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11.
Luis M. Reyes Geraldo M. Ribeiro Benedicto J.O. Franco JoséLuiz A. Alves Manoel L. de Siqueira 《Solid State Communications》1981,40(12):1031-1034
We have interpreted the KCl:Cu? spectrum in the context of the MS-Xα method. The results, considering only allowed electric-dipole transitions, are in good agreement with the experimental data. Four new absorption bands, peaking at 573, 390, ~324 and ~310nm, at room temperature, are reported. 相似文献
12.
The propagation of light along the axis of non-homogeneity of a structurally chiral medium (SCM) under the influence of a low-frequency (dc) electric field aligned along the same axis is investigated. The Pockels effect is assumed to occur, and the SCM is taken to possess locally a point group symmetry. The frequency-domain Maxwell curl equations are cast in a 4 × 4 matrix representation, and the Oseen transformation is invoked. The band structure is analyzed, as are also the eigenvectors as well as the transfer matrix. Finally, the reflection and transmission of a plane wave due to a SCM of finite thickness is considered. The low-frequency electric field is shown to control the bandgap. 相似文献
13.
Mg-AZ91E/TiCp composite was fabricated using a spontaneous infiltration technique at 950 °C under an argon atmosphere. The composites produced have 37 vol.% of metal matrix and 63 vol.% of TiC-like reinforcement. The obtained composites were subsequently solution heat-treated at 413 °C during 24 h, cold water quenched, and subsequently artificially aged at 168 and 216 °C during 16 h in an argon atmosphere. Effect of heat treatment on the microstructure and mechanical properties was evaluated. Microstructural characterization was analyzed using different techniques such as X-ray diffraction (XRD) and scanning electron microscopy (SEM). Interface between matrix and reinforcement was examined using transmission electron microscopy (TEM), and mechanical properties were evaluated by measuring the elastic modulus and hardness. Mg, TiC, Al, and Mg17Al12 phases through XRD were detected. Meanwhile, using TEM analysis in heat-treated composites MgAl2O4, MgO, and Al2O3 were identified. The as-fabricated composite have elastic modulus and hardness of 162 GPa and 316 Hv, respectively. After solution heat treatment and aging at 168 °C during 12 h, the composites reaches values of 178 GPa and 362 Hv for the elastic modulus and hardness, respectively. Time of aging was correlated with measures of elastic modulus and hardness. 相似文献
14.
A DC glow discharge produced in N2 gas can generate several species that are important in different applications, such as the modification of surface properties of materials. A low-pressure glow discharge apparatus was used for the the analysis of the Ar–N2 mixture at a total pressure of 2.0 Torr, a power of 20 W and 40 l/min flow rate of gases. The emission bands were measured in the wavelength range of 200–1100 nm. The principal elements are N2, N 2+ and Ar I. The electron temperature was found in the range of 1.72–2.08 eV, and the ion density was in the order of 1010 cm?3. 相似文献
15.
Link JM Reyes M Yager PM Anjos JC Bediaga I Göbel C Magnin J Massafferi A de Miranda JM Pepe IM dos Reis AC Carrillo S Casimiro E Cuautle E Sánchez-Hernández A Uribe C Vazquez F Agostino L Cinquini L Cumalat JP O'Reilly B Ramirez JE Segoni I Butler JN Cheung HW Gaines I Garbincius PH Garren LA Gottschalk E Kasper PH Kreymer AE Kutschke R Bianco S Fabbri FL Zallo A Cawlfield C Kim DY Rahimi A Wiss J Gardner R Kryemadhi A Chung YS Kang JS Ko BR Kwak JW Lee KB Park H Alimonti G Boschini M 《Physical review letters》2002,88(16):161801
A high statistics measurement of the Lambda(+)(c) lifetime from the Fermilab fixed-target FOCUS photoproduction experiment is presented. We describe the analysis technique with particular attention to the determination of the systematic uncertainty. The measured value of 204.6 +/- 3.4 (stat) +/- 2.5 (syst) fs from 8034 +/- 122 Lambda(+)(c)-->pK(-)pi(+) decays represents a significant improvement over the present world average. 相似文献
16.
Sigmund EE Calder ES Thomas GW Mitrović VF Bachman HN Halperin WP Kuhns PL Reyes AP 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2001,148(2):309-313
We have studied the temporal instability of a high field resistive Bitter magnet through nuclear magnetic resonance (NMR). This instability leads to transverse spin decoherence in repeated and accumulated NMR experiments as is normally performed during signal averaging. We demonstrate this effect via Hahn echo and Carr--Purcell--Meiboom--Gill (CPMG) transverse relaxation experiments in a 23-T resistive magnet. Quantitative analysis was found to be consistent with separate measurements of the magnetic field frequency fluctuation spectrum, as well as with independent NMR experiments performed in a magnetic field with a controlled instability. Finally, the CPMG sequence with short pulse delays is shown to be successful in recovering the intrinsic spin--spin relaxation even in the presence of magnetic field temporal instability. 相似文献
17.
N. Segovia M. A. Armienta C. Valdes M. Mena J. L. Seidel M. Monnin P. Pea M. B. E. Lopez A. V. Reyes 《Radiation measurements》2003,36(1-6):379-383
Soil radon has been monitored at two fixed stations in the northern flank of Popocatepetl Volcano, a high risk volcano located 60 km SE from Mexico City. Water samples from three springs were also studied for radon as well as major and trace elements. Radon in the soil was recorded using track detectors. Radon in the water samples was evaluated using the liquid scintillation method and an Alphaguard. The major elements were determined through conventional chemical methods and trace elements using an ICP-MS equipment. Soil radon levels were low, indicating a moderate diffuse degassing through the flanks of the volcano. Groundwater radon had almost no relation with the eruptive stages. Water chemistry was stable in the reported time (2000–2002). 相似文献
18.
On Covariant Phase Space and the Variational Bicomplex 总被引:1,自引:0,他引:1
Enrique G. Reyes 《International Journal of Theoretical Physics》2004,43(5):1267-1286
The notion of a phase space in classical mechanics is well known. The extension of this concept to field theory however, is a challenging endeavor, and over the years numerous proposals for such a generalization have appeared in the literature. In this paper We review a Hamiltonian formulation of Lagrangian field theory based on an extension to infinite dimensions of J.-M. Souriau's symplectic approach to mechanics. Following G. Zuckerman, we state our results in terms of the modern geometric theory of differential equations and the variational bicomplex. As an elementary example, we construct a phase space for the Monge–Ampere equation. 相似文献
19.
Daniel M. Reis Ernesto Reyes‐Gómez Luiz E. Oliveira Carlos A. A. de Carvalho 《Annalen der Physik》2018,530(7)
The electromagnetic propagation in a relativistic electron gas at finite temperatures and carrier densities is described. Using quantum electrodynamics at finite temperatures, electric and magnetic responses and general constitutive relations are obtained. Rewriting the propagator for the electromagnetic field in terms of the electric and magnetic responses, the modes that propagate in the gas are identified. As expected, the usual collective excitations are obtained, i.e., a longitudinal electric and two transverse magnetic plasmonic modes. In addition, a purely photonic mode is found, which satisfies the wave equation in vacuum, for which the electron gas is transparent. Dispersion relations for the plasmon modes at zero and finite temperatures are presented and the intervals of frequency and wavelength where both electric and magnetic responses are simultaneously negative are identified, a behavior previously thought not to occur in natural systems. The investigation of the electromagnetic responses of a relativistic electron gas shows that, apart from the usual longitudinal electric plasmon mode and the two transverse magnetic plasmon modes, there is also a pure photonic mode that propagates with the speed of light, as if the medium were transparent. Furthermore, there is a region of frequencies and wavenumbers of the external fields where both the longitudinal electric permittivity and magnetic permeability are simultaneously negative, a property found in artificially constructed metamaterials. 相似文献
20.
The extensive production and application of engineered silica nanoparticles (SiO2 NPs) will inevitably lead to their release into the environment. Granular media filtration, a widely used process in water and wastewater treatment plants, has the potential for NP abatement. In this work, laboratory-scale column experiments were performed to study the transport and retention of SiO2 NPs on three widely used porous materials, i.e., sand, anthracite, and granular activated carbon (GAC). Synthetic fluorescent core-shell SiO2 NPs (83 nm) were used to facilitate NP detection. Sand showed very low capacity for SiO2 filtration as this material had a surface with limited surface area and a high concentration of negative charge. Also, we found that the stability and transport of SiO2 NP were strongly dependent on the ionic strength of the solution. Increasing ionic strength led to NP agglomeration and facilitated SiO2 NP retention, while low ionic strength resulted in release of captured NPs from the sand bed. Compared to sand, anthracite and GAC showed higher affinity for SiO2 NP capture. The superior capacity of GAC was primarily due to its porous structure and high surface area. A process model was developed to simulate NP capture in the packed bed columns and determine fundamental filtration parameters. This model provided an excellent fit to the experimental data. Taken together, the results obtained indicate that GAC is an interesting material for SiO2 NP filtration. 相似文献