45 research outputs found
Strongly angle-dependent magnetoresistance in Weyl semimetals with long-range disorder
The chiral anomaly in Weyl semimetals states that the left- and right-handed
Weyl fermions, constituting the low energy description, are not individually
conserved, resulting, for example, in a negative magnetoresistance in such
materials. Recent experiments see strong indications of such an anomalous
resistance response; however, with a response that at strong fields is more
sharply peaked for parallel magnetic and electric fields than expected from
simple theoretical considerations. Here, we uncover a mechanism, arising from
the interplay between the angle-dependent Landau level structure and long-range
scalar disorder, that has the same phenomenology. In particular, we ana-
lytically show, and numerically confirm, that the internode scattering time
decreases exponentially with the angle between the magnetic field and the Weyl
node separation in the large field limit, while it is insensitive to this angle
at weak magnetic fields. Since, in the simplest approximation, the internode
scattering time is proportional to the anomaly-related conductivity, this
feature may be related to the experimental observations of a sharply peaked
magnetoresistance.Comment: 8 pages, 4 figure
Nodal-line semimetals from Weyl superlattices
The existence and topological classification of lower-dimensional Fermi
surfaces is often tied to the crystal symmetries of the underlying lattice
systems. Artificially engineered lattices, such as heterostructures and other
superlattices, provide promising avenues to realize desired crystal symmetries
that protect lower-dimensional Fermi surface, such as nodal lines. In this
work, we investigate a Weyl semimetal subjected to spatially periodic onsite
potential, giving rise to several phases, including a nodal-line semimetal
phase. In contrast to proposals that purely focus on lattice symmetries, the
emergence of the nodal line in this setup does not require small spin-orbit
coupling, but rather relies on its presence. We show that the stability of the
nodal line is understood from reflection symmetry and a combination of a
fractional lattice translation and charge-conjugation symmetry. Depending on
the choice of parameters, this model exhibits drumhead surface states that are
exponentially localized at the surface, or weakly localized surface states that
decay into the bulk at all energies.Comment: 11 pages, 8 figures, Editors' Suggestio
Thyroid Hormones Regulate Selenoprotein Expression and Selenium Status in Mice
Impaired expression of selenium-containing proteins leads to perturbed thyroid hormone (TH) levels, indicating the central importance of selenium for TH homeostasis. Moreover, critically ill patients with declining serum selenium develop a syndrome of low circulating TH and a central downregulation of the hypothalamus-pituitary-thyroid axis. This prompted us to test the reciprocal effect, i.e., if TH status would also regulate selenoprotein expression and selenium levels. To investigate the TH dependency of selenium metabolism, we analyzed mice expressing a mutant TH receptor α1 (TRα1+m) that confers a receptor-mediated hypothyroidism. Serum selenium was reduced in these animals, which was a direct consequence of the mutant TRα1 and not related to their metabolic alterations. Accordingly, hyperthyroidism, genetically caused by the inactivation of TRβ or by oral TH treatment of adult mice, increased serum selenium levels in TRα1+m and controls, thus demonstrating a novel and specific role for TRα1 in selenium metabolism. Furthermore, TH affected the mRNA levels for several enzymes involved in selenoprotein biosynthesis as well as serum selenoprotein P concentrations and the expression of other antioxidative selenoproteins. Taken together, our results show that TH positively affects the serum selenium status and regulates the expression of several selenoproteins. This demonstrates that selenium and TH metabolism are interconnected through a feed-forward regulation, which can in part explain the rapid parallel downregulation of both systems in critical illness
Landau levels, Bardeen polynomials and Fermi arcs in Weyl semimetals: the who's who of the chiral anomaly
Condensed matter systems realizing Weyl fermions exhibit striking
phenomenology derived from their topologically protected surface states as well
as chiral anomalies induced by electromagnetic fields. More recently,
inhomogeneous strain or magnetization were predicted to result in chiral
electric and magnetic fields, which modify and
enrich the chiral anomaly with additional terms. In this work, we develop a
lattice-based approach to describe the chiral anomaly, which involves Landau
and pseudo-Landau levels and treats all anomalous terms on equal footing, while
naturally incorporating Fermi arcs. We exemplify its potential by physically
interpreting the largely overlooked role of Fermi arcs in the covariant (Fermi
level) contribution to the anomaly and revisiting the factor of
difference between the covariant and consistent (complete band) contributions
to the term in the anomaly. Our framework
provides a versatile tool for the analysis of anomalies in realistic lattice
models as well as a source of simple physical intuition for understanding
strained and magnetized inhomogeneous Weyl semimetals.Comment: 10 pages, 7 figure
Characterization of pixelated silicon detectors for daily quality assurance measurements in proton therapy
The advanced imaging and delivery techniques in proton therapy allow
conformal high-dose irradiation of the target volume with high accuracy using
pencil beam scanning or beam shaping apertures. These irradiation methods
increasingly include small radiation fields with large dose gradients, which
require detector systems with high spatial resolution for quality assurance. In
addition the measurement of all success parameters for daily quality assurance
with only one proton field and one simple detector system would save a lot of
time in clinical usage. Based on their good spatial resolution and high rate
compatibility, pixelated silicon detectors could meet the new requirements. To
assess their applicability in proton therapy, ATLAS pixelated silicon detectors
are used to measure the lateral beam profile with high spatial resolution.
Furthermore, a dose dependent detector calibration is presented to allow the
measurement of the requested output constancy. A strategy to verify the proton
energy during the daily quality assurance is under study. Promising results
from proof-of-principle measurements at the West German Proton Therapy Centre
in Essen, Germany, have been obtained.Comment: 5 pages, 6 figures, accepted for publication in the proceedings of
TIPP 2021 to be published in Journal of Physics: Conference Serie
The radiosensitizing effect of platinum nanoparticles in proton irradiations is not caused by an enhanced proton energy deposition at the macroscopic scale
Objective. Due to the radiosensitizing effect of biocompatible noble metal nanoparticles (NPs), their administration is considered to potentially increase tumor control in radiotherapy. The underlying physical, chemical and biological mechanisms of the NPs' radiosensitivity especially when interacting with proton radiation is not conclusive. In the following work, the energy deposition of protons in matter containing platinum nanoparticles (PtNPs) is experimentally investigated. Approach. Surfactant-free monomodal PtNPs with a mean diameter of (40 ± 10) nm and a concentration of 300 μg ml−1, demonstrably leading to a substantial production of reactive oxygen species (ROS), were homogeneously dispersed into cubic gelatin samples serving as tissue-like phantoms. Gelatin samples without PtNPs were used as control. The samples' dimensions and contrast of the PtNPs were verified in a clinical computed tomography scanner. Fields from a clinical proton machine were used for depth dose and stopping power measurements downstream of both samples types. These experiments were performed with a variety of detectors at a pencil beam scanning beam line as well as a passive beam line with proton energies from about 56–200 MeV. Main results. The samples' water equivalent ratios in terms of proton stopping as well as the mean proton energy deposition downstream of the samples with ROS-producing PtNPs compared to the samples without PtNPs showed no differences within the experimental uncertainties of about 2%. Significance. This study serves as experimental proof that the radiosensitizing effect of biocompatible PtNPs is not due to a macroscopically increased proton energy deposition, but is more likely caused by a catalytic effect of the PtNPs. Thus, these experiments provide a contribution to the highly discussed radiobiological question of the proton therapy efficiency with noble metal NPs and facilitate initial evidence that the dose calculation in treatment planning is straightforward and not affected by the presence of sensitizing PtNPs
n-Doping of organic semiconductors for enhanced electron extraction from solution processed solar cells using alkali metals
To improve charge carrier injection into or extraction from organic optoelectronic devices, electrically doped layers are often employed. Whereas n-doping of organic semiconductors has been widely used in vacuum processed optoelectronic devices, adequate solution processes to enable future device printing are underdeveloped. In this work, we study n-doping of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) in a solution process, using sodium as the electron donor. Upon addition of elementary sodium to a clear TPBi/toluene solution, we observed a change in color, indicating charge carrier transfer between sodium and TPBi. The optical and electrical properties of doped and undoped TPBi were characterized in solution and in the corresponding thin-films. Electron Paramagnetic Resonance (EPR) measurements revealed an increase of the number of unpaired spins upon doping, indicating the presence of doping-induced charge carriers. Implementing TPBi:Na as electron extraction layers in organic solar cells, we found almost the same device performance as compared to state-of-the-art solar cells comprising zinc oxide electron extraction layers
Spermidine and resveratrol induce autophagy by distinct pathways converging on the acetylproteome
The acetylase inhibitor spermidine and the sirtuin-1 activator resveratrol disrupt the antagonistic network of acetylases and deacetylases that regulate autophagy
Visualizing the chiral anomaly in Dirac and Weyl semimetals with photoemission spectroscopy
Quantum anomalies are the breaking of a classical symmetry by quantum fluctuations. They dictate how physical systems of diverse nature, ranging from fundamental particles to crystalline materials, respond topologically to external perturbations, insensitive to local details. The anomaly paradigm was triggered by the discovery of the chiral anomaly that contributes to the decay of pions into photons and influences the motion of superfluid vortices in He3-A. In the solid state, it also fundamentally affects the properties of topological Weyl and Dirac semimetals, recently realized experimentally. In this work we propose that the most identifying consequence of the chiral anomaly, the charge density imbalance between fermions of different chirality induced by nonorthogonal electric and magnetic fields, can be directly observed in these materials with the existing technology of photoemission spectroscopy. With angle resolution, the chiral anomaly is identified by a characteristic note-shaped pattern of the emission spectra, originating from the imbalanced occupation of the bulk states and a previously unreported momentum dependent energy shift of the surface state Fermi arcs. We further demonstrate that the chiral anomaly likewise leaves an imprint in angle averaged emission spectra, facilitating its experimental detection. Thereby, our work provides essential theoretical input to foster the direct visualization of the chiral anomaly in condensed matter, in contrast to transport properties, such as negative magnetoresistance, which can also be obtained in the absence of a chiral anomaly.Peer reviewe