In pnictide RbEuFe4As4, superconductivity sets in at 36 K and coexists, below 15−19 K, with the long-range magnetic ordering of Eu 4f spins. Here we report scanning tunneling experiments performed on cold-cleaved single crystals of the compound. The data revealed the coexistence of large Rb-terminated and small Eu-terminated terraces, both manifesting 1 × 2 and \sqrt 2 × \sqrt 2 reconstructions. On \sqrt 2 × \sqrt 2 surfaces, a hidden electronic order with a period ∼5 nm was discovered. A superconducting gap of ∼7 meV was seen to be strongly filled with quasiparticle states. The tunneling spectra compared with density functional theory calculations confirmed that flat electronic bands due to Eu 4f orbitals are situated ∼1.8eV below the Fermi level and thus do not contribute directly to Cooper pair formation.
The existence problem for attractors of foliations with transverse linear connection is investigated. In general foliations with transverse linear connection do not admit attractors. A conditions that implies the existence of a global attractor which is a minimal set, is specified. An application to transversely similar pseudo-Riemannian foliations is obtained. The global structure of transversely similar Riemannian foliations is described. Different examples are constructed.
Energetic compounds 1–6, consisting of one or two furazan rings linked by azo or azoxy bridges and one or two nitro‐NNO‐azoxy substituents, were synthesized by nitration of the corresponding tert‐butyl‐NNO‐azoxy precursors with NO2BF4. Novel methods for the synthesis of 3,4‐bis(tert‐butyl‐NNO‐azoxy)furazan (7) and bis‐4,4′‐(tert‐butyl‐NNO‐azoxy)‐3,3′‐azoxyfurazan (21) were elaborated. The nitro‐NNO‐azoxy compounds obtained display high calculated detonation performance (vD=8.07–9.40 km s−1 and PC‐J=27.4–43.4 GPa) that is superior to the corresponding nitrofurazans (DNF, DNAzF, DNAF). The replacement of nitrofurazans with the corresponding (nitro‐NNO‐azoxy)furazans increases the specific impulse of the model solid composite propellant formulations by 2–10 s, which is due to high calculated heats of formation (600–892 kcal kg−1) and positive oxygen balance (0–20 %) of the latter compounds.
A smooth synthesis of 4H-[1,2,3]triazolo[4,5-c][1,2,5]oxadiazole 5-oxide 1 and its energetic salts (ammonium, hydroxylammonium, guanidinium, triaminoguanidinium) is reported. The compounds synthesized were characterized by multinuclear nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, mass spectrometry, elemental analysis, differential scanning calorimetry, single-crystal, and powder X-ray diffraction. All the compounds possess a beneficially high enthalpy of formation (88.9–168.0 kcal·mol–1). These data, in combination with the experimentally determined densities (1.702–1.934 g·cm–1), were used to calculate the detonation pressures (33.9–43.1 GPa) and velocities (8.86–9.31 km·s–1). The majority of the synthesized energetic salts had moderate impact and friction sensitivity, which made them promising candidates for various energy applications, including their use as components of solid composite propellants. It was shown that compounds 1 and 2c–f had higher energetic characteristics as components of solid composite propellants (the specific impulse was higher by 5–10 s) than HMX or CL-20 in propellant formulations.
A novel acceptor–donor–acceptor chromophore IDTT-HC2P with an indacenodithienothiophene core linked with two hydrazinylidenecyclopentadiene terminal acceptor groups was designed and synthesized via the reaction of dilithiated indacenodithienothiophene with the corresponding diazo compound. The frontier orbital energy levels as well as the bandgap which were estimated from both optical and electrochemical properties are very closely related to those of ITIC which in turn is associated with high power conversion efficiencies.
The low temperature transport of electron, or vibrational or electronic exciton towards polymer chains turns out to be dramatically sensitive to its interaction with transverse acoustic vibrations. We show that this interaction leads to substantial polaron eﬀect and decoherence, which are generally stronger than those associated with longitudinal vibrations. For site-dependent interactions transverse phonons form subohmic bath leading to the quantum phase transition accompanied by full suppression of the transport at zero temperature and fast decoherence characterized by temperature dependent rate k2 ∝ T3/4 at low temperature while k2 ∝ T2 for site-independent interactions. The latter dependence was used to interpret recent measurements of temperature dependent vibrational energy transport in polyethylene glycol oligomers.
An associative electron upconversion is proposed as a key step determining the selectivity of thiol–yne coupling. The developed synthetic approach provided an efficient tool to access a comprehensive range of products – four types of vinyl sulfides were prepared in high yields and selectivity. We report practically important transition-metal-free regioselective thiol–yne addition and formation of the demanding Markovnikov-type product by a radical photoredox process. The photochemical process was directly monitored by mass-spectrometry in a specially designed ESI-MS device with green laser excitation in the spray chamber. The proposed reaction mechanism is supported by experiments and DFT calculations.
4-Arylidene-1H-imidazol-5(4H)-ones react with N-benzylazomethine methylide under trifluoroacetic acid catalysis to form 9-aryl-1,3,7- triazaspiro[4.4]non-1-en-4-ones in 66–95% yields as one diastereomer in each case.
Spiro[imidazol-5-one-4,1′-cyclopropanes] behave as donor–acceptor (D–A) cyclopropanes in a formal cycloaddition reaction with aldehydes. The activation of such type of cyclopropanes is achieved with an equivalent of Brønsted acid. The reaction proceeds in high yields of 51–92% and demonstrates moderate diastereoselectivity at the quaternary stereocenter, which is determined by the electron-donating nature of the aldehyde partner. The ease of separation of stereoisomers allowed the creation of a library of 44 spiroannulated tetrahydrofurans with various substitution patterns.