294 research outputs found
Raman spectroscopy study of the interface structure in (CaCuO2)n/(SrTiO3)m superlattices
Raman spectra of CaCuO2/SrTiO3 superlattices show clear spectroscopic marker
of two structures formed in CaCuO2 at the interface with SrTiO3. For
non-superconducting superlattices, grown in low oxidizing atmosphere, the 425
cm-1 frequency of oxygen vibration in CuO2 planes is the same as for CCO films
with infinite layer structure (planar Cu-O coordination). For superconducting
superlattices grown in highly oxidizing atmosphere, a 60 cm-1 frequency shift
to lower energy occurs. This is ascribed to a change from planar to pyramidal
Cu-O coordination because of oxygen incorporation at the interface. Raman
spectroscopy proves to be a powerful tool for interface structure
investigation
Analysis of charge transfer mechanism on (Ba1-xNdxCuO2+d)2/(CaCuO2)n superconducting superlattices by thermoelectric power measurements
We have investigated the charge transfer mechanism in artificial
superlattices by Seebeck effect measurements. Such a technique allows a precise
determination of the amount of charge transferred on each CuO2 plane. A
systematic characterization of thermoelectric power in (BaCuO2+d)2/(CaCuO2)n
and (Ba0.9Nd0.1CuO2+d)2/(CaCuO2)n superlattices demonstrates that electrical
charge distributes uniformly among the CuO2 planes in the Ca-block. The
differences observed in the Seebeck effect behavior between the Nd-doped and
undoped superlattices are ascribed to the different metallic character of the
Ba-block in the two cases. Finally, the special role of structural disorder in
superlattices with n=1 is pointed out by such analysis.Comment: subitted to PRB, 15 pages, 3 figure
Charge localization at the interface between La1-xSrxMnO3 and the infinite layers cuprate CaCuO2
(CaCuO2)m/(La0.7Sr0.3MnO3)n superlattices, consisting of the infinite layers
cuprate CaCuO2 and the optimally doped manganite La1-xSrxMnO3, were grown by
pulsed laser deposition. The transport properties are dominated by the
manganite block. X-Ray Absorption spectroscopy measurements show a clear
evidence of an orbital reconstruction at the interface, ascribed to the
hybridization between the Cu 3d3z2-r2 and the Mn 3d3z2-r2 orbitals via
interface apical oxygen ions. Such a mechanism localizes holes at the
interfaces, thus preventing charge transfer to the CaCuO2 block. Some charge
(holes) transfer occurs toward the La0.7Sr0.3MnO3 block in strongly oxidized
superlattices, contributing to the suppression of the magnetotransport
properties.Comment: 20 pages, 6 figure
C-axis resistivity and high Tc superconductivity
Recently we had proposed a mechanism for the normal-state C-axis resistivity
of the high-T layered cuprates that involved blocking of the
single-particle tunneling between the weakly coupled planes by strong
intra-planar electron-electron scattering. This gave a C-axis resistivity that
tracks the ab-plane T-linear resistivity, as observed in the high-temperature
limit. In this work this mechanism is examined further for its implication for
the ground-state energy and superconductivity of the layered cuprates. It is
now argued that, unlike the single-particle tunneling, the tunneling of a
boson-like pair between the planes prepared in the BCS-type coherent trial
state remains unblocked inasmuch as the latter is by construction an eigenstate
of the pair annihilation operator. The resulting pair-delocalization along the
C-axis offers energetically a comparative advantage to the paired-up trial
state, and, thus stabilizes superconductivity. In this scheme the strongly
correlated nature of the layered system enters only through the blocking
effect, namely that a given electron is effectively repeatedly monitored
(intra-planarly scattered) by the other electrons acting as an environment, on
a time-scale shorter than the inter-planar tunneling time. Possible
relationship to other inter-layer pairing mechanisms proposed by several
workers in the field is also briefly discussed.Comment: typos in equations corrected, contents unchange
The London theory of the crossing-vortex lattice in highly anisotropic layered superconductors
A novel description of Josephson vortices (JVs) crossed by the pancake
vortices (PVs) is proposed on the basis of the anisotropic London theory. The
field distribution of a JV and its energy have been calculated for both dense
() PV lattices with distance
between PVs, and the nonlinear JV core size . It is shown that the
``shifted'' PV lattice (PVs displaced mainly along JVs in the crossing vortex
lattice structure), formed in high out-of-plane magnetic fields transforms into
the PV lattice ``trapped'' by the JV sublattice at a certain field, lower than
, where is the flux quantum, is the
anisotropy parameter and is the distance between CuO planes.
With further decreasing , the free energy of the crossing vortex lattice
structure (PV and JV sublattices coexist separately) can exceed the free energy
of the tilted lattice (common PV-JV vortex structure) in the case of with the in-plane penetration depth if the low
() or high ()
in-plane magnetic field is applied. It means that the crossing vortex structure
is realized in the intermediate field orientations, while the tilted vortex
lattice can exist if the magnetic field is aligned near the -axis and the
-plane as well. In the intermediate in-plane fields
, the
crossing vortex structure with the ``trapped'' PV sublattice seems to settle in
until the lock-in transition occurs since this structure has the lower energy
with respect to the tilted vortex structure in the magnetic field
oriented near the -plane.Comment: 15 pages, 6 figures, accepted for publication in PR
Energy and symmetry of excitations in undoped layered cuprates measured by Cu resonant inelastic x-ray scattering
We measured high resolution Cu edge resonant inelastic x-ray scattering
(RIXS) of the undoped cuprates LaCuO, SrCuOCl, CaCuO
and NdBaCuO. The dominant spectral features were assigned to
excitations and we extensively studied their polarization and scattering
geometry dependence. In a pure ionic picture, we calculated the theoretical
cross sections for those excitations and used them to fit the experimental data
with excellent agreement. By doing so, we were able to determine the energy and
symmetry of Cu-3 states for the four systems with unprecedented accuracy and
confidence. The values of the effective parameters could be obtained for the
single ion crystal field model but not for a simple two-dimensional cluster
model. The firm experimental assessment of excitation energies carries
important consequences for the physics of high superconductors. On one
hand, having found that the minimum energy of orbital excitation is always
eV, i.e., well above the mid-infrared spectral range, leaves to
magnetic excitations (up to 300 meV) a major role in Cooper pairing in
cuprates. On the other hand, it has become possible to study quantitatively the
effective influence of excitations on the superconducting gap in cuprates.Comment: 22 pages, 11 figures, 1 tabl
High Tc superconductivity in superlattices of insulating oxides
We report the occurrence of superconductivity, with maximum Tc = 40 K, in
superlattices (SLs) based on two insulating oxides, namely CaCuO2 and SrTiO3.
In these (CaCuO2)n/(SrTiO3)m SLs, the CuO2 planes belong only to CaCuO2 block,
which is an antiferromagnetic insulator. Superconductivity, confined within few
unit cells at the CaCuO2/SrTiO3 interface, shows up only when the SLs are grown
in a highly oxidizing atmosphere, because of extra oxygen ions entering at the
interfaces. Evidence is reported that the hole doping of the CuO2 planes is
obtained by charge transfer from the interface layers, which act as charge
reservoir.Comment: 18 pages, 8 figure
A case for systematic quality management in mosquito control programmes in Europe
The recent spread of invasive mosquito species, such as Aedes albopictus and the seasonal sporadic transmission of autochthonous cases of arboviral diseases (e.g., dengue, chikungunya, Zika) in temperate areas, such as Europe and North America, highlight the importance of effective mosquito-control interventions to reduce not only nuisance, but also major threats for public health. Local, regional, and even national mosquito control programs have been established in many countries and are executed on a seasonal basis by either public or private bodies. In order for these interventions to be worthwhile, funding authorities should ensure that mosquito control is (a) planned by competent scientific institutions addressing the local demands, (b) executed following the plan that is based on recommended and effective methods and strategies, (c) monitored regularly by checking the efficacy of the implemented actions, (d) evaluated against the set of targets, and (e) regularly improved according to the results of the monitoring. Adherence to these conditions can only be assured if a formal quality management system is adopted and enforced that ensures the transparency of effectiveness of the control operation. The current paper aims at defining the two components of this quality management system, quality assurance and quality control for mosquito control programs with special emphasis on Europe, but applicable over temperate areas
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