Resonant Pumping of d-d Crystal Field Electronic Transitions as a Mechanism of Ultrafast Optical Control of the Exchange Interactions in Iron Oxides.

The microscopic origin of ultrafast modification of the ratio between the symmetric (J) and antisymmetric (D) exchange interaction in antiferromagnetic iron oxides is revealed, using femtosecond laser excitation as a pump and terahertz emission spectroscopy as a probe. By tuning the photon energy of the laser pump pulse we show that the effect of light on the D/J ratio in two archetypical iron oxides FeBO_{3} and ErFeO_{3} is maximized when the photon energy is in resonance with a spin and parity forbidden d-d transition between the crystal-field split states of Fe^{3+} ions. The experimental findings are supported by a multielectron model, which accounts for the resonant absorption of photons by Fe^{3+} ions. Our results reveal the importance of the parity and spin-change forbidden, and therefore often underestimated, d-d transitions in ultrafast optical control of magnetism.

Characterization of the iron oxide phases formed during the synthesis of core-shell FexOy@C nanoparticles modified with Ag.

Core-shell FexOy@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static magnetic measurements, and optical magnetic circular dichroism (MCD). FexOy@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO3)3 · 9H2O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO3 in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe3O4 and γ-Fe2O3. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid FexOy@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe3O4@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the FexOy@C-Ag NPs is discussed.

From underdoped to overdoped cuprates: two quantum phase transitions.

Several experimental and theoretical studies indicate the existence of a critical point separating the underdoped and overdoped regions of the high-T(c) cuprates' phase diagram. There are at least two distinct proposals on the critical concentration and its physical origin. The first one is associated with the pseudogap formation for p < p, with p≈0.2. The other relies on the Hall effect measurements and suggests that the critical point and the quantum phase transition (QPT) take place at optimal doping, p(opt)≈0.16. Here we have performed a precise density of states calculation and found that there are two QPTs and the corresponding critical concentrations associated with the change of the Fermi surface topology upon doping.

[Stability of lepidopterous pheromone molecules of forest insects to environmental factors].

The interactions of the pheromones of some representatives of the Lepidoptera order with each other and with materials contained in the forest air, as well as the effect of electromagnetic radiation on pheromone have been studied. It was found that the reactions of pheromones with substances contained in the forest air are irreversible and proceed with the liberation of heat. Electromagnetic radiation affects very strongly the structure of pheromones. In this case, a pheromone molecule is activated and can readily enter into reaction.

Prediction of orientation relationships and interface structures between α-, ß-, γ-FeSi2 and Si phases.

A pure crystallogeometrical approach is proposed for predicting orientation relationships, habit planes and atomic structures of the interfaces between phases, which is applicable to systems of low-symmetry phases and epitaxial thin film growth. The suggested models are verified with the example of epitaxial growth of α-, γ- and ß-FeSi2 silicide thin films on silicon substrates. The density of near-coincidence sites is shown to have a decisive role in the determination of epitaxial thin film orientation and explains the superior quality of ß-FeSi2 thin grown on Si(111) over Si(001) substrates despite larger lattice misfits. Ideal conjunctions for interfaces between the silicide phases are predicted and this allows for utilization of a thin buffer α-FeSi2 layer for oriented growth of ß-FeSi2 nanostructures on Si(001). The thermal expansion coefficients are obtained within quasi-harmonic approximation from the DFT calculations to study the influence of temperature on the lattice strains in the derived interfaces. Faster decrease of misfits at the α-FeSi2(001)||Si(001) interface compared to γ-FeSi2(001)||Si(001) elucidates the origins of temperature-driven change of the phase growing on silicon substrates. The proposed approach guides from bulk phase unit cells to the construction of the interface atomic structures and appears to be a powerful tool for the prediction of interfaces between arbitrary phases for subsequent theoretical investigation and epitaxial film synthesis.

[The mechanism of formation of emitter by fluorescence of calcium-discharged obelin].

It is known that bioluminescence of obelin is triggered by Ca2+ the binding of which to the protein induces the decarboxylation of 2-hydroperoxycoelenterazine. The molecular mechanism of fluorescence of obelin, which determines the fluorescence of see hydroid Obelia Longissima, has been investigated with the use of quantum chemical calculations. According to quantum chemical calculations, the emitter of the reaction is the ion-pair state of phenolate-anion of coelenteramide. It has been shown that the fluorescence spectrum of this state depends on the position of the proton between the oxygen atoms of the phenol group of coelenteramide and the nitrogen atom of His22. The agreement of the calculated absorption and fluorescence spectra with the experimental spectrum shows the accuracy of the quantum chemical calculations and conclusions.

Geometric phases and quantum phase transitions in open systems.

The relationship is established between quantum phase transitions and complex geometric phases for open quantum systems governed by a non-Hermitian effective Hamiltonian with accidental crossing of the eigenvalues. In particular, the geometric phase associated with the ground state of the one-dimensional dissipative Ising model in a transverse magnetic field is evaluated, and it is demonstrated that the related quantum phase transition is of the first order.

Estimation of the thermal and photochemical stabilities of pheromones.

The correlation between the kinetic stability of molecules against temperature and variations in their geometric structure under optical excitation is investigated by the example of different organic pheromone molecules sensitive to temperature or ultraviolet radiation using the density functional theory. The kinetic stability is determined by the previously developed method based on the calculation of the probability of extension of any structural bond by a value exceeding the limit value Lмах corresponding to the breaking of the bond under temperature excitation. The kinetic stability calculation only requires the eigenfrequencies and vibrational mode vectors in the molecule ground state to be calculated, without determining the transition states. The weakest bonds in molecules determined by the kinetic stability method are compared with the bond length variations in molecules in the excited state upon absorption of light by a molecule. Good agreement between the results obtained is demonstrated and the difference between them is discussed. The universality of formulations within both approaches used to estimate the stability of different pheromone molecules containing strained cycles and conjugated, double, and single bonds allows these approaches to be applied for studying other molecules. Graphical Abstract Estimation of the thermal and photochemical stabilities of pheromones.

Lattice vibrations in an α- and ß-AgCuS superionic conductor: experimental time-of-flight inelastic neutron scattering studies.

Inelastic neutron scattering (INS) measurements of a powder specimen of cation conducting AgCuS have been performed in its ß-phase at 298 and 348 K and in its α-phase at 398 K by neutron time-of-flight (TOF) scattering. The neutron-weighted phonon density of states, G(ε), of the superionic and non-superionic phases of AgCuS has been obtained. G(ε) reveals a non-Debye behaviour at low energy that is caused by the presence of low-energy modes at 2.6 meV observed in the dynamic structure factor. The origin of this mode is not clear. The [Formula: see text] phase transition at 366 K is accompanied by considerable softening of several modes and simultaneous broadening of all peaks that result in a large increase of quasi-elastic intensity. Thermodynamic variables have been derived on the basis of G(ε).

The Fermi surface and the role of electronic correlations in Sm(2-x)Ce(x)CuO4.

Using a LDA+GTB (local density approximation+generalized tight-binding) hybrid scheme we investigate the band structure of the electron-doped high- T(c) material Sm(2-x)Ce(x)CuO(4). Parameters of the minimal tight-binding model for this system (the so-called three-band Emery model) were obtained within the NMTO (Nth-order muffin-tin orbital) method. The doping evolution of the dispersion and the Fermi surface in the presence of electronic correlations was investigated in two regimes of magnetic order: short-range (spin-liquid) and long-range (antiferromagnetic metal). Each regime is characterized by the specific topologies of the Fermi surfaces and we discuss their relation to recent experimental data.