共查询到20条相似文献,搜索用时 31 毫秒
1.
2.
The present communication reports the experimental values of NMR spin-lattice relaxation time (T1) and dielectric relaxation time (τ) of piperidine, pyrrole, pyridine, diethylamine, triethylamine and pyrrolidine. The values of activation energy (ΔEA) obtained using dielectric relaxation time, have been correlated with calculated values of ΔEA obtained using Arrhenius equation of NMR relaxation time (T1) for pyridine, diethylamine and pyrrole. Authors have also established a correlation between the experimental values of NMR spin-relaxation time (T1) with its calculated values obtained using different equations of dielectric relaxation time (τ). 相似文献
3.
G. Brammertz A. Golubov A. Peacock P. Verhoeve D. J. Goldie R. Venn 《Physica C: Superconductivity and its Applications》2001,350(3-4):227-236
We present an application of the generalised proximity effect theory. The theory has been used to determine the energy gap (Δg) in proximised transition metal/aluminium bilayer structures such as Nb/Al, Ta/Al, V/Al and Mo/Al. These bilayers have different film thicknesses ranging from 5 to 260 nm. For the cases of Nb/Al, Ta/Al and V/Al bilayers, the interface parameters γ and γBN (here we define γ as the ratio of the products of normal state resistivity and coherence length in each film of the bilayer while γBN is the ratio of the boundary resistance between films 1 and 2 to the product of the resistivity and coherence length in the second film), which were used as input parameters to the model, were inferred experimentally from an existing bilayer of each kind and then suitably modified for different film thicknesses. This experimental assessment is therefore based on a comparison of measurements of the critical temperature and the energy gap at 300 mK with the predictions from the model for various values of γ, γBN. The energy gap of the bilayer was experimentally determined by using symmetrical superconducting tunnel junctions (STJs) of the form S–Al–AlOx–Al–S, where each electrode corresponds to a proximised bilayer. However for the case of Mo/Al bilayers the interface parameters were determined theoretically since currently no STJ data for this configuration are available. The results for the Nb/Al, Ta/Al and V/Al bilayers have also then been compared to experimentally determined energy gaps found for a series of STJs with different film thicknesses. The correspondence between experiment and theory is very good. 相似文献
4.
We analyze the influence of residual two-photon excitation (2PE) in two-color (two-photon) optical beam induced current (2CE-OBIC) generation in wide band gap semiconductor samples. 2CE-OBIC generation is accomplished with two confocal excitation beams of separation angle θ and wavelengths λ1 and λ2 where , λe = hc/Eb, h is the Planck’s constant, c is speed of light in vacuum, and Eb is the energy band gap. Because the conduction band of the sample is a continuum, at least one excitation beam would also contribute an undesirable 2PE-OBIC signal that degrades the signal-to-noise ratio of the measured 2CE-OBIC response and broadens the effective OBIC distribution in the sample particularly when θ ≠ 0 or π. We show that the deleterious effects of crosstalk are reduced by a careful selection of λ1 and λ2 and the relative excitation beam intensities. λ1 and λ2 should be chosen to minimize the ratio of the two-photon absorption coefficients (β1, β2) to the 2CE absorption coefficient β12 or at least satisfy the constraint: β1 + β2 β12. Keeping the two excitation intensities equal is beneficial only when β1 = β2. Otherwise, it is advantageous to bias the intensity ratio towards the wavelength with a lower 2PE absorption coefficient. 相似文献
5.
The diffusion of W on a (211) plane of a W field emitter has been re-examined by means of the fluctuation autocorrelation method. Diffusion along channels yielded E = 16.8 ± 0.5 kcal, D0 = (3 ± 1) × 10−5 cm2 s−1. For diffusion across channels E =6.6 kcal, D0 = 4 × 10−9cm2 s−1 at T < 752 K, and E = 24 kcal, D0 = 5 × 10−4 cm2 s−1 at T > 752 K. The results for diffusion along channels yield E and D0 values intermediate between recent results by Wang and Ehrlich [Surf. Sci. 206 (1988) 451] using field ion microscopy (E = 19 kcal, D0 = 7.7 × 10−3 cm2 s−1) and Tringides and Gomer [J. Chem. Phys. 84 (1986) 4049], using the same method as the present work but a larger slit (E = 13.3 kcal, D0 = 7 × 10−7 cm2 s−1). The results for cross channel diffus good agreement with those of Tringides and Gomer below 752 K, where these authors stopped. The new high temperature results suggest that the channel wall exchange mechanism postulated by Tringides and Gomer for cross channel diffusion at low T gives way to diffusion by climbing over the channel walls with higher E but also higher D0 above 752 K. Possible reasons for the discrepancies between these three sets of results and the absence of cross channel diffusion in the work of Wang and Ehrlich are briefly discussed. 相似文献
6.
In this work,the equilibrium structure,electronic and elastic properties of L12-ordered Co-Al-W and Co-Al-W-X(X=Ti and Nb)phase were calculated,using first-principles calculations.Among six nonequivalent sites(Al1,Al2,Co3,Co4,W5,W6),Ti and Nb prefer to occupy the W6site,since the formation enthalpy of the system is lowest when Ti and Nb occupy the W。site.Both Ti and Nb most affect the density of states of Al atoms.Compared with the Al2 site,which is the sub-preference site of Ti and Nb,the density of states of Al atoms is higher with the addition of Ti and Nb in the W6site,which means that the latter system is more stable.According to the bulk modulus B,shear modulus G,Young's modulus E,hardness Hv and Poisson's ratioσ,for Co3(AI,W)alloy,the addition of Ti and Nb in the W6site decreases its hardness hut increases its duetility.This work cnnfirms that Ti and Nh can stahilize the Cag(Al,W)alloy and have a positive effect in solving the relatively poor ductility of this alloy,which has important implications for the development of cobalt.based alloys. 相似文献
7.
The isothermal desorption of SiO from the Si(100) and Si(111) surfaces was investigated by means of optical second-harmonic generation (SHG). Due to the high adsorbate sensitivity of this method, desorption rates could be measured over a wide range from 10−1 to 10−6 ML s−1. From their temperature dependence between 780 and 1000 K, activation energies of EA=3.4±0.2 eV and EA=4.0±0.3 eV and pre-exponential factors of ν0=1016±1 s−1 and ν0=1020±1 s−1 for SiO desorption were obtained for Si(100) and Si(111), respectively. In the case of the Si(100) surface, a pronounced decrease of the first-order rate constants was observed upon increasing the initial coverage from 0.02 to 0.6 ML. The results are interpreted in terms of coverage-dependent oxygen-binding configurations, which influence the stability of the oxide layer. 相似文献
8.
The energy during the process of self-diffusion in BCC transition metals Fe, W, Mo, Cr, Ta, Nb and V has been calculated by using modified analytic embedded-atom method (MAEAM). For each kind of three diffusion mechanisms nearest-neighbor (NN), next-nearest-neighbor (NNN) and third-nearest-neighbor (TNN), the energy curve is symmetric and the maximum value of the energy appears at the middle point of the diffusion path. Determined mono-vacancy formation energy , migration energy and activation energy Q1v for self-diffusion agree well with available experimental data of NN diffusion and are better than those obtained by the analytic embedded-atom method (AEAM) and Finnis–Sinclair models. Compared the energies corresponding to three diffusion mechanisms, the NN diffusion needs the lowest activation energy (and thus the lowest migration energy). So that, the NN mono-vacancy diffusion is favorable in BCC transition metals. 相似文献
9.
Ammar Sarem Bogdan J. Kowalski Elias Al-Khoury Nehme 《Journal of Electron Spectroscopy and Related Phenomena》2007,160(1-3):58-61
The electronic structures of Cd0.88Fe0.12Se and CdSe have been investigated by photoemission yield spectroscopy (PYS) in the photon energy range from 5 to 12 eV. The () surfaces were obtained by cleavage under ultrahigh vacuum (UHV) conditions. An Fe-related emission appeared at 0.58 eV above the valence band edge. The freshly cleaved surface of Cd0.88Fe0.12Se interacted with ambient atmosphere more strongly than CdSe crystal. Leaving the sample in an UHV chamber at room temperature enabled us to identify surface related features and to observe decrease of the ionization energy Ei, energy threshold Ed and the crystal affinity χ due to change of the surface conditions. Effective density of states, derived from the experimental spectra of Cd0.88Fe0.12Se exhibits, in contrast with CdSe, a surface-related feature degenerated with the bulk valence band. 相似文献
10.
Epoxy resin (ER)/graphite nanosheet (GN) composites with a low percolation threshold (owing to particular geometry of GN with the high aspect ratio) were fabricated. The nonlinear conduction behavior of ER/GN composites above the percolation threshold by the action of variable DC electrical field was investigated. For specimens, the current density or current reduces with decreasing graphite nanosheets concentrations, and the J–E curves are well fitted by a cubic, J=σ1E+σ3E3. Moreover, the crossover current density Jc, at which nonlinearity takes place, scales with the linear conductivity σ1 as with x≈1.390 and the third-order conductivity, σ3, also scales with Jc as with y≈1.175. Through the discussion of the nonlinearity within the framework of two theoretical models, the nonlinear random resistor network (NLRRN) and the dynamic random resistor network (DRRN), it is indicated that neither of these two models can fully explain our experimental results. Taking into account the microscopic structures and conduction processes of the composites, it is likely that a combination of these two models explain the nonlinear characteristics better. 相似文献
11.
We study the low-lying scalar mesons of light u, d, s flavors in the QCD sum rule. Having all possible combinations of tetraquark currents in the local form, QCD sum rule analysis has been carefully performed. We found that using the appropriate tetraquark currents, the masses of σ, κ, f0 and a0 mesons appear in the region of 0.6–1 GeV with the expected ordering. The results are compared with that of the conventional currents, where the masses are considerably larger. 相似文献
12.
N.V. Voshchinnikov H.K. Das 《Journal of Quantitative Spectroscopy & Radiative Transfer》2008,109(8):1527-1535
We calculate the wavelength dependence of the ratio of the linear polarization degree to extinction (polarizing efficiency) P(λ)/A(λ) from the ultraviolet to near-infrared. The prolate and oblate particles with aspect ratios from a/b=1.1 up to 10 are assumed to be rotating and partially aligned with the mechanism of paramagnetic relaxation (Davis–Greenstein). Size/shape/orientation effects are analyzed. It is found that the wavelength dependence of P(λ)/A(λ) is mainly determined by the particle composition and size whereas the values of P(λ)/A(λ) depend on the particle shape, degree and direction of alignment. 相似文献
13.
The current experimental determination of the absolute values of the CKM elements indicates that 2|Vub/VcbVus|=(1−z), with z given by z=0.19±0.14. This fact implies that irrespective of the form of the quark Yukawa matrices, the measured value of the SM CP phase β is approximately the maximum allowed by the measured absolute values of the CKM elements. This is for , which implies =π/2. Alternatively, assuming that β is exactly maximal and using the experimental measurement sin(2β)=0.726±0.037, the phase γ is predicted to be γ=(π/2−β)=66.3°±1.7°. The maximality of β, if confirmed by near-future experiments, may give us some clues as to the origin of CP violation. 相似文献
14.
V.P. Aleshin 《Nuclear Physics A》2005,760(3-4):234-262
We study the dissipation rate in systems of nucleons bound by a slowly time-dependent mean-field potential and slightly interacting between themselves. Starting from the many-body linear response formula we evaluate an expression for in terms of the pure shell-model quantities and the nucleon–nucleon collision rate Γ. The application of the classical sum rule leads then to an expression for in terms of the classical-path integral with the weighting function including Γ. For vanishing Γ this expression reduces to the Koonin–Randrup Knudsen-gas formula. For simplified Skyrme interactions the classical approximation for the Γ itself is obtained. In leptodermous systems the classical-path expression for decomposes into the wall formula and the multiple-reflection term owing to incomplete randomization of particle motion between consecutive encounters with the boundary. The mean-free path and temperature dependence of dissipation is analyzed for small-amplitude distortions of spherical cavities. 相似文献
15.
The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of H-terminated Pb/p-Si/Al contacts fabricated by us have been measured in the temperature range of 77–300 K. The experimental values of the barrier height (BH) Φbo and the ideality factor n for the device range from 0.674 and 1.072 eV (at 300 K) to 0.352 and 2.452 eV (at 77 K), respectively. The ideality factors become larger with lowering temperature while the barrier height decreases. The Φbo(n) plot shows a linear dependence in the temperature range of 77–300 K that can be explained by the barrier inhomogeneity at the metal/semiconductor interface. The extrapolation of the linear Φbo(n) plot to n = 1 has given a homogeneous barrier height of approximately 0.713 eV for the Pb/p-Si(1 0 0) contact. A Φbo versus 1/T plot was drawn to obtain evidence of a Gaussian distribution of the BHs, and values of and σs = 80.5 mV for the mean BH and zero-bias standard deviation have been obtained from this plot, respectively. Then, a modified versus 1/T plot gives and A* as 0.828 eV and 54.89 A/cm2 K2, respectively. Furthermore, an average value of −0.687 meV/K for the temperature coefficient has been obtained, the value of −0.687 meV/K for hydrogen terminated p-type Si differs from those given for p-type Si without hydrogen termination in the literature. 相似文献
16.
The desorption kinetics of halogens from Nb, Ta, Mo and W polycrystalline surfaces are studied, at low coverage (Θ ? 10?2 of a monolayer) and high temperature (1800–2400 K), using a pulsed ionic beam method. For a relatively high energy incident positive ion beam (? = 2500 eV), the diffusion kinetics of implanted Br and I ions towards the surface are observed. At low energy (? < 500 eV), a first order kinetic corresponding to the desorption of halogens from these surfaces is found. Preliminary results are given on the desorption energy and the preexponential factor of halogens adsorbed on Nb, Mo, Ta and W polycrystalline surfaces. The main result of this systematic study is that the mean adsorption lifetime for a given temperature, as the desorption energy decreases for F through I on all of the studied surfaces. A similarity between Nb and Ta, as between Mo and W surfaces for the desorption energies of halogens is also found. 相似文献
17.
H. Tamura K. Hosomi T. Koike Y. Ma M. Mimori K. Miwa K. Shirotori M. Ukai 《Nuclear Physics A》2008,804(1-4):73-83
A series of hypernuclear γ-ray spectroscopy experiments using a germanium detector array, Hyperball, have accumulated precise data on various p-shell Λ hypernuclei. Recently, 12ΛC and 11ΛB were studied at KEK using the (π+,K+γ) reaction, and a transition from a directly-populated spin-flip state, 16ΛO(2−), was observed in BNL E930 data. It is discussed whether the spin-dependent ΛN interaction parameters determined from previous experiments can consistently explain other γ spectroscopy data. At the J-PARC facility, further hypernuclear γ-ray spectroscopy experiments are planned, particularly to measure the g factor of a Λ in a nucleus from a spin-flip B(M1) value and to investigate the ΛN interaction more in detail. 相似文献
18.
First-principles investigations on structural stability,mechanical, and thermodynamic properties of LaT_2Al_(20)(T = Ti,V,Cr,Nb,and Ta) intermetallic cage compounds
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
《中国物理 B》2018,(12)
First principles calculations were used to explore the structural stability, mechanical properties, and thermodynamic properties of LaT_2 Al_(20)(T = Ti, V, Cr, Nb, and Ta) intermetallics. The calculated formation enthalpy and phonon frequencies indicate that LaT_2Al_(20) intermetallics exhibit the structural stability. The elastic moduli(B, G, E, and Hv) indicate that these intermetallics possess the better elastic properties than pure Al. The values of Poisson's ratio v and B/G demonstrate that LaT_2Al_(20) intermetallics are all brittle materials. The anisotropy of elasticity and Young's modulus(three-and two-dimensional figures) indicate that LaT_2Al_(20) compounds are anisotropic. Importantly, the calculated thermal quantities demonstrate that LaT_2Al_(20) intermetallics possess the better thermal physical properties than pure Al at high temperatures. 相似文献
19.
Structures of S=−2 many-body states (double-Λ and Ξ hypernuclei) are investigated on the basis of G-matrix interactions derived from the recently-developed extended-soft-core model (ESC04d). ΛΛ–ΞN mixing effects in 6ΛΛHe, 5ΛΛH and 5ΛΛHe are investigated with use of three-body models. Possible Ξ hypernuclei are investigated systematically with Ξ-core folding potentials derived from ΞN G-matrix interactions. A four-body narrow Ξ0–Ξ− mixed state, specific to ESC04d, is discussed. 相似文献
20.
Glasses in the system Li2O–GeO2–P2O5 have been prepared using melt quenching route. Li+ ion transport has been investigated in these glasses using both d.c. and a.c. conductivity measurements. Arrhenius plots of d.c. conductivity exhibits two different slopes, which has been discussed in the light of cluster-tissue model. A.c. conductivity data has been fitted to single power law. Dielectric relaxation has been analyzed based on the behaviour of moduli. Good collapse on to master plots is observed for both a.c. conductivity and moduli. The behaviour of Edc, s and β are found to be consistent when conductivity mechanism is considered as involving NBO–BO switching as the first step, which is followed by Li+ ion migration. 相似文献