Change In Internal Field Research Articles

Octahedral-shaped perovskite CaCu3Ti4O12 with dual defects and coexposed {(001), (111)} facets for visible-light photocatalysis

The finite light absorption and rapid charge carrier recombination are two major bottlenecks in semiconductor photolysis. Introducing defects and exposed facets at photocatalysts can significantly improve photoefficiency due to the synergetic effects of extending light response and effective usage of photoinduced charge carriers. However, the prime influential factors in surface defects and predominantly exposed facets that determine photocatalytic performance are still unclear. Herein, to combine the advantages of defects and exposed facets, we, for the first time, report a surfactant-free synthesis and boosted photolysis of a single-crystal CaCu3Ti4O12 octahedron with dual defects (oxygen and metal deficiencies of Cu+ and Ti3+) and coexposed ({001}, {111}) facets by a facile molten salt approach for visible light antibiotic decomposition. The utilized molten salt KCl acts as a structure directing agent, decreasing surface energy to form the octahedron shapes and coexposed facets. As-obtained sample displays a superior efficiency enhancement for antibiotic degradation, approximately 69 times faster than that of its counterpart, defect free CaCu3Ti4O12. Density functional theory (DFT) calculations and photodecomposition results show that existing of coexposed facets contributes to spatial separation of carriers, while dual defects improve light absorption, affect charge density and thus promote carrier transfer. The vacancy formation energies of oxygen and titanium defects on exposed facets were calculated. Moreover, dual defects, especially the Ti3+ species are greatly affected by defect dipoles of CaCu3Ti4O12 with distorted units of CuO4 and TiO6, in which TiO6 clusters make greater contributions on dipole-induced internal field changes. Finally, two concepts of "surface heterojunction" and "defect dipole" are revealed to unveil the synergetic roles of surface chemistry and defect engineering in tuning a photoactivity of crystal based inorganic semiconductors. With dual defects, predominantly coexposed facets and unique crystal shapes, this work reports a strategy and direction for rational synthesis of highly efficient visible light driven catalysts and provides new insights for unearthing the surface chemistry and defect engineering of inorganic photocatalysts.

Polarization of light backscattered by small particles

We study scattering of light by wavelength-scale spherical, cubic, and spheroidal particles as well as clusters of spherical particles for equal-volume-sphere size parameters 4 ≤ x ≤ 10 and refractive indices 1.1 ≤ m ≤ 2.0 . Such particles exhibit three specific features in the regime of backscattering: first, the intensity shows a backscattering peak; second, the degree of linear polarization for unpolarized incident light is negative; and, third, the depolarization ratio is double-lobed. We find that the overall characteristics of the scattering-matrix elements can be explained by an internal field composed of waves propagating in opposite directions near the particle perimeter and forming standing waves, as well as a wave propagating forward with the wavelength of the internal medium. When moving from the central axis of the particle toward its perimeter, the internal field changes from a forward-propagating wave with a wavelength dictated by the particle refractive index toward a standing wave with an apparent wavelength of the surrounding medium. The mapping of the internal field to the scattered far field is like an interference dial where rotation of the dial by a quarter of a wavelength on the particle perimeter results in a change from a destructive to constructive interference feature in the angular patterns (or vice versa). The dial is a manifestation of a well-known rule of thumb: the number of maxima or minima in the scattering-matrix elements is given by the size parameter. We explain the backscattering peak as deriving from the backward-propagating internal wave near the particle perimeter. Negative polarization follows from the spatial asymmetry of the internal fields: inside the particle, the fields are amplified near the central plane perpendicular to the polarization state of incident light, resulting in more pronounced interference effects for the perpendicular polarization than for the parallel polarization. The double-lobe feature in the depolarization results from the same internal-field structure with leading cross-polarized fields located slightly different from the copolarized fields. We discuss practical implications of these findings for the retrieval of particle sizes, shapes, and refractive indices from observations and laboratory experiments.

77Se NMR study of nonmagnetic–magnetic transition in (TMTSF)2X

77Se NMR measurements have been carried out on (TMTSF)2X (X=PF6 and AsF6) single crystals. For both compounds, NMR lines split into double-peaked spectra in the SDW state, which is explained with sinusoidal internal field at Se nucleus positions having the same incommensurate wave number with that of the SDW order. No change in the lineshape was observed at Tx at which the spin-relaxation rate shows a kink, suggesting that this anomaly does not cause significant static changes in internal field at the Se-site.

Gas Permeability of Fractured Sandstone/Coal Samples under Variable Confining Pressure

Rock fractures transmit underground gas effectively once they have sufficient widths and interconnection. However, the fracture geometries needed for gas transport are strongly influenced by surrounding pressure conditions. In order to inspect and quantify the influence of surrounding pressure, we design and manufacture a set of gas flow apparatus that can be connected to the MTS815 material testing system, which provides loads and exhibits external pressures in the experiment. With the apparatus and MTS815, we test the fractured samples of sandstone and coal and obtain their relationship between permeability and external pressure. In particular, our permeability calculation based on collection of gas flux and pressure difference has involved the influence of non-Darcian flow. In addition, our study also includes a numerical simulation in the RFPA software platform to display the internal field changes of cracks. The results show that fracture permeability strongly depends on confining pressure, and a critical pressure probably occurs, about 1.5–2 MPa in our experiments, to split each of the permeability curves into two stages, a slow climb and an exponential rush. As a complement, the numerical simulation also demonstrates one more stage for the permeability curve, the post-rush steady fluctuation.

Surface band-bending effects on the optical properties of indium gallium nitride multiple quantum wells

We report the use of selective wavelength excitation to examine the surface band-bending effects on the optical properties of 3.0-nm-thick indium gallium nitride (InGaN) multiple quantum wells (MQWs). Under a 355-nm excitation, the In0.18Ga0.82N well emission exhibits a linear dependence on the injected carrier density (Ninj) with a coefficient of (i) 8.5×10−18 meV cm3 for the spectral blueshift and (ii) 3×10−14 V cm2 for the change of internal field at a density up to Ninj∼1019 cm−3 at 77 K. When excited by a shorter wavelength at 248 nm, the emission from the thin GaN cap layer quenches, but that from the InGaN wells prevails. These observations are attributed to the transportation of photogenerated carriers from the bent GaN surface and redistribution in the InGaN wells. By solving the rate and Poisson equations with a Fermi-level pinning in the band-structure analysis, the emission from the InGaN/GaN MQWs is shown dominant by the recombination between the high-lying subbands and the screening of internal field effects.

Electroabsorption Spectroscopy ? Direct Determination of the Strong Piezoelectric Field in InGaN/GaN Heterostructure Diodes

We report on room temperature electroabsorption measurements on (In)GaN heterostructure p-i-n diodes. This technique allows a precise determination of the strong changes of internal fields at the InGaN/GaN hetero interface due to differences in the spontaneous polarisation and strain-induced piezoelectric fields. The measured fields vary between 1 and 1.4 MV/cm for thin InGaN layers with an In content ranging from approximately 7% to 9%. Absorption spectroscopy is also well suited to investigate the optical properties of GaN layers. In contrast to Si-doped or intrinsic material Mg-doped GaN films reveal a pronounced absorption tail below the band edge with absorption coefficients of about 30 cm -1 at typical emission wavelengths (λ 400 nm) of InGaN laser diodes.

Electroabsorption Spectroscopy — Direct Determination of the Strong Piezoelectric Field in InGaN/GaN Heterostructure Diodes

We report on room temperature electroabsorption measurements on (In)GaN heterostructure p-i-n diodes. This technique allows a precise determination of the strong changes of internal fields at the InGaN/GaN hetero interface due to differences in the spontaneous polarisation and strain-induced piezoelectric fields. The measured fields vary between 1 and 1.4 MV/cm for thin InGaN layers with an In content ranging from approximately 7% to 9%. Absorption spectroscopy is also well suited to investigate the optical properties of GaN layers. In contrast to Si-doped or intrinsic material Mg-doped GaN films reveal a pronounced absorption tail below the band edge with absorption coefficients of about 30 cm -1 at typical emission wavelengths (λ 400 nm) of InGaN laser diodes.

Magnetic domain patterns observed on synthetic Ti‐rich titanomagnetite as a function of temperature and in states of thermoremanent magnetization

The fundamental temperature dependence of magnetic domain structure has been recognized for several decades, but its relationship to thermoremanent magnetization (TRM) has been unexplored. As one step toward investigating this possible link, we have studied Bitter patterns on synthetic Ti‐rich titanomagnetite (Al0.1Mg0.1Ti0.6Fe2.2O4; Curie point of approximately 75°C) as a function of temperature in a weak field (0.42 Oe), in states of weak field TRM, and after alternating field demagnetization. Two different types of patterns were studied with the following goals: (1) simple Kittel‐like patterns of straight walls were studied in order to gain insight into the nature of TRM domain structures and the various possible mechanisms by which particles acquire weak field TRM; (2) mazelike patterns, which occupy regions of high surface stress, were studied in order to estimate the temperature dependence of magnetostriction constant in this material. Results of experiments to study Kittel‐like patterns suggest that two different mechanisms, and two resulting species of domain structure, could account for fundamental differences between multidomain and pseudosingle‐domain thermoremanence in titanomagnetite. In accordance with traditional models, typical multidomain TRM results from wall pinning in a particle that contains several domains whose volumes are approximately equal to each other. In contrast, pseudosingle‐domain TRM is acquired when denucleation of domains and domain walls yields a domain structure with a large associated “intrinsic” moment. The following observations support these conclusions. In a particle with a Kittel‐like pattern, the number of domains varies from one TRM experiment to the next. This variation indicates that a grain can acquire TRM in a range of local energy minimum states, rather than in one particular domain configuration of absolute minimum energy. Many such TRM patterns represent the near‐ideal local energy minimum domain states, consisting of several domains of approximately equal widths, usually envisioned in models of multidomain thermoremanence. Observations suggest that a particle can maintain such states during cooling if weakly pinned walls adjust readily to demagnetizing fields generated during the denucleation process. In this case, TRM results from small displacements of pinned walls with respect to a state of minimum magnetostatic energy; several observations suggest that blocking of walls at pinning centers in the classical manner occurs mainly after the final nucleation event as the particle cools toward room temperature from the Curie point. In contrast, we have observed examples in which denucleation leaves behind an anomalously large domain where several approximately uniform domains existed prior to denucleation; several smaller domains survive elsewhere in the particle, virtually unaffected by the transition. Evidently, strong wall‐pinning forces sometimes prevent surviving walls from responding to the change of internal field which must accompany the denucleation process. It is observed that denucleation can yield single‐domain‐like states during TRM acquisition. Thus denucleation, rather than complete failure to nucleate during TRM acquisition, is one mechanism for producing single‐domain‐like states in particles which ordinarily favor walls. Possibly, such states exist by virtue of strong pinning forces at surface defects, into which denucleating domains and domain walls collapse. Particles with an odd number of approximately uniform domains also are observed in TRM states, and such particles also may carry large intrinsic moments owing to domain imbalance; possibly, such domain configurations result from the final denucleation episode during cooling. Thus these observations indicate a strong link between TRM acquisition mechanisms, magnetic domain transitions, and the accessibility of domain states. We hypothesize that denucleation could account for the most desirable properties of TRM associated with pseudosingle‐domain titanomagnetite particles, namely, high intensity and high thermal stability. The second class of patterns, mazelike patterns, was studied to estimate the temperature dependence of domain wall energy and magnetostriction constant from the observed temperature dependence of domain width. The temperature dependence of magnetostriction constant obtained in this manner for AMTM60 is similar to that obtained for magnetite in earlier studies.

Rubber-modified epoxy resins: 3. Influence of filler on the dielectric relaxation properties

Dielectric measurements are reported on the effect of addition of glass (wool and beads) to a rubber modified epoxy resin. A parallel study of the influence of the addition of polydimethylsiloxane is also reported. It is found that there are significant shifts in the position of the dielectric relaxation associated with the acrylonitrile/butadiene phase. Analysis of the data indicates that the activation energy of the relaxation is little affected by the additives and the observed effects can be explained by the change in internal field brought about by the addition of filler through its effect on the permittivity of the material.

Solvent effects on some optical and spectroscopic properties of aromatic compounds

In this work we want of report some solvent effects observed during studies on benzenoid and heterocylic compounds by the Faraday effect and Depolarized Rayleigh Scattering (DRS), and their TCNE-complexes, by UV-visible spectrophotometry. The measured properties, respectively molecular magnetooptical rotation ([ϱ] M), molecular optical anisotropy (γ 2) and maximum absorption wavenumber ( − v max) undergo rather large changes when moving the gas phase to pure liquid or dissolved state, and vary weakly from a solvent to another one. As regards weak interactions, the Faraday effect allows essentially to observed internal field changes (Lorentz correction). It is likely that this bulk effect is not restricted to ‘aromatic’ solute. Moreover most solutesolvent interactions are masked by this non-specific medium effect, since even with polar solvents (CHCl 3, alcohols) no specific interaction given by π-donors and/or polar solute ( e.g. halobenzenes, anisole, …) is detected. So, for a given substance, the true value of [ϱ] M is obtained in the pure liquid state or in solvent as refractive as the solute. On the contrary, DRS, very sensitive to oriented interactions, can detect diatropism or paratropism in liquids. Therefore, to get comparable, ‘intrinsic’ γ 2 values one must now work with pure liquids but rather with dilute solutions in the same ‘inert’ solvent, that is a non-polar, weakly anisotropic one, containing bonds of the same kind as the solute. Accordingly, for organic compounds (H bonds), cyclohexane is preferred to CCl 4 (which may also give specific interactions). On the other hand the molar volume of the solvent need not be taken into account. The cases of benzene and some heterocyclics illustrate these principles. Finally, spectrophotometric studies of molecular complexes show that the influence of non-polar solvents on − v max generally reduces to a gross medium effect involving a linear function of (n 2 - 1)(2n 2 + 1). As regards polar solvents, more complex interactions with aromatic compounds and acceptor are likely to occur, and no correlation can be found with polarity parameters like Z or E T. TCNE-complexes of benzene, benzothiophene (after deconvolution of spectra) and dibenzothiophene are considered.

NMR Study in PdCo and PdNiCo Alloys

NMR of Co 59 has been performed in ferromagnetic PdCo alloys. The internal field changes from -215 kOe in pure Co metal to 230±5 kOe in alloys of Co diluted in Pd. The results are consistent with the previous measurements by nuclear polarization and Mössbauer effect experiments. The line width is very broad in alloys with low Co concentration. Even in 0.5% Co alloy, the width still has the value of 50 MHz. T 2 shows rapid decrease with decreasing Co concentration. The experiment has also been performed on Co 59 in ferromagnetic Pd x Ni 1- x Co system. The results are quite similar to those in PdCo alloys.

Application of Vibronic Model to Stress‐Induced Polarization of F‐Centre Emission

AbstractA vibronic model is applied to the stress‐induced polarization of the F‐center emission, and a comparison of calculations is made with experimental results by Hetrick and Compton [2]. A value obtained for a stress coupling constant with Eg, symmetry is consistent with parameters previously estimated from comparison of a model to experiments of magneticfield induced circular polarization and Stark effect. The temperature dependence of the stress‐induced polarization can be explained by assuming that an odd‐parity internal field changes with temperature.

Transition to paramagnetism in semiconductor plasmas

The internal axial magnetic field is calculated as a function of the amplitude of the helical oscillations. It is shown that for large enough amplitudes the internal field changes direction.

Field effects in insulating thin film secondary emitters

A horizontal shift in the collector characteristic curves suggests the presence of an electric field of 10 6V/cm or greater within insulating thin film secondary emitters. This assumption is supported by the fact that changes in secondary yield occur corresponding to changes in internal field. Furthermore, effects have been observed which suggest that the structure of insulating films may be an important factor relating to their properties because of the coplanar grid effect. This work was supported by the Wright Air Development Division, Air Research and Development Command, Wright-Patterson Air Force Base, Ohio, under Contract No. AF33(616)-6239.