216 research outputs found
Incommensurate Spin Ordering and Fluctuations in underdoped La_{2-x}Ba_{x}CuO_{4}
Using neutron scattering techniques, we have studied incommensurate spin
ordering as well as low energy spin dynamics in single crystal underdoped \LBCO
with x0.095 and 0.08; high temperature superconductors with T 27
K and 29 K respectively. Static two dimensional incommensurate magnetic order
appears below T=39.5 0.3 K in \LBCO (x=0.095) and a similar
temperature for x=0.08 within the low temperature tetragonal phase. The spin
order is unaffected by either the onset of superconductivity or the application
of magnetic fields of up to 7 Tesla applied along the c-axis in the x=0.095
sample. Such magnetic field {\it independent} behaviour is in marked contrast
with the field induced enhancement of the staggered magnetisation observed in
the related \LSCO system, indicating this phenomenon is not a universal
property of cuprate superconductors. Surprisingly, we find that
incommensurability is only weakly dependent on doping relative to
\LSCO. Dispersive excitations in \LBCO (x=0.095) at the same incommensurate
wavevector persist up to at least 60 K. The dynamical spin susceptibility of
the low energy spin excitations saturates below \tc, in a similar manner to
that seen in the superconducting state of LaCuO.Comment: 9 pages, 7 figures, submitted to PRB, figures update
Scattering phase shift for relativistic exponential-type separable potentials
The J-matrix method of scattering is used to obtain analytic expressions for
the phase shift of two classes of relativistic exponential-type separable
potentials whose radial component is either of the general form r^(n-1)exp(-r)
or r^(2n)exp(-r^2), where n = 0, 1, or 2. The rank of these separable
potentials is n + 1. The nonrelativistic limit is obtained and shown to be
identical to the nonrelativistic phase shift. An exact numerical evaluation for
higher order potentials (n > 2) can also be obtained in a simple way as
illustrated for the case n = 3.Comment: Accepted for publication in J. Phys. A, to appear in January 2002.
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Fermentación controlada de aceitunas verdes picholine marroquÃes sometidas a choque térmico e inoculadas sin sal
The present work reports the controlled fermentation of heat-shocked, unsalted and inoculated green olives. The effects of heat-shock (60, 70 and 80 °C three times for 5 min), inoculation with the oleuropeinolytic strain of L. plantarum FSO175 (L.p-FSO175) and the addition of Cell-Free Supernatant of C. pelliculosa L18 (CFS of C.p-L18) on the fermentation process of unsalted green olives were examined. The results showed a drastic reduction in the initial indigenous Enterobacteria, and an improvement in the acidification of heat-shocked olives at 70 and 80 °C, when compared to 60 °C. The inoculation with L.p-FSO175 and addition of CFS of C.p-L18 enhanced the fermentation and preservation of unsalted green olives, indicated by a significant decrease in pH, increase in free acidity and total disappearance of Enterobacteria. The heat-shock treatment at high temperature (80 °C), inoculation with L.p-FSO175 and addition of CFS of C.p-L18 led to the best reduction in bitterness, and favorable color changes (L, a, and b) in fermented olives. This sequential method led to more appreciated sensory characteristics (mainly bitterness and color) of fermented olives, lower spoilage incidence in olives, and reduced fermentation time to 50 days, and therefore may be suitable to control the fermentation of unsalted green olives of the Moroccan picholine variety.El presente trabajo reporta la fermentación controlada de aceitunas verdes sometidas a choque térmico, sin salar e inoculadas. Se estudian los efectos del choque térmico (60 °C, 70 °C y 80 °C tres veces durante 5 min), la inoculación con cepa oleuropeinolÃtica de L. plantarum FSO175 (L.p-FSO175) y la adición de sobrenadante libre de células de C. pelliculosa L18 (CFS de C.p-L18), sobre el proceso de fermentación de aceitunas verdes sin salar. Los resultados mostraron la drástica reducción de las enterobacterias autóctonas iniciales, y la mejora de la acidificación de las aceitunas sometidas a choque térmico de 70 °C y 80 °C, en comparación con 60 °C. La inoculación con L.p-FSO175 y la adición de CFS de C.p-L18 mejoró la fermentación y conservación de las aceitunas verdes sin salar, indicada por una disminución significativa del pH, aumento de la acidez libre y desaparición total de enterobacterias. El choque térmico a alta temperatura (80 °C), la inoculación con L.p-FSO175 y la adición de CFS de C.p-L18 condujeron a una mejor reducción del amargor y cambios de color favorables (L, a y b) en aceitunas fermentadas. Este método secuencial, que permitió apreciar las caracterÃsticas sensoriales (principalmente amargor y color) de las aceitunas fermentadas, y una menor incidencia de deterioro en las aceitunas, y redujo el tiempo de fermentación a 50 dÃas, puede ser adecuado para controlar la fermentación de aceitunas verdes sin salar de Marruecos, variedad picholine
Two Dimensional Incommensurate and Three Dimensional Commensurate Magnetic Order and Fluctuations in
We present neutron scattering measurements on single crystals of lightly
doped , with . These reveal the
evolution of the magnetism in this prototypical doped Mott insulator from a
three dimensional (3D) commensurate (C) antiferromagnetic ground state, which
orders at a relatively high TN, to a two dimensional (2D) incommensurate (IC)
ground state with finite ranged static correlations, which appear below a
relatively low effective TN. At low temperatures, the 2D IC magnetism co-exists
with the 3D C magnetism for doping concentrations as low as ? 0.0125. We find
no signal of a 3D C magnetic ground state by x ? 0.025, consistent with
the upper limit of x ? 0.02 observed in the sister family of doped Mott
insulators, . The 2D IC ground states observed for
are diagonal, and are rotated by 45 degrees within
the orthorhombic basal plane compared with those previously reported for
samples with superconducting ground states: , with $0.05
\leq? x \leq? 0.095. We construct a phase diagram based solely on magnetic
order parameter measurements, which displays much of the complexity of standard
high temperature superconductivity phase diagrams discussed in the literature.
Analysis of high energy-resolution inelastic neutron scattering at moderately
low temperatures shows a progressive depletion of the very low energy dynamic
magnetic susceptibility as x increases from 0.0125 to 0.035. This low energy,
dynamic susceptibility falls off? with increasing temperature on a scale much
higher than the effective 2D IC TN appropriate to these materials. Appreciable
dynamic 2D IC magnetic fluctuations inhabit much of the "pseudogap" regime of
the phase diagram.Comment: 12 pages, 10 figure
Co-existing Singlet and Ordered S=1/2 Moments in the Ground State of the Triclinic Quantum Magnet CuMoO4
CuMoO4 is a triclinic quantum magnet based on S = 1/2 moments at the Cu2+
site. It has recently attracted interest due to the remarkable changes in its
chromic and volumetric properties at high temperatures, and in its magnetic
properties at low temperatures. This material exhibits a first order structural
phase transition at T_C ~ 190 K as well as a magnetic phase transition at T_N ~
1.75 K. We report low temperature heat capacity measurements as well as
extensive elastic and inelastic neutron scattering measurements on powder
samples taken above and below T_N. We observe neutron diffraction consistent
with a simple (1/2, 0, 0) antiferromagnetic structure indicating a doubling of
the a-axis periodicity below T_N. In addition, inelastic neutron scattering
above a spin gap of ~ 2.3 meV is consistent with triplet excitations out of
paired S = 1/2 moments which form singlet dimers. Low lying spin wave
excitations are also observed and these originate from ordered S = 1/2 moments
below T_N. Taken together these measurements show the ground state of CuMoO4 to
display both non-magnetic singlets, and ferromagnetically-coupled spins
coexisting within an antiferromagnetic structure below T_N ~ 1.75 K.Comment: 7 pages, 7 figure
Magnetic Order and Fluctuations in the Presence of Quenched Disorder in the Kagome Staircase System (Co(1-x)Mg(x))3V2O8
Co3V2O8 is an orthorhombic magnet in which S=3/2 magnetic moments reside on
two crystallographically inequivalent Co2+ sites, which decorate a stacked,
buckled version of the two dimensional kagome lattice, the stacked kagome
staircase. The magnetic interactions between the Co2+ moments in this structure
lead to a complex magnetic phase diagram at low temperature, wherein it
exhibits a series of five transitions below 11 K that ultimately culminate in a
simple ferromagnetic ground state below T~6.2 K. Here we report magnetization
measurements on single and polycrystalline samples of (Co(1-x)Mg(x))3V2O8 for
x<0.23, as well as elastic and inelastic neutron scattering measurements on
single crystals of magnetically dilute (Co(1-x)Mg(x))3V2O8 for x=0.029 and
x=0.194, in which non-magnetic Mg2+ ions substitute for magnetic Co2+. We find
that a dilution of 2.9% leads to a suppression of the ferromagnetic transition
temperature by ~15% while a dilution level of 19.4% is sufficient to destroy
ferromagnetic long-range order in this material down to a temperature of at
least 1.5 K. The magnetic excitation spectrum is characterized by two spin-wave
branches in the ordered phase for (Co(1-x)Mg(x))3V2O8 (x=0.029), similar to
that of the pure x=0 material, and by broad diffuse scattering at temperatures
below 10 K in (Co(1-x)Mg(x))3V2O8 (x=0.194). Such a strong dependence of the
transition temperatures to long range order in the presence of quenched
non-magnetic impurities is consistent with two-dimensional physics driving the
transitions. We further provide a simple percolation model that
semi-quantitatively explains the inability of this system to establish
long-range magnetic order at the unusually-low dilution levels which we observe
in our experiments.Comment: 10 pages, 13 figure
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