227 research outputs found
Sedimentation stacking diagrams of binary mixtures of thick and thin hard rods
We use Onsager theory and the local density approximation to study
sedimentation-diffusion equilibrium density profiles of binary mixtures of
thick and thin hard rods. We construct stacking diagrams for three diameter
ratios, and find that even a simple spindle-shaped phase diagram with only
isotropic-nematic demixing can lead to counter-intuitive stacking sequences
such as an isotropic phase sandwiched between two nematic phases. For the most
complex phase diagram considered here, we find sixteen distinct stacking
sequences, including several with five sedimented layers. By adding
sedimentation paths to composition-pressure and density-density phase diagrams
and calculating density and composition profiles, we show that conclusions
about bulk phase diagrams of binary mixtures on the basis of
sedimentation-diffusion equilibria should be drawn warily.Comment: 9 pages, 8 figures, extended discussion in section 4, added
references, minor changes to figures (results unchanged
The influence of a weak magnetic field in the Renormalization-Group functions of (2+1)-dimensional Dirac systems
The experimental observation of the renormalization of the Fermi velocity
as a function of doping has been a landmark for confirming the
importance of electronic interactions in graphene. Although the experiments
were performed in the presence of a perpendicular magnetic field , the
measurements are well described by a renormalization-group (RG) theory that did
not include it. Here we clarify this issue, for both massive and massless Dirac
systems, and show that for the weak magnetic fields at which the experiments
are performed, there is no change in the renormalization-group functions. Our
calculations are carried out in the framework of the Pseudo-quantum
electrodynamics (PQED) formalism, which accounts for dynamical interactions. We
include only the linear dependence in , and solve the problem using two
different parametrizations, the Feynman and the Schwinger one. We confirm the
results obtained earlier within the RG procedure and show that, within linear
order in the magnetic field, the only contribution to the renormalization of
the Fermi velocity arises due to interactions. In addition, for gapped systems,
we observe a running of the mass parameter.Comment: Discussion about the fermionic mass has been added to the previous
versio
Antiferromagnetic magnons as highly squeezed Fock states underlying quantum correlations
Employing the concept of two-mode squeezed states from quantum optics, we
demonstrate a revealing physical picture for the antiferromagnetic ground state
and excitations. Superimposed on a N{\'e}el ordered configuration, a spin-flip
restricted to one of the sublattices is called a sublattice-magnon. We show
that an antiferromagnetic spin-up magnon is comprised by a quantum
superposition of states with spin-up and spin-down
sublattice-magnons, and is thus an enormous excitation despite its unit net
spin. Consequently, its large sublattice-spin can amplify its coupling to other
excitations. Employing von Neumann entropy as a measure, we show that the
antiferromagnetic eigenmodes manifest a high degree of entanglement between the
two sublattices, thereby establishing antiferromagnets as reservoirs for strong
quantum correlations. Based on these novel insights, we outline strategies for
exploiting the strong quantum character of antiferromagetic (squeezed-)magnons
and give an intuitive explanation for recent experimental and theoretical
findings in antiferromagnetic magnon spintronics
Tuning magnetic chirality by dipolar interactions
Chiral magnetism has gained enormous interest in recent years because of the
anticipated wealth of applications in nanoelectronics. The demonstrated
stabilization of chiral magnetic domain walls and skyrmions has been attributed
to the actively investigated Dzyaloshinskii-Moriya interaction. Recently,
however, predictions were made that suggest dipolar interactions can also
stabilize chiral domain walls and skyrmions, but direct experimental evidence
has been lacking. Here we show that dipolar interactions can indeed stabilize
chiral domain walls by directly imaging the magnetic domain walls using
scanning electron microscopy with polarization analysis. We further show that
the competition between the Dzyaloshinskii-Moriya and dipolar interactions can
reverse the domain-wall chirality. Finally, we suggest that this competition
can be tailored by a Ruderman-Kittel-Kasuya-Yosida interaction. Our work
therefore reveals that dipolar interactions play a key role in the
stabilization of chiral spin textures. This insight will open up new routes
towards balancing interactions for the stabilization of chiral magnetism
Chiral Spin Spirals at the Surface of the van der Waals Ferromagnet Fe3GeTe2
Topologically protected magnetic structures provide a robust platform for low
power consumption devices for computation and data storage. Examples of these
structures are skyrmions, chiral domain walls, and spin spirals. Here we use
scanning electron microscopy with polarization analysis to unveil the presence
of chiral counterclockwise N\'eel spin spirals at the surface of a bulk van der
Waals ferromagnet FeGeTe (FGT), at zero magnetic field. These N\'eel
spin spirals survive up to FGT's Curie temperature , with little change in the periodicity of the
spin spiral throughout the studied temperature range. The formation of a spin
spiral showing counterclockwise rotation strongly suggests the presence of a
positive Dzyaloshinskii-Moriya interaction in FGT, which provides the first
steps towards the understanding of the magnetic structure of FGT. Our results
additionally pave the way for chiral magnetism in van der Waals materials and
their heterostructures
Stabilizing chiral spin-structures via an alternating Dzyaloshinskii-Moriya interaction
The stabilization of chiral magnetic spin-structures in thin films is often
attributed to the interfacial Dzyaloshinskii-Moriya interaction (DMI). Very
recently, however, it has been reported that the chirality induced by the DMI
can be affected by dipolar interactions. These dipolar fields tend to form
N\'eel caps, which entails the formation of a clockwise chirality at the top of
the film and a counterclockwise chirality at the bottom. Here, we show that
engineering an alternating DMI that changes sign across the film thickness,
together with the tendency to form N\'eel caps, leads to an enhanced stability
of chiral spin-structures. Micromagnetic simulations for skyrmions demonstrate
that this can increase the effective DMI in a prototypical [Pt/Co/Ir]
multilayer system by at least \SI{0.6}{mJ.m^{-2}}. These gains are comparable
to what has been achieved using additive DMI, but more flexible as we are not
limited to a select set of material combinations. We also present experimental
results: by measuring equilibrium domain widths we quantify the effective DMI
in [Pt/Co/Ir] multilayer systems typically used for skyrmion stabilization.
Upon introducing an alternating DMI we demonstrate changes in the effective DMI
that agree with our simulations. Our results provide a route towards enhancing
the stability of chiral spin-structures that does not rely on enlarging the
chiral interactions.Comment: Includes supplementar
Magnetic chirality controlled by the interlayer exchange interaction
Chiral magnetism, wherein there is a preferred sense of rotation of the
magnetization, has become a key aspect for future spintronic applications. It
determines the chiral nature of magnetic textures, such as skyrmions, domain
walls or spin spirals, and a specific magnetic chirality is often required for
spintronic applications. Current research focuses on identifying and
controlling the interactions that define the magnetic chirality. The influence
of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) and, recently, the
dipolar interactions have previously been reported. Here, we experimentally
demonstrate that an indirect interlayer exchange interaction can be used as an
additional tool to effectively manipulate the magnetic chirality. We image the
chirality of magnetic domain walls in a coupled bilayer system using scanning
electron microscopy with polarization analysis (SEMPA). Upon increasing the
interlayer exchange coupling, we induce a transition of the magnetic chirality
from clockwise rotating N\'eel walls to degenerate Bloch-N\'eel domain walls
and we confirm our findings with micromagnetic simulations. In multi-layered
systems relevant for skyrmion research a uniform magnetic chirality across the
magnetic layers is often desired. Additional simulations show that this can be
achieved for reduced iDMI values when exploiting the interlayer exchange
interaction. This work opens up new ways to control and tailor the magnetic
chirality by the interlayer exchange interaction.Comment: Ms was off by a factor
Probing laser-induced spin-current generation in synthetic ferrimagnets using spin waves
Several rare-earth transition-metal ferrimagnetic systems exhibit all-optical magnetization switching upon excitation with a femtosecond laser pulse. Although this phenomenon is very promising for future optomagnetic data storage applications, the role of nonlocal spin transport in these systems is scarcely understood. Using Co/Gd and Co/Tb bilayers, we isolate the contribution of the rare-earth materials to the generated spin currents by using the precessional dynamics they excite in an adjacent ferromagnetic layer as a probe. By measuring terahertz (THz) standing spin-waves as well as GHz homogeneous precessional modes, we probe both the high- and low-frequency components of these spin currents. The low-frequency homogeneous mode indicates a significant contribution of Gd to the spin current but not from Tb, consistent with the difficulty in achieving all-optical switching in Tb-containing systems. Measurements on the THz frequency spin waves reveal the inability of the rare-earth generated spin currents to excite dynamics at the subpicosecond timescale. We present modeling efforts using the s-d model, which effectively reproduces our results and allows us to explain the behavior in terms of the temporal profile of the spin current
A multi-omics approach to delineate sputum microbiome-associated asthma inflammatory phenotypes
Asthma is a heterogeneous disease with multiple clinical presentations (phenotypes) [1]. Neutrophilic asthma is characterized by increased sputum neutrophils and generally has a poor response to corticosteroids and limited other therapeutic options. Neutrophilia originates from different factors including the defective resolution of inflammation or bacterial infections [2]. An association between airway bacterial imbalance (disturbance) and the neutrophilic phenotype has been reported [3], suggesting that airway microbiota composition is involved in neutrophilic asthma. Rather than being a separate entity [4], neutrophilic asthma may be in part, an alliance between innate immunity and microbiota composition that prompts protective mechanisms against invading pathogens [2]
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