48 research outputs found
Mean-field Study of Charge, Spin, and Orbital Orderings in Triangular-lattice Compounds ANiO2 (A=Na, Li, Ag)
We present our theoretical results on the ground states in layered
triangular-lattice compounds ANiO2 (A=Na, Li, Ag). To describe the interplay
between charge, spin, orbital, and lattice degrees of freedom in these
materials, we study a doubly-degenerate Hubbard model with electron-phonon
couplings by the Hartree-Fock approximation combined with the adiabatic
approximation. In a weakly-correlated region, we find a metallic state
accompanied by \sqroot3x\sqroot3 charge ordering. On the other hand, we obtain
an insulating phase with spin-ferro and orbital-ferro ordering in a wide range
from intermediate to strong correlation. These phases share many
characteristics with the low-temperature states of AgNiO2 and NaNiO2,
respectively. The charge-ordered metallic phase is stabilized by a compromise
between Coulomb repulsions and effective attractive interactions originating
from the breathing-type electronphonon coupling as well as the Hund's-rule
coupling. The spin-orbital-ordered insulating phase is stabilized by the
cooperative effect of electron correlations and the Jahn-Teller coupling, while
the Hund's-rule coupling also plays a role in the competition with other
orbital-ordered phases. The results suggest a unified way of understanding a
variety of low-temperature phases in ANiO2. We also discuss a keen competition
among different spin-orbital-ordered phases in relation to a puzzling behavior
observed in LiNiO2
Spin dynamics of frustrated easy-axis triangular antiferromagnet 2H-AgNiO2 explored by inelastic neutron scattering
We report inelastic neutron scattering measurements of the spin dynamics in
the layered hexagonal magnet 2H-AgNiO2 which has stacked triangular layers of
antiferromagnetically-coupled Ni2+ spins (S=1) ordered in a collinear
alternating stripe pattern. We observe a broad band of magnetic excitations
above a small gap of 1.8 meV and extending up to 7.5 meV, indicating strongly
dispersive excitations. The measured dispersions of the boundaries of the
powder-averaged spectrum can be quantitatively explained by a linear spin-wave
dispersion for triangular layers with antiferromagnetic nearest- and weak
next-nearest neighbor couplings, a strong easy-axis anisotropy and additional
weak inter-layer couplings. The resulting dispersion relation has global minima
not at magnetic Bragg wavevectors but at symmetry-related soft points and we
attribute this anomalous feature to the strong competition between the
easy-axis anisotropy and the frustrated antiferromagnetic couplings. We have
also calculated the quantum corrections to the dispersion relation to order 1/S
in spin-wave theory by extending the work of Chubukov and Jolicoeur [Phys. Rev.
B v46, 11137 (1992)] and find that the presence of easy-axis anisotropy
significantly reduces the quantum renormalizations predicted for the isotropic
model.Comment: 17 pages and 15 figures. To appear in Physical Review
Interplay between localized and itinerant d electrons in a frustrated metallic antiferromagnet, 2H-AgNiO2
We report the electronic and magnetic behaviour of the frustrated triangular
metallic antiferromagnet 2H-AgNiO2 in high magnetic fields (54 T) using
thermodynamic and transport measurements. Here localized d electrons are
arranged on an antiferromagnetic triangular lattice nested inside a honeycomb
lattice with itinerant d electrons. When the magnetic field is along the easy
axis we observe a cascade of field-induced transitions, attributed to the
competition between easy-axis anisotropy, geometrical frustration and coupling
of the localized and itinerant system. The quantum oscillations data suggest
that the Fermi surface is reconstructed by the magnetic order but in high
fields magnetic breakdown orbits are possible. The itinerant electrons are
extremely sensitive to scattering by spin fluctuations and a significant mass
enhancement (~ 3) is found.Comment: 4 page
Penetration of Meropenem into Epithelial Lining Fluid of Patients with Ventilator-Associated Pneumonia ▿
Antibiotic penetration to the infection site is critical for obtaining a good clinical outcome in patients with ventilator-associated pneumonia (VAP). Surprisingly few studies have quantified the penetration of β-lactam agents into the lung, as measured by the ratio of area under the concentration-time curve (AUC) in epithelial lining fluid (ELF) to AUC in plasma (AUCELF/AUCplasma ratio). These have typically involved noninfected patients. This study examines the penetration and pharmacodynamics of meropenem in the ELF among patients with VAP. Meropenem plasma and ELF concentration-time data were obtained from patients in a multicenter clinical trial. Concentration-time profiles in plasma and ELF were simultaneously modeled using a three-compartment model with zero-order infusion and first-order elimination and transfer (big nonparametric adaptive grid [BigNPAG]). A Monte Carlo simulation was performed to estimate the range of ELF/plasma penetration ratios one would expect to observe in patients with VAP, as measured by the AUCELF/AUCplasma ratio. The range of AUCELF/AUCplasma penetration ratios predicted by the Monte Carlo simulation was large. The 10th percentile of lung penetration was 3.7%, while the 90th percentile of penetration was 178%. The variability of ELF penetration is such that if relatively high ELF exposure targets are required to attain multilog kill or resistance suppression for bacteria like Pseudomonas aeruginosa, then even receiving the largest licensed dose of meropenem with an optimal prolonged infusion may not result in target attainment for a substantial fraction of the population
Interplay between localized and itinerant d electrons in a frustrated metallic antiferromagnet, 2H-AgNiO2
We report the electronic and magnetic behaviour of the frustrated triangular metallic antiferromagnet 2H-AgNiO2 in high magnetic fields (54 T) using thermodynamic and transport measurements. Here localized d electrons are arranged on an antiferromagnetic triangular lattice nested inside a honeycomb lattice with itinerant d electrons. When the magnetic field is along the easy axis we observe a cascade of field-induced transitions, attributed to the competition between easy-axis anisotropy, geometrical frustration and coupling of the localized and itinerant system. The quantum oscillations data suggest that the Fermi surface is reconstructed by the magnetic order but in high fields magnetic breakdown orbits are possible. The itinerant electrons are extremely sensitive to scattering by spin fluctuations and a significant mass enhancement (~ 3) is found