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New approaches for achieving more perfect transition metal oxide thin films
This perspective considers the enormous promise of epitaxial functional transition metal oxide thin films for future applications in low power electronic and energy applications since they offer wide-ranging and highly tunable functionalities and multifunctionalities, unrivaled among other classes of materials. It also considers the great challenges that must be overcome for transition metal oxide thin films to meet what is needed in the application domain. These challenges arise from the presence of intrinsic defects and strain effects, which lead to extrinsic defects. Current conventional thin film deposition routes often cannot deliver the required perfection and performance. Since there is a strong link between the physical properties, defects and strain, routes to achieving more perfect materials need to be studied. Several emerging methods and modifications of current methods are presented and discussed. The reasons these methods better address the perfection challenge are considered and evaluated
The behavior of grain boundaries in the Fe-based superconductors
The Fe-based superconductors (FBS) are an important new class of
superconducting materials. As with any new superconductor with a high
transition temperature and upper critical field, there is a need to establish
what their applications potential might be. Applications require high critical
current densities, so the usefulness of any new superconductor is determined
both by the capability to develop strong vortex pinning and by the absence or
ability to overcome any strong current-limiting mechanisms of which grain
boundaries in the cuprates are a cautionary example. In this review we first
consider the positive role that grain boundary properties play in the metallic,
low temperature superconductors and then review the theoretical background and
current experimental data relating to the properties of grain boundaries in FBS
polycrystals, bi-crystal thin films, and wires. Based on this evidence, we
conclude that grain boundaries in FBS are weak linked in a qualitatively
similar way to grain boundaries in the cuprate superconductors, but also that
the effects are a little less marked. Initial experiments with the textured
substrates used for cuprate coated conductors show similar benefit for the
critical current density of FBS thin films too. We also note that the
particular richness of the pairing symmetry and the multiband parent state in
FBS may provide opportunities for grain boundary modification as a better
understanding of their pairing state and grain boundary properties are
developed.Comment: To appear in Reports on Progress in Physic
Bond-length dependence of charge-transfer excitations and stretch phonon modes in perovskite ruthenates: Evidence of strong p â d hybridization effects
We reported the optical conductivity spectra of the Ruddlesden-Popper series ruthenates, i.e., Srn+1RunO3n+1
and Can+1RunO3n+1, where n=1, 2, and `. Among various optical transitions, we investigated two Ru-O
related modes, i.e., the charge-transfer excitation and the transverse stretching phonon. We found that their
frequency shifts are not much affected by a structural dimensionality, but are closely related to the Ru-O bond
length. Through the quantitative analysis of the charge-transfer excitation energy, we could demonstrate that
the pâd hybridization should play an important role in determining their electronic structure. In addition, we
discussed how the electronic excitation could contribute the lattice dynamics in the metallic ruthenate
Stabilization of monodomain polarization in ultrathin PbTiO3 films
Using in situ high-resolution synchrotron x-ray scattering, the Curie temperature T-C has been determined for ultrathin c-axis epitaxial PbTiO3 films on conducting substrates (SrRuO3 on SrTiO3), with surfaces exposed to a controlled vapor environment. The suppression of T-C was relatively small, even for the thinnest film (1.2 nm). We observe that 180 degrees stripe domains do not form, indicating that the depolarizing field is compensated by free charge at both interfaces. This is confirmed by ab initio calculations that find polar ground states in the presence of ionic adsorbates.open15511
New Fe-based superconductors: properties relevant for applications
Less than two years after the discovery of high temperature superconductivity
in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe
layers (1111, 122, 11, 111) are available. They share several characteristics
with cuprate superconductors that compromise easy applications, such as the
layered structure, the small coherence length, and unconventional pairing, On
the other hand the Fe-based superconductors have metallic parent compounds, and
their electronic anisotropy is generally smaller and does not strongly depend
on the level of doping, the supposed order parameter symmetry is s wave, thus
in principle not so detrimental to current transmission across grain
boundaries. From the application point of view, the main efforts are still
devoted to investigate the superconducting properties, to distinguish intrinsic
from extrinsic behaviours and to compare the different families in order to
identify which one is the fittest for the quest for better and more practical
superconductors. The 1111 family shows the highest Tc, huge but also the most
anisotropic upper critical field and in-field, fan-shaped resistive transitions
reminiscent of those of cuprates, while the 122 family is much less anisotropic
with sharper resistive transitions as in low temperature superconductors, but
with about half the Tc of the 1111 compounds. An overview of the main
superconducting properties relevant to applications will be presented. Upper
critical field, electronic anisotropy parameter, intragranular and
intergranular critical current density will be discussed and compared, where
possible, across the Fe-based superconductor families
Enhancement of the high-field critical current density of superconducting MgB2 by proton irradiation
A relatively high critical temperature, Tc, approaching 40 K, places the
recently-discovered superconductor magnesium diboride (MgB2) intermediate
between the families of low- and copper-oxide-based high-temperature
superconductors (HTS). Supercurrent flow in MgB2 is unhindered by grain
boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2
conductors may be easier to fabricate, and so could fill a potentially
important niche of applications in the 20 to 30 K temperature range. However,
one disadvantage of MgB2 is that in bulk material the critical current density,
Jc, appears to drop more rapidly with increasing magnetic field than it does in
the HTS phases. The magnitude and field dependence of Jc are related to the
presence of structural defects that can "pin" the quantised magnetic vortices
that permeate the material, and prevent them from moving under the action of
the Lorentz force. Vortex studies suggest that it is the paucity of suitable
defects in MgB2 that causes the rapid decay of Jc with field. Here we show that
modest levels of atomic disorder, induced by proton irradiation, enhance the
pinning, and so increase Jc significantly at high fields. We anticipate that
chemical doping or mechanical processing should be capable of generating
similar levels of disorder, and so achieve technologically-attractive
performance in MgB2 by economically-viable routes.Comment: 5 pages, 4 figures, to be published in Nature (in press
In situ epitaxial MgB2 thin films for superconducting electronics
A thin film technology compatible with multilayer device fabrication is
critical for exploring the potential of the 39-K superconductor magnesium
diboride for superconducting electronics. Using a Hybrid Physical-Chemical
Vapor Deposition (HPCVD) process, it is shown that the high Mg vapor pressure
necessary to keep the MgB phase thermodynamically stable can be achieved
for the {\it in situ} growth of MgB thin films. The films grow epitaxially
on (0001) sapphire and (0001) 4H-SiC substrates and show a bulk-like of
39 K, a (4.2K) of A/cm in zero field, and a
of 29.2 T in parallel magnetic field. The surface is smooth with a
root-mean-square roughness of 2.5 nm for MgB films on SiC. This deposition
method opens tremendous opportunities for superconducting electronics using
MgB
Molecular imaging of cell death in vivo by a novel small molecule probe
Apoptosis has a role in many medical disorders, therefore assessment of apoptosis in vivo can be highly useful for diagnosis, follow-up and evaluation of treatment efficacy. ApoSense is a novel technology, comprising low molecular-weight probes, specifically designed for imaging of cell death in vivo. In the current study we present targeting and imaging of cell death both in vitro and in vivo, utilizing NST-732, a member of the ApoSense family, comprising a fluorophore and a fluorine atom, for both fluorescent and future positron emission tomography (PET) studies using an 18F label, respectively. In vitro, NST-732 manifested selective and rapid accumulation within various cell types undergoing apoptosis. Its uptake was blocked by caspase inhibition, and occurred from the early stages of the apoptotic process, in parallel to binding of Annexin-V, caspase activation and alterations in mitochondrial membrane potential. In vivo, NST-732 manifested selective uptake into cells undergoing cell-death in several clinically-relevant models in rodents: (i) Cell-death induced in lymphoma by irradiation; (ii) Renal ischemia/reperfusion; (iii) Cerebral stroke. Uptake of NST-732 was well-correlated with histopathological assessment of cell-death. NST-732 therefore represents a novel class of small-molecule detectors of apoptosis, with potential useful applications in imaging of the cell death process both in vitro and in vivo
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