212 research outputs found
Neutron scattering study of novel magnetic order in Na0.5CoO2
We report polarized and unpolarized neutron scattering measurements of the
magnetic order in single crystals of Na0.5CoO2. Our data indicate that below
T_N=88 K the spins form a novel antiferromagnetic pattern within the CoO2
planes, consisting of alternating rows of ordered and non-ordered Co ions. The
domains of magnetic order are closely coupled to the domains of Na ion order,
consistent with such a two-fold symmetric spin arrangement. Magnetoresistance
and anisotropic susceptibility measurements further support this model for the
electronic ground state.Comment: 4 pages, 4 figure
Multiferroicity in the generic easy-plane triangular lattice antiferromagnet RbFe(MoO4)2
RbFe(MoO4)2 is a quasi-two-dimensional (quasi-2D) triangular lattice
antiferromagnet (TLA) that displays a zero-field magnetically-driven
multiferroic phase with a chiral spin structure. By inelastic neutron
scattering, we determine quantitatively the spin Hamiltonian. We show that the
easy-plane anisotropy is nearly 1/3 of the dominant spin exchange, making
RbFe(MoO4)2 an excellent system for studying the physics of the model 2D
easy-plane TLA. Our measurements demonstrate magnetic-field induced
fluctuations in this material to stabilize the generic finite-field phases of
the 2D XY TLA. We further explain how Dzyaloshinskii-Moriya interactions can
generate ferroelectricity only in the zero field phase. Our conclusion is that
multiferroicity in RbFe(MoO4)2, and its absence at high fields, results from
the generic properties of the 2D XY TLA.Comment: 5 pages, 5 figures, accepted in PRB as a Rapid Communicatio
Direct transition from a disordered to a multiferroic phase on a triangular lattice
Competing interactions and geometric frustration provide favourable
conditions for exotic states of matter. Such competition often causes multiple
phase transitions as a function of temperature and can lead to magnetic
structures that break inversion symmetry, thereby inducing ferroelectricity
[1-4]. Although this phenomenon is understood phenomenologically [3-4], it is
of great interest to have a conceptually simpler system in which
ferroelectricity appears coincident with a single magnetic phase transition.
Here we report the first such direct transition from a paramagnetic and
paraelectric phase to an incommensurate multiferroic in the triangular lattice
antiferromagnet RbFe(MoO4)2 (RFMO). A magnetic field extinguishes the electric
polarization when the symmetry of the magnetic order changes and
ferroelectricity is only observed when the magnetic structure has chirality and
breaks inversion symmetry. Multiferroic behaviour in RFMO provides a
theoretically tractable example of ferroelectricity from competing spin
interactions. A Landau expansion of symmetry-allowed terms in the free energy
demonstrates that the chiral magnetic order of the triangular lattice
antiferromagnet gives rise to a pseudoelectric field, whose temperature
dependence agrees with that observed experimentally.Comment: 16 pages pdf including 3 figure
Spin Gap and Resonance at the Nesting Wavevector in Superconducting FeSe0.4Te0.6
Neutron scattering is used to probe magnetic excitations in
FeSe_{0.4}Te_{0.6} (T_c=14 K). Low energy spin fluctuations are found with a
characteristic wave vector that corresponds to Fermi surface
nesting and differs from Q_m=(\delta,0,0.5) for magnetic ordering in
Fe_{1+y}Te. A spin resonance with \hbar\Omega_0=6.5 meV \approx 5.3 k_BT_c and
\hbar\Gamma=1.25 meV develops in the superconducting state from a normal state
continuum. We show that the resonance is consistent with a bound state
associated with s+/- superconductivity and imperfect quasi-2D Fermi surface
nesting.Comment: 4 pages, 4 figures, Submitted to Phys. Rev. Let
Na content dependence of superconductivity and the spin correlations in Na_{x}CoO_{2}\cdot 1.3H_{2}O
We report systematic measurements using the ^{59}Co nuclear quadrupole
resonance(NQR) technique on the cobalt oxide superconductors Na_{x}CoO_{2}\cdot
1.3H_{2}O over a wide Na content range x=0.25\sim 0.34. We find that T_c
increases with decreasing x but reaches to a plateau for x \leq0.28. In the
sample with x \sim 0.26, the spin-lattice relaxation rate 1/T_1 shows a T^3
variation below T_c and down to T\sim T_c/6, which unambiguously indicates the
presence of line nodes in the superconducting (SC) gap function. However, for
larger or smaller x, 1/T_1 deviates from the T^3 variation below T\sim 2 K even
though the T_c (\sim 4.7 K) is similar, which suggests an unusual evolution of
the SC state. In the normal state, the spin correlations at a finite wave
vector become stronger upon decreasing x, and the density of states at the
Fermi level increases with decreasing x, which can be understood in terms of a
single-orbital picture suggested on the basis of LDA calculation.Comment: version published in J. Phys. Condens. Matter (references updated and
more added
Lavado y desinfección con hipoclorito de sodio de lechuga contaminada con Escherichia coli O157:H7
Aims: to check the effectiveness of recommended hypochlorite concentrations to disinfection of raw vegetables.
Materials and methods: lettuce leaves contaminated with Shiga toxin-producing Escherichia coli O157: H7 were exposed to treatments based on NaClO concentration, pH and time. The measured response was the logarithm of N / N0.
Results: it was observed that chlorine concentration and time have a significant effect (p <0.05). The time was the most important linear variable, it has the highest regression coefficient. The largest reductions were obtained with 200 ppm at pH 7 and 8 in 30 minutes. No reduction reaches the five logarithms suggested for a good sanitizer.
Conclusions: it is not safe to wash with NaClO in the proportions that would be used by a housewife or food establishment. Pathogens as the studied, with low infective dose and adherence, could remain in the contaminated vegetables.Objetivos: comprobar la eficacia de lavado y desinfección en vegetales que se consumen crudos usando concentraciones de hipoclorito recomendadas.
Materiales y métodos: hojas de lechuga contaminadas con Escherichia coli O157:H7 productor de toxina Shiga fueron expuestas a tratamientos basados en tres variables concentración de NaClO, pH y tiempo, combinadas entre sÃ. La respuesta medida fue el logaritmo de N/N0.
Resultados: se observó que la concentración de cloro activo y el tiempo tienen efecto significativo (p<0.05). El tiempo fue la variable lineal más importante, posee el mayor coeficiente de regresión. Las mayores reducciones se obtuvieron con 200 ppm a pH 7 y 8 en 30 minutos. Ninguna reducción alcanza los cinco logaritmos sugeridos para un buen sanitizante.
Conclusiones: para un patógeno como el estudiado, con baja dosis infectiva y adherencia, no es seguro el lavado con NaClO en las proporciones que usarÃa un ama de casa o un establecimiento proveedor de comida
Emergent excitations in a geometrically frustrated magnet
Frustrated systems are ubiquitous and interesting because their behavior is
difficult to predict. Magnetism offers extreme examples in the form of spin
lattices where all interactions between spins cannot be simultaneously
satisfied. Such geometrical frustration leads to macroscopic degeneracies, and
offers the possibility of qualitatively new states of matter whose nature has
yet to be fully understood. Here we have discovered how novel composite spin
degrees of freedom can emerge from frustrated interactions in the cubic spinel
ZnCr2O4. Upon cooling, groups of six spins self-organize into weakly
interacting antiferromagnetic loops whose directors, defined as the unique
direction along which the spins are aligned parallel or antiparallel, govern
all low temperature dynamics. The experimental evidence comes from a
measurement of the magnetic form factor by inelastic neutron scattering. While
the data bears no resemblance to the atomic form factor for chromium, they are
perfectly consistent with the form factor for hexagonal spin loop directors.
The hexagon directors are to a first approximation decoupled from each other
and hence their reorientations embody the long-sought local zero energy modes
for the pyrochlore lattice.Comment: 10 pages, 4 figures upon reques
Structure and Magnetic Order in the NdFeAs(O,F) Superconductor System
The transition temperature Tc~26 K of the recently discovered superconductor
LaFeAs(O,F) has been demonstrated to be extremely sensitive to the lanthanide
ion, reaching 55 K for the Sm containing oxypnictides. Therefore, it is
important to determine how the moment on the lanthanide affects the overall
magnetism in these systems. Here we report a neutron diffraction study of the
Nd oxypnictides. Long ranged antiferromagnetic order is apparent in NdFeAsO
below 1.96 K. Rietveld refinement shows that both Fe and Nd magnetic ordering
are required to describe the observed data with the staggered moment 1.55(4)
Bohr magneton per Nd and 0.9(1) Bohr magneton per Fe at 0.3 K. The other
structural properties such as the tetragonal-orthorhombic distortion are found
to be very similar to those in LaFeAsO. Neither the magnetic ordering nor the
structural distortion occur in the superconducting sample NdFeAsO0.80F0.20 at
any temperatures down to 1.5 K.Comment: 4 pages, 5 figures, 1 table. Identical to v3, correct HTML front
matter; Scientific data and conclusions the same as in v
Thermodynamic properties of Ba1-xMxFe2As2 (M = La and K)
The specific heat of BaFeAs single crystal, electron-doped
BaLaFeAs and hole-doped BaKFeAs
polycrystals were measured. For undoped BaFeAs single crystal, a very
sharp specific heat peak was observed at 136 K. This is attributed to the
structural and antiferromagnetic transitions occurring at the same temperature.
of the electron-doped non-superconducting
BaLaFeAs also shows a small peak at 120 K, indicating a
similar but weaker structural/antiferromagnetic transition. For the hole-doped
superconducting BaKFeAs, a clear peak of was
observed at = 36 K, which is the highest peak seen at superconducting
transition for iron-based high- superconductors so far. The electronic
specific heat coefficient and Debye temperature of these
compounds were obtained from the low temperature data
- …