315 research outputs found
Magnetic frustration and spontaneous rotational symmetry breaking in PdCrO2
In the triangular layered magnet PdCrO2 the intralayer magnetic interactions
are strong, however the lattice structure frustrates interlayer interactions.
In spite of this, long-range, 120 antiferromagnetic order condenses at
~K. We show here through neutron scattering measurements under
in-plane uniaxial stress and in-plane magnetic field that this occurs through a
spontaneous lifting of the three-fold rotational symmetry of the nonmagnetic
lattice, which relieves the interlayer frustration. We also show through
resistivity measurements that uniaxial stress can suppress thermal magnetic
disorder within the antiferromagnetic phase.Comment: 9 pages, 9 figure
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Magnetoelastic coupling and ferromagnetic-type in-gap spin excitations in multiferroic α-Cu2V2O7
We investigate magnetoelectric coupling and low-energy magnetic excitations in multiferroic α-Cu2V2O7 by detailed thermal expansion, magnetostriction, specific heat and magnetization measurements in magnetic fields up to 15 T and by high-field/high-frequency electron spin resonance studies. Our data show negative thermal expansion in the temperature range ≤200 K under study. Well-developed anomalies associated with the onset of multiferroic order (canted antiferromagnetism with a significant magnetic moment and ferroelectricity) imply pronounced coupling to the structure. We detect anomalous entropy changes in the temperature regime up to ∼80 K which significantly exceed the spin entropy. Failure of Grüneisen scaling further confirms that several dominant ordering phenomena are concomitantly driving the multiferroic order. By applying external magnetic fields, anomalies in the thermal expansion and in the magnetization are separated. Noteworthy, the data clearly imply the development of a canted magnetic moment at temperatures above the structural anomaly. Low-field magnetostriction supports the scenario of exchange-striction driven multiferroicity. We observe low-energy magnetic excitations well below the antiferromagnetic gap, i.e., a ferromagnetic-type resonance branch associated with the canted magnetic moment arising from Dzyaloshinsii-Moriya (DM) interactions. The anisotropy parameter meV indicates a sizeable ratio of DM- and isotropic magnetic exchange
Role of crystal field ground state in the classical spin-liquid behavior of a quasi-one dimensional spin-chain system Sr3NiPtO6
The spin-chain compound Sr3NiPtO6 is known to have a nonmagnetic ground
state. We have investigated the nature of ground state of Sr3NiPtO6 using
magnetic susceptibility , heat capacity , muon spin
relaxation (SR) and inelastic neutron scattering (INS) measurements. The
and do not exhibit any pronounced anomaly that can be
associated with a phase transition to a magnetically ordered state. Our SR
data confirm the absence of long-range magnetic ordering down to 0.04 K.
Furthermore, the muon spin relaxation rate increases below 20 K and exhibits
temperature independent behavior at low temperature, very similar to that
observed in a quantum spin-liquid system. The INS data show a large excitation
near 8~meV, and the analysis of the INS data reveals a singlet CEF ground state
with a first excited CEF doublet state at = 7.7 meV. The
estimated CEF parameters reveal a strong planar anisotropy in the calculated
, consistent with the reported behavior of the of single
crystal Sr3NiPtO6. We propose that the nonmagnetic singlet ground state and a
large (much larger than the exchange interaction
) are responsible for the absence of long-range magnetic
ordering and can mimic a classical spin-liquid behavior in this quasi-1D spin
chain system Sr3NiPtO6. The classical spin-liquid ground state observed in
Sr3NiPtO6 is due to the single-ion property, which is different from the
quantum spin-liquid ground state observed in geometrically frustrated systems,
where two-ion exchanges play an important role.Comment: 11 pages, 10 figures, 1 tabl
Neisseria gonorrhoeae lipooligosaccharide glycan epitopes recognized by bactericidal IgG antibodies elicited by the meningococcal group B-directed vaccine, MenB-4C
IntroductionOuter membrane vesicles (OMVs) of Neisseria meningitidis in the group B-directed vaccine MenB-4C (BexseroR) protect against infections with Neisseria gonorrhoeae. The immunological basis for protection remains unclear. N. meningitidis OMV vaccines generate human antibodies to N. meningitidis and N. gonorrhoeae lipooligosaccharide (LOS/endotoxin), but the structural specificity of these LOS antibodies is not defined.MethodsTen paired human sera obtained pre- and post-MenB-4C immunization were used in Western blots to probe N. meningitidis and N. gonorrhoeae LOS. Post-MenB-4C sera (7v5, 19v5, and 17v5), representing individual human variability in LOS recognition, were then used to interrogate structurally defined LOSs of N. meningitidis and N. gonorrhoeae strains and mutants and studied in bactericidal assays.Results and discussionPost-MenB-4C sera recognized both N. meningitidis and N. gonorrhoeae LOS species, ~10% of total IgG to gonococcal OMV antigens. N. meningitidis and N. gonorrhoeae LOSs were broadly recognized by post-IgG antibodies, but with individual variability for LOS structures. Deep truncation of LOS, specifically a rfaK mutant without α-, β-, or γ-chain glycosylation, eliminated LOS recognition by all post-vaccine sera. Serum 7v5 IgG antibodies recognized the unsialyated L1 α-chain, and a 3-PEA-HepII or 6-PEA-HepII was part of the conformational epitope. Replacing the 3-PEA on HepII with a 3-Glc blocked 7v5 IgG antibody recognition of N. meningitidis and N. gonorrhoeae LOSs. Serum 19v5 recognized lactoneotetrose (LNT) or L1 LOS-expressing N. meningitidis or N. gonorrhoeae with a minimal α-chain structure of Gal-Glc-HepI (L8), a 3-PEA-HepII or 6-PEA-HepII was again part of the conformational epitope and a 3-Glc-HepII blocked 19v5 antibody binding. Serum 17v5 LOS antibodies recognized LNT or L1 α-chains with a minimal HepI structure of three sugars and no requirement for HepII modifications. These LOS antibodies contributed to the serum bactericidal activity against N. gonorrhoeae. The MenB-4C vaccination elicits bactericidal IgG antibodies to N. gonorrhoeae conformational epitopes involving HepI and HepII glycosylated LOS structures shared between N. meningitidis and N. gonorrhoeae. LOS structures should be considered in next-generation gonococcal vaccine design
Ab initio prediction of Boron compounds arising from Borozene: Structural and electronic properties
Structure and electronic properties of two unusual boron clusters obtained by
fusion of borozene rings has been studied by means of first principles
calculations, based on the generalized-gradient approximation of the density
functional theory, and the semiempirical tight-binding method was used for the
transport calculations. The role of disorder has also been considered with
single vacancies and substitutional atoms. Results show that the pure boron
clusters are topologically planar and characterized by (3c-2e) bonds, which can
explain, together with the aromaticity (estimated by means of NICS), the
remarkable cohesive energy values obtained. Such feature makes these systems
competitive with the most stable boron clusters to date. On the contrary, the
introduction of impurities compromises stability and planarity in both cases.
The energy gap values indicate that these clusters possess a semiconducting
character, while when the larger system is considered, zero-values of the
density of states are found exclusively within the HOMO-LUMO gap. Electron
transport calculations within the Landauer formalism confirm these indications,
showing semiconductor-like low bias differential conductance for these
stuctures. Differences and similarities with Carbon clusters are highlighted in
the discussion.Comment: 10 pages, 2 tables, 5 figure
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