70 research outputs found
Elementary Excitations of Ferromagnetic Metal Nanoparticles
We present a theory of the elementary spin excitations in transition metal
ferromagnet nanoparticles which achieves a unified and consistent quantum
description of both collective and quasiparticle physics. The theory starts by
recognizing the essential role played by spin-orbit interactions in determining
the energies of ferromagnetic resonances in the collective excitation spectrum
and the strength of their coupling to low-energy particle-hole excitations. We
argue that a crossover between Landau-damped ferromagnetic resonance and
pure-state collective magnetic excitations occurs as the number of atoms in
typical transition metal ferromagnet nanoparticles drops below approximately
, approximately where the single-particle level spacing, ,
becomes larger than, , where is the
ferromagnetic resonance frequency and is the Gilbert damping
parameter. We illustrate our ideas by studying the properties of semi-realistic
model Hamiltonians, which we solve numerically for nanoparticles containing
several hundred atoms. For small nanoparticles, we find one isolated
ferromagnetic resonance collective mode below the lowest particle-hole
excitation energy, at meV. The spectral weight of
this pure excitation nearly exhausts the transverse dynamical susceptibility
spectral weight. As approaches , the
ferromagnetic collective excitation is more likely to couple strongly with
discrete particle-hole excitations. In this regime the distinction between the
two types of excitations blurs. We discuss the significance of this picture for
the interpretation of recent single-electron tunneling experiments.Comment: 19 pages, 13 figure
Magnetization orientation dependence of the quasiparticle spectrum and hysteresis in ferromagnetic metal nanoparticles
We use a microscopic Slater-Koster tight-binding model with short-range
exchange and atomic spin-orbit interactions that realistically captures generic
features of ferromagnetic metal nanoparticles to address the mesoscopic physics
of magnetocrystalline anisotropy and hysteresis in nanoparticle quasiparticle
excitation spectra. Our analysis is based on qualitative arguments supported by
self-consistent Hartree-Fock calculations for nanoparticles containing up to
260 atoms. Calculations of the total energy as a function of magnetization
direction demonstrate that the magnetic anisotropy per atom fluctuates by
several percents when the number of electrons in the particle changes by one,
even for the largest particles we consider. Contributions of individual
orbitals to the magnetic anisotropy are characterized by a broad distribution
with a mean more than two orders of magnitude smaller than its variance and
with no detectable correlations between anisotropy contribution and
quasiparticle energy. We find that the discrete quasiparticle excitation
spectrum of a nanoparticle displays a complex non-monotonic dependence on an
external magnetic field, with abrupt jumps when the magnetization direction is
reversed by the field, explaining recent spectroscopic studies of magnetic
nanoparticles. Our results suggests the existence of a broad cross-over from a
weak spin-orbit coupling to a strong spin-orbit coupling regime, occurring over
the range from approximately 200- to 1000-atom nanoparticles.Comment: 39 pages, 18 figures, to be published in Physical Review
Chern Numbers for Spin Models of Transition Metal Nanomagnets
We argue that ferromagnetic transition metal nanoparticles with fewer than
approximately 100 atoms can be described by an effective Hamiltonian with a
single giant spin degree of freedom. The total spin of the effective
Hamiltonian is specified by a Berry curvature Chern number that characterizes
the topologically non-trivial dependence of a nanoparticle's many-electron
wavefunction on magnetization orientation. The Berry curvatures and associated
Chern numbers have a complex dependence on spin-orbit coupling in the
nanoparticle and influence the semiclassical Landau-Liftshitz equations that
describe magnetization orientation dynamics
Orbital and spin contributions to the -tensors in metal nanoparticles
We present a theoretical study of the mesoscopic fluctuations of -tensors
in a metal nanoparticle. The calculations were performed using a semi-realistic
tight-binding model, which contains both spin and orbital contributions to the
-tensors. The results depend on the product of the spin-orbit scattering
time and the mean-level spacing , but are
otherwise weakly affected by the specific shape of a {\it generic}
nanoparticle. We find that the spin contribution to the -tensors agrees with
Random Matrix Theory (RMT) predictions. On the other hand, in the strong
spin-orbit coupling limit , the
orbital contribution depends crucially on the space character of the
quasi-particle wavefunctions: it levels off at a small value for states of
character but is strongly enhanced for states of character. Our numerical
results demonstrate that when orbital coupling to the field is included, RMT
predictions overestimate the typical -factor of orbitals that have dominant
-character. This finding points to a possible source of the puzzling
discrepancy between theory and experiment.Comment: 21 pages, 6 figures; accepted for publication in Physical Review
There are three major Neisseria gonorrhoeae Ξ²-lactamase plasmid variants which are associated with specific lineages and carry distinct TEM alleles
Neisseria gonorrhoeaeΒ is a significant threat to global health with an estimated incidence of over 80βmillion cases each year and high levels of antimicrobial resistance. The gonococcal Ξ²-lactamase plasmid, pbla, carries the TEM Ξ²-lactamase, which requires only one or two amino acid changes to become an extended-spectrum Ξ²-lactamase (ESBL); this would render last resort treatments for gonorrhoea ineffective. Although pblaΒ is not mobile, it can be transferred by the conjugative plasmid, pConj, found inΒ N. gonorrhoeae. Seven variants of pblaΒ have been described previously, but little is known about their frequency or distribution in the gonococcal population. We characterised sequences of pblaΒ variants and devised a typing scheme, Ng_pblaST that allows their identification from whole genome short-read sequences. We implemented Ng_pblaST to assess the distribution of pblaΒ variants in 15β532 gonococcal isolates. This demonstrated that only three pblaΒ variants commonly circulate in gonococci, which together account for >99β% of sequences. The pblaΒ variants carry different TEM alleles and are prevalent in distinct gonococcal lineages. Analysis of 2758 pbla-containing isolates revealed the co-occurrence of pblaΒ with certain pConj types, indicating co-operativity between pblaΒ and pConj variants in the spread of plasmid-mediated AMR inΒ N. gonorrhoeae. Understanding the variation and distribution of pblaΒ is essential for monitoring and predicting the spread of plasmid-mediated Ξ²-lactam resistance inΒ N. gonorrhoeae
Transition-metal dimers and physical limits on magnetic anisotropy
Recent advances in nanoscience have raised interest in the minimum bit size
required for classical information storage, i.e. for bistability with
suppressed quantum tunnelling and energy barriers that exceed ambient
temperatures. In the case of magnetic information storage much attention has
centred on molecular magnets[1] with bits consisting of ~ 100 atoms, magnetic
uniaxial anisotropy energy barriers ~ 50 K, and very slow relaxation at low
temperatures. In this article we draw attention to the remarkable magnetic
properties of some transition metal dimers which have energy barriers
approaching ~ 500 K with only two atoms. The spin dynamics of these ultra small
nanomagnets is strongly affected by a Berry phase which arises from
quasi-degeneracies at the electronic Highest Occupied Molecular Orbital (HOMO)
energy. In a giant spin-approximation, this Berry phase makes the effective
reversal barrier thicker. [1] Gatteschi, D., Sessoli, R. & Villain, J.
Molecular Nanomagnets. (Oxford, New York 2006).Comment: 14 pages, 1 figur
A Three-Way Comparison of Tuberculin Skin Testing, QuantiFERON-TB Gold and T-SPOT.TB in Children
BACKGROUND: There are limited data comparing the performance of the two commercially available interferon gamma (IFN-gamma) release assays (IGRAs) for the diagnosis of tuberculosis (TB) in children. We compared QuantiFERON-TB gold In Tube (QFT-IT), T-SPOT.TB and the tuberculin skin test (TST) in children at risk for latent TB infection or TB disease. METHODS AND FINDINGS: The results of both IGRAs were compared with diagnosis assigned by TST-based criteria and assessed in relation to TB contact history. Results from the TST and at least one assay were available for 96 of 100 children. Agreement between QFT-IT and T-SPOT.TB was high (93% agreement, kappa = 0.83). QFT-IT and T-SPOT.TB tests were positive in 8 (89%) and 9 (100%) children with suspected active TB disease. There was moderate agreement between TST and either QFT-IT (75%, kappa = 0.50) or T-SPOT.TB (75%, kappa = 0.51). Among 38 children with TST-defined latent TB infection, QFT-IT gold and T-SPOT.TB assays were positive in 47% and 39% respectively. Three TST-negative children were positive by at least one IGRA. Children with a TB contact were more likely than children without a TB contact to have a positive IGRA (QFT-IT LR 3.9; T-SPOT.TB LR 3.9) and a positive TST (LR 1.4). Multivariate linear regression analysis showed that the magnitude of both TST induration and IGRA IFN-gamma responses was significantly influenced by TB contact history, but only the TST was influenced by age. CONCLUSIONS: Although a high level of agreement between the IGRAs was observed, they are commonly discordant with the TST. The correct interpretation of a negative assay in a child with a positive skin test in clinical practice remains challenging and highlights the need for longitudinal studies to determine the negative predictive value of IGRAs
Prospective Monitoring Reveals Dynamic Levels of T Cell Immunity to Mycobacterium Tuberculosis in HIV Infected Individuals
Monitoring of latent Mycobacterium tuberculosis infection may prevent disease. We tested an ESAT-6 and CFP-10-specific IFN-Ξ³ Elispot assay (RD1-Elispot) on 163 HIV-infected individuals living in a TB-endemic setting. An RD1-Elispot was performed every 3 months for a period of 3β21 months. 62% of RD1-Elispot negative individuals were positive by cultured Elispot. Fluctuations in T cell response were observed with rates of change ranging from β150 to +153 spot-forming cells (SFC)/200,000 PBMC in a 3-month period. To validate these responses we used an RD1-specific real time quantitative PCR assay for monokine-induced by IFN-Ξ³ (MIG) and IFN-Ξ³ inducible protein-10 (IP10) (MIG: rβ=β0.6527, pβ=β0.0114; IP-10: rβ=β0.6967, pβ=β0.0056; IP-10+MIG: rβ=β0.7055, pβ=β0.0048). During follow-up 30 individuals were placed on ARVs and 4 progressed to active TB. Fluctuations in SFC did not correlate with CD4 count, viral load, treatment initiation, or progression to active TB. The RD1-Elispot appears to have limited value in this setting
Identification of T-Cell Antigens Specific for Latent Mycobacterium Tuberculosis Infection
BACKGROUND: T-cell responses against dormancy-, resuscitation-, and reactivation-associated antigens of Mycobacterium tuberculosis are candidate biomarkers of latent infection in humans. METHODOLOGY/PRINCIPAL FINDINGS: We established an assay based on two rounds of in vitro restimulation and intracellular cytokine analysis that detects T-cell responses to antigens expressed during latent M. tuberculosis infection. Comparison between active pulmonary tuberculosis (TB) patients and healthy latently M. tuberculosis-infected donors (LTBI) revealed significantly higher T-cell responses against 7 of 35 tested M. tuberculosis latency-associated antigens in LTBI. Notably, T cells specific for Rv3407 were exclusively detected in LTBI but not in TB patients. The T-cell IFNgamma response against Rv3407 in individual donors was the most influential factor in discrimination analysis that classified TB patients and LTBI with 83% accuracy using cross-validation. Rv3407 peptide pool stimulations revealed distinct candidate epitopes in four LTBI. CONCLUSIONS: Our findings further support the hypothesis that the latency-associated antigens can be exploited as biomarkers for LTBI
Whole genome sequencing to investigate the emergence of clonal complex 23 Neisseria meningitidis serogroup Y disease in the United States
In the United States, serogroup Y, ST-23 clonal complex Neisseria meningitidis was responsible for an increase in meningococcal disease incidence during the 1990s. This increase was accompanied by antigenic shift of three outer membrane proteins, with a decrease in the population that predominated in the early 1990s as a different population emerged later in that decade. To understand factors that may have been responsible for the emergence of serogroup Y disease, we used whole genome pyrosequencing to investigate genetic differences between isolates from early and late N. meningitidis populations, obtained from meningococcal disease cases in Maryland in the 1990s. The genomes of isolates from the early and late populations were highly similar, with 1231 of 1776 shared genes exhibiting 100% amino acid identity and an average ΟN = 0.0033 and average ΟS = 0.0216. However, differences were found in predicted proteins that affect pilin structure and antigen profile and in predicted proteins involved in iron acquisition and uptake. The observed changes are consistent with acquisition of new alleles through horizontal gene transfer. Changes in antigen profile due to the genetic differences found in this study likely allowed the late population to emerge due to escape from population immunity. These findings may predict which antigenic factors are important in the cyclic epidemiology of meningococcal disease
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