82 research outputs found
Épocas de floração de cultivares de oliveira em Castelo Branco, Elvas e Santarém
Comunicação apresentada no III Simpósio Nacional de Olivicultura que decorreu em Castelo Branco, de 29 a 31 de Outubro, na Escola Superior Agrária do Instituto Politécnico de Castelo Branco.A época de floração da oliveira está determinada pelas características genéticas
inerentes a cada cultivar e é influenciada por factores climatéricos, variáveis de ano para
ano. Esta influência do clima pode provocar importantes modificações anuais nas
épocas de floração, aspecto fundamental para a calendarização das técnicas culturais do
olival. Para além desta influência, a caracterização das épocas de floração é, também,
importante para o estudo e conhecimento da adaptação de diferentes cultivares a uma
região e para os programas de melhoramento genético, entre outros. Neste trabalho
caracteriza-se a época de floração de cinco cultivares de oliveira, nas regiões de Castelo
Branco, Elvas e Santarém, durante os anos de 2000, 2001 e 2002. Os resultados obtidos
revelaram a existência de diferenças nas datas de floração e na duração de floração,
entre cultivares e regiões, mantendo-se constante a ordem de entrada das cultivares em
floração. As condições climatéricas prevalecentes no período de Janeiro a Maio,
nomeadamente as temperaturas, condicionaram o calendário da floração, exercendo
uma marcada influência no momento de início da floração e na duração do período de
floração
Competing Magnetic Phases on a "Kagome Staircase"
We present thermodynamic and neutron data on Ni_3V_2O_8, a spin-1 system on a
kagome staircase. The extreme degeneracy of the kagome antiferromagnet is
lifted to produce two incommensurate phases at finite T - one amplitude
modulated, the other helical - plus a commensurate canted antiferromagnet for T
->0. The H-T phase diagram is described by a model of competing first and
second neighbor interactions with smaller anisotropic terms. Ni_3V_2O_8 thus
provides an elegant example of order from sub leading interactions in a highly
frustrated systemComment: 4 pages, 3 figure
Magnetically driven ferroelectric order in NiVO
We show that for NiVO long-range ferroelectric and incommensurate
magnetic order appear simultaneously in a single phase transition. The
temperature and magnetic field dependence of the spontaneous polarization show
a strong coupling between magnetic and ferroelectric orders. We determine the
magnetic symmetry of this system by constraining the data to be consistent with
Landau theory for continuous phase transitions. This phenomenological theory
explains our observation the spontaneous polarization is restricted to lie
along the crystal b axis and predicts that the magnitude should be proportional
to a magnetic order parameter.Comment: 11 pages, 3 figure
Field dependence of magnetic ordering in Kagome-staircase compound Ni3V2O8
We present powder and single-crystal neutron diffraction and bulk
measurements of the Kagome-staircase compound Ni3V2O8 (NVO) in fields up to
8.5T applied along the c-direction. (The Kagome plane is the a-c plane.) This
system contains two types of Ni ions, which we call "spine" and "cross-tie".
Our neutron measurements can be described with the paramagnetic space group
Cmca for T < 15K and each observed magnetically ordered phase is characterized
by the appropriate irreducible representation(s). Our zero-field measurements
show that at T_PH=9.1K NVO undergoes a transition to an incommensurate order
which is dominated by a longitudinally-modulated structure with the spine spins
mainly parallel to the a-axis. Upon further cooling, a transition is induced at
T_HL=6.3K to an elliptically polarized incommensurate structure with both spine
and cross-tie moments in the a-b plane. At T_LC=4K the system undergoes a
first-order phase transition, below which the magnetic structure is a
commensurate antiferromagnet with the staggered magnetization primarily along
the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat
peak at T_CC'=2.3K indicates an additional transition, which we were however
not able to relate to a change of the magnetic structure. Neutron, specific
heat, and magnetization measurements produce a comprehensive temperature-field
phase diagram. The symmetries of the two incommensurate magnetic phases are
consistent with the observation that only one phase has a spontaneous
ferroelectric polarization. All the observed magnetic structures are explained
theoretically using a simplified model Hamiltonian, involving competing
nearest- and next-nearest-neighbor exchange interactions, spin anisotropy,
Dzyaloshinskii-Moriya and pseudo-dipolar interactions.Comment: 25 pages, 19 figure
Complex magnetic order in the kagome staircase compound Co3V2O8
Co3V2O8 (CVO) has a geometrically frustrated magnetic lattice, a Kagome
staircase. The crystal structure consists of two inequivalent Co sites,
one-dimensional chains of Co(2) spine sites, linked by Co(1) cross-tie sites.
Neutron powder diffraction has been used to solve the basic magnetic and
crystal structures of this system, while polarized and unpolarized single
crystal diffraction measurements have been used to reveal a variety of
incommensurate phases, interspersed with lock-in transitions to commensurate
phases. CVO initially orders magnetically at 11.3 K into an incommensurate,
transversely polarized, spin density wave state, with wave vector k=(0,delta,0)
with delta=0.55 and the spin direction along the a axis. Delta is found to
decrease monotonically with decreasing temperature, and then it locks into a
commensurate antiferromagnetic structure with delta=0.5 for 6.9<T<8.6 K. Below
6.9 K the magnetic structure becomes incommensurate again. Delta continues to
decrease with decreasing temperature, and locks-in again at delta=1/3 over a
narrow temperature range (6.2<T<6.5 K). The system then undergoes a strongly
first order transition to the ferromagnetic ground state (delta=0) at Tc=6.2 K.
A dielectric anomaly is observed around the ferromagnetic transition
temperature of 6.2 K, demonstrating a significant spin-charge coupling present
in CVO. A theory based on group theory analysis and a minimal Ising model with
competing exchange interactions can explain the basic features of the magnetic
ordering
Coupled magnetic and ferroelectric domains in multiferroic Ni3V2O8
Electric control of multiferroic domains is demonstrated through polarized
magnetic neutron diffraction. Cooling to the cycloidal multiferroic phase of
Ni3V2O8 in an electric field (E) causes the incommensurate Bragg reflections to
become neutron spin polarizing, the sense of neutron polarization reversing
with E. Quantitative analysis indicates the E-treated sample has handedness
that can be reversed by E. We further show close association between cycloidal
and ferroelectric domains through E-driven spin and electric polarization
hysteresis. We suggest that definite cycloidal handedness is achieved through
magneto-elastically induced Dzyaloshinskii-Moriya interactions.Comment: 4 pages, 5 figures. Updated affiliations and references. Minor
changes to text and figure
Strongly linked current flow in polycrystalline forms of the new superconductor MgB2
The discovery of superconductivity at 39 K in MgB2[1] raises many issues. One
of the central questions is whether this new superconductor resembles a
high-temperature-cuprate superconductor or a low-temperature metallic
superconductor in terms of its current carrying characteristics in applied
magnetic fields. In spite of the very high transition temperatures of the
cuprate superconductors, their performance in magnetic fields has several
drawbacks[2]. Their large anisotropy restricts high bulk current densities to
much less than the full magnetic field-temperature (H-T) space over which
superconductivity is found. Further, weak coupling across grain boundaries
makes transport current densities in untextured polycrystalline forms low and
strongly magnetic field sensitive[3,4]. These studies of MgB2 address both
issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature
of our samples, supercurrents flow throughout without the strong sensitivity to
weak magnetic fields characteristic of Josephson-coupled grains[3].
Magnetization measurements over nearly all of the superconducting H-T plane
show good temperature scaling of the flux pinning force, suggestive of a
current density determined by flux pinning. At least two length scales are
suggested by the magnetization and magneto optical (MO) analysis but the cause
of this seems to be phase inhomogeneity, porosity, and minority insulating
phase such as MgO rather than by weakly coupled grain boundaries. Our results
suggest that polycrystalline ceramics of this new class of superconductor will
not be compromised by the weak link problems of the high temperature
superconductors, a conclusion with enormous significance for applications if
higher temperature analogs of this compound can be discovered
Investigation of the Spin Density Wave in NaxCoO2
Magnetic susceptibility, transport and heat capacity measurements of single
crystal NaxCoO2 (x=0.71) are reported. A transition to a spin density wave
(SDW) state at Tmag = 22 K is observable in all measurements, except chi(ac)
data in which a cusp is observed at 4 K and attributed to a low temperature
glassy phase. M(H) loops are hysteretic below 15 K. Both the SDW transition and
low temperature hysteresis are only visible along the c-axis. The system also
exhibits a substantial (~40%) positive magnetoresistance below this
temperature. Calculations of the electronic heat capacity gamma above and below
Tmag and the size of the jump in C indicate that the onset of the SDW brings
about the opening of gap and the removal of part of the Fermi surface. Reduced
in-plane electron-electron scattering counteracts the loss of carriers below
the transition and as a result we see a net reduction in resistivity below
Tmag. Sodium ordering transitions at higher temperatures are observable as
peaks in the heat capacity with a corresponding increase in resistivity.Comment: 14 pages, 6 figure
Successive Magnetic Transitions of the Kagome Staircase Compound Co3V2O8 Studied in Various Magnetic Fields
For the spin-3/2 kagome staircase system Co3V2O8, magnetic field
(H)-temperature (T) phase diagrams have been constructed for the fields along
three principal directions up to 5 T, using results of various macroscopic
measurements on single crystal samples and also using neutron diffraction data
taken on both powder and single crystal samples under H along c. In zero
magnetic field, the system exhibits three transitions at temperatures Tc1~11.2
K, Tc2~8.8 K and Tc3~ (6.0-7.0) K. The single crystal data present clear
evidence for the noncollinear nature of the magnetic structures in all
magnetically ordered phases below Tc1. The sinusoidal nature of the
incommensurate modulation of the ordered moment reported in the former work has
been confirmed between Tc1 and Tc2, that is, no higher harmonics of the
modulation have been detected even for the present large single crystal. Even
in the phase of commensurate modulation between Tc2 and Tc3, we have not
detected any higher harmonics of the modulation. The phase diagrams show that
the magnetically ordered phases sensitively change to other phases with H,
indicating that the geometrical frustration inherent in this system is
important for the determination of the phase diagram. No evidence for
ferroelectric transitions has been observed in the measurements of the
dielectric constant applying the electric fields along three crystallographic
axes, a, b and c. Only small dielectric anomalies closely connected with the
magnetic phase transitions have been found.Comment: 5 pages, 10 figures, submitted to JPS
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