Low-temperature variation of magnetic order in a nonmagnetic n-Ge:As semiconductor in the vicinity of the metal-insulator phase transition

The phenomenon of the low-temperature transition from antiferro- to ferromagnetic ordering of impurity spins in a nonmagnetic compensated n-Ge:As semiconductor near the metal-insulator phase transformation has been experimentally observed. The effect is manifested by rather sharp changes in the spin density and g-factor in the electron spin resonance spectra. As the relative content of a compensating impurity (gallium) is reduced below 0.7, the transition temperature begins to decrease and, at a degree of compensation below 0.3, drops below the studied temperature range (i.e., below 2 K).     

Influence of disordering on the critical temperature of superconductors with a short coherence length

The influence of disordering on the critical temperature of s-wave superconductors with a short coherence length is studied by numerical simulation using the two-dimensional Hubbard model with attraction. The calculations are carried out within the Bogolyubov-De Gennes approach, which allows one to include the spatial inhomogeneity of the order parameter. The influence of the spatial inhomogeneity of physical quantities on the results obtained is studied. Disordering is shown to be able to cause a significant increase in the critical temperature.     

Photoelectrochemistry of conducting polymers modified with electron-acceptor moieties

The photoelectrochemical behavior of two polymers of the polythiophene series containing electron-acceptor groups in the main chain was studied and compared to that of the nonmodified polymer, poly(2,2'-bithiophene), PBT. The acceptor groups were 2,2'-bipyridine and biphenyl, which are electron-deficient as compared to the bithiophene unit. All three polymers demonstrated a pronounced photovoltaic effect, which for PBT was consistent with data reported earlier. The introduction of the electron-acceptor moieties was found to significantly enhance the magnitude of the steady-state photocurrent as well as to drastically alter the dependencies of the photocurrent on the polymer film thickness and the external bias. These observations indicated that the mechanism of photocurrent generation in the modified polymers differs from that in nonmodified polymer and involves the electron transfer to the electron-acceptor moieties in the polymer main chain. The values of the external quantum efficiency were estimated in a liquid-cell arrangement to be 0.27 and 0.19% for modified and nonmodified polymers, respectively.     

Observation of small changes in the energy of a neutron in an alternating magnetic field

A method for measuring small changes in the energy of a neutron has been proposed on the basis of the anomalous behavior of the dispersion of the neutron in the crystal near Bragg “resonance.” A high sensitivity of the method allows the observation of the acceleration of the neutron in the alternating magnetic field. It has been found that the small difference between the energies of two spin states of the neutron (parallel and antiparallel to the magnetic field) leads to significant spatial splitting of wave packets and, correspondingly, to the depolarization of the neutron beam.     

Mechanism and rate constants of the CH2 + CH2CO reactions in triplet and singlet states: A theoretical study

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH₂ with ketene CH₂CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice–Ramsperger–Kassel–Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH₃ and C₂H₄ + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C₂H₄ + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH₂COCH₂ intermediate or along the pathway of CO elimination from the initial CH₂CH₂CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C₂H₄ + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH₃CHCO and cyclic CH₂COCH₂ intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300–3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.     

Direct measurement of the delay time for a neutron in a crystal in the case of the Laue diffraction

The dynamical Laue diffraction has been studied for a direct beam diffracted from a thin (～3.5 cm) α-quartz crystal at the Bragg angles close to 90°. It is shown that diffraction occurs at Bragg angles up to 87°. The time-of-flight method is used to measure the time of the neutron presence in the crystal under the diffraction conditions. The time delay for a scattered neutron inside the crystal predicted earlier for the Bragg angles close to 90° confirmed experimentally. The effective velocity of the neutron propagation in a crystal measured at the incident-neutron velocity of 810 m/s and the diffraction angle of 87° equals (43±1) m/s. The result obtained confirms the possibility of increasing, by an order of magnitude, the sensitivity of the diffraction method of determining the dipole moment of a neutron at Bragg angles close to 90° predicted earlier theoretically.     

Effect of light on the poly-3-phenylthiophene properties

The electrochemical and photoelectrochemical behavior of a conducting polymer, poly-3-phenylthiophene (P3PhT), is studied in acetonitrile containing salts of tetrabutylammonium (TBA) and lithium. Like other thiophene polymers, P3PhT may or may not be doped cathodically in the presence of salts of TBA or lithium, respectively. In general, the photoelectrochemical behavior of P3PhT resembles that of polybithiophene and poly-3-methylthiophehe. In particular, P3PhT is photoactivated when exposed to light at negative potentials. Depending on the system in which measurements are taken (in the presence of salts of lithium or TBA), P3PhT undergoes photoelectrochemical undoping or photoelectrochemical cathodic doping, respectively, which is confirmed by measuring electrochemical impedance. However, the photoelectrochemical processes on P3PhT (at least in solutions containing lithium salts) proceed much slower than those on other thiophene polymers we studied earlier. The concentration of doping ions in P3PhT in the presence of lithium salts is higher than that in the presence of TBA salts even in the undoped state. This is due to a specific interaction between Li⁺ and the phenyl group in P3PhT. Deceased.     

Diffraction Enhancement of the Stern—Gerlach Effect for a Neutron in a Crystal

JETP Letters - The spatial splitting of an unpolarized neutron beam into two spin components in an inhomogeneous magnetic field (an analog of the Stern-Gerlach experiment) with small gradients has...     

THE EFFECT OF ACIDS ON THE INFRARED SPECTRA OF SCHIFF BASES—II. IMINES CONTAINING THE C=C-C=N AND C=C-C=C-C=N UNITS

Abstract— The Fourier-transform infrared spectra of chloroform-d solutions of conjugated imines CH₃CH=CHCH=NCH(CH₃)₂ and CH₃CH₂CH=CHCH=CHCH=NCH(CH₃)₂ and the related protonated species with HCl, HBr, HI, trichloro, dichloro, monobromo and monochloroacetic acids or propionic acid are presented. The effects of conjugation and protonation are examined. The results show that conjugation slightly increases the basicity of the Schiff bases. HCl, HBr and HI protonate the Schiff bases completely. The carboxylic acids protonate partially depending on their p K _a, values. When the Schiff base contains two (or more) C=C bonds conjugated with C=N, the main C=C stretching band undergoes a strong intensification showing that sizeable dipole moment variations occur along the conjugated chain.