49 research outputs found
Topological Defects and Non-homogeneous Melting of Large 2D Coulomb Clusters
The configurational and melting properties of large two-dimensional clusters
of charged classical particles interacting with each other via the Coulomb
potential are investigated through the Monte Carlo simulation technique. The
particles are confined by a harmonic potential. For a large number of particles
in the cluster (N>150) the configuration is determined by two competing
effects, namely in the center a hexagonal lattice is formed, which is the
groundstate for an infinite 2D system, and the confinement which imposes its
circular symmetry on the outer edge. As a result a hexagonal Wigner lattice is
formed in the central area while at the border of the cluster the particles are
arranged in rings. In the transition region defects appear as dislocations and
disclinations at the six corners of the hexagonal-shaped inner domain. Many
different arrangements and type of defects are possible as metastable
configurations with a slightly higher energy. The particles motion is found to
be strongly related to the topological structure. Our results clearly show that
the melting of the clusters starts near the geometry induced defects, and that
three different melting temperatures can be defined corresponding to the
melting of different regions in the cluster.Comment: 7 pages, 11 figures, submitted to Phys. Rev.
Generic properties of a quasi-one dimensional classical Wigner crystal
We studied the structural, dynamical properties and melting of a
quasi-one-dimensional system of charged particles, interacting through a
screened Coulomb potential. The ground state energy was calculated and,
depending on the density and the screening length, the system crystallizes in a
number of chains. As a function of the density (or the confining potential),
the ground state configurations and the structural transitions between them
were analyzed both by analytical and Monte Carlo calculations. The system
exhibits a rich phase diagram at zero temperature with continuous and
discontinuous structural transitions. We calculated the normal modes of the
Wigner crystal and the magneto-phonons when an external constant magnetic field
is applied. At finite temperature the melting of the system was studied via
Monte Carlo simulations using the (MLC). The
melting temperature as a function of the density was obtained for different
screening parameters. Reentrant melting as a function of the density was found
as well as evidence of directional dependent melting. The single chain regime
exhibits anomalous melting temperatures according to the MLC and as a check we
study the pair correlation function at different densities and different
temperatures, formulating a different criterion. Possible connection with
recent theoretical and experimental results are discussed and experiments are
proposed.Comment: 13 pages text, 21 picture
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
Bodily tides near spin-orbit resonances
Spin-orbit coupling can be described in two approaches. The method known as
"the MacDonald torque" is often combined with an assumption that the quality
factor Q is frequency-independent. This makes the method inconsistent, because
the MacDonald theory tacitly fixes the rheology by making Q scale as the
inverse tidal frequency.
Spin-orbit coupling can be treated also in an approach called "the Darwin
torque". While this theory is general enough to accommodate an arbitrary
frequency-dependence of Q, this advantage has not yet been exploited in the
literature, where Q is assumed constant or is set to scale as inverse tidal
frequency, the latter assertion making the Darwin torque equivalent to a
corrected version of the MacDonald torque.
However neither a constant nor an inverse-frequency Q reflect the properties
of realistic mantles and crusts, because the actual frequency-dependence is
more complex. Hence the necessity to enrich the theory of spin-orbit
interaction with the right frequency-dependence. We accomplish this programme
for the Darwin-torque-based model near resonances. We derive the
frequency-dependence of the tidal torque from the first principles, i.e., from
the expression for the mantle's compliance in the time domain. We also explain
that the tidal torque includes not only the secular part, but also an
oscillating part.
We demonstrate that the lmpq term of the Darwin-Kaula expansion for the tidal
torque smoothly goes through zero, when the secondary traverses the lmpq
resonance (e.g., the principal tidal torque smoothly goes through nil as the
secondary crosses the synchronous orbit).
We also offer a possible explanation for the unexpected frequency-dependence
of the tidal dissipation rate in the Moon, discovered by LLR
Ad26 vector-based COVID-19 vaccine encoding a prefusion-stabilized SARS-CoV-2 Spike immunogen induces potent humoral and cellular immune responses
Development of effective preventative interventions against SARS-CoV-2, the etiologic agent of COVID-19 is urgently needed. The viral surface spike (S) protein of SARS-CoV-2 is a key target for prophylactic measures as it is critical for the viral replication cycle and the primary target of neutralizing antibodies. We evaluated design elements previously shown for other coronavirus S protein-based vaccines to be successful, e.g., prefusion-stabilizing substitutions and heterologous signal peptides, for selection of a S-based SARS-CoV-2 vaccine candidate. In vitro characterization demonstrated that the introduction of stabilizing substitutions (i.e., furin cleavage site mutations and two consecutive prolines in the hinge region of S2) increased the ratio of neutralizing versus non-neutralizing antibody binding, suggestive for a prefusion conformation of the S protein. Furthermore, the wild-type signal peptide was best suited for the correct cleavage needed for a natively folded protein. These observations translated into superior immunogenicity in mice where the Ad26 vector encoding for a membrane-bound stabilized S protein with a wild-type signal peptide elicited potent neutralizing humoral immunity and cellular immunity that was polarized towards Th1 IFN-gamma. This optimized Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in a phase I clinical trial (ClinicalTrials.gov Identifier: NCT04436276).Molecular basis of virus replication, viral pathogenesis and antiviral strategie
Immunogenicity and efficacy of one and two doses of Ad26.COV2.S COVID vaccine in adult and aged NHP
Safe and effective coronavirus disease-19 (COVID-19) vaccines are urgently needed to control the ongoing pandemic. While single-dose vaccine regimens would provide multiple advantages, two doses may improve the magnitude and durability of immunity and protective efficacy. We assessed one-and two-dose regimens of the Ad26.COV2.S vaccine candidate in adult and aged nonhuman primates (NHPs). A two-dose Ad26.COV2.S regimen induced higher peak binding and neutralizing antibody responses compared with a single dose. In one-dose regimens, neutralizing antibody responses were stable for at least 14 wk, providing an early indication of durability. Ad26.COV2.S induced humoral immunity and T helper cell (Th cell) 1-skewed cellular responses in aged NHPs that were comparable to those in adult animals. Aged Ad26.COV2.S-vaccinated animals challenged 3 mo after dose 1 with a SARS-CoV-2 spike G614 variant showed near complete lower and substantial upper respiratory tract protection for both regimens. Neutralization of variants of concern by NHP sera was reduced for B.1.351 lineages while maintained for the B.1.1.7 lineage independent of Ad26.COV2.S vaccine regimen.Molecular basis of virus replication, viral pathogenesis and antiviral strategie
A Delphi-method-based consensus guideline for definition of treatment-resistant depression for clinical trials
Criteria for treatment-resistant depression (TRD) and partially responsive depression (PRD) as subtypes of major depressive disorder (MDD) are not unequivocally defined. In the present document we used a Delphi-method-based consensus approach to define TRD and PRD and to serve as operational criteria for future clinical studies, especially if conducted for regulatory purposes. We reviewed the literature and brought together a group of international experts (including clinicians, academics, researchers, employees of pharmaceutical companies, regulatory bodies representatives, and one person with lived experience) to evaluate the state-of-the-art and main controversies regarding the current classification. We then provided recommendations on how to design clinical trials, and on how to guide research in unmet needs and knowledge gaps. This report will feed into one of the main objectives of the EUropean Patient-cEntric clinicAl tRial pLatforms, Innovative Medicines Initiative (EU-PEARL, IMI) MDD project, to design a protocol for platform trials of new medications for TRD/PRD. © 2021, The Author(s).EU/EFPIA/Innovative Medicines Initiative 2 Joint Undertaking