Electrostriction in a uniaxial ferroelectric with second order phase transition

The electrostriction constant γ(m²V^?2) of a uniaxial ferroelectric with a second order phase transition has been calculated as a function of the dielectric constant (?P/?E)₀. The results have been experimentally verified on triglycine sulphate (TGS) by measuring γ and (?P/?E)₀. A sign reversal of γ above the Curie temperature is presented.     

Investigation of gain characteristics in mixed crystals LiMeF4 (Me = Y,Lu, Yb) doped by Ce3+ ions

Differential gain spectra in the range 295–335 nm were measured in crystals of scheelite structure LiY_{1 ? x} Lu _x F₄ (x = 0–1), doped by Ce³⁺ ions. It is shown that variation of Lu³⁺ and Y³⁺ ions relative content in LiY_{1 ? x} Lu _x F₄ crystals allows to manipulate the spectral width of the amplification band. Cross-sections of excited-state absorption at the wavelengths of Ce³⁺ luminescence, probability ratios of formation and thermal destruction of color centers depending on the Y³⁺ ions content in LiY_{1 ? x} Lu _x F₄ crystals were estimated. Even better gain characteristics have been demonstrated by LiLuF₄:Ce³⁺, doped by Yb³⁺ ions. The highest optical gain coefficient with a wide amplification band among studied samples was observed in LiLuF₄:Ce³⁺ crystal, codoped by Yb³⁺ ions.     

Linear temperature dependence of low temperature pyroelectric coefficient

Due to the deformation of electronic charge clouds, the Dick-Overhauser exchange charge in ionic materials is shown to have quadratic temperature dependence as T tends to zero in non-centrosymmetric crystals. It follows that the pyroelectric coefficient π has a linear temperature dependence in the same low temperature limit. The order of magnitude of π obtained theoretically by a simple model at T = 5 K is 2 × 10^?7 μC cm^?2 K^?1, which is in fairly good agreement with that obtained experimentally on LiTaO₃ by Lines.     

Measurement of the Baryon-exchange reactions K−p→Λη, K−p→Λπ0 at 6 GeV/c and the value of the ηNN coupling constant

In order to determine the ηNN coupling constant we have measured the two reactions K^?p→Λη and K^?p→Λπ⁰ with a magnetic wire chamber spectrometer which contained a gamma counter for the γγ decays of π⁰ and η. The Λ polarization and the differential cross sections are given. The latter have quite different u dependences. Their ratio is interpreted, in terms of a nucleon-Regge exchange model, as the effect of a small ηNN coupling constant for which we obtain G_ηNN² = G_πNN² · (0.26 ± 0.10) as allowed by SU(3). The large value given by Heisenberg's non-linear field theory, G_ηNN² = G_πNN² · 0.9, is excluded by this measurement if the characteristic u dependence of the Λπ⁰ channel is attributed to N_α Regge exchange.     

Waveguide structures written in SF57 glass with fs-laser pulses above the critical self-focusing threshold

Waveguide writing with femtosecond (fs) laser pulses in glasses is compromised by uncontrollable non-linear propagation phenomena for powers exceeding a certain threshold, P_cr, which is inversely proportional to the material linear and non-linear refractive indices, n₀ and n₂, respectively. We have studied the behaviour of a commercial glass, Schott SF57, with high values of n₀ and n₂ when it is processed above P_cr with 800 and 1260 nm, 100 fs laser pulses to produce waveguide structures. Two types of structures, longitudinal (l-) and transversal (t-) ones depending on the sample translation, were obtained. In both of them, material damage surrounded by guiding regions was generated. We attribute the formation of the guiding regions of increased refractive index to the high local pressure reached in the focal volume where catastrophic damage occurs. This pressure can be high enough to compress the material nearby thus generating regions with a permanent refractive index increase up to 5 × 10⁻⁴. The efficiency of the guiding region is conditioned by the pulse energy. Mode profile analyses reveal double-sided exponential guided modes in the l-waveguides. As an alternative route to obtain suitable mode shapes and, simultaneously, to increase the coupling efficiency, the use of multiple structures has been investigated. Various l-structures have been written close to each other generating a region of increased refractive index large enough to effectively support a Lorentzian mode at 633 nm.     

NIR luminescence from Er/Yb, Tm/Yb, Er/Tm and Nd ions-doped zincborotellurite glasses for optical amplification

In this paper, we report the near-infrared luminescence from the Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, Er³⁺/Tm³⁺ and Nd³⁺ ions-doped TeO₂-ZnO-B₂O₃-Li₂O-Na₂O glasses for optical amplification. The X-ray diffraction (XRD) and differential scanning calorimetry (DSC) profiles of the host glass matrix have been carried out. From the DSC thermogram, glass transition (T_g), crystallization (T_c) and melting (T_m) temperatures have been evaluated. The near-infrared spectra of Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, Er³⁺/Tm³⁺ and Nd³⁺ ions-doped glasses have shown full-width at half-maxima (FWHM) around 58, 127, 87 and 35 nm, respectively. These glasses with better thermal stability and broad near-infrared emissions should have potential applications in broadly tunable laser sources and broadband optical amplification at low-loss telecommunication windows.     

Mechanisms of off-center ion behavior in oxygen-octahedral perovskite-type ferroelectrics

A mechanism of the off-center impurity-ion behavior induced by the quadratic Jahn-Teller effect on polar distortions and (or) joint action of the local electrostriction and linear Jahn-Teller effect on nonpolar distortions is proposed. The off-center behavior of Ni⁵⁺ and Fe⁴⁺ ions in the incipient ferroelectric KTaO₃ is substantiated. It is shown that, in the limit of neglecting the correlation between off-center Li⁺ ions, their displacements from KNbO₃ lattice sites decrease, whereas, under the conditions of strong correlation, a critical increase in their displacements from lattice sites occurs with approaching the Curie point.     

Tm and Tm-Tb-doped germanate glasses for S-band amplifiers

The mechanism involved in the Tm³⁺(³F₄)→Tb³⁺(⁷F_0,1,2) energy transfer as a function of the Tb concentration was investigated in Tm:Tb-doped germanate (GLKZ) glass. The experimental transfer rate was determined from the best fit of the ³F₄ luminescence decay due to the Tm→Tb energy transfer using the Burshtein model. The result showed that the 1700 nm emission from ³F₄ can be completely quenched by 0.8 mol% of Tb³⁺. As a consequence, the ⁷F₃ state of Tb³⁺ interacts with the ³H₄ upper excited state of Tm³⁺ slighting decreasing its population. The effective amplification coefficient β(cm⁻¹) that depends on the population density difference Δn=n(³H₄)-n(³F₄) involved in the optical transition of Tm³⁺ (S-band) was calculated by solving the rate equations of the system for continuous pumping with laser at 792 nm, using the Runge-Kutta numerical method including terms of fourth order. The population density inversion Δn as a function of Tb³⁺ concentration was calculated by computational simulation for three pumping intensities, 0.2, 2.2 and 4.4 kWcm⁻². These calculations were performed using the experimental Tm→Tb transfer rates and the optical constants of the Tm (0.1 mol%) system. It was demonstrated that 0.2 mol% of Tb³⁺ propitiates best population density inversion of Tm³⁺ maximizing the amplification coefficient of Tm-doped (0.1 mol%) GLKZ glass when operating as laser intensity amplification at 1.47 μm.     

Quadratic electrostrictive effects in NaCl and KA1(SO4)2·12H2O derived from stress dependence of dielectric constants

The equivalence of quadratic electrostrictive coefficients and certain stress derivatives of dielectric constants provides a chance for an independent determination of the tensor of quadratic electrostriction. The experimental values obtained by the aid of stress-induced shifts of the dielectric constants allow a clarification of the contradictory results of different authors on NaCl and KA1(SO₄)₂·12H₂O. The new values for 293 K are: NaCl: d₁₁₁₁ = 0.68, d₁₁₂₂ =-0.215, d₁₂₁₂ = 0.12; KA1(SO₄)₂·12H₂O: d₁₁₁₁ = 0.63, (d₁₁₂₂ + d₁₂₁₁)/2 = -0.15, d₁₂₁₂ = 0.46 (Unit: 10^-20 m² V^-2). These crystals exhibit a mechanical dilatation parallel to an electric field and a contraction in perpendicular directions. This is accompanied by by a small increase in volume. Further, it is shown that certain relations between different electrostrictive effects of second order given in literature are erroneous in respect to sign..     

Non-linear optical properties of silver nanoparticles prepared by hydrogen reduction method

Silver nanoparticles have been prepared using hydrogen gas as the reducing agent for silver nitrate and poly(vinyl pyrrolidone) as the capping agent; the reaction was carried out at 70 °C for 3 h. The size of the nanoparticles was found to be about 20 nm as analyzed using transmission electron micrographs. The X-ray diffraction pattern revealed the face-centered cubic (fcc) structure of silver nanoparticles. The linear absorption of Ag nanoparticles, α, is obtained about 3.71 cm⁻¹. The non-linear refractive indices of silver nanoparticles were defined by the z-scan technique using CW He-Ne laser (λ = 632.8 nm) at different incident intensities. The magnitude of non-linear refractive index (n₂) was measured to be in the order of 10⁻⁷ (cm²/W) with a negative sign. Therefore self-defocusing phenomena is taking placed for Ag nanoparticles.     

Effective field theory for massive gravitons and gravity in theory space

We introduce a technique for restoring general coordinate invariance into theories where it is explicitly broken. This is the analog for gravity of the Callan-Coleman-Wess-Zumino formalism for gauge theories. We use this to elucidate the properties of interacting massless and massive gravitons. For a single graviton with a Planck scale M_Pl and a mass m_g, we find that there is a sensible effective field theory which is valid up to a high-energy cutoff Λ parametrically above m_g. Our methods allow for a transparent understanding of the many peculiarities associated with massive gravitons, among them the need for the Fierz-Pauli form of the Lagrangian, the presence or absence of the van Dam-Veltman-Zakharov discontinuity in general backgrounds, and the onset of non-linear effects and the breakdown of the effective theory at large distances from heavy sources. The natural sizes of all non-linear corrections beyond the Fierz-Pauli term are easily determined. The cutoff scales as Λ∼(m_g⁴M_Pl)^1/5 for the Fierz-Pauli theory, but can be raised to Λ∼(m_g²M_Pl)^1/3 in certain non-linear extensions. Having established that these models make sense as effective theories, there are a number of new avenues for exploration, including model building with gravity in theory space and constructing gravitational dimensions.     

Far-infrared laser oscillation in p-Ge

Time-resolved measurements, together with spectroscopic study with a grating monochromator, are made on far-infrared stimulated emission from p-Ge in crossed electric and magnetic fields. Gain saturation is confirmed, for the first time, to occur to establish laser oscillation. Small-signal gain per unit length deduced from the time constant of light amplification is 7.4 × 10^-3 cm^-1 and 2.7 × 10^-2 cm^-2, respectively, for samples with N_A − N_D ≅ 4.5 × 10¹³cm⁻³ and 1.7 × 10¹⁴ cm^-3. Output power detected at distance 28 cm from the sample is of order 1–10 W, but higher power over 100W is suggested for the total output from the sample.     

Pyroelectric effect in lead-magnoniobate-based solid solutions

A coordinated study of the dielectric, electrostriction, and pyroelectric properties of the lead-magnoniobate-based ceramic solid solutions 0.9PbMg_1/3Nb_2/3O₃-0.1PbTiO₃ and 0.55PbMg_1/3Nb_2/3O₃-0.45PbSc_1/2Nb_1/2O₃ conducted at the relaxor transition temperatures is reported. Electromechanical coupling stimulated by the giant electrostriction effect is shown to play an essential part in the pyroelectric effect.     

Prediction of accidental cancellation of different deformation components and optimum fusion orientations

In view of the role of nuclear deformations in the fusion between spherical and deformed nuclei to form super-heavy elements, we try to understand how a cancellation of the different nuclear deformations could arise. We first investigated the correlation between the orientation variation of the deformed nucleus radius and the orientation Coulomb barrier distribution in presence of the higher order deformation components, β₆ and β₈, in addition to the lower order ones. This correlation has been reported in our previous work (Ismail and Seif, 2010) [1] in presence of the lower order (β₂, β₃ and β₄) deformations. Even if there are higher deformations, we found here that the simple expression which describes the deformed target nucleus can be used to predict with good accuracy the behavior of the fusion Coulomb barrier with both orientation and deformation as well as the optimum (cold or hot) fusion configurations. It can predict the orientations of compact and elongated configurations of the interaction and whether they are equatorial or polar or none of them. The value and sign of the deformation parameters ratios with respect to one of them have been used to classify these configurations. We applied the same correlation to predict successfully the mutual cancellation effects between the different deformation components up to β₈. Illustrative examples are given in which the cancellation, at some orientations, brings the fusion barrier back to the spherical case or keeps only the effect of quadrupole deformation, or the effects of both β₂ and β₄.     

Convective amplification of gas turbine engine internal noise sources

The acoustic field of a noise source is altered when the source is in motion. The change in the acoustic field introduced by the source motion, caused by source alteration and propagation effects, is defined as convective amplification. Previous studies of this phenomenon have been based on analytical models that did not incorporate the physical features necessary for calculation of the convective amplification factor for the internal noise sources of a gas turbine engine, which is required to predict in-flight noise levels from static engine noise measurements. An improved theoretical model was developed. At low frequencies, this model resulted in a convective amplification factor of (1?M₀ cos θ_e)^?4, which is identical with the factor established in earlier studies. At high frequencies, however, convective amplification is a function of flight speed, radiation angle, and source geometry.     

Microscopic analysis of octupole shape transitions in neutron-rich actinides with relativistic energy density functional

Quadrupole and octupole deformation energy surfaces, low-energy excitation spectra, and electric transition rates in eight neutron-rich isotopic chains – Ra, Th, U, Pu, Cm, Cf, Fm, and No – are systematically analyzed using a quadrupole-octupole collective Hamiltonian model, with parameters determined by constrained reflectionasymmetric and axially-symmetric relativistic mean-field calculations based on the PC-PK1 energy density functional.The theoretical results of low-lying negative-parity bands, odd-even staggering, average octupole deformations β_3,and B(E3;3_1~- →0_1~+) show evidence of a shape transition from nearly spherical to stable octupole-deformed, and finally octupole-soft equilibrium shapes in the neutron-rich actinides. A microscopic mechanism for the onset of stable octupole deformation is also discussed in terms of the evolution of single-nucleon orbitals with deformation.     

Collisionally pumped hybrid soft X-ray laser in Ne-like sulphur

We describe an experiment demonstrating XUV amplification following collisional excitation in a capillary discharge plasma irradiated by a picosecond IR laser pulse. Guiding and temporally resolved transmission of the pump laser beam are also demonstrated and analysed. The short pump laser pulse heated rapidly the electrons producing amplification in the 3p¹S₀–3s¹P₁ transition of Ne-like sulphur at 60.84 nm. The estimated gain–length product was equal to 6.8, while the beam divergence reached 2.5 mrad for 30 mm capillary. This new, hybridly pumped collisional soft X-ray laser with the transient gain offers a new way towards efficient table-top XUV sources.     

On the coexistence of oblate and prolate deformed bands in83Zr

Deformation parameters of the positive parity yrast band and negative parity bands in⁸³Zr are deduced from lifetimes andE2/M1 mixing ratios. Lifetimes of high spin states have been determined from recoil distance Doppler shift and Doppler shift attenuation measurements using the⁵⁴Fe(³²S,2pnγ) ⁸³Zr reaction. Ten lifetimes and five lifetimes limits were determined. The positive parity band, built on theg _9/2 K=5/2 orbital has an average deformation ¦β ₂¦=0.28(2), and shows a reduction ofE2 transition strengths in the observed backbend region at I^π≈21/2⁺. In contrast, theE2 strengths in the negative parity states show a steady increase up to I^π≈=15/2^?. These states are more strongly deformed than the positive parity states (¦β ₂¦=0.33(3)). TheE2/M1 mixing ratios show that the negative parity band hasK=3/2 and is prolate, and favour oblate deformation for the positive parity yrast band. In theK=1/2^? band theE2 strength of the 7/2^?→3/2^? transition yields a deformation ¦β ₂¦=0.26(5). The band structure is compared with calculations within the Hartree-Fock Bogoliubov cranking model.     

Application of X-ray absorption spectroscopy to the investigation of charge states of iron ions in iron borate nanoceramics

The method of X-ray absorption spectroscopy has been used for the investigation of charge states of iron ions in iron borate nanoceramics prepared by shear deformation under pressure. The experimental Fe 2p X-ray absorption spectra have been presented in comparison with the calculation of atomic multiplets of iron ions taking into account the charge transfer from the 2p orbitals of oxygen to the 3d orbitals of iron and the crystal-field splitting of the 3d orbitals of iron. Our results indicate that, in addition to iron ions in the ground charge state Fe³⁺, nanostructured FeBO₃ contains a few percent of Fe²⁺ ions. It has been found that an increase in the degree of plastic deformation (the rotation angle of the anvils) leads to a decrease in the size of crystallites and to an increase in the concentration of Fe²⁺ ions without the formation of new phases. The results of this work agree with the magnetic and optical measurements and confirm high defectness of FeBO₃ nanoceramics.