Second-order Band Research Articles

Influence of the growth temperature on the first and second-order Raman band ratios and widths of carbon nanotubes and fibers

Multi-walled carbon nanotubes (MWCNT) and carbon nanofibers (CNF) were created using chemical vapor deposition at growth temperatures between 500 and 750°C, which have increasing crystallinity with increasing growth temperature. We used Raman spectroscopy to analyze the samples. The intensity ratios compared to the G-band, and full-width at half-maximum, of all observable Raman bands in both the first and second-order region were investigated. Good match was observed for the defect related bands of the MWCNT samples and data found in the literature. Several second-order bands display a strong dependency to growth temperature. Similar growth temperature (and thus defect) dependencies were found between several first and second-order bands, which might aid in determining the physical causes of these bands. CNF show much weaker Raman features due to their low crystallinity, making them more difficult to analyse. The results of this work are used to give recommendations on how to investigate MWCNT and CNF crystallinity using Raman spectroscopy. Finally, we demonstrate that Raman spectroscopy can be used to distinguish between the MWCNT root and tip growth mechanism.

Postcompensation for nonlinearity of Mach–Zehnder modulator in radio-over-fiber system based on second-order optical sideband processing

A postcompensation technique for nonlinearity of a Mach-Zehnder modulator in a radio-over-fiber (ROF) system is proposed and experimentally demonstrated based on second-order optical sideband processing and optical carrier band attenuation. The phase of the second-order optical sideband is shifted to suppress the third-order intermodulation distortion (IMD3) in a direct detection ROF link. The optical carrier band is attenuated to make two kinds of origins of IMD3 have equal intensity and cancel each other out. A spurious-free dynamic range of 124.8 dB·Hz(2/3) is achieved, which is about 25 dB more than that without compensation.

A SECOND-ORDER BPF USING A MINIATURIZED-ELEMENT FREQUENCY SELECTIVE SURFACE

A new type of low-proflle frequency selective surface (FSS) with an overall thickness of ‚=40 and a second-order band pass frequency response is presented. The proposed FSS is composed of two metal layers, separated by a thin dielectric substrate. Each layer is a two-dimensional periodic structure with sub-wavelength periodic unit cells. By printing the same topology on each side of the substrate, a second-order frequency response is realized. To provide a physical insight into the operating mechanism, equivalent circuit networks are also investigated in each step of design procedure. Using the proposal technique, low proflle and reduced sensitivity to angle of incident wave for both TE and TM polarizations are obtained and the overall thickness of the substrate is fairly thin. FSS samples are designed, fabricated, and installed in waveguide operating at X-band and a good agreement between the simulated and measured results is achieved.

Structural, electronic, elastic and thermodynamical properties of BaxSr1−xLiH3

In this paper, we aim to study the structural, electronic, elastic and thermodynamical properties of BaxSr1−xLiH3 using ab initio calculations within the generalized gradient approximation and local density approximation. In particular, the lattice constant, bulk modulus, second-order elastic constants (Cij) and electronic band structures are calculated and compared with the available experimental and other theoretical values. In addition, we have also predicted the variation of Young's modulus (E), Poisson's ratio (v), anisotropy factor (A), sound velocities, Debye temperature (θD) and melting temperature (Tm) as a function of the Ba concentration (x).

Elliptical pollen corona from North American boreal paper birch trees (Betula papyrifera): strong fall orientations for near-spherical particles

It has only recently been realized that solar corona can be generated by dispersions of tree pollen grains suspended in the atmosphere, and these studies have come almost exclusively from Scandinavia. Using corona photographic and surface pollen analyses, it is shown here that paper birch trees in the interior of Alaska regularly generate solar corona during the boreal green-out in mid-May. Although near-spherical in shape, these ~27 μm average diameter particles have three surface protrusions involved in germination that are indicated to aid in the generation of elliptical corona, for which a strong preferential particle orientation is needed. For observations at solar elevation angles of ~35°-40°, an axis ratio of about 1.2 and average radius of 2.5° (for the second-order red band) are found. Because oriented particles of a particular shape tend to fall slower than randomly oriented ones, this microdesign promotes the lateral spread of pollen and enhances tree reproductive opportunities, an especially important trait for pioneering species.

Double-diamond NaAl via pressure: Understanding structure through Jones zone activation

Under normal conditions, sodium forms a 1:1 stoichiometric compound with indium, and also with thallium, both in the double-diamond structure. But sodium does not combine with aluminum at all. Could NaAl exist? If so, under what conditions and in which structural types? Instead of beginning with a purely computational and first-principles structure search, we are led to apply the early Brillouin and higher (Jones) zone ideas of the physics determining structural selection. We begin with a brief recapitulation of the higher zone concept as applied to the stability of metals and intermetallic compounds. We then discuss the extension of this concept to problems where density becomes a primary variable, within the second-order band structure approximation. An analysis of the range of applicability of pressure-induced Jones zone activation is presented. The simple NaAl compound serves us as a numerical laboratory for the application of this concept. Higher zone arguments and chemical intuition lead quite naturally to the suggestion that 1:1 compound formation between sodium and aluminum should be favored under pressure and specifically in the double-diamond structure. This is confirmed computationally by density functional theoretic methods within the generalized gradient approximation.

THE DIFFERENTIAL DIFFERENCE OPERATIONAL FLOATING AMPLIFIER: NEW CMOS REALIZATIONS AND APPLICATIONS

This paper presents different novel CMOS realizations for the differential difference operational floating amplifier (DDOFA). The DDOFA was first introduced in Ref. 1 and was used to realize different analog circuits like integrators, filters and variable gain amplifiers. New CMOS realizations for the DDOFA are introduced in this literature. Furthermore the DDOFA is modified to realize a fully differential current conveyor (FDCC). Novel CMOS realizations of the FDCC are presented. The FDCC is used to realize second-order band pass–low-pass filter. Performance comparisons between the different realizations of the DDOFA and FDCC are given in this literature. PSPICE simulations of the overall proposed circuits are given using 0.25 μm CMOS Technology from TMSC MOSIS model and dual supply voltages of ±1.5 V.

Classifying nanostructured carbons using graphitic indices derived from Raman spectra

Raman scattering is known to be a unique characterization tool for carbon-based materials such as graphite, diamond, graphene and nanostructured carbons. In this article, we propose a new parameter for estimating the crystallinity of nanostructured carbons. This new parameter estimates the length of the curved graphene planes. To take into account the effect of curvature, the importance of the second-order band at about 2700 cm −1 is emphasized. Using this new parameter, we propose a classification of different nanostructured carbons. The relevance of this new parameter for characterizing nanostructured carbons is confirmed by transmission electron microscopy.

The structural, thermodynamical and elastic properties of TiO

In this paper, we have studied the structural, elastic, electronic and thermodynamical properties of TiO by performing ab initio calculations within the local density approximation (LDA). In particular, the lattice constant, bulk modulus, cohesive energy, phase transition pressure (Pt) from NaCl (B1) to CsCl (B2) structure, second-order elastic constants (Cij) and electronic band structures are calculated and compared with available experimental and other theoretical values. In order to gain further information, we have also predicted Young's modulus, Poisson's ratio (ν), anisotropy factor (A), sound velocities, Debye temperature (θD) and their pressure-dependent behaviours in the B1 phase.

Cholesteric helix: topological problem, photonics and electro-optics

Using numerical simulations and the results of our experiments we discuss the behaviour of a defect-free cholesteric liquid crystal in rather a strong electric field. The smooth helix unwinding predicted by de Gennes (Solid State Commun. 1968, 6, 163,) occurs under the thermodynamic equilibrium conditions and can easily proceed with the assistance of multiple defects. The defect-free helix in the field strictly perpendicular to the helical axis cannot be unwound for topological reasons. The field deforms the helix without a change of its period and induces higher harmonics of the helical structure. Due to the latter, the shape of the Bragg photonic stop-band changes and a second-order photonic band is observed in the optical transmission. Moreover, novel laser effects are predicted by modelling for the dye-doped, distorted cholesteric structure. The same field-induced anharmonicity results in a strong change of the polarisation state of the beam passing the helical structure along its axis. Due to fast relaxation of the higher harmonics, the switching time of the devices using the corresponding electro-optical effect (called in-plane switching) can be improved significantly. This is shown both experimentally and by numerical calculations.

Current mode high-frequency KHN filter employing differential class AB log domain integrator

In this study, a current mode log domain differential Class AB biquad filter based on Kerwin–Huelsman–Newcomb (KHN) structure has been synthesized by using the state-space method and by adopting translinear circuits. The proposed circuit can produce second-order low pass, band pass, high pass, all pass, and notch filter characteristics. The circuit is synthesized for high-frequency applications, i.e. around 100 MHz. The natural frequency and Q quality factor of the filter can be tuned electronically by varying the currents of current sources. Moreover, by varying currents of selected current sources, one can change the characteristics of the notch filter to generate general, low pass, and high pass notch filters. The designed circuit is simulated in both time domain and frequency domain in PSpice by using both idealized and NE 600 series type real transistors that are suitable for high-frequency operations. The frequency as well as time domain responses are found to be as expected. In addition to these simulations, THD and noise analysis are carried out. The details of obtained results are given.

Wave speeds, shear bands and the second-order work for incrementally nonlinear constitutive models

The paper discusses the consequences of the incremental nonlinearity of a constitutive model of a solid for the analyses of characteristic wave speeds, acceleration waves, the second-order work criterion and shear band formation. Incremental nonlinearity may entail qualitative changes in the results as compared to incrementally linear models. Certain well-known correlations cannot be established if the constitutive equation is assumed to be incrementally nonlinear. In particular, the spectra of the characteristic wave speeds and the acceleration wave speeds become continuous and are described by different equations. The second-order work criterion as a sufficient condition of uniqueness of the incremental boundary value problem loses its applicability in bifurcation analyses, unless the applicability can be proved for a particular type of nonlinearity. The singularity of the acoustic tensor in the general nonlinear case correlates neither with the vanishing of the second-order work nor with the shear band formation.

Universal current-mode biquad filter using dual output current differencing transconductance amplifier

A new universal current-mode biquad filter using current differencing transconductance amplifier (CDTA) is proposed. The proposed configuration can realize the second-order low pass, high pass, band pass, and band reject responses simultaneously. The circuit employs both grounded capacitors (ideal for integrated circuit implementation), and offers low active and passive sensitivities. The workability of the proposed configuration has been verified using SPICE simulation.

Effect of phosphorus content on structural properties of phosphorus incorporated tetrahedral amorphous carbon films

With phosphorus incorporated tetrahedral amorphous carbon (ta-C:P) films prepared using filtered cathodic vacuum arc technique with PH3 as the dopant source, we investigate the effect of phosphorus content on the structural properties of the films by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS analysis indicates that a function is established between the atomic fraction of phosphorus in the samples and the flow rate of PH3 during deposition, and that phosphorus implantation increases the graphite-like trihedral sp2 bonds deduced from fitted C 1s and P 2p core level spectra. Raman spectra of a broad range show that there are two notable features for all ta-C:P films: the first-order band centered at about 1560 cm-1 and the second-order band between 2400 and 3400 cm-1. The broad first-order band demonstrates that the amorphous structure of all samples does not remarkably change when a lower flow rate of PH3 is implanted, while a higher concentration of phosphorus impurity enhances the clustering of sp2 sites dispersed in sp3 skeleton and the evolution of structural ordering. Furthermore, the second-order Raman spectra confirm the formation of small graphitic crystallites in size due to a finite-crystal-size effect.

Slow to superluminal light waves in thin 3D photonic crystals

Phase measurements on self-assembled three-dimensional photonic crystals show that the group velocity of light can flip from small positive (slow) to negative (superluminal) values in samples of a few mum size. This phenomenon takes place in a narrow spectral range around the second-order stop band and follows from coupling to weakly dispersive photonic bands associated with multiple Bragg diffraction. The observations are well accounted for by theoretical calculations of the phase delay and of photonic states in the finite-sized systems.

Diameter dependence of second-order Raman features of graphene tubes

The second-order Raman band ( ω G ′ ) for single wall carbon nanotubes (SWNTs) in the range ∼2600–2700 cm −1 depends on the excitation energy and the diameter. However, the diameter dependence is not well established and understood. Here we propose that the two different physical dependencies are separable and can be taken into account by a phenomenological here derived equation. Using this methodology we have been able to find consistency between data for double-WNTs and large diameter SWNTs. Further, the determined diameter dependence of G′ correctly predicts the frequency for an infinitely large tube diameter, which is equivalent to a planar graphene sheet.

Shear banding analysis of geomaterials by strain gradient enhanced damage model

Shear band localization is investigated by a strain-gradient-enhanced damage model for quasi-brittle geomaterials. This model introduces the strain gradients and their higher-order conjugate stresses into the framework of continuum damage mechanics. The influence of the strain gradients on the constitutive behaviour is taken into account through a generalized damage evolutionary law. A weak-form variational principle is employed to address the additional boundary conditions introduced by the incorporation of the strain gradients and the conjugate higher-order stresses. Damage localization under simple shear condition is analytically investigated by using the theory of discontinuous bifurcation and the concept of the second-order characteristic surface. Analytical solutions for the distributions of strain rates and strain gradient rates, as well as the band width of localised damage are found. Numerical analysis demonstrates the shear band width is proportionally related to the internal length scale through a coefficient function of Poisson’s ratio and a parameter representing the shape of uniaxial stress–strain curve. It is also shown that the obtained distributions of strains and strain gradients are well in accordance with the underlying assumptions for the second-order discontinuous shear band boundary and the weak discontinuous bifurcation theory.

Statolith shape and microstructure as indicators of ontogenetic shifts in the squid Gonatus fabricii (Oegopsida, Gonatidae) from the Norwegian Sea

Statolith shape and microstructure were studied in 151 specimens of the common arctic squid Gonatus fabricii (7.3–322 mm pen length) collected in the southern Norwegian Sea. Statolith development and growth both comprised two main periods, which corresponded with the epipelagic and meso-bathypelagic ontogenetic periods of G. fabricii. During the epipelagic period (pen length range from 3 to 50–60 mm), statoliths quickly developed from the droplet-like form in early paralarvae to the pre-definite stage in juveniles (30–50 mm pen length). Paralarval and juvenile statoliths grew with high growth rates, and their microstructure contained narrow first-order growth increments. Three main growth zones (Z1, Z2, Z3) developed during this period, being well distinguished from each other by specific patterns of microstructure and separated from each other by distinct checks. During the meso-bathypelagic period (from 50–60 to 322 mm pen length), statoliths hardly changed their shape and grew very slowly. Only one growth zone (Z4) was formed within the statolith microstructure, characterized by disappearance of the first growth increments and formation of specific second-order bands. Each second-order band consisted of approximately seven first-order increments. If the assumption “one increment-one day” is true for G. fabricii, the squid would then be a slow-growing animal with a life span for both sexes not exceeding 2 years.

Raman Scattering in the Intermetallic Compound PtGa2

Raman seattering from the (111) surface of a single-crystal PtGa 2 sample was measured at 300 K. The first-order spectrum yields energy for the k 0 optical phonon mode of 118 cm -1 . The full width at half intensity was found to be 3 cm -1 . The second-order Raman seattering band was observed at 236 cm -1 . These values are compared with those obtained from the isostructural compounds AuAl 2 , AuGa 2 , and AuIn 2 .

New current-mode universal biquad filter

A new current-mode universal biquadratic filter using only two second generation current conveyors and a minimum number of passive components is presented. The proposed configuration can realize the second-order low pass, high pass, band pass and band reject responses simultaneously. The validity of the proposed formulation is confirmed by PSpice simulation results.