1,681 research outputs found
Effect of Sr substitution on superconductivity in Hg2(Ba1-ySry)2YCu2O8-d (part2): bond valence sum approach of the hole distribution
The effects of Sr substitution on superconductivity, and more particulary the
changes induced in the hole doping mechanism, were investigated in
Hg2(Ba1-ySry)2YCu2O8-d by a "bond valence sum" analysis with Sr content from y
= 0.0 to y = 1.0. A comparison with CuBa2YCu2O7-d and Cu2Ba2YCu2O8 systems
suggests a possible explanation of the Tc enhancement from 0 K for y = 0.0 to
42 K for y = 1.0. The charge distribution among atoms of the unit cell was
determined from the refined structure, for y = 0.0 to 1.0. It shows a charge
transfer to the superconducting CuO2 plane via two doping channels pi(1) and
pi(2), i.e. through O2(apical)-Cu and Ba/Sr-O1 bonds respectively.Comment: 13 pages, 5 figures, accepted for publication in Journal of Physics:
Condensed Matte
A method for the location of noise-polluted area
This paper deals with the working out of a method suitable to locate the critical areas from an acoustic point of view inside the pertinence zones of the roads. We have applied our method to about one thousand kilometres of major roads managed by ANAS in Lombardia. The procedure is based on the coupling of a Geographic Information System (GIS) with an acoustics simulation model. In order to characterize the noise sources in the prediction software, it has been necessary to estimate in every significant section of the streets the day and night average fluxes of vehicles, the vehicle typology and their average velocity. This study started from a statistical analysis of the experimental data obtained by sound measurements and by counting the vehicle fluxes. As a result, we have obtained on a GIS the acoustics map of the whole Lombardia road network with information on where the noise limits classes are exceeded
Can't get no learning: the Brexit fiasco through the lens of policy learning
It seems paradoxical to suggest that theories of learning might be used to explain policy failure. Yet the Brexit fiasco connects with recent approaches linking varieties of policy learning to policy pathologies. This article sets out to explain the UK government’s (mis)management of the Brexit process from June 2016 to May 2019 from a policy learning perspective. Drawing on interviews with UK policy-makers and stakeholders, we ask how did the UK government seek to learn during the Brexit negotiations? We consider four modes of learning: reflexivity, epistemic, hierarchical, and bargaining. By empirically tracing the policy process and scope conditions for each of these, we argue that learning through the first three modes proved highly dysfunctional. This forced the government to rely on bargaining between competing factions, producing a highly political form of learning which stymied the development of a coherent Brexit strategy. We argue that the analysis of Brexit as a policy process – rather than a political event – reveals how policy dynamics play an important role in shaping the political context within which they are located. The article concludes that public policy analysis can, therefore, add significant value to our understanding of Brexit by endogenising accounts of macro political developments
Errors evaluation in the estimate of the noise from the road traffic
Specific algorithms together with noise data acquired during a measurement campaign, consisting of approximately 80 one-hour records, were utilized to model the noise levels of a road network. Experimental measurements were used to evaluate the reliability of the model by analyzing the differences between the measured values and the estimated ones. We think that these differences have to
be especially ascribed to an imperfect representation of the combined effects of the attenuation due to acoustic wave diffraction and the attenuation produced by the
ground effect
Polarization memory in the nonpolar magnetic ground state of multiferroic CuFeO2
We investigate polarization memory effects in single-crystal CuFeO2, which
has a magnetically-induced ferroelectric phase at low temperatures and applied
B fields between 7.5 and 13 T. Following electrical poling of the ferroelectric
phase, we find that the nonpolar collinear antiferromagnetic ground state at B
= 0 T retains a strong memory of the polarization magnitude and direction, such
that upon re-entering the ferroelectric phase a net polarization of comparable
magnitude to the initial polarization is recovered in the absence of external
bias. This memory effect is very robust: in pulsed-magnetic-field measurements,
several pulses into the ferroelectric phase with reverse bias are required to
switch the polarization direction, with significant switching only seen after
the system is driven out of the ferroelectric phase and ground state either
magnetically (by application of B > 13 T) or thermally. The memory effect is
also largely insensitive to the magnetoelastic domain composition, since no
change in the memory effect is observed for a sample driven into a
single-domain state by application of stress in the [1-10] direction. On the
basis of Monte Carlo simulations of the ground state spin configurations, we
propose that the memory effect is due to the existence of helical domain walls
within the nonpolar collinear antiferromagnetic ground state, which would
retain the helicity of the polar phase for certain magnetothermal histories.Comment: 9 pages, 7 figure
Non-collinear long-range magnetic ordering in HgCr2S4
The low-temperature magnetic structure of \HG has been studied by
high-resolution powder neutron diffraction. Long-range incommensurate magnetic
order sets in at T22K with propagation vector
\textbf{k}=(0,0,0.18). On cooling below T, the propagation vector
increases and saturates at the commensurate value \textbf{k}=(0,0,0.25). The
magnetic structure below T consists of ferromagnetic layers in the
\textit{ab}-plane stacked in a spiral arrangement along the \textit{c}-axis.
Symmetry analysis using corepresentations theory reveals a point group symmetry
in the ordered magnetic phase of 422 (D), which is incompatible with
macroscopic ferroelectricity. This finding indicates that the spontaneous
electric polarization observed experimentally cannot be coupled to the magnetic
order parameter
Essential Role of the Cooperative Lattice Distortion in the Charge, Orbital and Spin Ordering in doped Manganites
The role of lattice distortion in the charge, orbital and spin ordering in
half doped manganites has been investigated. For fixed magnetic ordering, we
show that the cooperative lattice distortion stabilize the experimentally
observed ordering even when the strong on-site electronic correlation is taken
into account. Furthermore, without invoking the magnetic interactions, the
cooperative lattice distortion alone may lead to the correct charge and orbital
ordering including the charge stacking effect, and the magnetic ordering can be
the consequence of such a charge and orbital ordering. We propose that the
cooperative nature of the lattice distortion is essential to understand the
complicated charge, orbital and spin ordering observed in doped manganites.Comment: 5 pages,4 figure
Universal magneto-orbital ordering in the divalent -site quadruple perovskite manganites MnO ( = Ca, Sr, Cd, and Pb)
Through analysis of variable temperature neutron powder diffraction data, we
present solutions for the magnetic structures of SrMnO,
CdMnO, and PbMnO in all long-range ordered phases. The
three compounds were found to have magnetic structures analogous to that
reported for CaMnO. They all feature a higher temperature lock-in
phase with \emph{commensurate} magneto-orbital coupling, and a delocked,
multi-\textbf{k} magnetic ground state where \emph{incommensurate}
magneto-orbital coupling gives rise to a constant-moment magnetic helix with
modulated spin helicity. CdMnO represents a special case in which
the orbital modulation is commensurate with the crystal lattice and involves
stacking of fully and partially polarized orbital states. Our results provide a
robust confirmation of the phenomenological model for magneto-orbital coupling
previously presented for CaMnO. Furthermore, we show that the model
is universal to the quadruple perovskite manganites synthesised to
date, and that it is tunable by selection of the -site ionic radius
Ferroelectricity Induced by Acentric Spin-Density Waves in YMn2O5
The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn2O5, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition
Temperature and field dependence of the phase separation, structure, and magnetic ordering in LaCaMnO, (, 0.50, and 0.53)
Neutron powder diffraction measurements, combined with magnetization and
resistivity data, have been carried out in the doped perovskite
LaCaMnO (, 0.50, and 0.53) to elucidate the structural,
magnetic, and electronic properties of the system around the composition
corresponding to an equal number of Mn3+ and Mn4+. At room temperature all
three samples are paramagnetic and single phase, with crystallographic symmetry
Pnma. The samples then all become ferromagnetic (FM) at K. At
K, however, a second distinct crystallographic phase (denoted A-II)
begins to form. Initially the intrinsic widths of the peaks are quite large,
but they narrow as the temperature decreases and the phase fraction increases,
indicating microscopic coexistence. The fraction of the sample that exhibits
the A-II phase increases with decreasing temperature and also increases with
increasing Ca doping, but the transition never goes to completion to the lowest
temperatures measured (5 K) and the two phases therefore coexist in this
temperature-composition regime. Phase A-II orders antiferromagnetically (AFM)
below a N\'{e}el temperature K, with the CE-type magnetic
structure. Resistivity measurements show that this phase is a conductor, while
the CE phase is insulating. Application of magnetic fields up to 9 T
progressively inhibits the formation of the A-II phase, but this suppression is
path dependent, being much stronger for example if the sample is field-cooled
compared to zero-field cooling and then applying the field. The H-T phase
diagram obtained from the diffraction measurements is in good agreement with
the results of magnetization and resistivity.Comment: 12 pages, 3 tables, 11 figure
- …