343 research outputs found
Surface site density, silicic acid retention and transport properties of compacted magnetite powder
International audienc
Equidistribution of Heegner Points and Ternary Quadratic Forms
We prove new equidistribution results for Galois orbits of Heegner points
with respect to reduction maps at inert primes. The arguments are based on two
different techniques: primitive representations of integers by quadratic forms
and distribution relations for Heegner points. Our results generalize one of
the equidistribution theorems established by Cornut and Vatsal in the sense
that we allow both the fundamental discriminant and the conductor to grow.
Moreover, for fixed fundamental discriminant and variable conductor, we deduce
an effective surjectivity theorem for the reduction map from Heegner points to
supersingular points at a fixed inert prime. Our results are applicable to the
setting considered by Kolyvagin in the construction of the Heegner points Euler
system
Iterative Phase Retrieval Algorithms for Scanning Transmission Electron Microscopy
Scanning transmission electron microscopy (STEM) has been extensively used
for imaging complex materials down to atomic resolution. The most commonly
employed STEM imaging modality of annular dark field produces
easily-interpretable contrast, but is dose-inefficient and produces little to
no contrast for light elements and weakly-scattering samples. An alternative is
to use phase contrast STEM imaging, enabled by high speed detectors able to
record full images of a diffracted STEM probe over a grid of scan positions.
Phase contrast imaging in STEM is highly dose-efficient, able to measure the
structure of beam-sensitive materials and even biological samples. Here, we
comprehensively describe the theoretical background, algorithmic implementation
details, and perform both simulated and experimental tests for three iterative
phase retrieval STEM methods: focused-probe differential phase contrast,
defocused-probe parallax imaging, and a generalized ptychographic gradient
descent method implemented in two and three dimensions. We discuss the
strengths and weaknesses of each of these approaches using a consistent
framework to allow for easier comparison. This presentation of STEM phase
retrieval methods will make these methods more approachable, reproducible and
more readily adoptable for many classes of samples.Comment: 25 pages, 11 figures, 1 tabl
Encoding multistate charge order and chirality in endotaxial heterostructures
Intrinsic resistivity changes associated with charge density wave (CDW) phase
transitions in 1T-TaS hold promise for non-volatile memory and computing
devices based on the principle of phase change memory (PCM). High-density PCM
storage is proposed for materials with multiple intermediate resistance states,
which have been observed in 1T-TaS. However, the metastability responsible
for this behavior makes the presence of multistate switching unpredictable in
1T-TaS devices. Here, we demonstrate the synthesis of nanothick
verti-lateral 1H-TaS/1T-TaS heterostructures in which the number of
endotaxial metallic 1H-TaS monolayers dictates the number of
high-temperature resistance transitions in 1T-TaS lamellae. Further, we
also observe optically active heterochirality in the CDW superlattice
structure, which is modulated in concert with the resistivity steps. This
thermally-induced polytype conversion nucleates at folds and kinks where
interlayer translations that relax local strain favorably align 1H and 1T
layers. This work positions endotaxial TaS heterostructures as prime
candidates for non-volatile device schemes implementing coupled switching of
structure, chirality, and resistance
Modular symbols in Iwasawa theory
This survey paper is focused on a connection between the geometry of
and the arithmetic of over global fields,
for integers . For over , there is an explicit
conjecture of the third author relating the geometry of modular curves and the
arithmetic of cyclotomic fields, and it is proven in many instances by the work
of the first two authors. The paper is divided into three parts: in the first,
we explain the conjecture of the third author and the main result of the first
two authors on it. In the second, we explain an analogous conjecture and result
for over . In the third, we pose questions for general
over the rationals, imaginary quadratic fields, and global function fields.Comment: 43 page
Denominators of Eisenstein cohomology classes for GL_2 over imaginary quadratic fields
We study the arithmetic of Eisenstein cohomology classes (in the sense of G.
Harder) for symmetric spaces associated to GL_2 over imaginary quadratic
fields. We prove in many cases a lower bound on their denominator in terms of a
special L-value of a Hecke character providing evidence for a conjecture of
Harder that the denominator is given by this L-value. We also prove under some
additional assumptions that the restriction of the classes to the boundary of
the Borel-Serre compactification of the spaces is integral. Such classes are
interesting for their use in congruences with cuspidal classes to prove
connections between the special L-value and the size of the Selmer group of the
Hecke character.Comment: 37 pages; strengthened integrality result (Proposition 16), corrected
statement of Theorem 3, and revised introductio
Towards the empowered commuter : exploring information behaviour of informal public transport users
Papers presented virtually at the 41st International Southern African Transport Conference on 10-13 July 2023.Information is emerging as one of the key dimensions of mobility, and as an important
lever through which sustainable access can be promoted in Sub-Saharan African (SSA)
cities. Information acts as an enabler of transport, by allowing travellers to make informed
decisions on whether, when, and how they travel. The lack of information can also act as a
barrier to accessing desired services, with consequences for the attainment of equity and
human development goals. Little is known about what information passengers need and
how they find and use information. This paper reports on a qualitative mixed-methods
study to explore information behaviour amongst informal transport passengers in
Tshwane, South Africa. We use the theory and methods of information science (with
information behaviour as a subdiscipline) to provide a lens for the investigation. We
collected mobility data through travel diaries, user-submitted photos of the travel context
and voice messages to explain these, and in-depth individual interviews combined with
drawings of situations where respondents used information and its sources (called
information horizon mappings). The drawings and interviews revealed the information-poor
environment within which informal transport operates, and the strong reliance on other
people as well as tacit embedded knowledge for gaining access to information. The paper
concludes by offering directions for further research around digital information
Defending the genome from the enemy within:mechanisms of retrotransposon suppression in the mouse germline
The viability of any species requires that the genome is kept stable as it is transmitted from generation to generation by the germ cells. One of the challenges to transgenerational genome stability is the potential mutagenic activity of transposable genetic elements, particularly retrotransposons. There are many different types of retrotransposon in mammalian genomes, and these target different points in germline development to amplify and integrate into new genomic locations. Germ cells, and their pluripotent developmental precursors, have evolved a variety of genome defence mechanisms that suppress retrotransposon activity and maintain genome stability across the generations. Here, we review recent advances in understanding how retrotransposon activity is suppressed in the mammalian germline, how genes involved in germline genome defence mechanisms are regulated, and the consequences of mutating these genome defence genes for the developing germline
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