58 research outputs found
Can we observationally test the weak cosmic censorship conjecture?
In general relativity, gravitational collapse of matter fields ends with the
formation of a spacetime singularity, where the matter density becomes infinite
and standard physics breaks down. According to the weak cosmic censorship
conjecture, singularities produced in the gravitational collapse cannot be seen
by distant observers and must be hidden within black holes. The validity of
this conjecture is still controversial and at present we cannot exclude that
naked singularities can be created in our Universe from regular initial data.
In this paper, we study the radiation emitted by a collapsing cloud of dust and
check whether it is possible to distinguish the birth of a black hole from the
one of a naked singularity. In our simple dust model, we find that the
properties of the radiation emitted in the two scenarios is qualitatively
similar. That suggests that observational tests of the cosmic censorship
conjecture may be very difficult, even in principle.Comment: 1+19 pages, 4 figures. v2: minor change
Gravitational blueshift from a collapsing object
We discuss a counterintuitive phenomenon of classical general relativity, in
which a significant fraction of the radiation emitted by a collapsing object
and detected by a distant observer may be blueshifted rather than redshifted.
The key-point is that when the radiation propagates inside the collapsing body
it is blueshifted, and this time interval may be sufficiently long for the
effect to be larger than the later redshift due to the propagation in the
vacuum exterior, from the surface of the body to the distant observer.
Unfortunately, the phenomenon can unlikely have direct observational
implications, but it is interesting by itself as a pure relativistic effect.Comment: 6 pages, 3 figures. v2: refereed versio
Can we observationally test the weak cosmic censorship conjecture?
In general relativity, gravitational collapse of matter fields ends with the formation
of a spacetime singularity, where the matter density becomes infinite and standard physics breaks down. According to the weak cosmic censorship conjecture, singularities produced in the gravitational collapse cannot be seen by distant observers and must be hidden within black holes. The validity of this conjecture is still controversial and at present we cannot exclude that naked singularities can be created in our Universe from regular initial data. In this paper, we study the radiation emitted by a collapsing cloud of dust and check whether it is possible to distinguish the birth of a black hole from the one of a naked singularity. In our simple dust model, we find that the properties of the radiation emitted in the two scenarios is qualitatively similar. That suggests that observational tests of the cosmic censorship conjecture may be very difficult, even in principle
Skyrmion-Bubble Bundles in an X-type Sr2Co2Fe28O46 Hexaferrite above Room Temperature
Magnetic skyrmions are spin swirls that possess topological nontriviality and
are considered particle-like entities. They are distinguished by an integer
topological charge Q. The presence of skyrmion bundles provides an opportunity
to explore the range of values for Q, which is crucial for the advancement of
topological spintronic devices with multi-Q properties. In this study, we
present a new material candidate, Sr2Co2Fe28O46 hexaferrite of the X-type,
which hosts small dipolar skyrmions at room temperature and above. By
exploiting reversed magnetic fields from metastable skyrmion bubbles at zero
fields, we can incorporate skyrmion-bubble bundles with different interior
skyrmion/bubble numbers, topological charges, and morphologies at room
temperature. Our experimental findings are consistently supported by
micromagnetic simulations. Our results highlight the versatility of topological
spin textures in centrosymmetric uniaxial magnets, thereby paving the way for
the development of room-temperature topological spintronic devices with multi-Q
characteristics.Comment: https://doi.org/10.1002/adma.20230611
Edge-Mediated Skyrmion Chain and Its Collective Dynamics in a Confined Geometry
The emergence of a topologically nontrivial vortex-like magnetic structure,
the magnetic skyrmion, has launched new concepts for memory devices. There,
extensive studies have theoretically demonstrated the ability to encode
information bits by using a chain of skyrmions in one-dimensional nanostripes.
Here, we report the first experimental observation of the skyrmion chain in
FeGe nanostripes by using high resolution Lorentz transmission electron
microscopy. Under an applied field normal to the nanostripes plane, we observe
that the helical ground states with distorted edge spins would evolves into
individual skyrmions, which assemble in the form of chain at low field and move
collectively into the center of nanostripes at elevated field. Such skyrmion
chain survives even as the width of nanostripe is much larger than the single
skyrmion size. These discovery demonstrates new way of skyrmion formation
through the edge effect, and might, in the long term, shed light on the
applications.Comment: 7 pages, 3 figure
Electrical Probing of Field-Driven Cascading Quantized Transitions of Skyrmion Cluster States in MnSi Nanowires
Magnetic skyrmions are topologically stable whirlpool-like spin textures that
offer great promise as information carriers for future ultra-dense memory and
logic devices1-4. To enable such applications, particular attention has been
focused on the skyrmions properties in highly confined geometry such as one
dimensional nanowires5-8. Hitherto it is still experimentally unclear what
happens when the width of the nanowire is comparable to that of a single
skyrmion. Here we report the experimental demonstration of such scheme, where
magnetic field-driven skyrmion cluster (SC) states with small numbers of
skyrmions were demonstrated to exist on the cross-sections of ultra-narrow
single-crystal MnSi nanowires (NWs) with diameters, comparable to the skyrmion
lattice constant (18 nm). In contrast to the skyrmion lattice in bulk MnSi
samples, the skyrmion clusters lead to anomalous magnetoresistance (MR)
behavior measured under magnetic field parallel to the NW long axis, where
quantized jumps in MR are observed and directly associated with the change of
the skyrmion number in the cluster, which is supported by Monte Carlo
simulations. These jumps show the key difference between the clustering and
crystalline states of skyrmions, and lay a solid foundation to realize
skyrmion-based memory devices that the number of skyrmions can be counted via
conventional electrical measurements
Observation of Hybrid Magnetic Skyrmion Bubbles in Fe3Sn2 Nanodisks
It is well known that there are two types of magnetic bubbles in uniaxial
magnets. Here, using Lorentz-transimission electronic microscopy magnetic
imaging, we report the direct experimental observation of 3D type-III hybrid
bubbles, which comprise N\'eel-twisted skyrmion bubbles with topological charge
Q = -1 in near-surface layers and type-II bubbles with Q = 0 in interior
layers, in Fe3Sn2 nanodisks. Using the tilted magnetic field, we further show
the controlled topological magnetic transformations of three types of bubbles
in a confined ferromagnetic nanodisk. Our observations are well reproduced
using micromagnetic simulations based on measured magnetic parameters. Our
results advance fundamental classification and understanding of magnetic
bubbles, which could propel the applications of three-dimensional magnetism.Comment: https://doi.org/10.1103/PhysRevB.107.17442
Current-Controlled Skyrmion Number in Confined Ferromagnetic Nanostripes
Skyrmions are vortex-like localized magnetic structures that possess an
integer-valued topological index known as the skyrmion number or topological
charge. Skyrmion number determines the topology-related emergent magnetism,
which is highly desirable for advanced storage and computing devices. In order
to achieve device functions, it is necessary to manipulate the skyrmion number
in confined nanostructured geometries using electrical methods. Here, we report
the reliable current-controlled operations for manipulating the skyrmion number
through reversible topological transformations between skyrmion chains and
stripe domains in confined Fe3Sn2 nanostripes. The results of micromagnetic
simulations are successful in numerically reproducing our experiments and
explaining them through the combined effect of current-induced Joule heating
and magnetic hysteresis. These findings hold the potential to advance the
development of topological spintronic devices.Comment: https://doi.org/10.1002/adfm.20230404
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