalcogenides (TMDs)
are more sensitive to defects than traditional bulk materials. The
technological adoption of TMDs is dependent on the mitigation of deleterious
defects and guided incorporation of functional foreign atoms. The first step
towards impurity control is the identification of defects and assessment of
their electronic properties. Here, we present a comprehensive study of point
defects in monolayer tungsten disulfide (WS) grown by chemical vapor
deposition (CVD) using scanning tunneling microscopy/spectroscopy, CO-tip
noncontact atomic force microscopy, Kelvin probe force spectroscopy, density
functional theory, and tight-binding calculations. We observe four different
substitutional defects: chromium (Cr) and molybdenum
(Mo) at a tungsten site, oxygen at sulfur sites in both bottom and
top layers (O top/bottom), as well as two negatively charged
defects (CDs). Their electronic fingerprints unambiguously corroborate the
defect assignment and reveal the presence or absence of in-gap defect states.
The important role of charge localization, spin-orbit coupling, and strain for
the formation of deep defect states observed at substitutional defects in
WS as reported here will guide future efforts of targeted defect
engineering and doping of TMDs
An Integrated TCGA Pan-Cancer Clinical Data Resource to Drive High-Quality Survival Outcome Analytics
For a decade, The Cancer Genome Atlas (TCGA) program collected clinicopathologic annotation data along with multi-platform molecular profiles of more than 11,000 human tumors across 33 different cancer types. TCGA clinical data contain key features representing the democratized nature of the data collection process. To ensure proper use of this large clinical dataset associated with genomic features, we developed a standardized dataset named the TCGA Pan-Cancer Clinical Data Resource (TCGA-CDR), which includes four major clinical outcome endpoints. In addition to detailing major challenges and statistical limitations encountered during the effort of integrating the acquired clinical data, we present a summary that includes endpoint usage recommendations for each cancer type. These TCGA-CDR findings appear to be consistent with cancer genomics studies independent of the TCGA effort and provide opportunities for investigating cancer biology using clinical correlates at an unprecedented scale. Analysis of clinicopathologic annotations for over 11,000 cancer patients in the TCGA program leads to the generation of TCGA Clinical Data Resource, which provides recommendations of clinical outcome endpoint usage for 33 cancer types
Control of prostate cancer associated with withdrawal of a supplement containing folic acid, L-methyltetrahydrofolate and vitamin B12: a case report
<p>Abstract</p> <p>Introduction</p> <p>This is the first report of possible direct stimulation of hormone-resistant prostate cancer or interference of docetaxel cytotoxicity of prostate cancer in a patient with biochemical relapse of prostatic-specific antigen. This observation is of clinical and metabolic importance, especially at a time when more than 80 countries have fortified food supplies with folic acid and some contemplate further fortification with vitamin B<sub>12</sub>.</p> <p>Case presentation</p> <p>Our patient is a 71-year-old Caucasian man who had been diagnosed in 1997 with prostate cancer, stage T1c, and Gleason score 3+4 = 7. His primary treatment included intermittent androgen deprivation therapy including leuprolide + bicalutamide + deutasteride, ketoconazole + hydrocortisone, nilandrone and flutamide to resistance defined as biochemical relapse of PSA. While undergoing docetaxel therapy to treat a continually increasing prostate-specific antigen level, withdrawal of 10 daily doses of a supplement containing 500 μg of vitamin B<sub>12 </sub>as cyanocobalamin, as well as 400 μg of folic acid as pteroylglutamic acid and 400 μg of L-5-methyltetrahydrofolate for a combined total of 800 μg of mixed folates, was associated with a return to a normal serum prostatic-specific antigen level.</p> <p>Conclusion</p> <p>This case report illustrates the importance of the effects of supplements containing large amounts of folic acid, L-5-methyltetrahydrofolate, and cyanocobalamin on the metabolism of prostate cancer cells directly and/or B vitamin interference with docetaxel efficacy. Physicians caring for patients with prostate cancer undergoing watchful waiting, hormone therapy, and/or chemotherapy should consider the possible acceleration of tumor growth and/or metastasis and the development of drug resistance associated with supplement ingestion. We describe several pathways of metabolic and epigenetic interactions that could affect the observed changes in serum levels of prostate-specific antigen.</p
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