131 research outputs found
Revealing the correlation between real-space structure and chiral magnetic order at the atomic scale
We image simultaneously the geometric, electronic and magnetic structure of a
buckled iron bilayer film that exhibits chiral magnetic order. We achieve this
by combining spin-polarized scanning tunneling microscopy and magnetic exchange
force microscopy (SPEX), to independently characterize the geometric as well as
the electronic and magnetic structure of non-flat surfaces. This new SPEX
imaging technique reveals the geometric height corrugation of the
reconstruction lines resulting from strong strain relaxation in the bilayer,
enabling the decomposition of the real-space from the eletronic structure at
the atomic level, and the correlation with the resultant spin spiral ground
state. By additionally utilizing adatom manipulation, we reveal the chiral
magnetic ground state of portions of the unit cell that were not previously
imaged with SP-STM alone. Using density functional theory (DFT), we investigate
the structural and electronic properties of the reconstructed bilayer and
identify the favorable stoichiometry regime in agreement with our experimental
result
Electrical detection of magnetic skyrmions by non-collinear magnetoresistance
Magnetic skyrmions are localised non-collinear spin textures with high
potential for future spintronic applications. Skyrmion phases have been
discovered in a number of materials and a focus of current research is the
preparation, detection, and manipulation of individual skyrmions for an
implementation in devices. Local experimental characterization of skyrmions has
been performed by, e.g., Lorentz microscopy or atomic-scale tunnel
magnetoresistance measurements using spin-polarised scanning tunneling
microscopy. Here, we report on a drastic change of the differential tunnel
conductance for magnetic skyrmions arising from their non-collinearity: mixing
between the spin channels locally alters the electronic structure, making a
skyrmion electronically distinct from its ferromagnetic environment. We propose
this non-collinear magnetoresistance (NCMR) as a reliable all-electrical
detection scheme for skyrmions with an easy implementation into device
architectures
The first Re(V) Complex with a Ligand from the Nature: Non-Covalent Interactions from Crystal Structure
A lot of scientific effort was dedicated to research into a cure for cancer over past
decades. Many of them failed due to a low solubility of synthetized compounds or low
selectivity between healthy and pathogenic cells. It was shown that apigenin, a natural
pigment of chamomile, showed some cytotoxic activity itself,[1] however, its extremely
low solubility in water is a limiting factor in its use as a potential anticancer drug.
Here, we present a crystal structure of the first Re(V) complex containing apigenin
– a natural occurring ligand. Various types of non-covalent interactions were found in
the crystal structure (Figure 1). Namely, there are a couple of different hydrogen bonds
(i.e. mono and bifurcated O-H∙∙∙O; O-H∙∙∙Cl; C-H∙∙∙Cl), T-shaped C-H∙∙∙π and π∙∙∙π
stacking interactions. At least some of these interactions could be responsible for
compounds’ final mechanism of anticancer action
Femtosecond control of electric currents at the interfaces of metallic ferromagnetic heterostructures
The idea to utilize not only the charge but also the spin of electrons in the
operation of electronic devices has led to the development of spintronics,
causing a revolution in how information is stored and processed. A novel
advancement would be to develop ultrafast spintronics using femtosecond laser
pulses. Employing terahertz (10 Hz) emission spectroscopy, we
demonstrate optical generation of spin-polarized electric currents at the
interfaces of metallic ferromagnetic heterostructures at the femtosecond
timescale. The direction of the photocurrent is controlled by the helicity of
the circularly polarized light. These results open up new opportunities for
realizing spintronics in the unprecedented terahertz regime and provide new
insights in all-optical control of magnetism.Comment: 3 figures and 2 tables in the main tex
Supervised machine learning on Galactic filaments. Revealing the filamentary structure of the Galactic interstellar medium
Context. Filaments are ubiquitous in the Galaxy, and they host star formation. Detecting them in a reliable way is therefore key towards our understanding of the star formation process.
Aims: We explore whether supervised machine learning can identify filamentary structures on the whole Galactic plane.
Methods: We used two versions of UNet-based networks for image segmentation. We used H2 column density images of the Galactic plane obtained with Herschel Hi-GAL data as input data. We trained the UNet-based networks with skeletons (spine plus branches) of filaments that were extracted from these images, together with background and missing data masks that we produced. We tested eight training scenarios to determine the best scenario for our astrophysical purpose of classifying pixels as filaments.
Results: The training of the UNets allows us to create a new image of the Galactic plane by segmentation in which pixels belonging to filamentary structures are identified. With this new method, we classify more pixels (more by a factor of 2 to 7, depending on the classification threshold used) as belonging to filaments than the spine plus branches structures we used as input. New structures are revealed, which are mainly low-contrast filaments that were not detected before. We use standard metrics to evaluate the performances of the different training scenarios. This allows us to demonstrate the robustness of the method and to determine an optimal threshold value that maximizes the recovery of the input labelled pixel classification.
Conclusions: This proof-of-concept study shows that supervised machine learning can reveal filamentary structures that are present throughout the Galactic plane. The detection of these structures, including low-density and low-contrast structures that have never been seen before, offers important perspectives for the study of these filaments
Signatures of pressure-enhanced helimagnetic order in van der Waals multiferroic NiI
The van der Waals (vdW) type-II multiferroic NiI has emerged as a
candidate for exploring non-collinear magnetism and magnetoelectric effects in
the 2D limit. Frustrated intralayer exchange interactions on a triangular
lattice result in a helimagnetic ground state, with spin-induced improper
ferroelectricity stabilized by the interlayer interactions. Here we investigate
the magnetic and structural phase transitions in bulk NiI, using
high-pressure Raman spectroscopy, optical linear dichroism, and x-ray
diffraction. We obtain evidence for a significant pressure enhancement of the
antiferromagnetic and helimagnetic transition temperatures, at rates of
K/GPa, respectively. These enhancements are attributed to a
cooperative effect of pressure-enhanced interlayer and third-nearest-neighbor
intralayer exchange. These results reveal a general path for obtaining
high-temperature type-II multiferroicity via high pressures in vdW materials
CONCERTO: Extracting the power spectrum of the [C II ] emission line
CONCERTO is the first experiment to perform a [CII] line intensity mapping
survey to target . Measuring the [CII] power spectrum allows us to study
the role of dusty star-forming galaxies in the star formation history during
the Reionization and post-Reionization. The main obstacle to this measurement
is the contamination by bright foregrounds. We evaluate our ability to retrieve
the [CII] signal in mock observations using the Simulated Infrared Dusty
Extragalactic Sky. We compared two methods for dealing with the dust continuum
emission from galaxies: the standard PCA and the arPLS method. For line
interlopers, the strategy relies on masking low-redshift galaxies using
external catalogues. As we do not have observations of CO or classical CO
proxies ,we relied on the COSMOS stellar mass catalogue. To measure the power
spectrum of masked data, we adapted the P of K EstimatoR and discuss its use on
LIM data. The arPLS method achieves a reduction of the continuum background to
a sub-dominant level of the [CII] at z=7 by a factor of>70. When using PCA,
this factor is only 0.7. The masking lowers the power amplitude of line
contamination down to This residual level is dominated
by faint undetected sources. For our [CII] model, this results in a detection
at z = 5.2 with a power ratio [CII]/(residual interlopers) = for a
22 % area survey loss. However, at z = 7, [C II ] / (residual interlopers). Thanks to the large area covered by SIDES-Uchuu, we show that the
power amplitude of line residuals varies by 12-15% for z=5.2-7. We present an
end-to-end simulation of the extragalactic foreground removal that we ran to
detect the [CII] at high redshift via its power spectrum. We show that dust
continuum emission are not a limiting foreground for [CII] LIM. Residual CO and
[CI] limits our ability to measure the [CII] power spectrum at z>7.Comment: 15 pages, 12 figures, to be published in Astronomy & Astrophysic
A Compressed Sensing Approach to 3D Weak Lensing
(Abridged) Weak gravitational lensing is an ideal probe of the dark universe.
In recent years, several linear methods have been developed to reconstruct the
density distribution in the Universe in three dimensions, making use of
photometric redshift information to determine the radial distribution of lensed
sources. In this paper, we aim to address three key issues seen in these
methods; namely, the bias in the redshifts of detected objects, the line of
sight smearing seen in reconstructions, and the damping of the amplitude of the
reconstruction relative to the underlying density. We consider the problem
under the framework of compressed sensing (CS). Under the assumption that the
data are sparse in an appropriate dictionary, we construct a robust estimator
and employ state-of-the-art convex optimisation methods to reconstruct the
density contrast. For simplicity in implementation, and as a proof of concept
of our method, we reduce the problem to one-dimension, considering the
reconstruction along each line of sight independently. Despite the loss of
information this implies, we demonstrate that our method is able to accurately
reproduce cluster haloes up to a redshift of z=1, deeper than state-of-the-art
linear methods. We directly compare our method with these linear methods, and
demonstrate minimal radial smearing and redshift bias in our reconstructions,
as well as a reduced damping of the reconstruction amplitude as compared to the
linear methods. In addition, the CS framework allows us to consider an
underdetermined inverse problem, thereby allowing us to reconstruct the density
contrast at finer resolution than the input data.Comment: Submitted to A&A (6 July 2011
Emergent Phenomena Induced by Spin-Orbit Coupling at Surfaces and Interfaces
Spin-orbit coupling (SOC) describes the relativistic interaction between the
spin and momentum degrees of freedom of electrons, and is central to the rich
phenomena observed in condensed matter systems. In recent years, new phases of
matter have emerged from the interplay between SOC and low dimensionality, such
as chiral spin textures and spin-polarized surface and interface states. These
low-dimensional SOC-based realizations are typically robust and can be
exploited at room temperature. Here we discuss SOC as a means of producing such
fundamentally new physical phenomena in thin films and heterostructures. We put
into context the technological promise of these material classes for developing
spin-based device applications at room temperature
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