N.M.R. solid echoes in systems of dipolar-coupled inequivalent spins-1 or dipolar-coupled inequivalent pairs of spins-1/2

Proton solid-echo transverse relaxation functions for many thermotropic and lyotropic mesophases, mapped by measurement of the echo amplitude S_yx (t′ = τ) as a function of τ using a P_y (90°)-τ-P_x (90°)-t′ sequence, yield gaussian behaviour of the form exp [-½M ^E ₂τ²] for decays up to 20–30 per cent of the value at τ = 0. M ^E ₂, the second moment for the dipolar interactions between the spin-½ pairs, is related to the van Vleck second moment M ^VV ₂ through a factor f. Whilst experiments suggested a value of 0·70–0·72 for f = M ^E ₂/M ^VV ₂, simple models that ignored the non-equivalence of the dipolar-coupled spin-pairs had predicted f = 0·65. In this paper we derive an exact analytic expression for the spin response of a model of two dipolar-coupled inequivalent spins-1 to the pulse sequence P_y (90°)-τ-P _α(β)-t′, and show that the present model, with the quenching of the spin-flip terms of the dipolar hamiltonian, resolves the afore-mentioned discrepancy. We also reconcile the differences between the experimental and the earlier predicted values of f for deuteron N.M.R. spin echoes in perdeuterated solids.     

N.M.R. solid echoes in systems of coupled pairs of spins-1/2

Proton solid echo transverse relaxation functions, mapped by measurement of the echo amplitude S _{y x} (t′ = τ) as a function of τ using a P _y (90°)-τ-P _x (90°)-t′ sequence, are reported for a variety of thermotropic mesogens and mesophases (see Table 1). The relationship of the shapes of this function to the structure and dynamics of the mesogen molecule and the mesophase structure is explored. A heuristic approach is adopted. The spin response for an alkyl chain moiety of a mesogen molecule in a uniaxial mesophase is considered to be the sum of the responses for an assembly of representative subsystems each of which consists of two equivalent pairs of spins-½. The response for the latter is calculated by introducing the effects of motional averaging into the expression obtained in Paper I and estimating an ordering matrix from deuterium quadrupole splittings measured on the corresponding deuteriated mesogen. This model, albeit physically unrealistic, qualitatively reproduces the salient features of the observed relaxation functions. The shape functions are found to be rather insensitive to the structure of the mesogen molecule and the type of mesophase involved, but distinguished by the morphological structure of the sample. In homogeneously aligned mesophases, the shape is mainly determined by the distribution in the intersegmental (CH₂) dipolar interactions, whilst in unaligned mesophases the shape is determined by the orientational distribution of the director. A value for M ₂ ^vv(inter-pair) can be obtained from the decay of the relaxation function of unaligned mesophases, but not for aligned ones. There is an exchange process which contributes to the damping of both the FID signal and solid echo relaxation function in unaligned smectic-A mesophases which is absent in the corresponding aligned mesophases. This process must be a consequence of the defect structure of the smectic-A phase and involve rapid diffusion of molecules between regions distinguished by different orientations of the mesophase director.     

Natural orbitals,diatomic molecules and ERD

The effects of molecular motions on spin-echo signals (2n + 1)90°_y—τ—90°_x:—Acq(t) in nuclear spin systems with dipolar interactions have been investigated. It was found that in the case of a rigid lattice (M₂τ² _c: >> 1) and a motionally narrowed NMR line (M₂τ² _c,<< 1) a maximum of echo signals is observed at t_e = 2τ + t₂—t₁/2, where t₁ and t₂ are the widths of the RF pulses. It also was found that in the slow motion region (M₂τ² _c ? 1) an amplitude of the dipolar echo signal is reduced and the maximum of the echo signal is shifted to the end of the second pulse. The theory developed is confirmed by the experimental results obtained for C₆H₆.     

Leading logarithmic corrections to the weak leptonic and semi-leptonic low-energy hamiltonian

If one defines the parameters of the Weinberg-Salam theory at a momentum scale M = O(M_W, M_Z), the weak effective hamiltonian at a momentum scale μ ? M has logarithmically enhanced corrections, of order αln(M²/μ²). We present a computation of these corrections, for that part of the hamiltonian which leads to detectable weak-electromagnetic interference effects. The largest correction can be absorbed into a running sin²θ(μ). Other, smaller, corrections are estimated, taking into account the effect of strong interactions.An estimate of the non-logarithmically enhanced corrections is also given, by evaluating them in the limit sin²θ → 0. From the SLAC e - d asymmetry it was found sin²θ = 0.224 ± 0.020 at μ² ? 2 GeV². In correspondence, we find sin²θ(M) = 0.217 ± 0.020. This value, however, is subject to uncertainties deriving from the effect of the strange and of the antiquark parton sea.     

Results from the NOMAD experiment

The NOMAD experiment has sought ν_μ ? ν_τ oscillations by looking for the emergence of τ^? in events from the CERN SPS neutrino beam. With some improvements in the techniques of analysis in relation to the results published previously and with the inclusion of data from the 1998 run, no evidence for the oscillations has been found, which results in an updated limit on the oscillation probability [P(ν_μ → ν_τ) < 0.5 × 10^?3 at a 90% C.L.]. The corresponding limit on the oscillation mixing angle is given by sin²2θ_μτ < 1.0 × 10^?3 for large Δm ². By using a 1% contamination of ν_e in the neutrino beam, we can also rule out ν _e ? ν_τ oscillations and constrain the probability of the relevant transition as P(ν _e → ν_τ) < 3 × 10^?2 at a 90% C.L. (sin²2θ _eτ < 6 × 10^?2 at large Δm ²).     

How to compare the SU(5) value of sin2θw with experiments?

We establish the relation between κsin²θ_w to be found from neutral-current experiments and sin²θ_w(Q) for Q=M_W predicted by grand unified theories. We then calculate sin²θ_W(M_W) in the minimal SU(5) model taking the M_W as well as M_x threshold effects into account. We find that these two threshold effects on sin²θ_W(M_W) cancel with each other and sin²θ_W(M_W)=0.211± 0.005.     

Electromagnetic corrections to neutrino processes in the leading logarithmic approximation,the value of sin θ and the W and Z masses

The parameters of the Weinberg-Salam model can be defined by amplitudes at a momentum scale M = O(M_W, M_Z). We derive the leading logarithmic e.m. correction to the relations giving the neutrino amplitudes at a momentum scale μ ? M in terms of sin²θ(M), α(M), M_W and M_Z. For leptonic processes, the Fermi constant is not corrected, but a running, universal, sin²θ(μ) > sin²θ(M) should be used. The Fermi constant for semileptonic processes is renormalized by a factor $\text{?(μ) > 1}$, for charged currents, and is not renormalized, for neutral current processes. The latter are described by the same sin²θ(μ) as the leptonic ones. We estimate that sin²θ(M) is about 0.013 smaller than the value of sin²θ obtained from semileptonic data with no correction, thereby improving the agreement with grand unified theories. The prediction for W (Z) masses and widths in terms of the low energy parameters are discussed. Using previous calculations at order α, we obtain predictions for the masses which are accurate up to and including terms of order (αlnM²)².     

Cancellation of energy divergences in Coulomb gauge QCD

Electroweak unification is obtained in anSU(7) model at a mass scale 3×10¹⁰≦M≦3×10¹⁶ GeV's, with left-right symmetric subgroups and sin² θ _w (M)=3/8. BelowM, the model reduces toSU(3) _L ×SU(3) _R , the flavor sector of the “trinification theory” of Glashow et al., or of theE ₆ grand unified theory. This model predicts a natural massless neutrino, and fractionally charged leptons with masses in theM regime.     

Estimation ofv μ→v τ oscillation parameters in the E-564 hybrid experiment

The limits on the parameters of thev _μ→v _τ oscillations are obtained in the E-564 hybrid experiment: Δm ²≦4.5 eV² and sin²(2θ)≦6.0·10^?2 (90% CL). A possibility of direct observation of τ-lepton decays in the nuclear emulsion vertex detector allowed the sensitivity of this experiment to the parameters of the oscillations to be increased.     

Radio-frequency μSR experiments at the ISIS pulsed muon facility

This paper explores the use of pulsed radio-frequency (RF) techniques to remove the frequency limitations imposed on conventional transverse muon spin rotation (μSR) experiments at a pulsed muon source by the finite muon pulse width. The implementation of the 90° pulse technique is demonstrated by observing the free precession signal of diamagnetic muons implanted in polythene, the change in signal amplitude as a function of RF pulse length is plotted and the precise condition for a 90° pulse determined. The technique is evaluated by comparing measurements made using conventional spin rotation experiments to those employing pulsed RF methods. The potential for applying standard NMR multiple-pulse methods to the μSR experiment is considered and the use of two-pulse RF sequences (90° _x ?τ?90° _x and 90° _x ?τ?180° _x ) to form a muon spin echo demonstrated.     

Expressions for the computer-evaluation of the four kernel functions for line formation with Doppler and Lorentz profiles

Rational approximations for the kernel functions K₁(τ), K₂(τ), M₁(τ) and M₂(τ) that describe the transfer of radiation scattered with complete redistribution over Doppler and Lorentz profiles have been obtained from their series and asymptotic expressions by the techniques of Padé approximants with a maximum relative error of less than 10^-4.     

On the theory of NMR spin echoes in solids

A general expression in terms of two-time correlation functions is derived for the spin echo responses to 90°-τ-β⁰_φ pulse sequences of quadrupolar spins coupled through dipolar interactions. The second moments of the correlation function are calculated for a system of spin one nuclei and shown to be in accordance with the experimental observations. Furthermore, results are presented for the fourth moments.     

粉末样品自旋3/2四极核中心跃迁的高分辨核磁共振谱

本文利用脉冲对自旋3/2四极核中心跃迁激发的选择性,在粉末样品中用一个90°脉冲将电场梯度张量在空间某一取向的核自旋磁化矢量转到旋转坐标系中XY平面进行检测,从而得到与单晶样品类似的高分辨谱线,在理论上用密度算符公式作了分析,实验上用NaNO₂和Na₂SO₄粉末样品进行了验证。 关键词：     

Investigation of AlGaN/GaN fluorine plasma treatment enhancement-mode high electronic mobility transistors by frequency-dependent capacitance and conductance analysis

This paper reports fluorine plasma treatment enhancement-mode HEMTs (high electronic mobility transistors) EHEMTs and conventional depletion-mode HEMTs DHEMTs fabricated on one wafer using separate litho-photography technology. It finds that fluorine plasma etches the AlGaN at a slow rate by capacitance--voltage measurement. Using capacitance--frequency measurement, it finds one type of trap in conventional DHEMTs with τ_T=(0.5-6) ms and D_T= (1 - 5) × 10¹³ cm^-2·eV^-1. Two types of trap are found in fluorine plasma treatment EHEMTs, fast with τ_T(f)=(0.2-2) μs and slow with τ_T(s)=(0.5-6) ms. The density of trap states evaluated on the EHEMTs is D_T(f)=(1 - 3) × 10¹² cm^-2·eV^-1 and D_T(s)=(2 - 6) × 10¹² cm^-2·eV^-1 for the fast and slow traps, respectively. The result shows that the fluorine plasma treatment reduces the slow trap density by about one order, but introduces a new type of fast trap. The slow trap is suggested to be a surface trap, related to the gate leakage current.     

Measurement of the spectral functions of vector current hadronic tau decays

A measurement of the spectral functions of non-strange τ vector current final states is presented, using 124 358 τ pairs recorded by the ALEPH detector at LEP during the years 1991 to 1994. The spectral functions of the dominant two- and four-pion τ decay channels are compared to published results of e ⁺ e ^- annihilation experiments via isospin rotation. A combined fit of the pion form factor from τ decays and e ⁺ e ^- data is performed using different parametrizations. The mass and the width of the ρ ^±(770) and the ρ ⁰(770) are separately determined in order to extract possible isospin violating effects. The mass and width differences are measured to be M _ρ ^± ₍₇₇₀₎ - M _ρ ⁰ ₍₇₇₀₎ = (0.0 ± 1.0) MeV/c ² and Γ _ρ ^± ₍₇₇₀₎ - Γ _ρ ⁰ ₍₇₇₀₎ = (0.1 ± 1.9) MeV/c ².     

An SO(10) model with 54 + 126 + 10 higgs

We find an absolute minimum of an SO(10) symmetric potential with SU(3) × U(1) invariance. By fixing the higher scales M_R(〈126〉) ? M_x(〈54〉), the model is consistent with the experimental knowledge about matter stability and the value of sin²θ_w. We determine the spectrum of scalar particles and show that their tree-diagram contributions to nucleon decay amplitudes are proportional to 1/M_x².     

Geometry,strength of binding and Br2 charge redistribution in the complex OC ··· Br2 determined by rotational spectroscopy

The ground-state rotational spectra of the six isotopomers ¹⁶O¹²C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁸¹Br, ¹⁶O¹²C ··· ⁷⁹Br⁸¹Br, ¹⁶O¹³C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹³C ··· ⁸¹Br⁷⁹Br, were observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectroscopic constants B _O, D _J, χ _aa (Br_i), χ _aa (Br_o), M_bb (Br_i) and M _bb (Br_o), where i = inner and o = outer, were determined for each isotopomer. The complex is linear, with the weak bond between the C atom of CO and Br_i. The rotational constants were used to determine the distance r(C ··· Br_i) = 3.1058Å and to show that the Br—Br bond lengthens by ～0.005–0.01Å on complex formation. The intermolecular stretching force constant kσ = 5.0 Nm^?1 was obtained from D_J and the Br nuclear quadrupole coupling constants were interpreted to reveal that a fraction δ = 0.02 of an electronic charge is transferred from Br_i to Br_o when Br₂ is subsumed into the complex. Properties of the two series OC ··· XY and H₃N ··· XY, where XY = C1₂, Br₂ and BrC1, are compared.     

Correlation between phase structure and electrical conduction in BISNVOX system for intermediate temperature solid oxide fuel cells (IT-SOFC)

Samples of Sn⁴⁺-substituted bismuth vanadate, formulated as Bi₄Sn _x V_{2? x} O_{11?( x /2)? δ} in the composition range 0.07 ≤ x ≤ 0.30, were prepared by standard solid-state reactions. Sample characterization and the principal phase transitions (α ? β, β ? γ and γ′ ? γ) were investigated by FT-IR spectroscopy, X-ray powder diffraction, differential thermal analysis (DTA) and AC impedance spectroscopy. For composition x = 0.07, the α ? β and β ? γ phase transitions were observed at temperatures of 451 and 536°C, respectively. DTA thermograms and Arrhenius plots of conductivities revealed the γ′ ? γ phase transition at 411 and 423°C for x = 0.20 and 0.30, respectively. AC impedance plots showed that conductivity is mainly due to the grain contribution, which is evident in the enhanced short-range diffusion of oxide ion vacancy in the grains with increasing temperature. The highest ionic conductivity (5.03 × 10^?5 S cm^?1 at 300°C) was observed for the x = 0.17 solid solution with less pronounced thermal hysteresis.     

129Xe contact shift in oxygen gas

We have determined the temperature dependence of σ₁ of ¹²⁹Xe in oxygen gas. These results were obtained by measurement of the resonance frequency of ¹²⁹Xe in gas samples of known densities in Xe and O₂. The shift of the resonance frequency due to Xe-Xe interactions has been measured in pure Xe gas samples with improved precision. This allows the determination of σ₁(Xe-O₂) by subtracting out the known effect of Xe-Xe interactions in mixed Xe-O₂ samples. σ₁(Xe-O₂) values are reported here for the temperature range 220 to 440 K. The values of σ₁(Xe-O₂) are adequately described by the polynomial function in p.p.m. amagat^-1 σ₁(Xe-O₂) = - 1·061 + 3·64 × 10^-3τ - 2·19 × 10^-5τ² + 9·58 × 10^-8τ³ - 2·08 × 10^-10τ⁴, where τ = (T - 300 K). It is found that the temperature dependence of σ₁(Xe-O₂) can be interpreted in terms of a contact interaction between Xe and the paramagnetic O₂ molecule.     

Conventional and collision photon echo in ytterbium vapor

The polarization properties of the photon echo generated by two linearly polarized pulses of resonant radiation at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition of ¹⁷⁴Yb are investigated. A complicated polarization behavior of the photon echo versus an angle between the polarization vectors of the excitation pulses is revealed in a mixture of ytterbium vapor with inert gas. For the angles ranging from 0° to 75°, a conventional echo with its linear polarization coinciding with the second excitation pulse dominates and the echo amplitude decreases with an increasing angle. For the angles ranging from 75° to 89°, the photon echo is elliptically polarized. Finally, for an angle of 90°, the conventional echo disappears and the collision echo becomes linearly polarized along the first excitation pulse.