Crystal field of Yb<Superscript>3+</Superscript> tetragonal centers in the YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript> intermetallic compound

The spectra of electron paramagnetic resonance and inelastic neutron scattering in crystals of the heavy-fermion intermetallic compound YbRh₂Si₂ are interpreted. The phenomenological potentials of the crystal electric field of Yb³⁺ tetragonal centers and the parameter of the Hamiltonian for the spin-orbit interaction of electrons are determined from the experimental energy level schemes. A comparison of the results obtained from experimental data on electron paramagnetic resonance, inelastic neutron scattering, and Mössbauer spectroscopy shows that the most probable ground state of Yb³⁺ ions in the YbRh₂Si₂ crystal is the Kramers doublet Γ _t6 ^? .     

Electron paramagnetic resonance of Yb<Superscript>3+</Superscript> ions in a concentrated YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript> compound with heavy fermions

The EPR signal from localized ytterbium ions was observed in an undoped YbRh₂Si₂ compound with heavy fermions in the temperature range from 1.5 to 25 K. The exponential contribution dominating the temperature dependence of EPR line width at temperatures above 15 K was shown to be caused by the random transitions from the ground to the first excited Stark sublevel of the Yb³⁺(4f¹³) ion with the activation energy Δ=115 K.     

Magnetic phases in UNi<Subscript>2</Subscript>Si<Subscript>2</Subscript>

The tetragonal compound UNi₂Si₂ exhibits in zero magnetic field three different antiferromagnetic phases belowT _N =124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q _z ). Two additional order-order magnetic phase transitions are observed below T _N , namely atT ₁=108 K and T ₂=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi₂Si₂ single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi₂Si₂.     

Spin dynamics of the intermediate-valence compound EuCu<Subscript>2</Subscript>Si<Subscript>2</Subscript>

The dynamic magnetic response of the intermediate-valence compound EuCu₂Si₂ has been studied using inelastic neutron scattering. At low temperatures, strong renormalization of the ⁷ F ₀ → ⁷ F ₁ spin-orbit transition energy is detected; it is likely to be related to partial delocalization of the f electrons of Eu. An increase in the temperature increases the valence instability of europium and results in further changes in the magnetic excitation spectrum parameters and the appearance of an intense quasi-elastic component.     

The Optical Gain of a Si-Based Lattice-Matched Si<Subscript>0.15</Subscript>Ge<Subscript>0.621</Subscript>Sn<Subscript>0.229</Subscript>/Si<Subscript>0.637</Subscript>Ge<Subscript>0.018</Subscript>Sn<Subscript>0.345</Subscript> MQW Laser

We study the optical-gain characteristics of a Si-based MQW laser, in which the active region has 20 Si_0.15Ge_0.621Sn_0.229 quantum wells separated by 20 Si_0.637Ge_0.018Sn_0.345 barriers. We reach a maximum optical gain of 2300 cm^?1 with an estimated carrier concentration of 5·10¹⁸ cm^?3, which is equivalent to the transparent current density equal to 0.5 kA/cm². Furthermore, we discuss the optical confinement factor and modal gain. The modal gain depends sensitively on the number of the quantum wells (QWs), and this fact restricts the optical confinement factor. The modal gain of the model we proposed can reach 1500 cm^?1 at the injection current density equal to 3 kA/cm². We hope that our results show the possibility to obtain a Si-based near-infrared laser.     

Structure and dynamics of the Lu<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> lattice: Ab initio calculation

The ab initio calculations have been carried out for the crystal structure and Raman spectrum of a single crystal of lutetium pyrosilicate Lu₂Si₂O₇. The types of fundamental vibrations and their frequencies and intensities in the Raman spectrum for two polarizations of the crystal have been determined. The calculations have been performed within the framework of the density functional theory (DFT) using the hybrid functionals. The ions involved in the vibrations have been identified using the method of isotopic substitution. The results of the calculations are in good agreement with the experiment.     

On the toughening mechanisms of MoSi<Subscript>2</Subscript> reinforced Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

The toughness increment occurring in Si₃N₄-based composites due to the addition of MoSi₂ particles was compared to the predictions of theoretical models based on the combination of residual stresses and crack deflection toughening mechanisms. A direct application of theoretical models led to a substantial discrepancy between predicted and observed values. For this reason, the basic parameters of the theoretical models were experimentally evaluated. The residual stresses were assessed by measuring the strain in the reinforcing particles by X-ray diffraction. Moreover, the MoSi₂ interparticle distance was calculated by image analysis and the crack paths were analyzed in order to check the actual extent of crack deflection. The overall toughness increase recalculated as the sum of the newly estimated values of residual stresses and crack deflection contributions, was shown to be in good agreement with the experimental results. PACS 81.05.Je; 81.40.Np     

Electronic excitations in BeAl<Subscript>2</Subscript>O<Subscript>4</Subscript>, Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>, and Be<Subscript>3</Subscript>Al<Subscript>2</Subscript>Si<Subscript>6</Subscript>O<Subscript>18</Subscript> crystals

Low-temperature (T = 7 K) time-resolved selectively photoexcited luminescence spectra (2–6 eV) and luminescence excitation spectra (8–35 eV) of wide-bandgap chrysoberyl BeAl₂O₄, phenacite Be₂SiO₄, and beryl Be₃Al₂Si₆O₁₈ crystals have been studied using time-resolved VUV spectroscopy. Both the intrinsic luminescence of the crystals and the luminescence associated with structural defects were assigned. Energy transfer to impurity luminescence centers in alexandrite and emerald was investigated. Luminescence characteristics of stable crystal lattice defects were probed by 3.6-MeV accelerated helium ion beams.     

Study of the heavy-fermion compound CeRu<Subscript>2</Subscript>Si<Subscript>2</Subscript> by the small-angle neutron scattering method

In order to directly observe neutron scattering by heavy fermion quasiparticles at low temperatures, a CeRu₂Si₂ single crystal has been studied by the small-angle neutron scattering method. In the experiment, neutron scattering is observed at T = 0.85 K for momentum transfers q ≤ 0.04 Å^?1, which is treated as the orbital component of magnetic scattering by heavy fermion quasiparticles. It has been found that the application of a magnetic field H = 1 T leads to both an increase in the observed scattering and its anisotropy with respect to the field direction. Moreover, measurements in the magnetic field reveal additional scattering for q > 0.04 Å^?1, which is well described by a Lorentzian and is interpreted as neutron magnetic scattering by spin-density fluctuations with a correlation radius R_c ≈ 30 Å.     

Growth and optical activity of Ca<Subscript>3</Subscript>NbGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> single crystal

Crystals of Ca₃NbGa₃Si₂O₁₄ (CNGS) with ordered langasite structure were grown using the Czochralski method along the Cartesian X axis [110]. The as-grown crystals exhibit high optical quality and structure perfection. Optical activities were obtained by measuring polarised transmission at various wavelengths between crossed polarisers using a TU-1900 spectrophotometer and we found that CNGS crystals showed very large values of . PACS 81.10.-h; 42.79.Ci; 78.20.Ek     

Structural transformations in the Al<Subscript>85</Subscript>Ni<Subscript>6.1</Subscript>Co<Subscript>2</Subscript>Gd<Subscript>6</Subscript>Si<Subscript>0.9</Subscript> amorphous alloy during multiple rolling

The effect of multiple rolling at room temperature on the structure and crystallization of the Al₈₅Ni_6.1Co₂Gd₆Si_0.9 amorphous alloy has been studied using transmission electron microscopy, differential scanning calorimetry, and X-ray diffraction. The total plastic strain is 33%. It has been shown that the deformation results in the formation of aluminum nanocrystals with the average size that does not exceed 10–15 nm. The nanocrystals are formed in regions of localization of plastic deformation. The deformation decreases the thermal effect of nanocrystallization (∼15%) as compared to the heat release at the first stage of crystallization of the unstrained sample. The morphology, structure, and distribution of precipitates have been investigated. Possible mechanisms of the formation of nanocrystals during the deformation have been discussed.     

Electronic structure and half-metallic property of Si<Subscript>3</Subscript>CaC<Subscript>4</Subscript>

The electronic structures and magnetic properties of Si₃CaC₄ in zinc-blende phase has been studied by employing the first-principles method based on density functional theory (DFT). The calculations predict stable ferromagnetic ground state in Si₃CaC₄, resulting from calcium substitution for silicon. The calculated total magnetic moment is 2.00 μ _B per supercell, which mainly arises from the Ca and neighboring C atoms. Band structures and density of states studies show half-metallic (HM) ferromagnetic property for Si₃CaC₄. The ferromagnetic coupling is generally observed between the Ca and C atoms. The ferromagnetism of Si₃CaC₄ can be explained by the hole-mediated double exchange mechanism. The sensitivity of half-metallicity of Si₃CaC₄ as a function of lattice constant is also discussed, and the half-metallicity can be kept in a wider lattice constant range.     

Properties of ferromagnetic resonance in Fe<Subscript>73.5</Subscript>CuNb<Subscript>3</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript> nanocrystalline alloys

The structural properties and parameters of ferromagnetic resonance have been studied for Fe_73.5CuNb₃Si_13.5B₉ nanocrystalline alloys produced from the initial amorphous state via annealing under different conditions. The dependence of the linewidth of the ferromagnetic resonance on the grain size ΔH ∼ D ⁶ has been found. The result is discussed within the framework of the random magnetic anisotropy model.     

Silicon isotope separation utilizing infrared multiphoton dissociation of Si<Subscript>2</Subscript>F<Subscript>6</Subscript> irradiated with two-frequency CO<Subscript>2</Subscript> laser lights

Silicon isotope separation has been performed utilizing the infrared multiphoton dissociation (IRMPD) of Si₂F₆ irradiated with two-frequency CO₂ laser lights. The two-frequency excitation method improved the separation efficiency by keeping the high enrichment factors. For example, Si₂F₆ with the ²⁸Si fraction of 99.4% was obtained at 40.0% dissociation of Si₂F₆ after the simultaneous irradiation of 100 pulses with 966.23 cm^-1 photons (0.089 J/cm²) and 954.55 cm^-1 photons (0.92 J/cm²), while 1000 pulses were needed to obtain 99.0% of ²⁸Si at 27.2% dissociation in the case of single frequency irradiation at 954.55 cm^-1 (0.92 J/cm²). The single-step enrichment factors of ²⁹Si and ³⁰Si increased with increasing Si₂F₆ pressure. The reason for this enhancement has been discussed in terms of the rotational and vibrational relaxations by collisions with ambient gases. PACS 42.62.Cf; 82.30.Lp; 82.50.Bc     

Pseudogap behavior in Bi<Subscript>2</Subscript>Ca<Subscript>2</Subscript>SrCu<Subscript>2</Subscript>O<Subscript>8</Subscript>: Results of the generalized dynamical mean-field approach

Pseudogap phenomena are observed for the normal underdoped phase of different high-T _c cuprates. Among others, the Bi₂Sr₂CaCu₂O_{8 − δ} (Bi2212) compound is one of the most studied experimentally. To describe the pseudogap regime in Bi2212, we use a novel generalized ab initio LDA + DMFT + Σ_k hybrid scheme. This scheme is based on the strategy of one of the most powerful computational tools for real correlated materials: the local density approximation (LDA) + dynamical mean-field theory (DMFT). Conventional LDA + DMFT equations are here supplied with an additional (momentum-dependent) self-energy Σ_k in the spirit of our recently proposed DMFT + Σ_k approach taking into account pseudogap fluctuations. In the present model, Σ_k describes nonlocal correlations induced by short-range collective Heisenberg-like antiferromagnetic spin fluctuations. The effective single-impurity problem of the DMFT is solved by the numerical renormalization group (NRG) method. Material-specific model parameters for the effective x ² − y ² orbital of Cu-3d shell of the Bi2212 compound, e.g., the values of intra-and interlayer hopping integrals between different Cu sites, the local Coulomb interaction U, and the pseudogap potential Δ were obtained within the LDA and LDA + DMFT schemes. Here, we report on the theoretical LDA + DMFT + Σ_k quasiparticle band dispersion and damping, Fermi surface renormalization, momentum anisotropy of (quasi)static scattering, densities of states, spectral densities, and angular-resolved photoemission (ARPES) spectra, taking into account pseudogap and bilayer splitting effects for normal (slightly) underdoped Bi2212 (δ = 0.15). We show that LDA + DMFT + Σ_k successfully describes strong (pseudogap) scattering close to Brillouin zone boundaries. Our calculated LDA + DMFT + Σ_k Fermi surfaces and ARPES spectra in the presence of pseudogap fluctuations are almost insensitive to the bilayer splitting strength. However, our LDA-calculated value of bilayer splitting is rather small to describe the experimentally observed peak-dip-hump structure. The results obtained are in good semiquantitative agreement with various recent ARPES experiments. The article was submitted by the authors in English.     

Quasi-one-dimensional quantum spin liquid in the Cu(C<Subscript>4</Subscript>H<Subscript>4</Subscript>N<Subscript>2</Subscript>)(NO<Subscript>3</Subscript>)<Subscript>2</Subscript> insulator

We analyze measurements of the magnetization, differential susceptibility and specific heat of quasi-onedimensional insulator Cu(C₄H₄N₂)(NO₃)₂ (CuPzN) subjected to magnetic fields. We show that the thermodynamic properties are defined by quantum spin liquid formed with spinons, with the magnetic field tuning the insulator CuPzN towards quantum critical point related to fermion condensation quantum phase transition (FCQPT) at which the spinon effective mass diverges kinematically. We show that the FCQPT concept permits to reveal and explain the scaling behavior of thermodynamic characteristics. For the first time, we construct the schematic T–H (temperature-magnetic field) phase diagram of CuPzN that contains Landau–Fermi-liquid, crossover and non-Fermi liquid parts, thus resembling that of heavy-fermion compounds.     

Energy levels of rare-earth ions in Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S

The energies of the ground 4f ⁿ levels of tri- and divalent rare-earth ions with respect to the conduction and valence bands of Gd₂O₂S crystal has been determined. It is shown that the Pr³⁺, Tb³⁺, and Eu³⁺ ions can be luminescence centers in Gd₂O₂S. The levels of the Nd³⁺, Dy³⁺, Er³⁺, Tm³⁺, Sm³⁺, and Ho³⁺ ions lie in the valence band; therefore, these ions cannot play the role of activators. The ground 4f level of the Ce³⁺ ion is near the midgap, due to which Ce³⁺ effectively captures holes from the valence band and electrons from the conduction band and significantly decreases the afterglow level of the Gd₂O₂S:Pr and Gd₂O₂S:Tb phosphors.     

Investigations of silicon nitride and boron carbide monolayers and Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/B<Subscript>4</Subscript>C multilayers

Many different coating concepts for improving mechanical properties have been worked out. One of the advanced coating concepts is the multilayer and superlattice concept, mainly with one or two metallic components. Previous work has shown that the mechanical properties of the covalent-bonded Si₃N₄ and SiC could be improved when combining them in a multilayer system. In the present work the silicon nitride monolayer from the earlier work was combined with boron carbide instead of silicon carbide. First, the boron carbide thin films deposited at different substrate temperatures were examined. Then the number of monolayers in the multilayer system with a constant layer thickness was varied in order to investigate the influence of the interfaces on film properties of the multilayer system. PACS 81.05.Je; 81.15.Cd; 82.80.Pv; 87.64.Je     

Computer calculations for thermal behavior of Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Li<Subscript>2</Subscript>CO<Subscript>3</Subscript> melt

The thermal behavior of Na₂CO₃+Li₂CO₃ melt is studied by the method of thermodynamic simulation. The equilibrium compositions of the gas and salt phases are calculated at different temperatures in the initial argon atmosphere. Basic trends of the variation in the compositions of the melts and the gas phase above the melts in the presence of carbon are determined. The obtained results characterizing the stability of carbonate components in the melt are analyzed.     

Chemical Structure of Fe<Subscript>78</Subscript>B<Subscript>13</Subscript>Si<Subscript>9</Subscript> Alloy Surface Layers in the Solid and Liquid States

The chemical structure of Fe₇₈B₁₃Si₉ alloy in the solid and liquid states and local atomic environment are studied in situ by X-ray photoelectron spectroscopy (XPS). The chemical bonds between elements in the melt are analyzed during a temperature increase. Two temperature regions are identified. The liquid surface in the first temperature region is shown to contain clusters of Fe-Si and (Fe-O _x )-Si types. In the second one, clusters of Fe-B and (Fe-O _x )-B types dominate. It is impossible to determine the composition of the clusters definitively using XPS data only. A jump-like change in the composition of the surface layers of the melt is detected, which is interpreted as structural transformations within the liquid state.