(1 + 2)-Dimensional sub-strongly nonlocal spatial optical solitons: Perturbation method

By extending the (1 + 1)-dimensional [(1 + 1)-D] perturbation method suggested by Ouyang et al. [S. Ouyang, Q. Guo, W. Hu, Phys. Rev. E. 74 (2006) 036622] to the (1 + 2)-D case, we obtain a fundamental soliton solution to the (1 + 2)-D nonlocal nonlinear Schrödinger equation (NNLSE) with a Gaussian-type response function for the sub-strongly nonlocal case. Numerical simulations show that the soliton solution obtained in this paper can describe the soliton states in both the sub-strongly nonlocal case and the strongly nonlocal case. It is found that the phase constant and the power of the (1 + 2)-D strongly nonlocal spatial optical soliton with a Gaussian-type response function are both in inverse proportion to the 4th power of its beam width.     

Phase transitions in CsHSO4 up to 2.5 GPa: Impedance spectroscopy under pressure

Phase transitions in CsHSO₄ at pressures up to 2.5 GPa have been studied with the help of electrical impedance measurements. The phase boundaries have been identified with the help of calculated activation energies of electrical conductivity and dielectric relaxation time. The derived temperatures of phase transition from the low conductive phase II into super ionic phase I at pressure less than 1 GPa confirm the previous results of Ponyatovski? et al. (1985) [4] and Friesel et al. (1989) [27]. The phase diagram derived in this study for pressure larger than 1 GPa differs from the data of Ponyatovski? et al. (1985) [4]. The phase transitions IV-VI and VI-I occur at higher temperatures having significantly larger Clapeyron slope. The phase VII was not identified from heating cycle and appears only under cooling between phases I and VI. The phase VIII was detected at 2.5 GPa at T<350 K and only during heating.     

Ion-acoustic dressed solitons in electron-positron-ion plasmas

Expanding the Sagdeev potential to include fourth-order nonlinearities of electric potential and integrating the resulting energy equation, an exact soliton solution is determined for ion-acoustic waves in an electron-positron-ion (e-p-i) plasma system. This exact solution reduces to the dressed soliton solution obtained for the system using renormalization procedure in the reductive perturbation method (RPM), when Mach number (M) is expanded in terms of soliton velocity (λ) and terms up to order of λ² are retained in the analysis. Variation of shape, velocity, width and product (P) of amplitude (A) and square of width (W²) for the KdV soliton, core structure, dressed soliton, and exact soliton are graphically represented for different values of fractional positron concentration (p). It is found that for a given value of the fractional positron concentration (p) and amplitude of soliton, the velocity of the dressed soliton is faster and width is narrower than the KdV or exact soliton, and agrees qualitatively with the experimental observations of Ikezi et al. for small amplitude solitons in the plasma free from positron component. Among all these structures, the product P(AW²) is found to be lowest for the dressed soliton and it decreases as Mach number of soliton or fractional positron concentration in the plasma increases.     

Angle-of-arrival variance’s dependence on the aperture size for indoor convective turbulence

We analyze the angle-of-arrival variance of an expanded and collimated laser beam once it has traveled through an indoor convective turbulence. A continuous position detector is set at the focus of a lens collecting the laser beam. The effect of the different turbulent scales, between the inner and the outer scales, is studied by changing the diameter of a circular pupil before the collector lens. The experimental optical setup follows the design introduced by Masciadri and Vernin [Appl. Opt., 36(6) (1997) 1320]. Tilt data measurements are studied using the fractional Brownian motion model for the turbulent wave-front phase introduced in a previous paper [Pérez et al., J. Opt. Soc. Am. A 21(10) (2004) 1962]. The Hurst exponents associated to different strengths of turbulence are obtained from the here proposed D^2H−2 dependence.     

A high pressure distorted α-uranium (Pnma) structure in plutonium

Under pressure many rare earths and actinide metals transform to α-U type structure or its lower symmetry distorted forms. We have reinterpreted the diffraction data of Dabos et al. for Pu [S. Dabos et al. J. Alloys Compd. 190 (1993) 237] and find that an Am IV type distorted α-U structure in Pnma space group can explain its high pressure phase. The structures of both the high pressure Am IV type phase and α-Pu, the 0.1 MPa phase, are shown to have a distorted hcp topology. The upturn in the atomic volume of Pu at 0.1 MPa can also be rationalized on the basis of this proposal.     

Constant-cutoff approach to radiative decays of hyperons: E2/M1 transition ratios

We suggest a quantum stabilization method for theSU(2) σ-model, based on the constant-cutoff limit of the cutoff quantization method developed by Balakrishnaet al., which avoids the difficulties with the usual soliton boundary conditions pointed out by Iwasaki and Ohyama. We investigate the baryon numberB=1 sector of the model and show that after the collective coordinate quantization it admits a stable soliton solution which depends on a single dimensional arbitrary constant. We then study the radiative decays ofJ ^π=3/2⁺ baryons using the constant-cutoff approach to theSU(3) collective treatment of the Skyrme model for hyperons. Thus we evaluate the widths and E2/M1 ratios, showing that there is a general qualitative agreement with the results obtained using the complete Skyrme model, as well as the nonrelativistic quark model and quenched lattice model, for the total widths.     

Twining effects in the magnetism of small Pd clusters

We report a theoretical study of the magnetic behavior of symmetrical twined Pd_N (N≤220) clusters. The twined Pd_N particles were built from two equal Pd_M seed-clusters with fcc-like structure for M=38, 55, 79 and 116. The optimized geometrical structures of Pd_N (with N<2M) were obtained from an uniform relaxation of the fcc-like twined configurations using the embedded atom method (EAM). The spin-polarized electronic structure and related magnetic properties of those optimized geometries were calculated by solving self-consistently a spd tight-binding Hamiltonian. We observe that, in some cases the twining process may induce and/or enhance the magnetic moment of the clusters even in the case when the seed-clusters are non-magnetic. Our results also suggest a strong dependence on the twining orientation, providing further support to the influence of symmetry effects on the magnetic properties of finite transition-metal systems. We discuss our results in comparison with some recent experimental observations for Pd nanoparticles [Shinohara et al., Phys. Rev. Lett. 91 (2003) 197201. [14]; Sampedro et al., Phys. Rev. Lett. 91 (2003) 237203. [13]].     

Phase diagram of the relaxor (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 investigated by dielectric and Raman spectroscopies

Lead zinc niobate-lead titanate[(1−x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃] (PZN-PT) crystals with x=4.5% and x=12% have been investigated using dielectric and Raman measurements over a range of temperatures. Above room temperature, dielectric measurements show that both compositions exhibit structural phase transitions according to the phase diagram proposed by Kuwata et al. [Ferroelectrics 387 (1981) 579]. Below room temperature, an anomaly at around 180 K for the x=12% sample is observed, suggesting another phase transition. Raman measurements are used to study all phase transitions.     

On quantum solitons and their classical relatives: Bethe Ansatz states in soliton sectors of the sine-Gordon system

Previously we have found that the semiclassical sine-Gordon/Thirring spectrum can be received in the absence of quantum solitons via the spin $\text{1}\text{2}$ approximation of the quantized sine-Gordon system on a lattice. Later on, we have recovered the Hilbert space of quantum soliton states for the sine-Gordon system. In the present paper we present a derivation of the Bethe Ansatz eigenstates for the generalized ice model in this soliton Hilbert space. We demonstrate that via “Wick rotation” of a fundamental parameter of the ice model one arrives at the Bethe Ansatz eigenstates of the quantum sine-Gordon system. The latter is a “local transition matrix” ancestor of the conventional sine-Gordon /Thirring model, as derived by Faddeev et al. within the quantum inverse-scattering method. Our result is essentially based on the N < ∞, Δ = 1, m ? 1 regime. Consequently, the spectrum received, though resembling the semiclassical one, does not coincide with it at all.     

Photorefractive hologram writing with high modulation depth in photovoltaic media under different boundary conditions

Photorefractive recording response to sinusoidal intensity pattern in photovoltaic media under different boundary conditions, namely, open-circuit, crystal-resistor-circuit, short-circuit and crystal-voltage supply-circuit, are investigated by solving steady-state Kukhtarev equations numerically. In high modulation depth m limit and when photovoltaic field becomes sufficiently large, we find that both the magnitude and imaginary part of fundamental space-charge field E₁ as a function of m may transit from superlinear increment to sublinear increment for short-circuit condition. The role of photovoltaic field in holographic storage and two-wave mixing is also discussed in comparison with an earlier experimental literature [G. Cook et al., Opt. Commun. 192 (2001) 393]. We further generalize Gu et al.’s remark on photovoltaic field and dc space-charge field, which is for small m only [C. Gu et al., J. Appl. Phys. 69 (1991) 1167], to whole range of m in the limit donor density much larger than ionized donor density that the effect of photovoltaic field cannot be cancelled or considered as an externally applied field for an arbitrary non-zero m.     

Physics and mathematics of dispersion-managed optical solitons

We review the main physical and mathematical properties of dispersion-managed (DM) optical solitons. Theory of DM solitons can be presented at two levels of accuracy: first, simple, but nevertheless, quantitative models based on ordinary differential equations governing evolution of the soliton width and phase parameter (the so-called chirp); and second, a comprehensive path-average theory that is capable of describing in detail both the fine structure of DM soliton form and its evolution along the fiber line. An analogy between DM soliton and a macroscopic nonlinear quantum oscillator model is also discussed. To cite this article: S.K. Turitsyn et al., C. R. Physique 4 (2003).     

Fluorescence of jet-cooled naphthalene: Emission spectra, lifetimes and quantum yields

Fluorescence spectra of naphthalene, C₁₀H₈, were obtained in the laboratory under conditions which provide an appropriate simulation of the cometary conditions: super-cooled gas phase molecules in a collision-free environment. Five spectra were recorded, the excitation energies ranging from 1422 to 5293 cm⁻¹ above the first electronic state S₁ at 32 020 cm⁻¹. A comparison with previous jet-cooled naphthalene fluorescence spectra obtained by Beck et al. [1] and Hermine [2] shows that the former results are not consistent with the present ones. Spectra obtained by Beck et al. show weaker intensities at greater wavelengths compared to those obtained by Hermine and ourselves. We also measured the fluorescence lifetimes by recording the fluorescence decay as a function of time after the excitation of the molecules by monochromatic lasers and deduced the fluorescence quantum yields. A synthetic emission spectrum under solar irradiation is obtained for astrophysical implications.     

Improving unrestricted imaging quality of a triangular lattice photonic crystal slab by modifying surface configuration

We propose a photonic crystal (PC) structure only by replacing square lattice [Luo et al. (2002) [12]] with triangular lattice to obtain an unrestricted imaging. Equal-frequency contours (EFCs) analysis shows that this triangular lattice two-dimensional PC exhibits an effective isotropic refractive index n_eff=−1 at a normalized frequency ω=0.291×2πc/a. Imaging quality of this triangular lattice PC slab involving both power intensity and full-width at half-maximum intensity of the image is studied using the finite-difference time-domain (FDTD) simulation. In order to achieve a high-quality image, an appropriate surface termination is chosen. In addition, by adjusting the surface air-hole radius of the PC slab, the imaging quality can be further improved. Coupled-mode theory analysis shows that the optimized surface termination and the adjusted surface air-hole can excite two kinds of surface modes that can couple with the Bloch wave in the PC. With the help of these surface modes, both the intensity of image and the super-resolution capacity of this triangular lattice PC slab can be improved greatly.     

Electronic structure and superconductivity of europium

We have calculated the electronic structure of Eu for the bcc, hcp, and fcc crystal structures for volumes near equilibrium up to a calculated 90 GPa pressure using the augmented-plane-wave method in the local-density approximation. The frozen-core approximation was used with a semi-empirical shift of the f-states energies in the radial Schrödinger equation to move the occupied 4f valence states below the Γ₁ energy and into the core. This shift of the highly localized f-states yields the correct europium phase ordering with lattice parameters and bulk moduli in good agreement with experimental data. The calculated superconductivity properties under pressure for the bcc and hcp structures are also found to agree with and follow a T_c trend similar to recent measurement by Debessai et al. [1].     

Theoretical study of the structural phase transition and elastic properties of HfN under high pressures

The effect of hydrostatic pressure on the structures of HfN at 0 K was investigated by using the projector augmented wave (PAW) within the Perdew–Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA). The transition pressure between NaCl (B1) and CsCl (B2) structures is predicted to be 277.3 GPa. This value is consistent with that reported by Kroll, while in contrast to the results obtained by Ojha et al. and Meenaatci et al. Moreover, the elastic properties of B1-HfN and B2-HfN under high pressures are successfully obtained. It is found that the elastic constants, bulk modulus B, shear modulus G, compressional and shear wave velocities increase monotonically with increasing pressure. The Debye temperature Θ calculated from the elastic constants of HfN is in good agreement with the experimental values. The anisotropies of B1-HfN and B2-HfN at zero pressure have also been discussed.     

Analytical approach to quantum phase transitions of ultracold Bose gases in bipartite optical lattices using the generalized Green’s function method

In order to investigate the quantum phase transitions and the time-of-flight absorption pictures analytically in a systematic way for ultracold Bose gases in bipartite optical lattices, we present a generalized Green’s function method. Utilizing this method, we study the quantum phase transitions of ultracold Bose gases in two types of bipartite optical lattices, i.e., a hexagonal lattice with normal Bose–Hubbard interaction and a d-dimensional hypercubic optical lattice with extended Bose–Hubbard interaction. Furthermore, the time-of-flight absorption pictures of ultracold Bose gases in these two types of lattices are also calculated analytically. In hexagonal lattice, the time-of-flight interference patterns of ultracold Bose gases obtained by our analytical method are in good qualitative agreement with the experimental results of Soltan-Panahi, et al. [Nat. Phys. 7, 434 (2011)]. In square optical lattice, the emergence of peaks at \(\left( { \pm \frac{\pi }{a}, \pm \frac{\pi }{a}} \right)\) in the time-of-flight absorption pictures, which is believed to be a sort of evidence of the existence of a supersolid phase, is clearly seen when the system enters the compressible phase from charge-density-wave phase.     

Phase structure and critical behavior of multi-Higgs U(1) lattice gauge theory in three dimensions

We study the three-dimensional (3D) compact U(1) lattice gauge theory coupled with N-flavor Higgs fields by means of the Monte Carlo simulations. This model is relevant to multi-component superconductors, antiferromagnetic spin systems in easy plane, inflational cosmology, etc. It is known that there is no phase transition in the N = 1 model. For N = 2, we found that the system has a second-order phase transition line in the c₂ (gauge coupling)-c₁ (Higgs coupling) plane, which separates the confinement phase and the Higgs phase. Numerical results suggest that the phase transition belongs to the universality class of the 3D XY model as the previous works by Babaev et al. and Smiseth et al. suggested. For N = 3, we found that there exists a critical line similar to that in the N = 2 model, but the critical line is separated into two parts; one for c₂<c_2tc=2.4±0.1 with first-order transitions, and the other for c_2tc<c₂ with second-order transitions, indicating the existence of a tricritical point. We verified that similar phase diagram appears for the N = 4 and N = 5 systems. We also studied the case of anistropic Higgs coupling in the N = 3 model and found that there appear two second-order phase transitions or a single second-order transition and a crossover depending on the values of the anisotropic Higgs couplings. This result indicates that an “enhancement” of phase transition occurs when multiple phase transitions coincide at a certain point in the parameter space.     

Effect of cation doping on low-temperature specific heat of LaMnO3 manganite

We have investigated the thermodynamic properties of perovskite manganite LaMnO₃, the parent compound of colossal magnetoresistive manganites, with the Ca²⁺ doping at the A-site. As strong electron-phonon interactions are present in these compounds, the lattice part of the specific heat deserves proper attention. We have described the temperature dependence of the lattice contribution to the specific heat at constant volume (C_v(lattice)) of La_1−xCa_xMnO₃ (x=0.125, 0.175, 0.25, 0.35, 0.50, 0.67, 0.75) as a function of temperature (1 K–20 K) by means of a rigid ion model (RIM).The trends of specific heat variations with temperature are almost similar at all the composition. The Debye temperatures obtained from the lattice contributions are found to be in somewhat closer agreement with the experimental data. The specific heat values revealed by using RIM are in closer agreement with the available experimental data, particularly at low temperatures for some concentrations (x) of La_1−xCa_xMnO₃. The theoretical results at higher temperatures can be improved by including the effects of the charge ordering, van der Waals attraction and anharmonicity in the framework of RIM.     

Characteristics of surface nano-structural modifications in nitrogen ion implanted W as a function of temperature

The surface modifications of tungsten massive samples (0.5 mm foils) made by nitrogen ion (30 keV; 1 × 10¹⁸ N⁺ cm⁻²) implantation are studied by XRD, AFM, and SIMS. XRD patterns clearly showed WN₂ (0 1 8) (rhombohedral) very close to W (2 0 0) line. Crystallite sizes obtained from WN₂ (0 1 8) line, showed an increase with substrate temperature. AFM images showed the formation of grains on W samples, which grew in size with temperature. These morphological changes are similar to those observed for thin films by increasing substrate temperature (i.e. structure zone model (SZM)). Surface roughness variation with temperature, showed a decrease with increasing temperature. The density of implanted nitrogen ions, and the depth of nitrogen ion implantation in W are studied by SIMS. The results show a minimum for N⁺ density at a certain temperature consistent with XRD results (i.e. I_W (2 0 0)/I_W (2 1 1)). This minimum in XRD results is again similar to that obtained for different thin films by Savaloni et al. [Physica B, 349 (2004) 44; Vacuum, 77 (2005) 245] and Shi and Player [Vacuum, 49 (1998) 257].