Intrinsic Localized Spin-Wave Modes in an Anisotropic Ferromagnet with Dzyaloshinsky-Moriya Interaction

The existence and properties of intrinsic localized spin-wave modes in a ferromagnetic XXZ spin chain with Dzyaloshinsky-Moriya interaction are investigated analytically in the semiclassical limit. The model Hamiltonian is quantized by introducing the Dyson-Maleev transformation and the coherent state representation is chosen as the basic representation of the system. By making use of the method of multiple scales combined with a quasidiscreteness approximation, the equation of motion for the coherent-state amplitude is reduced to the nonlinear Schrödinger equation. It is shown that a bright intrinsic localized spin-wave mode whose eigenfrequency lies below the bottom of the magnon frequency band can exist in the ferromagnetic system. We also show that the system can produce a dark intrinsic localized spin-wave mode, i.e., nonpropagating kink, whose eigenfrequency is below the upper of the magnon frequency band. In addition, we find that the introduction of the Dzyaloshinsky-Moriya interaction changes wave numbers in the Brillouin-zone corresponding to the appearance of intrinsic localized spin-wave modes.     

Magnon satellite bands in the optical spectrum of antiferromagnetic Rb<Subscript>2</Subscript>MnCl<Subscript>4</Subscript>

The evolution of optical absorption in a two-dimensional antiferromagnet is investigated in the range of the transition ⁶A_1g → ⁴A_1g, ⁴E _g (⁴G) observed in manganese ions in an external magnetic field inducing noncollinearity of the magnetic structure. It is revealed that hot and cold satellites of the exciton-magnon bands appear in the optical absorption spectrum and then increase in intensity. The shapes of the magnon satellite bands corresponding to a two-dimensional magnetic structure are calculated. It is demonstrated that magnons at the inner points of the Brillouin zone appreciably contribute to the absorption. The zero-point magnetic oscillations play a decisive role in the absorption associated with the magnon decay at low temperatures.     

On the Theory of Plasmon Dispersion in Electron-Doped Cuprates

An explicit expression for the dynamic charge susceptibility for electron-doped cuprates has been derived. This expression accurately reproduces the wave vector dependence of the plasmon frequency observed in inelastic X-ray scattering experiments for Nd_{2 – x}Ce_xCuO₄. The imaginary part of the charge susceptibility along the triangular path in the Brillouin zone is plotted. It is demonstrated that the spectral weight of the plasmon mode near q = 0 is negligibly low. The calculated frequencies of the plasmon mode for all wave vectors in the Brillouin zone turn out to lie outside the range of damping related to electron?hole excitations. A formula for the charge susceptibility is derived within the t?t′?t″?J model supplemented by the Coulomb interaction operator and three-site terms. The derivation is performed by the Green’s function technique employing the formalism of composite Hubbard operators and the Mori projection method, which have proved themselves in the analysis of collective spin excitations. The used Fourier transform of the Coulomb interaction corresponds to the monolayer model with a spatially periodic structure, which is embedded in a three-dimensional crystal lattice.     

Congratulations to Akusticheskii Zhurnal on the occasion of its 50th anniversary. Vibrations of spherical inclusions in elastic solids

Variational principles are derived for the analysis of dynamical phenomena associated with spherical inclusions embedded in homogeneous isotropic elastic solids. The starting point is Hamilton’s principle, with the material properties assumed to vary only with the radial distance r from the origin. Attention is restricted to disturbances that are symmetric about the polar (z) axis, such that the nonzero displacement components in spherical coordinates, u _r and u_θ, are independent of the polar coordinate φ. The symmetry allows for a decoupling of the polar components, the nth of which is described by U _{r, n} (r, t)P _n (cosθ) and U_{θ, n}(r, t)dP _n /dθ. A variational principle is subsequently derived for the field quantities U _{r, n} and U_{θ, n}. Concepts analogous to those of the theory of matched asymptotic expansions are used to embellish the principle in order to allow for the damping associated with the outward radiation of elastic waves. Examples illustrating the use of the variational principle for formulating plausible lumped-parameter models are given for the cases of n = 0 and n = 1.     

Simulation of the dynamic properties of Hg<Subscript>2</Subscript><Emphasis Type="Italic">Hal</Emphasis><Subscript>2</Subscript> crystals (<Emphasis Type="Italic">Hal</Emphasis>=Cl,Br, I)

The dispersion curves for crystal lattices of univalent mercury halides Hg₂Hal₂ (Hal=Cl, Br, I) are calculated from the experimental frequencies at singular points of the Brillouin zone and the velocities of sound in the framework of the valence-force field model. The results of calculating the dispersion branch of the TA soft mode along the Γ-X direction of the Brillouin zone and the calculated elastic moduli are presented. The calculated values are in good agreement with the experimental data.     

Role of initial correlation in coarsening of a ferromagnet

We study the dynamics of ordering in ferromagnets via Monte Carlo simulations of theIsing model, employing the Glauber spin-flip mechanism, in space dimensionsd = 2 and3, on square and simplecubic lattices. Results for the persistence probability and the domain growth arediscussed for quenches to various temperatures (T _f ) below the criticalone (T _c ), from differentinitial temperatures T _i ≥T _c . In long timelimit, for T _i >T _c ,the persistence probability exhibits power-law decay with exponents θ ? 0.22 and? 0.18 in d = 2 and 3, respectively. For finite T _i , the early timebehavior is a different power-law whose life-time diverges and exponent decreases asT _i →T _c . The two steps areconnected via power-law as a function of domain length and the crossover to the secondstep occurs when this characteristic length exceeds the equilibrium correlation length atT =T _i . T _i =T _c is expected toprovide a new universality class for which we obtain θ ≡θ _c ? 0.035 ind = 2 and?0.105 in d = 3. The time dependenceof the average domain size ?, however, is observed to be rather insensitive tothe choice of T _i .     

Angular correlation measurements in the Be9(d,p, γ)Be10 reaction

Thep-γ angular correlations in the Be⁹(d, p, γ)Be¹⁰ reaction have been measured in the reaction plane atE _d =1.3, 1.5, 1.9, 2.3 and 2.45 MeV for proton anglesθ _p =35°, 80° and 120°. The anisotropy of the angular correlations measured forθ _p =35° is independent of deuteron energy. A systematic shift of the symmetry axis from the recoil direction has been observed. The shift is 20.5° at E_d=2.45 MeV. The anisotropy decreases with increasing proton angle and vanishes atθ _p =120°.     

Analytic prediction for planar turbulent boundary layers

Analytic predictions of mean velocity profile(MVP) and streamwise(x) development of related integral quantities are presented for flows in channel and turbulent boundary layer(TBL), based on a symmetry analysis of eddy length and total stress. Specific predictions include the relations for momentum Reynolds number(Reθ) with friction Reτ and streamwise Re_x: Re_θ≈ 3.27Re_τ,and Re_x/Re_θ = 4.94(lnRe_θ + 1.88)~2 + 1; the streamwise development of the friction velocity u_τ: U_e/u_τ≈ 2.22 lnRe_x + 2.86. 3.83ln(lnRe_x), and of the boundary layer thickness δ_e: x/δ_e ≈ 7.27 lnRe_x.5.18.12.52ln(lnRe_x), which are fully validated by recent reliable data.     

Limiting rate of secret-key generation in quantum cryptography in spacetime

The fundamental restrictions on the maximum admissible rate of secret-key commitment in quantum cryptography in real time are discussed. It is shown that the maximum rate in a quantum channel with limited transmission band is achieved in a cryptosystem on orthogonal states. The dimensionless rate (the number of bits per unit time frequency band through unit of the channel) is determined by the universal function C(λ₀(ΔkT))/ΔkT [where C(λ₀(ΔkT)) is the transmission capacity of a classical binary channel, Δk is the transmission band width, 1/T is the transmission frequency of quantum states, and λ₀ is the maximum eigenvalue of a certain integral equation].     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Spin-phonon interactions in cuprates

The parameter ζ of spin-phonon coupling is obtained through the analysis of the spin-wave dynamics of magnetic systems. It is shown that the parameter of spin-phonon coupling is greater, the greater the relative electron-ion potential g ₁, the smaller the exchange correlation radius r _c , and the smaller the mass M of ions that form a crystalline lattice.     

Spectroscopy of resonant excitation of exciton luminescence of GaSe–GaTe solid solutions

The luminescence excitation spectra of localized excitons in GaSe_0.85Te_0.15 solid solutions have been investigated at the temperature T = 2 K. It has been shown that the excitation spectra of excitons with the localization energy ε > 10 mV exhibit an additional maximum M _E located on the low-energy side of the maximum corresponding to the free exciton absorption band with n = 1. It has been found that the shift in the position of the maximum M _E in the excitation spectrum with respect to the energy of detected photons increases as the energy of detected photons decreases, i.e., with an increase in the localization energy of excitons. Under the resonant excitation of localized excitons by a monochromatic light from the region of the exciton emission band, in the exciton luminescence spectrum on the low-energy side from the excitation line, there is also a maximum of the luminescence (M _L ). The energy distance between the position of the excitation line and the position of the maximum in the luminescence spectrum increases with a decrease in the frequency of the excitation light. The possible mechanisms of the formation of the described structure of the luminescence excitation and exciton luminescence spectra of GaSe_0.85Te_0.15 have been considered. It has been concluded that the maximum M _E in the excitation spectrum and the maximum M _L in the luminescence spectrum are attributed to electronic–vibrational transitions with the creation and annihilation of localized excitons, respectively.     

Molecular dynamics calculations of anharmonic properties of the vibrational spectrum of BCC zirconium under pressure

The structural stability and lattice dynamics of the high-pressure bcc phase of Zr at a constant temperature T = 500 K are studied for various volumes using molecular dynamics simulation with the Animalu pair pseudopotential. Dispersion curves of the vibrational spectrum calculated by the molecular dynamics method for various volumes are compared to the phonon spectrum obtained in the harmonic approximation. It is demonstrated that, as the volume decreases, all frequencies of the vibrational spectrum increase gradually and bcc zirconium remains strongly anharmonic along all high-symmetry directions of the Brillouin zone over the entire range of volumes studied. The strongly anharmonic N _T1 phonon is significantly softened near the point of structural instability of bcc-Zr at T = 500 K and V = 0.87V ₀. As the volume decreases to V = 0.73V ₀ under pressure, the anharmonic corrections for this phonon decrease by almost an order of magnitude and the phonons near the H point of the Brillouin zone become anharmonic. The damping of the T ₁ phonon mode along the [110] direction is calculated as a function of pressure.     

Energy spectrum of charge carriers in Bi<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

Analysis of the quantum oscillations of magnetoresistance (the Shubnikov-de Haas effect) in Bi_{1 ? x} Sb _x alloys with an antimony content in the range 0.255 < x < 0.260 has revealed a Lifshitz electronic-topological transition, which quite possibly can be explained in terms of the existence of a saddle point in the energy spectrum of these compositions. Such a peculiarity comes into existence when the direct band gap at the L point of the Brillouin zone in the semiconductor region of the compounds with x > 0.04 becomes negative. This compel one to revise essentially all earlier calculations based on the previously obtained values of the band parameters. In order to check the agreement between the new values of the band parameters and the data on the density of states obtained from measurements of the thermopower in the classical limit of strong magnetic fields, theoretical calculations of the charge carrier concentration n and the density of states at the Fermi level ρ(E _F) have been performed for the case of negative values of the direct band gap at the L point E _gL. The calculations of the parameters n and ρ(E _F) have demonstrated that the change in E _gL and the corresponding correction of the band parameters ensure good agreement with the experimental data. According to these calculations, one electronic-topological transition occurs at an antimony content x ～ 0.165, when a saddle point appears in the energy spectrum. The second transition is associated with the transformation of the six ellipsoids of the Fermi surface into three dumbbell-like figures at antimony concentrations in the range 0.255 < x < 0.260.     

High precision determination of the low-energy constants for the two-dimensional quantum Heisenberg model on the honeycomb lattice

The low-energy constants, namely the staggered magnetization density M? _s per spin, the spin stiffness ρ _s , and the spinwave velocity c of the two-dimensional (2-d) spin-1/2 Heisenberg model on the honeycomb lattice are calculated using first principles Monte Carlo method. The spinwave velocity c is determined first through the winding numbers squared. M? _s and ρ _s are then obtained by employing the relevant volume- and temperature-dependence predictions from magnon chiral perturbation theory. The periodic boundary conditions (PBCs) implemented in our simulations lead to a honeycomb lattice covering both a rectangular and a parallelogram-shaped region. Remarkably, by appropriately utilizing the predictions of magnon chiral perturbation theory, the numerical values of M? _s , ρ _s , and c we obtain for both the considered periodic honeycomb lattice of different geometries are consistent with each other quantitatively. The numerical accuracy reached here is greatly improved. Specifically, by simulating the 2-d quantum Heisenberg model on the periodic honeycomb lattice overlaying a rectangular area, we arrive at M? _s = 0.26882(3), ρ _s  = 0.1012(2)J, and c = 1.2905(8)Ja. The results we obtain provide a useful lesson for some studies such as simulating fermion actions on hyperdiamond lattice and investigating second order phase transitions with twisted boundary conditions.     

Quasi-normal Modes of Rerssner-Nordström Black Hole

The minimum interval of event horizon area of Rerssner-Nordström black hole was calculated via using the loop quantum gravity theory. Based on the first law of black hole thermodynamics, the real part of quasi-normal modes frequency of the black hole was calculated. The expression of asymptotically quasi-normal mode frequency of Rerssner-Nordström black hole was deduced strictly. By analyzing the value of the minimum spin j _{m i n} , the two families of quasi-normal mode spectra of the charged black hole were obtained for j _{m i n} = 1/2 and j _{m i n} = 1 respectively. Our conclusion is in complete agreement with the analytical results of Hod. Our results provide the theoretical basis for the source of the real part of the quasi-normal mode frequency of the black hole.     

Angular correlation measurements in the Cr53(p,n γ)Mn53 reaction

Then-γ angular correlation measurements for the Cr⁵³(p, nγ) Mn⁵³ reaction were performed atE _p=2.3, 2.4 and 2.5 MeV. The neutron detector was located in the reaction plane atθ _n=45?, 90? and 120? and perpendicularly to this plane atθ _n=Φ=90?. The experimental results obtained are in good agreement with predictions of the statistical compound-nucleus theory.     

Mean-square displacements of metal and boron atoms in crystal lattices of rare-earth hexaborides

The intensities of the I₄₁₀ and I₄₁₁ reflections of nine rare-earth hexaborides MB₆ (M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy) are experimentally studied in the temperature range 4.2–300 K. The mean-square displacements of metal and boron atoms are calculated from the temperature dependences of the intensities I₄₁₀(T) and I₄₁₁(T). The characteristic temperatures of the metal (θ_M) and boron (θ_B) sublattices of rare-earth hexaborides are determined in the Debye approximation. It is found that the characteristic temperatures decrease with an increase in the atomic number of the metal.     

Effect of intrinsic defects on the electronic structure of BN nanotubes

The effect of intrinsic defects on the electronic structure of boron-nitrogen nanotubes (5, 5) and (9, 0) is investigated by the method of linearized associated cylindrical waves. Nanotubes with extended defects of substitution N _B of a boron atom by a nitrogen atom and, vice versa, nitrogen by boron BN with an impurity concentration of 1.5 to 5% are considered. It is shown that the presence of such defects significantly affects the band structure of boron-nitrogen nanotubes. A defect band D^π(B, N) is formed in the bandgap, which sharply reduces the width of the gap. The presence of impurities also affects the valence band: the widths of s, sp, and p_π bands change and the gap between s and sp bands is partially filled. These effects may be detected experimentally by, e.g., optical and photoelectron spectroscopy.     

Probing the sign-changeable interaction between dark energy and dark matter with current observations

We consider the models of vacuum energy interacting with cold dark matter in this study, in which the coupling can change sigh during the cosmological evolution. We parameterize the running coupling b by the form b(a) = b_0 a + b_e(1-a), where at the earlytime the coupling is given by a constant b_e and today the coupling is described by another constant b_0. We explore six specific models with(i) Q = b(a)H_0ρ_0,(ii) Q = b(a)H_0ρ_(de),(iii) Q = b(a)H_0ρ_c,(iv) Q = b(a)Hρ_0,(v) Q = b(a)Hρ_(de), and(vi) Q = b(a)Hρ_c.The current observational data sets we use to constrain the models include the JLA compilation of type Ia supernova data, the Planck 2015 distance priors data of cosmic microwave background observation, the baryon acoustic oscillations measurements,and the Hubble constant direct measurement. We find that, for all the models, we have b_0 0 and b_e 0 at around the 1σ level,and b_0 and b_e are in extremely strong anti-correlation. Our results show that the coupling changes sign during the evolution at about the 1σ level, i.e., the energy transfer is from dark matter to dark energy when dark matter dominates the universe and the energy transfer is from dark energy to dark matter when dark energy dominates the universe.