686 research outputs found
Magnetic impurity coupled to interacting conduction electrons
We consider a magnetic impurity which interacts by hybridization with a
system of weakly correlated electrons and determine the energy of the ground
state by means of an 1/N_f expansion. The correlations among the conduction
electrons are described by a Hubbard Hamiltonian and are treated to lowest
order in the interaction strength. We find that their effect on the Kondo
temperature, T_K, in the Kondo limit is twofold: First, the position of the
impurity level is shifted due to the reduction of charge fluctuations, which
reduces T_K. Secondly, the bare Kondo exchange coupling is enhanced as spin
fluctuations are enlarged. In total, T_K increases. Both corrections require
intermediate states beyond the standard Varma-Yafet ansatz. This shows that the
Hubbard interaction does not just provide quasiparticles, which hybridize with
the impurity, but also renormalizes the Kondo coupling.Comment: ReVTeX 19 pages, 3 uuenconded postscript figure
The importance of XY anisotropy in Sr2IrO4 revealed by magnetic critical scattering experiments
The magnetic critical scattering in SrIrO has been characterized
using X-ray resonant magnetic scattering (XRMS) both below and above the 3D
antiferromagnetic ordering temperature, T. The order parameter
critical exponent below T is found to be \beta=0.195(4), in the
range of the 2D XYh universality class. Over an extended temperature range
above T, the amplitude and correlation length of the intrinsic
critical fluctuations are well described by the 2D Heisenberg model with XY
anisotropy. This contrasts with an earlier study of the critical scattering
over a more limited range of temperature which found agreement with the theory
of the isotropic 2D Heisenberg quantum antiferromagnet, developed to describe
the critical fluctuations of the conventional Mott insulator LaCuO and
related systems. Our study therefore establishes the importance of XY
anisotropy in the low-energy effective Hamiltonian of SrIrO, the
prototypical spin-orbit Mott insulator.Comment: 6 pages, 4 figure
The Magnetic Structure of DyMn2O5 Determined by Resonant X-ray Scattering
Resonant magnetic x-ray scattering has been used to investigate the magnetic
structure of the magnetoelectric multiferroic DyMn2O5. We have studied the
magnetic structure in the ferroelectric phase of this material, which displays
the strongest ferroelectric polarisation and magnetodielectric effect of the
RMn2O5 (where R is a rare earth ion, Y or Bi) family. The magnetic structure
observed is similar to that of the other members of the series, but differs in
the direction of the ordered moments. In DyMn2O5 both the Dy and Mn moments lie
close to the b-axis, whereas in other RMn2O5 they lie close to the a-axis.Comment: 8 pages, 8 figure
Identification of the Moving Junction Complex of Toxoplasma gondii: A Collaboration between Distinct Secretory Organelles
Apicomplexan parasites, including Toxoplasma gondii and Plasmodium sp., are obligate intracellular protozoa. They enter into a host cell by attaching to and then creating an invagination in the host cell plasma membrane. Contact between parasite and host plasma membranes occurs in the form of a ring-shaped moving junction that begins at the anterior end of the parasite and then migrates posteriorly. The resulting invagination of host plasma membrane creates a parasitophorous vacuole that completely envelops the now intracellular parasite. At the start of this process, apical membrane antigen 1 (AMA1) is released onto the parasite surface from specialized secretory organelles called micronemes. The T. gondii version of this protein, TgAMA1, has been shown to be essential for invasion but its exact role has not previously been determined. We identify here a trio of proteins that associate with TgAMA1, at least one of which associates with TgAMA1 at the moving junction. Surprisingly, these new proteins derive not from micronemes, but from the anterior secretory organelles known as rhoptries and specifically, for at least two, from the neck portion of these club-shaped structures. Homologues for these AMA1-associated proteins are found throughout the Apicomplexa strongly suggesting that this moving junction apparatus is a conserved feature of this important class of parasites. Differences between the contributing proteins in different species may, in part, be the result of selective pressure from the different niches occupied by these parasites
Magnetic excitations of Fe_{1+y}Se_xTe_{1-x} in magnetic and superconductive phases
We have used inelastic neutron scattering and muon-spin rotation to compare
the low energy magnetic excitations in single crystals of superconducting
Fe1.01Se0.50Te0.50 and non-superconducting Fe1.10Se0.25Te0.75. We confirm the
existence of a spin resonance in the superconducting phase of
Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of (1/2,1/2,0). The
non-superconducting sample exhibits two incommensurate magnetic excitations at
(1/2,1/2,0)\pm(0.18,-0.18,0) which rise steeply in energy, but no resonance is
observed at low energies. A strongly dispersive low-energy magnetic excitation
is also observed in Fe1.10Se0.25Te0.75 close to the commensurate
antiferromagnetic ordering wavevector (1/2-\delta,0,1/2) where \delta \approx
0.03. The magnetic correlations in both samples are found to be quasi-two
dimensional in character and persist well above the magnetic
(Fe1.10Se0.25Te0.75) and superconducting (Fe1.01Se0.50Te0.50) transition
temperatures.Comment: 10 pages, 4 figure
Ultrafast Spin-To-Charge Conversion at the Surface of Topological Insulator Thin Films
Strong spin-orbit coupling, resulting in the formation of
spin-momentum-locked surface states, endows topological insulators with
superior spin-to-charge conversion characteristics, though the dynamics that
govern it have remained elusive. Here, we present an all-optical method that
enables unprecedented tracking of the ultrafast dynamics of spin-to-charge
conversion in a prototypical topological insulator BiSe/ferromagnetic
Co heterostructure, down to the sub-picosecond timescale. Compared to pure
BiSe or Co, we observe a giant terahertz emission in the
heterostructure than originates from spin-to-charge conversion, in which the
topological surface states play a crucial role. We identify a 0.12-picosecond
timescale that sets a technological speed limit of spin-to-charge conversion
processes in topological insulators. In addition, we show that the
spin-to-charge conversion efficiency is temperature independent in BiSe
as expected from the nature of the surface states, paving the way for designing
next-generation high-speed opto-spintronic devices based on topological
insulators at room temperature.Comment: 19 pages, 4 figure
Human Placental Syncytiotrophoblasts Restrict Toxoplasma gondii Attachment and Replication and Respond to Infection by Producing Immunomodulatory Chemokines
Toxoplasma gondii is a major source of congenital disease worldwide, but the cellular and molecular factors associated with its vertical transmission are largely unknown. In humans, the placenta forms the key interface between the maternal and fetal compartments and forms the primary barrier that restricts the hematogenous spread of microorganisms. Here, we utilized primary human trophoblast (PHT) cells isolated from full-term placentas and human midgestation chorionic villous explants to determine the mechanisms by which human trophoblasts restrict and respond to T. gondii infection. We show that placental syncytiotrophoblasts, multinucleated cells that are in direct contact with maternal blood, restrict T. gondii infection at two distinct stages of the parasite lytic cycle—at the time of attachment and also during intracellular replication. Utilizing comparative transcriptome sequencing (RNA-seq) transcriptional profiling, we also show that human placental trophoblasts from both the second and third trimesters respond uniquely to T. gondii infection compared to trophoblast cell lines, typified by the upregulation of several immunity-related genes. One of the most differentially induced genes was the chemokine CCL22, which relies on the secretion of a parasite effector(s) either during or after invasion for its induction. Collectively, our findings provide new insights into the mechanisms by which the human placenta restricts the vertical transmission of T. gondii at early and late stages of human pregnancy and demonstrate the existence of at least two interferon-independent pathways that restrict T. gondii access to the fetal compartment.
IMPORTANCE Toxoplasma gondii is a major source of congenital disease worldwide and must breach the placental barrier to be transmitted from maternal blood to the developing fetus. The events associated with the vertical transmission of T. gondii are largely unknown. Here, we show that primary human syncytiotrophoblasts, the fetus-derived cells that comprise the primary placental barrier, restrict T. gondii infection at two distinct stages of the parasite life cycle and respond to infection by inducing a unique immunomodulatory transcriptional profile. Collectively, our findings provide important insights into the mechanisms by which human syncytiotrophoblasts restrict T. gondii infection at early and late stages of human pregnancy, identify both permissive and resistant human placental cell types, and identify the placenta-enriched signaling pathways induced in response to infection
Positron-neutrino correlation in the 0^+ \to 0^+ decay of ^{32}Ar
The positron-neutrino correlation in the decay of
Ar was measured at ISOLDE by analyzing the effect of lepton recoil on
the shape of the narrow proton group following the superallowed decay. Our
result is consistent with the Standard Model prediction. For vanishing Fierz
interference we find , which yields improved
constraints on scalar weak interactions
Microstructural analysis of phase separation in iron chalcogenide superconductors
The interplay between superconductivity, magnetism and crystal structure in
iron-based superconductors is a topic of great interest amongst the condensed
matter physics community as it is thought to be the key to understanding the
mechanisms responsible for high temperature superconductivity. Alkali metal
doped iron chalcogenide superconductors exhibit several unique characteristics
which are not found in other iron-based superconducting materials such as
antiferromagnetic ordering at room temperature, the presence of ordered iron
vacancies and high resistivity normal state properties. Detailed
microstructural analysis is essential in order to understand the origin of
these unusual properties. Here we have used a range of complementary scanning
electron microscope based techniques, including high-resolution electron
backscatter di raction mapping, to assess local variations in composition and
lattice parameter with high precision and sub-micron spatial resolution. Phase
separation is observed in the Csx Fe2-ySe2 crystals, with the minor phase
distributed in a plate-like morphology throughout the crystal. Our results are
consistent with superconductivity occurring only in the minority phase.Comment: Accepted for publication in a special edition of Supercond. Sci.
Techno
- …