Magnetoelectric Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> and relativity theory

Based on the dielectric continuum phonon model, uniaxialmodel and force balance equation the mobility of two dimensional electrongas in wurtzite Al_xGa_1-xN/GaN/Al_xGa_1-xN quantum wells isdiscussed theoretically within the temperature range dominated by opticalphonons. The dependences of the electron mobility on temperature, Al molarfraction and electron sheet density are presented including hydrostaticpressure effect. The built-in electric field is also taken into account. Itis found that under normal pressure the main contribution to the mobility isfrom the scattering of interface optical phonons in narrow (for well widthd < 12 Å) and wide (for d > 117 Å and d > 65 Å for finitelythick barriers and infinitely thick ones, respectively) wells, whereas thatis from the scattering of confined optical phonons in a well with anintermediate width. It is shown that the electron mobility decreases withincreasing Al molar fraction and temperature, whereas increases obviouslywith increasing electron sheet density. The theoretical calculated electronmobility is 978 cm²/V?s which is higher than an available experimentaldata 875 cm²/V?s when x equals to 0.58 at room temperature. Theresults under hydrostatic pressure considering the modification of strainindicate that the mobility increases slightly as hydrostatic pressureincreases from 0 to 10 GPa.     

Phonon linewidths in YNi<Subscript>2</Subscript>B<Subscript>2</Subscript>C

Phonons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron-phonon coupling. YN₂B₂C was chosen as a test example because it is a conventional superconductor with a fairly high T _c (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron-phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth.      

Structural,optical, and thermal properties of MAX-phase Cr<Subscript>2</Subscript>AlB<Subscript>2</Subscript>

First-principles calculations of the structural, optical, and thermal properties of Cr₂AlB₂ are performed using the pseudopotential plane-wave method within the generalized gradient approximation (GGA). Calculation of the elastic constant and phonon dispersion indicates that Cr₂AlB₂ is mechanically and thermodynamically stable. Analysis of the band structure and density of states indicates that Cr₂AlB₂ is metallic. The thermal properties under increasing temperature and pressure are investigated using the quasi-harmonic Debye model. The results show that anharmonic effects on Cr₂AlB₂ are important at low temperature and high pressure. The calculated equilibrium primitive cell volume is 95.91 Å3 at T = 300 K, P = 0 GPa. The ability of Cr₂AlB₂ to resist volume changes becomes weaker with increasing temperature and stronger with increasing pressure. Analysis of optical properties of Cr₂AlB₂ shows that the static dielectric function of Cr₂AlB₂ is 53.1, and the refractive index n ₀ is 7.3. If the incident light has a frequency exceeding 16.09 eV, which is the plasma frequency of Cr₂AlB₂, Cr₂AlB₂ changes from metallic to dielectric material.     

Phonons in ZrB<Subscript>12</Subscript>

The first results of measuring dispersion curves of acoustic phonons in the ZrB₁₂ superconductor are reported. The measurements have been conducted by the inelastic neutron scattering method on an ATOS three-axis spectrometer (IR-8, Russian Research Centre “Kurchatov Institute,” Moscow) at room temperature with a single crystal grown using a 99.5%-enriched ¹¹B isotope. The results obtained have been analyzed in terms of the lattice dynamics model proposed earlier for rare-earth dodecaborides. In this model, the constants of the force interaction of Zr atoms with one another and with the boron framework have been determined.     

Effect of uniaxial pressure on the infrared spectra of (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The infrared reflectance spectra of a mechanically free or uniaxial-pressure-confined (NH₄)₂SO₄ crystal were studied for the first time in the spectral range 800–1700 cm^?1 in three crystallographic directions. Using the Kramers-Kronig relations, the dispersion and pressure dependences of the following quantities are obtained: the index of refraction n, the real (?₁) and imaginary (?₂) parts of the permittivity, the frequencies of longitudinal (ω_LO) and transverse (ω_TO) optical vibrations, the damping constant γ, and the oscillator strength f of the mechanically free or clamped (NH₄)₂SO₄ crystal. A considerable change in the main reflection bands with pressure was observed, which is due to the effect of uniaxial pressure on the NH₄ and SO₄ tetrahedral frames.     

Transport characteristics of phonons and the specific heat of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:ZrO<Subscript>2</Subscript> solid solution single crystals

The temperature dependences of the specific heat and transport characteristics of phonons in single crystals of yttrium-stabilized zirconium dioxide Y₂O₃:ZrO₂ solid solutions have been studied. It has been shown that the temperature dependences of the specific heat at T > 5 K are almost identical at the degree of stabilization of a solid solution with an Y₂O₃ content of 5–20 mol %. Differences in the temperature dependences of the specific heat of samples from different sources at T < 5 K are due to the presence of low-energy two-level systems. The features of the transport characteristics of thermal phonons at liquid helium temperatures reflect not only the presence of two-level systems but also the scattering of phonons on low-dimensional domains of another phase coherently conjugate to the main phase of the Y₂O₃:ZrO₂ solid solution.     

Spectroscopic study of the behavior of the order parameter in model ferroelastic crystals of Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript>

Information is obtained about the temperature behavior of the order parameter of a phase transition by theoretical and experimental investigation of odd (acoustic and IR-active) phonons that appear in the Raman scattering spectra from the X points of the Brillouin zone (BZ) boundary in the paraphase of Hg₂Cl₂ crystals and are induced by the phase transition, unit-cell doubling, and the X → Γ folding in the BZ. The relevant critical exponents are determined, whose values are in agreement with the results of X-ray diffraction measurements and, within the Landau phenomenological theory of phase transitions, indicate that the phase transition in these crystals is close to the tricritical point.     

Acoustical investigations of a La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> single crystal

The acoustical, resistive, and magnetic properties of a La_0.75Sr_0.25MnO₃ lanthanum manganite single crystal are investigated in the temperature range involving the second-order magnetic phase transition. The acoustical measurements are performed by the pulse-echo method in the frequency range 14–90 MHz. It is found that, as the temperature decreases, the velocity of a longitudinal acoustic wave propagating along the [111] axis in the single crystal drastically increases at temperatures below the critical point of the magnetic phase transition. No dispersion of the acoustic velocity is revealed. A sharp increase in the acoustic velocity is accompanied by the appearance of an acoustical absorption peak. The observed effects are discussed with due regard for the interaction of acoustic waves with the magnetic moments of the manganese ions.     

Phonons and their dispersion in model ferroelastics Hg2Hal2

Dispersion relations of the acoustic and optical phonon frequencies have been calculated and plotted, and the density of states of the phonon spectrum of Hg₂Cl₂ and Hg₂Br₂ crystals has been derived. The effect of hydrostatic pressure on the frequencies of acoustic and optical phonons and their dispersion has been theoretically analyzed. It has been found that an increase in the pressure leads to a strong softening of the slowest acoustic TA branch (the soft mode) at the X point of the Brillouin zone boundary, which is consistent with the phenomenological Landau theory and correlates with experiment.     

Femtosecond SRS-generation of coherent optical phonons in vanadate crystals m-LaVO<Subscript>4</Subscript> and t-GdVO<Subscript>4</Subscript> (SRS-phononics of the crystals)

Excitation of multicomponent trains of coherent optical phonons with different frequencies in monoclinic m-LaVO₄ and tetragonal t-GdVO₄ crystals under impulsive femtosecond SRS-lasing is reported.     

Phonon spectra and the one-phonon and two-phonon densities of states of UO<Subscript>2</Subscript> and PuO<Subscript>2</Subscript>

The vibrational spectra of uranium dioxide UO₂ and plutonium dioxide PuO₂, as well as the one-phonon densities of states and thermal occupation number weighted two-phonon densities of states, have been calculated within the framework of the phenomenological rigid ion model. It has been shown that the acoustic and optical branches of the spectra are predominantly determined by vibrations of the metal and oxygen atoms, respectively, because the atomic masses of the metal and oxygen differ from each other by an order of magnitude. On this basis, the vibrational spectra can be represented in two Brillouin zones, i.e., in the Brillouin zone of the crystal and the Brillouin zone of the oxygen sublattice. In this case, the number of optical branches decreases by a factor of two. The two-phonon densities of states consist of two broad structured peaks. The temperature dependences of the upper peak exhibit a thermal broadening of the phonon lines L01 and L02 in the upper part of the optical branches. The lower peak is responsible for the thermal broadening of the lowest two optical (T02, T01) and acoustic (LA, TA) branches.     

Linear and nonlinear optical properties of yttrium formate dihydrate,Y(HCOO)<Subscript>3</Subscript>·2H<Subscript>2</Subscript>O

Yttrium formate dihydrate, Y(HCOO)₃·2H₂O (point group 222), a promising new material for Raman laser frequency converters, has been reinvestigated with respect to the linear and nonlinear optical (second-harmonic generation, SHG) properties. High-precision data of refractive indices and their dispersion in the wavelength range 365–1083 nm are given, together with the three independent components of the tensor of nonlinear optical susceptibility (SHG) [d_ijk ^SHG]. A calculation of the macroscopic nonlinear optical susceptibility from the hyperpolarizabilities of the formate groups results in good agreement with the experimental values. This signals that the nonlinear optical interaction in Y(HCOO)₃·2H₂O can mainly be attributed to the formate groups. PACS 42.65.Ky; 42.70.Mp; 78.20.Ci     

Synthesis and structural characterization of uranium-doped Ca<Subscript>2</Subscript>CuO<Subscript>3</Subscript>, a one-dimensional quantum antiferromagnet

The technological settings of a modified sol-gel method for the preparation of highly fine homogeneous powder of Ca₂CuO₃ doped with uranium 238 (x= 0.0–0.05) is presented. The analysis of structure, purity of phases and the justification for the role of uranium in the given compounds are provided, together with an almost complete classification of the observed optical phonons by means of Raman, IR measurements and ab initio calculations. A significant reduction in particle size was achieved by doping, and a strong correlation between resistivity and doping concentration was observed and explained using the phonon-assisted electron hopping conduction model. The persistence of a covalent insulation state in all compounds is an interesting feature of this doping.     

Acoustogyration diffraction of optical waves: case of SiO<Subscript>2</Subscript> and TeO<Subscript>2</Subscript> crystals

Interaction of acoustic and light waves with accounting for elastooptic and elastogyration effects is theoretically described. Collinear acoustogyration diffraction in quartz and paratellurite crystals is experimentally investigated and thoroughly analyzed. Piezogyration effect is experimentally studied for TeO₂ crystals. The acoustogyration efficiency and the acoustogyration figure of merit calculated for a number of crystals (GaAs, TeO₂ and SiO₂) are shown to be too small for experimental detection. On the other hand, we demonstrate that the light diffraction at periodical distribution of the imaginary part of dielectric permittivity related to the piezogyration effect should, in principle, be observed for the case of interaction of optical waves with enantiomorphous ferroelastic domain structure and in cholesteric liquid crystals.     

Baric changes in refractive indices of K<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystals

The influence of uniaxial pressure applied along the principal crystallophysical directions on the dispersion and temperature dependences of the refractive indices n _i of K₂ZnCl₄ crystals has been investigated. The n _i values are found to be fairly sensitive to uniaxial pressure, whereas an uniaxial stress does not change the behavior of the dispersion and temperature dependences of n _i . The baric changes in n _i have been studied. The electronic polarizability α _i , refractions R, and parameters of UV oscillators (λ_0i , B _1i ) of mechanically deformed K₂ZnCl₄ crystals have been calculated. The contributions of UV and IR oscillators to n _i (λ) have been estimated for different temperatures, spectral regions, and stresses. A significant baric shift of the points of the paraelectric phase-incommensurate phase-commensurate phase transitions to different temperature ranges, depending on the direction of pressure application, is found; this shift is due to the effect of uniaxial stress on the K₂ZnCl₄ crystal structure.     

Electronic structure,ferroelectricity and optical properties of CaBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript>

Using first-principles calculations based on density-functional theory in its local-density approximation, we investigated the Electronic structure, ferroelectricity and optical properties of CaBi₂Ta₂O₉ (CBT) for the first time. It is found that CBT compound has an indirect band gap of 3.114 eV and the O 2s and 2p states are strongly hybridized with the 6s states of Bi which belong to the (Bi₂O₂)²⁺ planes. The quite strong Ta–O and Bi–O hybridization is the primary source for ferroelectricity. Our results imply that the interaction between Bi and O is highly covalent. The anisotropy occurs mainly above 4 eV in the optical properties. The different optical properties have been discussed.     

Electronic and optical properties of single-layer MoS<Subscript>2</Subscript>

The electronic structures of a MoS₂ monolayer are investigated with the all-electron first principle calculations based on the density functional theory (DFT) and the spin-orbital couplings (SOCs). Our results show that the monolayer MoS₂ is a direct band gap semiconductor with a band gap of 1.8 eV. The SOCs and d-electrons in Mo play a very significant role in deciding its electronic and optical properties. Moreover, electronic elementary excitations are studied theoretically within the diagrammatic self-consistent field theory. Under random phase approximation, it shows that two branches of plasmon modes can be achieved via the conduction-band transitions due to the SOCs, which are different from the plasmons in a two-dimensional electron gas and graphene owing to the quasi-linear energy dispersion in single-layer MoS₂. Moreover, the strong optical absorption up to 10⁵ cm^-1 and two optical absorption edges I and II can be observed. This study is relevant to the applications of monolayer MoS₂ as an advanced photoelectronic device.     

Photoacoustic spectroscopy of thin films of As<Subscript>2</Subscript>S<Subscript>3</Subscript>, As<Subscript>2</Subscript>Se<Subscript>3</Subscript> and GeSe<Subscript>2</Subscript>

Photoacoustic spectroscopy (PAS) is one of the important branches of spectroscopy, which enables one to detect light-induced heat production following the absorption of pulsed radiation by the sample. As₂S₃, As₂Se₃ and GeSe₂ exhibit a wide variety of photo-induced phenomena that enable them to be used as optical imaging or storage medium and various electronic devices, including electro-optic information storage devices and optical mass memories. Therefore, accurate measurement of thermal properties of semiconducting films is necessary to study the memory density. The thermal conductivity of thin films of As₂S₃ (thickness 100 μm and 80 μm), As₂Se₃ (thickness 100 μm and 80 μm) and GeSe₂ (thickness 120 μm and 100 μm) has been measured using PAS technique. Our result shows that the thermal conductivity of thicker films is larger than the thinner films. This can be explained by the thermal resistance effect between the film and the surface of the substrate.      

Evolution of elemental distribution in free-standing structures of Zr/ZrSi<Subscript>2</Subscript> with MoSi<Subscript>2</Subscript> and ZrSi<Subscript>2</Subscript> protective coatings under annealing

An analysis of the in-depth distribution of elements in annealed samples of multilayer Zr/ZrSi₂ EUV filters with protective layers of MoSi₂ and ZrSi₂ was performed by secondary ion mass spectrometry (SIMS). The vacuum annealing of the samples was performed at temperatures of up to 720–950°C for 3–7 h and at a residual pressure of ∼10⁻⁷ torr. It was demonstrated that MoSi₂ coatings effectively inhibit the accumulation of oxygen by the heated film, which is the process that determines the optical degradation of the filter.