Zakharov equations with zeroth harmonic and a new type of modulation instability

It has been shown that the system of Zakharov equations for the amplitudes of the first and zeroth harmonics of the waves on the surface of an ideal liquid describes not only the known type of the modulation instability of the envelope of the main harmonic with respect to harmonic perturbations with small wave vectors κ (Benjamin-Feier modulation instability), but also the modulation instability of a combination of the main and zeroth harmonics at κ values on the order of the wave vector k ₀ of the main harmonic. In contrast to the Benjamin-Feier modulation instability typical for large depths, the described modulation instability does not disappear at k ₀ h < 1.363.     

Modulation instability with incoherent white light in self-defocusing photorefractive crystal

We report on the first experimental observation of modulation instability and spontaneous pattern formation with incoherent white light emitted from an incandescent lamp in self-defocusing photorefractive LiNbO₃:Fe crystal. Experimental results show that the modulation instability of white light in self-defocusing medium is related to the input intensity, illumination time and the direction of the crystalline c-axis with respect to that of the lamp filament. At the illumination time t = 0 and the steady-state of MI, we give the spatial distribution of the optical Fourier power spectrum experimentally and the corresponding Fourier transformation of the output intensity numerically, and observe the emergence of the high frequency component along the c-axis.     

Modulation instability of dispersive electromagnetic waves in a Josephson junction in a finite-thickness plate

Modulation instability of finite-amplitude dispersive electromagnetic waves is studied in the framework of nonlocal electrodynamics of a Josephson junction in a finite-thickness plate. The dispersion equation is derived for the increment of small amplitude perturbations. For this type of waves, the regions of development of modulation instability and stability of waves are determined. It is shown that modulation instability of waves develops in a finite range of wavevectors Q _B1(k, D, L) < Q < Q _B2(k, A, D, L), while waves are stable in the important long-wave region 0 ≤ Q ≤ Q _B1(k, D, L) and for Q ≥ Q _B2(k, A, D, L). The unique possibility of controlling the modulation instability region by dispersion parameter k, viz., the wave vector (or frequency ω(k)) of the modes in the linear approximation.     

Factors that influence the modulation instability in self-defocusing photorefractive crystal

We report on the experimental observation of the modulation instability in self-defocusing photorefractive LiNbO₃: Fe crystal. Experimental results show that the modulation instability is related to the illumination time; and it is also related to the angle that between the direction of c-axis and the direction of the threadlike beam that illuminate the crystal. And the modulation instability is also related to the shape of the incident light.     

Ab initio investigation of phonon spectra in GdLiF4 compound under hydrostatic pressure

Employing density functional theory (DFT) within the generalized gradient approximation, the GdLiF₄ structure has been studied for a pressure range from 0 to 12 GPa. The influence of pressure on the lattice vibrational spectrum of the scheelite phase (I4₁/a, Z = 4) has been evaluated by means the “direct” approach, i.e., using force constants calculated from atomic displacements. As a result the Raman and infrared modes have been identified and their dependencies on pressure have been investigated and compared with available experimental data. It has been found that instability of the crystal structure appears at pressures above 6 GPa.     

Control of the perturbation-wave-vector range of modulation instability of dispersive electromagnetic waves in the nonlocal Josephson electrodynamics of a thin superconducting film

Modulation instability of dispersive electromagnetic waves propagating through a Josephson junction in a thin superconducting film is investigated in the framework of the nonlocal Josephson electrodynamics. A dispersion relation is found for the time increment of small perturbations of the amplitude. For dispersive waves, it is first established that spatial nonlocality suppresses the modulation instability in the range of perturbation wave vectors 0≤Q≤Q_B1(k), i.e., in the long-wavelength range of experimental interest. The modulation instability range Q_B1(k)<Q<Q_B2(k, A, L) can be controlled (which is a unique possibility) by varying a dispersion parameter, namely, the wave vector k [or the frequency ω(k)] of linear-approximation waves. In the wave-vector ranges 0≤Q≤Q_B1(k) and Q≤Q_B2(k, A, L), waves are shown to be stable.     

New aspects on URu2Si2 and CeTIn5 (T=Rh, Ir, Co) observed by high pressure NMR and NQR

NMR and NQR studies on two interesting systems (URu₂Si₂, CeTIn₅) were performed under high pressure. (1) URu₂Si₂: In the pressure range 3.0 to 8.3 kbar, we have observed new ²⁹Si NMR signals arising from the antiferromagnetic (AF) region besides the previously observed ²⁹Si NMR signals which come from the paramagnetic (PM) region in the sample. This gives definite evidence for spatially-inhomogeneous development of AF ordering below T ₀ of 17.5 K. The volume fraction is enhanced by applied pressure, whereas the value of internal field (∼91 mT) remains constant up to 8.3 kbar. In the AF region, the ordered moment is about one order of magnitude larger than 0.03 μB. (2) CeTIn₅: The pressure and temperature (T) dependences of nuclear spin-lattice relaxation rate 1/T ₁ of ¹¹⁵In in CeTIn₅ have shown that the superconductivity (SC) occurs close to an AF instability. From the T dependences of 1/T ₁ and Knight shift below T _c. CeTIn₅ has been found to exhibit non-s wave (probable d wave) SC with even parity and line nodes in the SC energy gap.     

Explosive instability of quasi-phonon triads in antiferromagnet under frequency modulated electromagnetic field

Compensation of nonlinear frequency shift of hybrid magnetoelastic waves (quasi-phonons) in the process of parametric three-boson coupling is studied theoretically and experimentally. The singular frequency modulation of electromagnetic pumping is proposed for the observation of explosive instability of quasi-phonon triads. The explosive supercritical dynamics is simulated theoretically on the basis of strong nonlinear equations of magnetoelastic dynamics and observed in α-Fe₂O₃ magnetoacoustic resonator.     

Unstable equilibrium and radiation defects in solids

A new type of point defect in solids genetically related with Frenkel's pairs which appears when there exists the instability zone of pair vacancy (v) - interstitial atom (i). It is shown that, at large instability zones typical of ‘loose’ crystalline structures, equilibrium concentration of unstable pairs i-v may exceed the concentration of usual Frenkel's pairs. In the example of the semiconducting crystals In₂Te₃ and Ga₂Te₃ having loose structure it is experimentally demonstrated that equilibrium point defects cannot be preserved by specimen quenching and that irradiation by large doses of high-energy particles does not result in stable radiation defects formation.     

Theoretical investigation of EPR and optical spectra of Mo(V) in [Mo6O19][N(C4H9)4]3 salt

The electron paramagnetic resonance (EPR) spectral data (the g factors and hyperfine structure constants) and d–d transition spectra for the tetragonal Mo⁵⁺ centre in [Mo₆O₁₉][N(C₄H₉)₄]₃ salt are theoretically investigated from the complete diagonalization method (CDM) for a 4d¹ ion in tetragonally compressed octahedron. The theoretical results are in good agreement with the experimental data. The dependency of the g factors of the ground state on the R_∥(MoO bond length) has been studied. It is shown that the g factors varied with the R_∥ approximately in a linear way.     

Appearance of the dielectric instability and its coexistence with the superconductivity in ultrathin metallic filaments with decreasing diameter

Some dielectric instability appears before the superconducting (s/c) transition of Hg, In, Ga, Sn filaments when their diameters are decreased to 20–30 Å. Above the s/c transition the differential conductivity of the filaments may be approximated by the formula σ = σ_a + σ_b exp [-E₀(T)/E], where the electric field E₀ is equal toabout 10–60 mV cm^-1, and falls abruptly before the s/c transition. There are conductivity oscillations along E in the s/c transition region. The results are interpreted in terms of Peierls instability and charge-density wave (CDW) conductivity.     

Spin dynamics in the stripe phase of high-T c cuprates with the modulation of superconducting order

Possible superconducting order modulation and its effect on the spin susceptibility in the coexisting phase of the stripe and superconducting orders are investigated. It is shown that the superconducting order modulation is mainly caused by the spin-domain-derived scattering, while the charge-domain-derived scattering tends to suppress it in a wide parameter region. The modulation leads to a two-peak structure in the spin excitation spectrum, which is qualitatively consistent with the recent experimental observations in La_{2- x} Sr _x CuO₄. This result suggests the importance of the superconducting order modulation for the understanding of the multiform spin excitations in these cuprate superconductors.     

Cross-phase modulation of surface magnetostatic spin waves

This paper explains the modulation instability of two surface magnetostatic spin waves simultaneously propagating in a ferromagnetic film. Self-modulation of the spin waves appears when their power reaches a threshold, and this is a sign of cross-phase modulation. The parameters of the unstable process are calculated, and the gains of the perturbation amplitudes are determined. The results published earlier on the experimental detection of the cross-phase modulation of spin waves are explained. Zh. éksp. Teor. Fiz. 116, 2058–2068 (December 1999)     

Elastic and electronic properties of YNi2B2C under pressure from first principles

The elastic and electronic structure properties of YNi₂B₂C under pressure are investigated by performing the generalized gradient approximation (GGA) and local density approximation (LDA) correction scheme in the frame of density functional theory (DFT). The pressure dependences of the normalized lattice parameters a/a₀ and c/c₀, the ratio c/a, and the normalized primitive volume V/V₀ of YNi₂B₂C are also obtained. The lattice constants and bulk modulus obtained are in agreement with the available experimental and other theoretical data. We have also studied the pressure dependences of elastic properties. It is found that, as pressure increases, the elastic constants C₁₁, C₃₃, C₆₆, C₁₂, and C₁₃ increase, the variation of elastic constant C₄₄ is not obvious. Moreover, our compressional and shear wave velocities V_L=6.99 km/s and V_S=3.67 km/s as well as the Debye temperature Θ=549.7 K at 0 GPa compare favorably with the available experimental data. The pressure dependences of band structures, energy gap and density of states are also investigated.     

The Peierls instability and superconductivity in quasi-2-d La2-x(Ba,Sr)xCuO4 compounds

The Peierls instability in the quasi-2-d La_2-x(Ba,Sr)_xCuO₄ is reexamined by taking the weak interlayer coupling into account. A two-step Peierls transition theory is developed, the general misunderstandings on the Peierls instability are therefore clarified, and a number of experimental anomalies observed in La₂CuO₄ are well explained. By suggesting an additional change of space group symmetries at the lower transition point, the calculated band structures are also intepreted. The possibility of CDW coexistence with the high-T_c superconductivity, the possible evidence for the 3-d superconductivity of the high-T_c materials are discussed with this theory.     

Studies of the optical spectrum band positions and spin-Hamiltonian parameters for Pa ion in ThX4 (X=Cl, Br) crystals

The optical spectrum band positions and spin-Hamiltonian (SH) parameters (g factors g_∥ and g_⊥ and hyperfine structure constants A_∥ and A_⊥) for 5f¹ ion Pa⁴⁺ at the tetragonal Th⁴⁺ site of ThX₄ (X=Cl, Br) crystals are calculated from a complete diagonalization (of energy matrix) method (CDM). In the CDM, the magnetic (or Zeeman) interaction and hyperfine interaction terms are added to the Hamiltonian in the conventional CDM and so the optical and EPR spectra data can be studied in a unified way. The calculated results are in reasonable agreement with experimental values. The possible misprints or small errors in the experimental g factors for Pa⁴⁺ in ThX₄ crystals are pointed out. The results are discussed.     

BaVS3晶格动力学研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论研究了具有六角结构的BaVS₃化合物的晶格动力学性质,得到了整个声子谱.计算得到的Γ点拉曼频率和实验数据进行了比较,其中E²_2g模、A_1g模和实验测量值符合得比较好.对于E³_2g模,采用线性响应计算的结果与实验值差别较大.对该模应用冻结声子方法研究后认为差异主要是由于E³_2g模的较强的非谐性引起的.此外V原子在平面内的振动模E¹_2u出现了虚频.虚频的出现预示着六角相的BaVS₃结构的不稳定性,从而很好地解释了该材料由六角相到正交相的结构相变. 关键词： 晶格动力学 密度泛函 赝势方法 3')" href="#">BaVS₃     

Forced ferromagnetic ordering in systems with magnetic ions of two types

The distribution of magnetization in domain walls of ferromagnetic (FM) crystals with two order parameters interacting with each other—F_1z and F_2z, is considered. In one case the interaction is represented as (F_1zF_2z) and in another—(F_1z²F_2z²). It is shown that in the former case the magnetization distribution has space amplitude modulation, and in latter case appears space frequency modulation. Changes of the paramagnetic-ferromagnetic phase transition temperature in view of interaction of two FM subsystems have been studied.     

Metal carbide-induced negative flatband voltage shift in TaCx and HfCx/HfO2 gate stacks

We systematically investigated the role of the top interface for TaC_x and HfC_x/HfO₂ gate stacks on the effective work function (Φ_m,eff) shift by inserting a SiN layer at the gate/HfO₂ top interface or HfO₂/SiO₂ bottom interface. We found that Φ_m,eff of the TaN gate electrode on HfO₂ was larger than that on SiO₂ because of the HfO₂/SiO₂-bottom-interface dipole. On the other hand, we found that Φ_m,eff values of the TaC_x and HfC_x gate electrodes on HfO₂ agree with Φ_m,eff on SiO₂. This is because the potential offset of the opposite direction with respect to the bottom interface dipole appears at the metal carbide/HfO₂ interface. It is thus concluded that the top interface in the metal carbide/HfO₂ gate stacks causes the negative Φ_m,eff shift.     

First-principles calculations on the role of Ni-doping in Cun clusters: From geometric and electronic structures to chemical activities towards CO2

First-principles calculations have been applied to investigate the role of Ni-doping in modifying the geometric and electronic structures of Cu_n clusters (n=1-12), and the chemical activity of clusters towards CO₂. The results reveal that Ni-doping introduces a significant modulation of the electronic structures, such as the density of states, HOMO-LUMO gaps and the d-band centers, which dominate their chemical activities towards CO₂ adsorption. Our findings contribute valuable information for high efficient Cu-Ni alloy nano-catalyst design.