Domain sizes in heterogeneous polymers by spin diffusion using single-quantum and double-quantum dipolar filters

1H spin-diffusion experiments employing a double-quantum (DQ) dipolar filter were performed for the characterization of the microdomain structure of heterogeneous samples. For this purpose the NMR spin-diffusion process was analysed based on a model morphology of three different domains with arbitrary sizes, diffusivities, and filter efficiency. General analytical solutions for z magnetization source and sink were obtained valid for a one-dimensional lamellar morphology in the full range of spin-diffusion times. These solutions of the spin-diffusion equations were used for determining the crystalline, interface, and amorphous domain sizes in polystyrene-poly(ethylene oxide) (PS-PEO) and poly(hydroxyethylmethacrylate)-poly(ethylene oxide) (PHEMA-PEO). The DQ dipolar filter has a good efficiency for PS-PEO but is only partially efficient in filtering the signal of the mobile domains in the PHEMA-PEO diblock copolymer. The domain sizes measured by the DQ filter method are compared to those obtained using the traditional dipolar filter creating z magnetization in the mobile domains.     

One- and two-dimensional 15N exchange CP/MAS NMR studies of the structure and electronic properties of the intermolecular N-H...N hydrogen bond in imidazole crystal

The hydrogen bond of the type N-H...N in imidazole crystal has been studied by one and two-dimensional 15N exchange CP/MAS NMR measurements as well as the powder NMR spectrum. The chemical shift anisotropies for -N= and -N< were determined from the powder 1D spectrum. In 2D exchange CP/MAS NMR spectrum, the cross peaks between the 15N main resonance peaks for -N= and -N< were observed, implying that magnetization exchange between -N= and -N< takes place. The 1D exchange CP/MAS NMR measurements determined the exchange rate of magnetization at 289 K to be 1.3 and 1.5 s(-1) for -N= and -N<, respectively. The proton-driven spin-diffusion model interprets the experimental values, and the exchange rate depends strongly on the RF power of the proton decoupling field, suggesting that the magnetization transfer between -N= and -N< takes place by the 1H-driven spin-diffusion mechanism.     

Measurement of the local 1H spin-diffusion coefficient in polymers

Proton spin diffusion is widely used to determine domain sizes in heterogeneous organic solids. For an accurate analysis, spin diffusion coefficients are required. However, in most cases they are not directly measured, but instead derived from model systems. The effects of magic-angle spinning (MAS), mobility, or spin-lock fields on spin-diffusion coefficients have also been difficult to quantify. In this work, direct measurement of local (1)H spin-diffusion coefficients in any rigid polymer is achieved in experiments with heteronuclear dephasing of the (1)H magnetization, a mixing time for (1)H spin diffusion, and (13)C detection after cross-polarization. In the presence of (1)H homonuclear decoupling and (13)C 180 degrees-pulse recoupling, each (13)C spin dephases a significant number (3-20) of protons, depending on the dephasing time. For (13)C and other sufficiently dilute heteronuclei, the dephasing of the protons is described by simple spin-pair REDOR curves. As a result, every (13)C nucleus will "burn" a spherical hole of known diameter and profile into the proton magnetization distribution. (1)H spin diffusion into the hole during the mixing time can be monitored and simulated accurately for every resolved (13)C site, with the spin-diffusion coefficient as the only significant unknown parameter. By varying the dephasing time, holes with diameters of 0.4-0.8 nm can be burned into the proton magnetization profile and thus the dependence of the local spin-diffusion coefficients on the proton density or partial mobility can be explored. The effects of transverse or magic-angle spin-lock fields on spin diffusion can be quantified conveniently by this method. Analytical and numerical fits yield short-range spin-diffusion coefficients of 0.2-0.5 nm(2)/ms on the 0.5-nm scale, which is smaller than the value of 0.8 nm(2)/ms for organic solids previously measured on the 10-nm scale.     

Large spin accumulation in a permalloy-silver lateral spin valve

A large spin accumulation due to the electrical spin injection has been observed in Permalloy-silver lateral spin-valve structures. The observed resistance change is the largest among the reported metallic lateral spin valves with Ohmic junctions. The spin-diffusion length deduced from the experimental results is also found to be the longest among the normal metals reported so far. All the results can be quantitatively explained by the common spin-diffusion model without any discrepancies unlike the results of Godfrey and Johnson.     

Temperature evolution of spin relaxation in a NiFe/Cu lateral spin valve

Temperature dependence of spin relaxation process in a Cu wire has been studied by means of nonlocal spin-valve measurements. The spin-diffusion length of the Cu wire is found to take maximum at the characteristic temperature, below which the spin-diffusion length is reduced. The mechanism of the reduction can be explained by considering the spin-flip scattering due to the oxidized surface of the Cu wire. The thickness dependence of the characteristic temperature supports the interpretation with the surface oxidation.     

Untersuchung von elektrischer Hyperfeinstruktur-Wechselwirkung in Hafniumammoniumhexafluorid-Einkristallen durch γγ-Winkelkorrelationsmessungen

The angular correlation of the 133 keV-482 keV-yy-cascade in the decay of Hf¹⁸¹ is strongly attenuated if solid sources of hafniumammoniumhexafluoride are used. The unperturbed correlation was observed however when a single crystal of hafniumammoniumhexafluoride was used whose main axis pointed into the direction of one of the two detectors. This proves that the perturbation is static and that the maximum component of the electric field gradient at the position of the hafnium nucleus coincides with the direction of the main axis of the crystal. The anisotropy of the angular correlation was measured as a function of the direction of the crystal axis. The results agree with the theoretical predicted functions for a strong electric quadrupole interaction. Then we combined the intrinsic electric field with an external magnetic field. The magnetic field direction was chosen parallel to thez-axis of the electric field gradient and perpendicular to the plane of the detectors. The theory for axially symmetric field gradients predicts a maximum of the anisotropy of the angular correlation for a magnetic field strength at which resonance exists between electric and magnetic precession. For a strong electric interaction the maxium anisotropy has half the value of the unperturbed correlation. In our case the electric quadrupole interaction was so strong that we could not reach the resonance even when we applied external magnetic fields up to 48000 gauss. The observed anisotropies were too large however to be fitted by theoretical curves which were calculated under the assumption that the field gradient has axial symmetry. Therefore we developed the theory for non-axially symmetric electric field gradients. Now a fit was possible and gave unique solutions for the strength of the electric hyperfine interaction as well as for the asymmetry coefficient of the electric field gradient tensor. The accuray of these results was not very high but the strength of the electric hyperfine interaction was found to be small enough to make a direct observation of the electric spin rotation by the differential angular correlation method possible. The observed pattern confirmed the non-axially symmetry of the electric field gradient and we derived the following parameters:

$$\omega _{E_0 } = \left( {570 \pm 30} \right)MHz\left( {\omega _{E_0 } = electric interaction frequency = \frac{{6eQ \cdot \left| {V_{zz} } \right|}}{{4I \cdot \left( {2I - 1} \right) \cdot \rlap{--} h}}} \right)$$     

Spin-diffusion NMR at low field for the study of multiphase solids

The use of spin-diffusion NMR for the measurement of domain sizes in multiphase materials is becoming increasingly popular, in particular for the study of heterogeneous polymers. Under conditions where T(1) relaxation can be neglected, which is mostly the case at high field, analytical and approximate solutions to the evolution of spin diffusion are available. In order to extend the technique to more general conditions, we performed a comprehensive study of the diffusion of magnetization in a model copolymer at low field, where T(1) tends to be of the same order of magnitude as the typical spin-diffusion time. In order to study the effects of T(1) and to delineate the optimal T(1) values for back correction prior to applying the initial-rate approximation, we developed a numerical simulation based on the diffusion equation and including longitudinal relaxation. We present and discuss the limits of simple correction strategies for initial-slope analysis based on apparent relaxation times from saturation-recovery experiments or the spin-diffusion experiments themselves. Our best strategy faithfully reproduces domain sizes obtained by both TEM investigations and full simultaneous fitting of spin-diffusion and saturation-recovery curves. Full fitting of such independent data sets not only yields correct domain sizes, but also the true longitudinal relaxation times, as well as spin-diffusion coefficients. Effects of interphases with distinct mobility on spin-diffusion curves, as well as practical hints concerning the reliable component decomposition of the detected low-resolution FID signal by help of different magnetization filters are also discussed in detail.     

First-principles study on the effect of Hf content onmartensitic transformation temperatureof TiNiHf alloy

In this paper a first-principles study of the electronic structure and stability of B2 TiDFT TiNiHf 电子结构 马氏体转化温度 平面波DFT, TiNiHf, electronic structure, martensitic transformation temperatureProject supported by the National Natural Science Foundation of China (Grant No 50471018).3/3/2006 12:00:00 AM6/7/2006 12:00:00 AMIn this paper a first-principles study of the electronic structure and stability of B2 Ti1-xNiHfx （x = 0.2, 0.4, 0.6） and B19′ Ti1-xNiHfx（x = 0, 0.5） alloys is presented. The calculations are performed by the plane-wave pseudopotential method in the framework of the density functional theory with the generalized gradient approximation. This paper calculates the lattice parameters, density of states, charge density, and heats of formation. The results show that the electronic structure and stability of B2 Ti1-xNiHfx change gradually with Hf content. However, Hf content has little effect on the electronic structure and stability of B19′ Ti1-xNiHfx. The mechanism of the effect of Hf content on martensitic transformation temperature of TiNiHf alloys is studied from the electronic structure.     

Generation of Plasma Wave at Pump-Wave Frequency and Second Harmonic Generation at Ultrarelativistic Laser Power

In this paper, the influence of ponderomotive and relativistic nonlinearities on the filamentation of an ultraintense laser pulse is investigated in three dimensions within the paraxial ray approximations. Generation of electron plasma wave (EPW) structure at pump-wave frequency and the second harmonic generation in these filamentary structures are reported. The generation of the plasma wave is due to intensity gradient (in the transverse direction of the laser beam in filamentary structure) and density gradient (due to ponderomotive-force effect). For typical laser–plasma parameters: The $hbox{laser intensity} = 2.5timesbreak 10^{20} hbox{W/cm}^{2}$; the $hbox{particles density} = 1.9 times 10^{19} hbox{cm}^{-3}$; and it is found that the maximum intensity of EPW is in the range of $2.0 times 10^{13} hbox{W}/hbox{cm}^{2}$. Interaction of the plasma wave with the incident laser beam leads to second harmonic generation, and the yield comes out to be $approx! 2.1 times 10^{-7}$.      

High-Speed Magic-Angle SpinningC MAS NMR Spectra of Adamantane: Self-Decoupling of the Heteronuclear Scalar Interaction and Proton Spin Diffusion

We have investigated the carbon line shape of solid adamantane under high-speed magic-angle sample spinning (MAS) acquired without proton decoupling. The CH-group shows a spinning-speed-dependent line broadening while the CH₂-group consists of a spinning-speed-independent sharp component and a spinning-speed-dependent broader part. These phenomena can be explained by self-decoupling of theJ-interaction due to proton spin diffusion. Such a self-decoupling process can be described by a magnetization exchange process between the multiplet lines. Changing the spin-diffusion rate constant by off-resonance irradiation of the protons allows us to observe the full range from slow exchange to coalescence to fast exchange of the carbon spectra. One of the multiplet components in the CH₂-group corresponds to a group spin of the protons of zero and therefore does not couple to the other protons. This gives rise to the sharp central line. The magnetization exchange rate constant between the different multiplet lines can be determined from the spectra and is a measure for the spinning-speed-dependent proton spin-diffusion rate constant. Even at an MAS speed of 30 kHz, proton spin diffusion is still observable despite the relatively weak intermolecular proton dipolar-coupling network in adamantane which results in a static proton line width of only 14 kHz (full width at half height).     

Imaging intramolecular coulomb repulsions in multiply charged anions

The properties of multiply charged anions are dominated by intramolecular Coulomb repulsion (ICR). Using photoelectron imaging, we show the effect of ICR on photoelectron angular distributions for a series of dianions, -O2C(CH2)_{n}CO_{2};{-} (D_{n};{2-}). The observed photoemission band of D_{n};{2-} was due to a perpendicular transition from the charged end group. However, photoemission intensities were observed to peak along the laser polarization for smaller n due to the strong ICR that forces electrons to be emitted along the molecular axis. This emission pattern weakens with increasing n and at D112- the angular distribution reverses back to peak at the perpendicular direction due to the reduced ICR.     

Optical transitions in Er^3＋ doped lead germanate glasses

Er^{3+}- and Er^{3+}/Yb^{3+}-doped lead germanate glasses that are suitable for use in fibre lasers and optical amplifiers as well as optical waveguide devices have been fabricated and characterized. The absorption spectra from near-infrared to visible were obtained and the Judd－Ofelt parameters were determined from the absorption band. Intense and broad 1.53μm infrared fluorescence and visible upconversion luminescence were observed under 976 nm diode laser excitation. For 1.53μm emission band, the full widths at half-maximum are 36, 37, 51 nm for GPE, GPYE and GPFE samples, respectively. For frequency upconversion emission, the intense bands centred at around 524, 545, 657nm are due to the {}^4S_{3/2}+{}^2H_{11/2}→{}^4I_{15/2} and {}^4F_{9/2}→{}^4I_{15/2} transitions of Er^{3+} ions. The quadratic dependence of the green and red emissions on excitation power indicates that the two-photon absorption process occurs under the 976nm excitation.     

Inclusive K(S);(0)K(S);(0) resonance production in ep collisions at HERA

Inclusive K_{S};{0}K_{S};{0} production in ep collisions at the DESY ep collider HERA was studied with the ZEUS detector using an integrated luminosity of 0.5 fb;{-1}. Enhancements in the mass spectrum were observed and are attributed to the production of f_{2}(1270)/a_{2};{0}(1320), f_{2};{'}(1525) and f_{0}(1710). Masses and widths were obtained using a fit which takes into account theoretical predictions based on SU(3) symmetry arguments, and are consistent with the Particle Data Group values. The f_{0}(1710) state, which has a mass consistent with a glueball candidate, was observed with a statistical significance of 5 standard deviations. However, if this state is the same as that seen in gammagamma-->K_{S};{0}K_{S};{0}, it is unlikely to be a pure glueball state.     

Effects of charge on the structures and spin moments of Nil3 cluster

The structural stability and magnetic properties of the icosahedral Ni13, Ni13^＋1 and Ni13^-1 clusters have been obtained by utilizing all-electron density functional theory with the generalized gradient approximations for the exchange-correlation energy. The calculated results show that the ground states of neutral and charged clusters all favour a D3d structure, a distorted icosahedron, due to the Jahn-Teller effect. The radial distortions caused by doping one electron and by doping one hole are opposite to each other. Doping one electron will result in a 1/2 decrease and doping one hole will result in a 1/2 increase of the total spin. Both increasing interatomic spacing and decreasing coordination will lead to an enhancement of the spin magnetic moments for Nil3 clusters.     

First-principles study of electronic and optical properties in wurtzite Zn1-xCuxO

We perform a first-principles simulation to study the electronic and optical properties of wurtzite Zn1 xCuxO.The simulations are based upon the Perdew-Burke-Ernzerhof form of generalised gradient approximation within the density functional theory.Calculations are carried out in different concentrations.With increasing Cu concentration,the band gap of Zn1 xCuxO decreases due to the shift of valence band.The imaginary part of the dielectric function indicates that the optical transition between O 2p states in the highest valence band and Zn 4s states in the lowest conduction band shifts to the low energy range as the Cu concentration increases.Besides,it is shown that the insertion of Cu atom leads to redshift of the optical absorption edge.Meanwhile,the optical constants of pure ZnO and Zn0.75Cu0.25O,such as loss function,refractive index and reflectivity,are discussed.     

First-principles investigation of BAs and BxGa1-xAs alloys

First-principles investigation of BAs and BxGa1-xAs alloys Using first-principles calculations in the generalized gradient approximation, the electronic properties of BAs and BxGa1-xAs alloys are studied. At the Brillouin-zone centre, the lowest conduction band is the three-degenerate p-like Г15c state rather than s-like Г1c state, and the conduction band minimum （CBM） is along the A line between the Г and X points-at approximately 11/14（1,0,0）2π/a. With boron content at 0%-18.75%, BxGa1-xAs alloys have a small （2.6 eV） and relatively composition-independent band-gap bowing parameter, the band-gap increases monotonically by -18meV/B% with increasing boron content. In addition, the formation enthalpies of mixing for BxGa1-xAs alloys with boron content at 6.25% and 12.5% are calculated, and the large formation enthalpies may explain the difficulty in alloying boron to GaAs.     

A novel yellow emitting phosphor Dy3+, Bi3+ co-doped YVO4 potentially for white light emitting diodes

Novel Y1 x yVO4:xDy3+,yBi3+(0.01 ≤ x ≤ 0.05,0 ≤ y ≤ 0.20) phosphors for light emitting diode(LED) were successfully synthesised by solid-state reaction.The calculation results of electronic structure show that YVO4 has a direct band gap with 3 eV at G.The top of the valence band is dominated by O 2p state and the bottom of the conduction band is mainly composed of O 2p and V 3d states.An efficient yellow emission under near-ultraviolet(365 nm) excitation is observed.Compared with the pure YVO4:Dy3+ samples,the Dy3+,Bi3+ co-doped samples show a more intensive emission peak(at 574 nm) and a new broad emission band(450-770 nm),due to the 4F9/2 6H13/2 transition of Dy3+ and the emission of the VO3 4 Bi3+ complex respectively.The optimum chromaticity index of Y1 x yVO4:xDy3+,yBi3+(0.01 ≤ x ≤ 0.05,0 ≤ y ≤ 0.20) is(0.447,0.497),which indicates that YVO4:Dy3+,Bi3+ has higher colour saturation than the commercial phosphor YAG:Ce3+.The effects of concentration of Dy3+,Bi3+,electric states and the photoluminescence properties are discussed in details.     

Giant magnetocaloric effect in Tb5Ge2-xSi2-xMn2x compounds

The crystal structure,magnetic and magnetocaloric characteristics of the pseduo ternary compounds of Tb5Ge2 xSi2 xMn2x(0 ≤ 2x ≤ 0.1) were investigated by x-ray powder diffraction and magnetization measurements.The x-ray powder diffraction results show that all compounds preserve the monoclinic phase as the majority phase and all the synthesized compounds were observed to be ferromagnetic from magnetization measurements.Magnetic phase transitions were interpreted in terms of Landau theory.Maximum isothermal magnetic entropy change value(20.84 J.kg-1.K-1) was found for Tb5Ge1.95Si1.95Mn0.1 at around 123 K in the magnetic field change of 5 T.     

Magnetic field production during preheating at the electroweak scale

We study the generation of magnetic fields during preheating within a scenario of hybrid inflation at the electroweak scale. We find that the nonperturbative and strongly out-of-equilibrium process of generation of magnetic fields with a nontrivial helicity occurs along the lines predicted by Vachaspati many years ago. The magnitude (rho_{B}/rho_{EW} approximately 10{-2}) and correlation length of these helical magnetic fields grow linearly with time during preheating and are consistent with the possibility that these seeds gave rise to the microgauss fields observed today in galaxies and clusters of galaxies.     

Searching for isotropic stochastic gravitational-wave background in the international pulsar timing array second data release

We search for isotropic stochastic gravitational-wave background (SGWB) in the International Pulsar Timing Array second data release. By modeling the SGWB as a power-law, we find very strong Bayesian evidence for a common-spectrum process, and further this process has scalar transverse (ST) correlations allowed in general metric theory of gravity as the Bayes factor in favor of the ST-correlated process versus the spatially uncorrelated common-spectrum process is 30 ± 2. The median and the 90% equal-tail amplitudes of ST mode are ${{ \mathcal A }}_{\mathrm{ST}}={1.29}_{-0.44}^{+0.51}\times {10}^{-15}$, or equivalently the energy density parameter per logarithm frequency is ${{\rm{\Omega }}}_{\mathrm{GW}}^{\mathrm{ST}}={2.31}_{-1.30}^{+2.19}\times {10}^{-9}$, at frequency of 1 year⁻¹. However, we do not find any statistically significant evidence for the tensor transverse (TT) mode and then place the 95% upper limits as ${{ \mathcal A }}_{\mathrm{TT}}\lt 3.95\times {10}^{-15}$, or equivalently ${{\rm{\Omega }}}_{\mathrm{GW}}^{\mathrm{TT}}\lt 2.16\times {10}^{-9}$, at frequency of 1 year⁻¹.