20 research outputs found
Some New Reactions and Properties of Xanthane Hydride (5-Amino-1,2,4-dithiazole-3-thione)
Aminomethylation of xanthane hydride (5-amino-1,2,4-dithiazole-3-thione) with the RNH2–HCHO system has resulted in the formation of the derivatives of new heterocyclic system (3,7-dihydro-5H-[1,2,4]-dithiazolo[4,3-a][1,3,5]triazine) in low yields. The reaction of xanthane hydride with dicyandiamide has led to thioammeline [4,6-diamino-1,3,5-triazine-2(5Н)-thione]. Some practically important properties of xanthane hydride and its derivatives have been investigated. Xanthane hydride has efficiently exhibited carbon steel corrosion in neutral aqueous media. The prepared compounds have not exhibited growth-regulating or antidote activity to herbicide 2,4-D
"Head-to-head" and "tail-to-tail" 180-degree domain walls in an isolated ferroelectric
"Head-to-head" and "tail-to-tail" 180-degree domain-walls in a finite
isolated ferroelectric sample are theoretically studied using Landau theory.
The full set of equations, suitable for numerical calculations is developed.
The explicit expressions for the polarization profile across the walls are
derived for several limiting cases and wall-widths are estimated. It is shown
analytically that different regimes of screening and different dependences for
width of charged domain walls on the temperature and parameters of the system
are possible, depending on spontaneous polarization and concentration of
carriers in the material. It is shown that the half-width of charged domain
walls in typical perovskites is about the nonlinear Thomas-Fermi
screening-length and about one order of magnitude larger than the half-width of
neutral domain-walls. The formation energies of "head-to-head" walls under
different regimes of screening are obtained, neglecting the poling ability of
the surface. It is shown that either "head-to-head" or "tail-to-tail"
configuration can be energetically favorable in comparison with the monodomain
state of the ferroelectric if the poling ability of the surface is large
enough. If this is not the case, the existence of charged domain walls in bulk
ferroelectrics is merely a result of the domain-growth kinetics. Size-effect
corresponding to the competition between state with charged domain wall, single
domain state, multidomain state, and the state with the zero polarization is
considered. The results obtained for the case of an isolated ferroelectric
sample were compared with the results for an electroded sample. It was shown
that charged domain wall in electroded sample can be either metastable or
stable, depends on the work function difference between electrodes and
ferroelectric and the poling ability of the electrode/ferroelectric interface.Comment: 47 pages, 10 figure
Ultrafast Suppression of the Ferroelectric Instability in KTaO
We use an x-ray free-electron laser to study the ultrafast lattice dynamics
following above band-gap photoexcitation of the incipient ferroelectric
potassium-tantalate, \kto. %
We use ultrafast near-UV (central wavelength 266\,nm and 50 fs pulse
duration) laser light to photoexcite charge carriers across the gap and probe
the ultrafast lattice dynamics by recording the x-ray diffuse intensity
throughout multiple Brillouin zones using pulses from the Linac Coherent Light
Source (LCLS) (central wavelength 1.3\,\AA\, and ~fs pulse duration). We
observe changes in the diffuse intensity that we conclude are associated with a
hardening of the soft transverse optical and transverse acoustic phonon
branches along to and to . Using ground- and
excited-state interatomic force constants from density functional theory (DFT)
and assuming the phonon populations can be described by a time-dependent
temperature, we fit the quasi-equilibrium thermal diffuse intensity to the
experimental time-dependent intensity. We obtain the instantaneous lattice
temperature and density of photoexcited charge carriers as a function of time
delay. The DFT calculations demonstrate that photoexcitation transfers charge
from oxygen derived -bonding orbitals to Ta derived antibonding
orbitals, further suppressing the ferroelectric instability and increasing the
stability of the cubic, paraelectric structure.Comment: 8 pages, 4 figure
Synthesis and Reactions of Substituted 3-amino-2-furyl(aryl)-thieno[2,3-b]pyridines
New substituted thieno[2,3-b]pyridines which contain 4-nitropehnyl and 5-nitro-, carboxy-, methoxycarbonyl-2-furyl groups in the 2 position have been obtained
Direct Observation of Coherent Longitudinal and Shear Acoustic Phonons in TaAs Using Ultrafast X-ray Diffraction
Using femtosecond time-resolved X-ray diffraction, we investigated optically
excited coherent acoustic phonons in the Weyl semimetal TaAs. The low symmetry
of the (112) surface probed in our experiment enables the simultaneous
excitation of longitudinal and shear acoustic modes, whose dispersion closely
matches our simulations. We observed an asymmetry in the spectral lineshape of
the longitudinal mode that is notably absent from the shear mode, suggesting a
time-dependent frequency chirp that is likely driven by photoinduced carrier
diffusion. We argue on the basis of symmetry that these acoustic deformations
can transiently alter the electronic structure near the Weyl points and support
this with model calculations. Our study underscores the benefit of using
off-axis crystal orientations when optically exciting acoustic deformations in
topological semimetals, allowing one to transiently change their crystal and
electronic structures.Comment: 6 pages, 4 figures for manuscript, and 15 pages and 11 figures for
supplemental material