602 research outputs found
Perpendicular Reading of Single Confined Magnetic Skyrmions
Thin-film sub-5 nm magnetic skyrmions constitute an ultimate scaling
alternative for future digital data storage. Skyrmions are robust non-collinear
spin-textures that can be moved and manipulated by small electrical currents.
We show here an innovative technique to detect isolated nanoskyrmions with a
current-perpendicular-to-plane geometry, which has immediate implications for
device concepts. We explore the physics behind such a mechanism by studying the
atomistic electronic structure of the magnetic quasiparticles. We investigate
how the isolated skyrmion local-density-of-states which tunnels into the
vacuum, when compared to the ferromagnetic background, is modified by the
site-dependent spin-mixing of electronic states with different relative canting
angles. Local transport properties are sensitive to this effect, as we report
an atomistic conductance anisotropy of over 20% for magnetic skyrmions in
Pd/Fe/Ir(111) thin-films. In single skyrmions, engineering this spin-mixing
magnetoresistance possibly could be incorporated in future magnetic storage
technologies
Pseudo spin-orbit coupling of Dirac particles in graphene spintronics
We study the pseudo spin-orbital (SO) effects experienced by massive Dirac
particles in graphene, which can potentially be of a larger magnitude compared
to the conventional Rashba SO effects experienced by particles in a 2DEG
semiconductor heterostructure. In order to generate a uniform vertical pseudo
SO field, we propose an artificial atomic structure, consisting of a graphene
ring and a charged nanodot at the center which produces a large radial electric
field. In this structure, a large pseudo SO coupling strength can be achieved
by accelerating the Dirac particles around the ring, due to the small energy
gap in graphene and the large radial electric field emanating from the charged
nanodot. We discuss the theoretical possibility of harnessing the pseudo SO
effects in mesoscopic applications, e.g. pseudo spin relaxation and switching.Comment: 12 pages, 1 figur
Graph Q-Learning for Combinatorial Optimization
Graph-structured data is ubiquitous throughout natural and social sciences,
and Graph Neural Networks (GNNs) have recently been shown to be effective at
solving prediction and inference problems on graph data. In this paper, we
propose and demonstrate that GNNs can be applied to solve Combinatorial
Optimization (CO) problems. CO concerns optimizing a function over a discrete
solution space that is often intractably large. To learn to solve CO problems,
we formulate the optimization process as a sequential decision making problem,
where the return is related to how close the candidate solution is to
optimality. We use a GNN to learn a policy to iteratively build increasingly
promising candidate solutions. We present preliminary evidence that GNNs
trained through Q-Learning can solve CO problems with performance approaching
state-of-the-art heuristic-based solvers, using only a fraction of the
parameters and training time
Group equivariant neural posterior estimation
Simulation-based inference with conditional neural density estimators is a powerful approach to solving inverse problems in science. However, these methods typically treat the underlying forward model as a black box, with no way to exploit geometric properties such as equivariances. Equivariances are common in scientific models, however integrating them directly into expressive inference networks (such as normalizing flows) is not straightforward. We here describe an alternative method to incorporate equivariances under joint transformations of parameters and data. Our method -- called group equivariant neural posterior estimation (GNPE) -- is based on self-consistently standardizing the "pose" of the data while estimating the posterior over parameters. It is architecture-independent, and applies both to exact and approximate equivariances. As a real-world application, we use GNPE for amortized inference of astrophysical binary black hole systems from gravitational-wave observations. We show that GNPE achieves state-of-the-art accuracy while reducing inference times by three orders of magnitude
Measurement of the charged pion mass using X-ray spectroscopy of exotic atoms
The transitions in pionic nitrogen and muonic oxygen were measured
simultaneously by using a gaseous nitrogen-oxygen mixture at 1.4\,bar. Due to
the precise knowledge of the muon mass the muonic line provides the energy
calibration for the pionic transition. A value of
(139.57077\,\,0.00018)\,MeV/c (\,1.3ppm) is derived for the
mass of the negatively charged pion, which is 4.2ppm larger than the present
world average
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