714 research outputs found
On stoichiometry and intermixing at the spinel/perovskite interface in CoFe2O4/BaTiO3 thin films
The performance of complex oxide heterostructures depends primarily on the interfacial coupling of the two component structures. This interface character inherently varies with the synthesis method and conditions used since even small composition variations can alter the electronic, ferroelectric, or magnetic functional properties of the system. The focus of this article is placed on the interface character of a pulsed laser deposited CoFe2O4/BaTiO3 thin film. Using a range of state-of-the-art transmission electron microscopy methodologies, the roles of substrate morphology, interface stoichiometry, and cation intermixing are determined on the atomic level. The results reveal a surprisingly uneven BaTiO3 substrate surface formed after the film deposition and Fe atom incorporation in the top few monolayers inside the unit cell of the BaTiO3 crystal. Towards the CoFe2O4 side, a disordered region extending several nanometers from the interface was revealed and both Ba and Ti from the substrate were found to diffuse into the spinel layer. The analysis also shows that within this somehow incompatible composite interface, a different phase is formed corresponding to the compound Ba2Fe3Ti5O15, which belongs to the ilmenite crystal structure of FeTiO3 type. The results suggest a chemical activity between these two oxides, which could lead to the synthesis of complex engineered interfaces
Target approach for prevention activities in dental service of closed government systems. Ukrainian concept
Rate constant for the reaction NH2 + NO from 216 to 480 K
The absolute rate constant was measured by the technique of flash photolysis-laser induced fluorescence (FP-LIF). NH2 radicals were produced by the flash photolysis of ammonia and the fluorescent NH2 photons were measured by multiscaling techniques. At each temperature, the results were independent of variations in total pressure, and flash intensity. The results are compared with previous determinations using the techniques of mass spectrometry, absorption spectroscopy, laser absorption spectroscopy, and laser induced fluorescence. The implications of the results are discussed with regard to combustion, post combustion, and atmospheric chemistry. The results are also discussed theoretically
Ground State Properties of Anderson Impurity in a Gapless Host
Using the Bethe ansatz method, we study the ground state properties of a
Anderson impurity in a ``gapless'' host, where a density of band
states vanishes at the Fermi level as . As
in metals, the impurity spin is proven to be screened at arbitrary parameters
of the system. However, the impurity occupancy as a function of the bare
impurity energy is shown to acquire novel qualitative features which
demonstrate a nonuniversal behavior of the system. The latter explains why the
Kondo screening is absent (or exists only at quite a large electron-impurity
coupling) in earlier studies based on scaling arguments.Comment: 5 pages, no figure, RevTe
Dense, Fe-rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?
We present observations of two LMC supernova remnants (SNRs), DEM L238 and
DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central
emission, surrounded by a faint shell, is present in both remnants. The central
emission has an entirely thermal spectrum dominated by strong Fe L-shell lines,
with the deduced Fe abundance in excess of solar and not consistent with the
LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and
DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission
originates in gas swept up and heated by the blast wave. A standard Sedov
analysis implies about 50 solar masses in both swept-up shells, SNR ages
between 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, and
subluminous explosions with energies of 3x10^50 ergs. The central Fe-rich
supernova ejecta are close to collisional ionization equilibrium. Their
presence is unexpected, because standard Type Ia SNR models predict faint
ejecta emission with short ionization ages. Both SNRs belong to a previously
unrecognized class of Type Ia SNRs characterized by bright interior emission.
Denser than expected ejecta and/or a dense circumstellar medium around the
progenitors are required to explain the presence of Fe-rich ejecta in these
SNRs. Substantial amounts of circumstellar gas are more likely to be present in
explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar
SNRs could be remnants of ``prompt'' Type Ia explosions with young (~100 Myr
old) progenitors.Comment: 24 pages, 8 figures, ApJ, in pres
Nuclear spin-lattice relaxation rate in the D+iD superconducting state: implications for CoO superconductor
We calculated the nuclear spin-lattice relaxation rate for the D+iD
superconducting state with impurities. We found that small amount of unitary
impurities quickly produces the residual density of states inside the gap. As a
result, the T-linear behavior in 1/T is observed at low temperatures. Our
results show that the D+iD pairing symmetry of the superconducting state of
NaCoOH O is compatible with recent Co 1/T
experiments of several groups.Comment: 5 pages, 4 figures, minor change
Bethe ansatz approach to thermodynamics of superconducting magnetic alloys
We derive thermodynamic Bethe ansatz equations for a model describing an
Anderson impurity embedded in a BCS superconductor. The equations
are solved analytically in the zero-temperature limit, T=0. It is shown that
the impurities depress superconductivity in the Kondo limit, however at T=0 the
system remains in the superconducting state for any impurity concentration. In
the mixed-valence regime, an impurity contribution to the density of states of
the system near the Fermi level overcompensates a Cooper pairs weakening, and
superconductivity is enhanced.Comment: 4 pages, RevTex, to appear in PR
Energy Gap Induced by Impurity Scattering: New Phase Transition in Anisotropic Superconductors
It is shown that layered superconductors are subjected to a phase transition
at zero temperature provided the order parameter (OP) reverses its sign on the
Fermi-surface but its angular average is finite. The transition is regulated by
an elastic impurity scattering rate . The excitation energy spectrum,
being gapless at the low level of scattering, develops a gap as soon as the
scattering rate exceeds some critical value of .Comment: Revtex, 11 page
In situ transmission electron microscopy of resistive switching in thin silicon oxide layers
Silicon oxide-based resistive switching devices show great potential for applications in nonvolatile random access memories. We expose a device to voltages above hard breakdown and show that hard oxide breakdown results in mixing of the SiOx layer and the TiN lower contact layers. We switch a similar device at sub-breakdown fields in situ in the transmission electron microscope (TEM) using a movable probe and study the diffusion mechanism that leads to resistance switching. By recording bright-field (BF) TEM movies while switching the device, we observe the creation of a filament that is correlated with a change in conductivity of the SiOx layer. We also examine a device prepared on a microfabricated chip and show that variations in electrostatic potential in the SiOx layer can be recorded using off-axis electron holography as the sample is switched in situ in the TEM. Taken together, the visualization of compositional changes in ex situ stressed samples and the simultaneous observation of BF TEM contrast variations, a conductivity increase, and a potential drop across the dielectric layer in in situ switched devices allow us to conclude that nucleation of the electroforming—switching process starts at the interface between the SiOx layer and the lower contact
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