We report on the quantum yield (eta) and decay time (tau) measurements at room temperature for the bright red-orange (602 nm) luminescence from new germanium-vacancy (Ge-V) centers in nano- and microcrystalline diamonds synthesized at high pressure and high temperature. The values eta = 3 +/- 1% and tau = 6.2 +/- 0.2 ns were found. The Stokes shift measured as the energy difference between the maxima of the luminescence and luminescence excitation spectra is negligible. The relative intensity of the zero-phonon line constitutes up to 70% from the total intensity of the luminescence. Results of our ab initio DFT calculations for the ground-state electronic and vibrational structure of (Ge-V)(-) in diamond are presented and discussed.
We report an observation of coherent phonons of E-g(1), E-u(1), A(1g)(1), and E-g(2) symmetry generated in a single-crystal film of Bi2Se3 by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the E-g(2) mode. The generation of E-g(2) phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear E-u(1) oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of E-g(2). The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of E-g(1) (1.1 THz) and A(1g)(1) (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in Bi2Se3.
The intercalation of H2O, CO2, and other fluid species in expandable clay minerals (smectites) may play a significant role in controlling the behavior of these species in geological C-sequestration and enhanced petroleum production and has been the subject of intensive study in recent years. This paper reports the results of a computational study of the effects of the properties of the charge balancing, exchangeable cations on H2O and CO2 intercalation in the smectite mineral, hectorite, in equilibrium with an H2O-saturated supercritical CO2 fluid under reservoir conditions using Grand Canonical Molecular Dynamics (GCMD) methods. The results show that the intercalation behavior is greatly different with cations with relatively low hydration energies and high affinities for CO2 (here Cs+) than with cations with higher hydration energies (here Ca2+). With Cs+, CO2 intercalation occurs in a 1-layer structure and does not require H2O intercalation, whereas with Ca2+ the presence of a sub-monolayer of H2O is required for CO2 intercalation. The computational results provide detailed structural, dynamical and energetic insight into the differences in intercalation behavior and are in excellent agreement with in situ experimental XRD, IR, quartz crystal microbalance, and NMR results for smectite materials obtained under reservoir conditions.
The FeSe wires and tapes with Fe (S22 steel) and Cu/Nb sheath were fabricated by various modifications of the powder-in-tube (PIT) method. The superconducting critical parameters (upper critical field, critical current, critical temperature) depending on heat treatment time were investigated using resistive R(T) and transport measurements I (V) in magnetic fields up to 9T. The low-temperature annealing up to 72 h improve superconducting properties of superconducting wire with steel sheath. However, the annealing is the cause of superconducting wires degradation due to deterioration of contacts between the steel shell and the FeSe core. We show that industrial PIT technology of manufacturing Nb3Sn wires and tapes might be adopted for iron-based superconductors. Moreover, industrial PIT is a promising technique for fabricating large scale and high-quality superconducting wire and tapes. In addition, we show that fine grinding of the FeSe bulks leads mainly to a phase transition from the tetragonal to the hexagonal crystal structure that exhibits no superconductivity.
The structural and spectroscopic features of the EuAl3(BO3)4 individual skeletal microcrystals synthesized by a melt solution method have been studied. Their infrared spectra taken from the as-grown microcrystal surfaces mainly contain the lines of the rhombohedral modification of EuAl3(BO3)4 and additional peaks of its monoclinic modification. TEM and X-ray diffraction studies confirm that these additional peaks in the IR spectra belong to the monoclinic C2/c polytype of the EuAl3(BO3)4 compound. We are the first to demonstrate the presence of coherent monoclinic domains in rhombohedral EuAl3(BO3)4 crystals by TEM. Cathodoluminance spectroscopy shows that the microcrystals generate strong emission lines in the range 580–630 nm, and their intensities are strongly influenced by the crystal orientation.
We construct a distribution function of the strain-tensor components induced by point defects in an elastically anisotropic continuum, which can be used to account quantitatively for many effects observed in different branches of condensed matter physics. Parameters of the derived six-dimensional generalized Lorentz distribution are expressed through the integrals computed over the array of strains. The distribution functions for the cubic diamond and elpasolite crystals and tetragonal crystals with the zircon and scheelite structures are presented. Our theoretical approach is supported by a successful modeling of specific line shapes of singlet-doublet transitions of the Tm3+ ions doped into ABO4 (A=Y, Lu; B=P, V) crystals with zircon structure, observed in high-resolution optical spectra. The values of the defect strengths of impurity Tm3+ ions in the oxygen surroundings, obtained as a result of this modeling, can be used in future studies of random strains in different rare-earth oxides.
We report a comprehensive study of physical properties of the binary superconductor compound SnAs. The electronic band structure of SnAs was investigated using both angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and density functional theory (DFT) within generalized gradient approximation (GGA). The DFT/GGA calculations were done including spin-orbit coupling for both bulk and (111) slab crystal structures. Comparison of the DFT/GGA band dispersions with ARPES data shows that (111) slab much better describes ARPES data than just bulk bands. Superconducting properties of SnAs were studied experimentally by specific heat, magnetic susceptibility, magnetotransport measurements and Andreev reflection spectroscopy. Temperature dependences of the superconducting gap and of the specific heat were found to be well consistent with those expected for the single band BCS superconductors with an isotropic s-wave order parameter. Despite spin-orbit coupling is present in SnAs, our data shows no signatures of a potential unconventional superconductivity, and the characteristic BCS ratio 2/Tc = 3.48 − 3.73 is very close to the BCS value in the weak coupling limit.
A novel triphenylamine derivative-linked ionic liquid unit, 1-(6-((4-(bis(4-(thiophen-2-yl)phenyl)amino)- benzoyl)oxy)hexyl)-3-methyl-imidazolium tetrafluoroborate (TTPAC6IL-BF4), was designed and synthesized successfully, and its corresponding polymer PTTPAC6IL-BF4 was obtained by the electropolymerization method. The highest occupied molecular orbital energy band of TTPAC6IL-BF4 is higher and the onset oxidative potential lower compared with that of 6-bromohexyl 4-(bis(4-(thiophen-2-yl)phenyl)amino) benzoate (TTPAC6Br) without modifying the ionic liquid unit. The results imply that introducing an ionic liquid unit to the side chain is an efficient method to improve the switching time of conjugated polymers and would be inspirational for the design and preparation of novel bifunctional electrochromic polymeric electrolytes.
We report on the first high-resolution optical spectroscopy study of LiYF4:Ho in an external magnetic field. Peculiarities in the hyperfine structure of holmium spectral lines are discussed for the cases H||c and H⊥c (H = 0.53 and 0.87 T). The spectra reveal a strong interaction between crystal-field levels, mediated by Zeeman and hyperfine terms in the Hamiltonian. A study of the magnetic-field-dependent isotope shifts in 7Li0.1 6Li0.9YF4:Ho (0.1 at.%) single crystals delivers an estimate of the difference in magnetic g factors for holmium centers with all 6Li isotopes in the nearest surrounding of Ho3+ (g(0)) and the centers having one 7Li isotope there (g(1)):g(1) − g(0) = 0.01 ± 0.005.
In the paper the content of individual elements (Fe, Co, Zr, Ca and F) contained in nanocomposites FeCoZr ferromagnetic alloy in the CaF2 transparent ceramics dielectric matrix, depending on the content of the metal phase x was determined by the X-ray diffusion microanalysis (EDX) method. The nanocomposites were made by sputtering by argon ions. Investigations of changes in the chemical composition of nanocomposites under the influence of high-temperature treatments were carried out using the thermogravimetry method in the temperature range from 25 °C to 1000 °C with a temperature increase rate of 10 °C/min. On the basis of the research, a model of changes of the structural-phase state of nanogranular layers of ferromagnetic alloy Fe45Co45Zr10 in the transparent ceramics CaF2 matrix occurring under the influence of high-temperature treatments was proposed.
The damage and structural state of the surface layer of Al–Li–Mg samples composed of Al–5% Mg–2% Li (wt %) under pulsed action of power streams of high-temperature deuterium plasma and highenergy deuterium ions in the Plasma Focus (PF) device have been investigated. The radiation power density was q ~ 106 W/cm2; the pulse duration was 50–100 ns. Pulsed thermal heating and rapid cooling is established to lead to the melting and solidification of a thin surface layer of the alloy for several tens of nanoseconds. At the same time, in the superheated surface layer of the alloy, microcavities of a spherical shape are formed which is associated with intense evaporation of lithium into micropores within the heated layer. Thermal stresses caused by abrupt heating, melting, and cooling of a thin surface layer of metal result in formation of microcracks in the near-surface zone of the samples. The evaporation by the power electron beam of the elements of the anode material of the PF device (copper and tungsten) and their subsequent deposition onto the irradiated surface of the investigated samples in the form of droplets of submicron size are noted. It is shown that the thermal and radiationstimulated processes generated in the alloy under the action of pulsed energy fluxes in the implemented irradiation regime lead to the redistribution of elements in the surface layer of the aluminum solution, contributing to an increase in magnesium content and the formation of magnesium oxide on the surface.
A model of the thermo-field electron emission from the metal cathode with a thin insulating surface film at temperatures of 200–400 K is developed. An expression for the film emission efficiency in the gas discharge is obtained. The efficiency is equal to the fraction of electrons emitted into the film from the metal substrate, which enter the discharge volume and increase the effective secondary-electron emission yield of the cathode. It is shown that the thermo-field mechanism of electron emission influences noticeably the ignition voltage of the low-current discharge with such cathode at rather low temperatures exceeding the room temperature by less than 100 K.
Mechanical performances of titanium biomedical implants manufactured by superplastic forming are strongly related to the process parameters: the thickness distribution along the formed sheet has a key role in the evaluation of post-forming characteristics of the prosthesis. In this work, a finite element model able to reliably predict the thickness distribution after the superplastic forming operation was developed and validated in a case study. The material model was built for the investigated titanium alloy (Ti6Al4V-ELI) upon results achieved through free inflation tests in different pressure regimes. Thus, a strain and strain rate dependent material behaviour was implemented in the numerical model. It was found that, especially for relatively low strain rates, the strain rate sensitivity index of the investigated titanium alloy significantly decreases during the deformation process. Results on the case study highlighted that the strain rate has a strong influence on the thickness profile, both on its minimum value and on the position in which such a minimum is found.
Copper borate Cu3(BO3)2 is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper Cu2+ ions occupy 16 nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of 3d9 copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low P1 space-group symmetry and a large unit cell with Z = 10. An unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 cm−1, which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at TN = 10 K, thus evidencing magnetoelastic interaction. No new phonons were found below TN, which excludes the spin-Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at ∼1.8 eVis observed, which we assign to overlapping of transitions between the 3d9 states of Cu2+ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around ∼2.8−3.0 eV.
The work presents a study of manganese-doped copper metaborate (Cu, Mn) B2O4 using optical spectroscopy. The temperature of the antiferromagnetic phase transition T-N = 19 K has been defined according to the absorption spectra. Polarization studies (Cu, Mn) B2O4 in isotropic ab-plane show the presence of linear antiferromagnetic dichroism in the magnetically ordered state previously observed in pure copper metaborate CuB2O4. This measurement allows to find the magnetic phase transition into an elliptical structure at the temperature T* = 7.0 K.
A method for the construction of microwave devices with longitudinal interaction is proposed. Devices of the present type create uniform cross-sectional distribution of temperature of rods made of polymer composite materials. Results from theoretical and experimental investigations of the cross-sectional distribution of the temperature of the material of the rod as well as the parameters of the microwave device are presented.
The mixture of argon and mercury vapor with temperature-dependent composition is used as the background gas in different types of gas discharge illuminating lamps. The aim of this work was to develop a model of the low-current discharge in an argon-mercury mixture at presence of a thin insulating film on the cathode and to investigate the influence of film on the discharge ignition voltage at low ambient temperatures. When discharge modeling, we used the obtained earlier expression which describes dependence of the mixture ionization coefficient on temperature. When there was a thin insulating film on the cathode the model took into account that positive charges are accumulated on its surface during the discharge. They generate an electric field in the film sufficient for the field emission of electrons from the metal substrate of the electrode into the insulator and some of them can overcome the potential barrier at the film outer boundary and go out in the discharge volume improving emission characteristics of the cathode. Calculations showed that at a temperature decrease the electric field strengthes in the discharge gap and the voltage in it are increased due to reduction of the saturated mercury vapor density in the mixture followed by the decrease of its ionization coefficient. Existence of a thin insulating film on the cathode surface results in an increase of the cathode effective secondary electron emission yield which compensates the reduction of the mixture ionization coefficient value. The results of discharge characteristics modeling demonstrate that in case of the cathode with an insulating film the discharge ignition becomes possible at a lower inter-electrode voltage. This ensures outdoor mercury lamp turning on at a reduced supply voltage and increases its reliability under low ambient temperatures.
The paper considers an influence of different kinds of radio-frequency plasma treatments onto modification of MIS structures with a thermal SiO2 film which is aimed at improvement of electro-physical parameters of the film. It was found that for the modification of MIS structures it is more preferable to utilize the oxygen plasma radio-frequency plasma treatment performed by a setup with the parallel-plate-type reactor. This is due to the fact that setup allows to have lesser degradation of charge characteristics of the gate dielectric in comparison with a setup with the cylindrical quartz reactor. The radio-frequency plasma treatment stimulates restructuring of SiO2 film and, as a result, diminishes possibility of sample breakdown and raises injection and radiation stability of the samples.
In this study the process of a hot laboratory rolling of a round bar on the flat rolls was studied by laboratory
experiments and numerical modelling in order to evaluate the effect of boundary conditions and simulation techniques
on the model predictions. The computer simulations of rolling process were performed by two different techniques
based on Finite Element Method (FEM). The first technique solves three dimensional problems. The second one is
based on the sequential solution of a series of generalized plane problems. Each technique was used for solving of
isothermal forming task and non-isothermal one. The results were compared with laboratory rolling performed with
different reductions and at different temperatures. It was found that the difference of initial temperature is incon-
sequential to the prediction of strain and strain-rate distributions. This observation was confirmed experimentally.
Single crystal of TlCl was doped with NIR photoluminescent univalent bismuth cations by prolonged immersion in liquid bismuth metal. The ion exchange Tl+ + Bi0 ↔ Tl0 + Bi+ at the crystal surface with subsequent Bi+ migration to the bulk are expected to drive the doping process. Contrary with Bi‐doped TlCl crystals, grown by Bridgman method, the ion exchange does not produce the additional nonluminescent bismuth‐containing centers. The investigation of photoluminescence emission and excitation spectra lead to the conclusion, that Bi+ is the main NIR emissive center in Bi‐doped TlCl.
The aim of this work was a comparative study of the wear resistance of a sample of an aluminum alloy (EN AW-2024, an aluminum alloy of the Al-Cu-Mg system) modified with ultrafine particles of minerals using the sclerometry method, which makes it possible to measure the physicomechanical properties of the material at the microscale, as well as determining some tribological parameters (hardness and elastic modulus) of a duralumin sample with a mineral coating. Wear resistance was measured using a NanoScan-4D scanning hardness tester using the multi-cycle friction method using a sapphire sphere with control of the pressing force and the deepening of the tip into the sample. The use of such a measurement system is especially important when testing thin modified layers, when the layer thickness is comparable with the surface roughness parameters and the influence of the substrate is excluded. The measurement results showed that the wear resistance of the surface of an aluminum alloy sample modified with ultrafine mineral particles increased by more than 12 times compared to the wear resistance of an aluminum alloy surface without modification.