1,428 research outputs found
Intraplanar couplings in the CuO_2 lattice of cuprate superconductors
We have investigated the doping dependencies of the basal areas in
single-layer high-T_c cuprates La_{2-x}Sr_xCuO_4 and HgBa_2CuO_x, as well as in
two-layer Y_{1-y}Ca_yBa_2Cu_3O_x and HgBa_2CaCu_2O_x. The basal areas not only
tend to shrink on hole doping, as expected from single electron quantum
chemistry, but exhibit also a "bulge'' around optimum doping. We attribute the
"bulge'' to the effects of the strongly correlated quantum liquid on the CuO_2
lattice, rendering it nearly incompressible around optimum doping, but highly
compressible in the weakly overdoped regime. Inhomogenous doping cannot account
for this anomaly in the electronic compressibility of the CuO_2 lattice.Comment: 2 pages, 2 figures, to appear in Physica C (Proc. M2S-HTSC-VII
Micromagnetic modelling and imaging of vortex/merons structures in an oxide | metal heterostructure
Using micromagnetic simulations, we have modelled the formation of imprinted merons and anti-merons in cobalt overlayers of different thickness (1-8 nm), stabilised by interfacial exchange with antiferromagnetic vortices in α-Fe2O3. Structures similar to those observed experimentally could be obtained with reasonable exchange parameters, also in the presence of surface roughness. We produce simulated meron/antimeron images by magnetic force microscopy (MFM) and nitrogen-vacancy (N-V) centre microscopy, and established signatures of these topological structures in different experimental configurations
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
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
Can’t Get No Learning: The Brexit Fiasco through the Lens of Policy Learning
This is the final version. Available on open access from Taylor & Francis via the DOI in this recordIt seems paradoxical to suggest that theories of learning might be used to explain policy failure. Yet the Brexit fiasco connects with recent approaches linking four varieties of policy learning to policy pathologies (Dunlop, 2017; Dunlop and Radaelli, 2013, 2018). This article sets out to explain the UK government’s (mis)management of the Brexit process from June 2016 to May 2019. 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 short-termist 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 serve to endogenise existing accounts of macro political developments like Brexit.European Commissio
Low-Temperature Permittivity of Insulating Perovskite Manganites
Measurements of the low-frequency (f<=100 kHz) permittivity and conductivity
at T<= 150 K are reported for La(1-x)Ca(x)MnO(3) (0<=x<=1) and
Ca(1-y)Sr(y)MnO(3) (0<=y<=0.75) having antiferromagnetic, insulating ground
states covering a broad range of Mn valencies from Mn(3+) to Mn(4+). Static
dielectric constants are determined from the low-T limiting behavior. With
increasing T, relaxation peaks associated with charge-carrier hopping are
observed in the real part of the permittivities and analyzed to determine
dopant binding energies. The data are consistent with a simple model of
hydrogenic impurity levels and imply effective masses m*/m_e~3 for the Mn(4+)
compounds. Particularly interesting is a large dielectric constant (~100)
associated with the C-type antiferromagnetic state near the composition
La(0.2)Ca(0.8)MnO(3).Comment: 6 pages, 8 figures, PRB in pres
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
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
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