4,373 research outputs found
Resonant Metalenses for Breaking the Diffraction Barrier
We introduce the resonant metalens, a cluster of coupled subwavelength
resonators. Dispersion allows the conversion of subwavelength wavefields into
temporal signatures while the Purcell effect permits an efficient radiation of
this information in the far-field. The study of an array of resonant wires
using microwaves provides a physical understanding of the underlying mechanism.
We experimentally demonstrate imaging and focusing from the far-field with
resolutions far below the diffraction limit. This concept is realizable at any
frequency where subwavelength resonators can be designed.Comment: 4 pages, 3 figure
Zero Temperature Phase Transition in Spin-ladders: Phase Diagram and Dynamical studies of Cu(Hp)Cl
In a magnetic field, spin-ladders undergo two zero-temperature phase
transitions at the critical fields Hc1 and Hc2. An experimental review of
static and dynamical properties of spin-ladders close to these critical points
is presented. The scaling functions, universal to all quantum critical points
in one-dimension, are extracted from (a) the thermodynamic quantities
(magnetization) and (b) the dynamical functions (NMR relaxation). A simple
mapping of strongly coupled spin ladders in a magnetic field on the exactly
solvable XXZ model enables to make detailed fits and gives an overall
understanding of a broad class of quantum magnets in their gapless phase
(between Hc1 and Hc2). In this phase, the low temperature divergence of the NMR
relaxation demonstrates its Luttinger liquid nature as well as the novel
quantum critical regime at higher temperature. The general behaviour close
these quantum critical points can be tied to known models of quantum magnetism.Comment: few corrections made, 15 pages, to be published in European Journal
of Physics
Comment on "Localized behavior near the Zn impurity in YBa2Cu4O8 as measured by nuclear quadrupole resonance"
Williams and Kramer [Phys. Rev. B {\bf 64}, 104506 (2001)] have recently
argued against the existence of staggered magnetic moments residing on several
lattice sites around Zn impurities in YBCO superconductors. This claim, which
is in line with an earlier publication by Williams, Tallon and Dupree [Phys.
Rev. B {\bf 61}, 4319 (2000)], is however in contradiction with a large body of
experimental data from different NMR groups. On the contrary, the authors argue
in favor of a very localized spin and charge density on Cu sites first
neighbors to Zn. We show that the conclusions of Williams and Kramer arise from
erroneous interpretations of NMR and NQR data.Comment: 4 page
Field-induced local moments around nonmagnetic impurities in metallic cuprates
We consider a defect in a strongly correlated host metal and discuss, within
a slave boson mean field formalism for the model, the formation of an
induced paramagnetic moment which is extended over nearby sites. We study in
particular an impurity in a metallic band, suitable for modelling the optimally
doped cuprates, in a regime where the impurity moment is paramagnetic. The form
of the local susceptibility as a function of temperature and doping is found to
agree well with recent NMR experiments, without including screening processes
leading to the Kondo effect.Comment: 7 pages, submitted to Phys Rev
An apodizing phase plate coronagraph for VLT/NACO
We describe a coronagraphic optic for use with CONICA at the VLT that
provides suppression of diffraction from 1.8 to 7 lambda/D at 4.05 microns, an
optimal wavelength for direct imaging of cool extrasolar planets. The optic is
designed to provide 10 magnitudes of contrast at 0.2 arcseconds, over a
D-shaped region in the image plane, without the need for any focal plane
occulting mask.Comment: 9 pages, 5 figures, to appear in Proc. SPIE Vol. 773
Neuromorphic computing for attitude estimation onboard quadrotors
Compelling evidence has been given for the high energy efficiency and update
rates of neuromorphic processors, with performance beyond what standard Von
Neumann architectures can achieve. Such promising features could be
advantageous in critical embedded systems, especially in robotics. To date, the
constraints inherent in robots (e.g., size and weight, battery autonomy,
available sensors, computing resources, processing time, etc.), and
particularly in aerial vehicles, severely hamper the performance of
fully-autonomous on-board control, including sensor processing and state
estimation. In this work, we propose a spiking neural network (SNN) capable of
estimating the pitch and roll angles of a quadrotor in highly dynamic movements
from 6-degree of freedom Inertial Measurement Unit (IMU) data. With only 150
neurons and a limited training dataset obtained using a quadrotor in a real
world setup, the network shows competitive results as compared to
state-of-the-art, non-neuromorphic attitude estimators. The proposed
architecture was successfully tested on the Loihi neuromorphic processor
on-board a quadrotor to estimate the attitude when flying. Our results show the
robustness of neuromorphic attitude estimation and pave the way towards
energy-efficient, fully autonomous control of quadrotors with dedicated
neuromorphic computing systems
High resolution coherent population trapping on a single hole spin in a semiconductor
We report high resolution coherent population trapping on a single hole spin
in a semiconductor quantum dot. The absorption dip signifying the formation of
a dark state exhibits an atomic physics-like dip width of just 10 MHz. We
observe fluctuations in the absolute frequency of the absorption dip, evidence
of very slow spin dephasing. We identify this process as charge noise by,
first, demonstrating that the hole spin g-factor in this configuration
(in-plane magnetic field) is strongly dependent on the vertical electric field,
and second, by characterizing the charge noise through its effects on the
optical transition frequency. An important conclusion is that charge noise is
an important hole spin dephasing process
Rotational modes in molecular magnets with antiferromagnetic Heisenberg exchange
In an effort to understand the low temperature behavior of recently
synthesized molecular magnets we present numerical evidence for the existence
of a rotational band in systems of quantum spins interacting with
nearest-neighbor antiferromagnetic Heisenberg exchange. While this result has
previously been noted for ring arrays with an even number of spin sites, we
find that it also applies for rings with an odd number of sites as well as for
all of the polytope configurations we have investigated (tetrahedron, cube,
octahedron, icosahedron, triangular prism, and axially truncated icosahedron).
It is demonstrated how the rotational band levels can in many cases be
accurately predicted using the underlying sublattice structure of the spin
array. We illustrate how the characteristics of the rotational band can provide
valuable estimates for the low temperature magnetic susceptibility.Comment: 14 pages, 7 figures, to be published in Phys. Rev.
Long-range/short-range separation of the electron-electron interaction in density functional theory
By splitting the Coulomb interaction into long-range and short-range
components, we decompose the energy of a quantum electronic system into
long-range and short-range contributions. We show that the long-range part of
the energy can be efficiently calculated by traditional wave function methods,
while the short-range part can be handled by a density functional. The analysis
of this functional with respect to the range of the associated interaction
reveals that, in the limit of a very short-range interaction, the short-range
exchange-correlation energy can be expressed as a simple local functional of
the on-top pair density and its first derivatives. This provides an explanation
for the accuracy of the local density approximation (LDA) for the short-range
functional. Moreover, this analysis leads also to new simple approximations for
the short-range exchange and correlation energies improving the LDA.Comment: 18 pages, 14 figures, to be published in Phys. Rev.
A high-affinity antibody against the CSP N-terminal domain lacks Plasmodium falciparum inhibitory activity
Malaria is a global health concern and research efforts are ongoing to develop a superior vaccine to RTS,S/AS01. To guide immunogen design, we seek a comprehensive understanding of the protective humoral response against Plasmodium falciparum circumsporozoite protein (PfCSP). In contrast to the well-studied responses to the repeat region and the C-terminus, the antibody response against the N-terminal domain of PfCSP (N-CSP) remains obscure. Here, we characterized the molecular recognition and functional efficacy of the N-CSP-specific monoclonal antibody 5D5. The crystal structure at 1.85 Åresolution revealed that 5D5 binds an α-helical epitope in N-CSP with high affinity through extensive shape and charge complementarity, and the unusual utilization of an N-linked glycan. Nevertheless, functional studies indicated low 5D5 binding to live Pf sporozoites, and lack of sporozoite inhibition in vitro and in mosquitoes. Overall, our data on low recognition and inhibition of sporozoites do not support the inclusion of the 5D5 epitope into the next generation of CSP-based vaccines.Summary Statement The Plasmodium falciparum sporozoite surface protein, PfCSP, is an attractive vaccine target, but the antibody response against the CSP N-terminal domain has remained understudied. Here, to guide immunogen design, Thai et al. provide insights into the binding motif and functional efficacy of the N-terminal domain-specific monoclonal antibody, 5D5
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