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The Sn-doping effects on the electrical conduction and optical properties of GaAs(631)A epilayers grown by molecular beam epitaxy were investigated. We found that the conduction type conversion, frequently observed in the doping of layers grown on high-index substrates, is avoided when tin-doping is implemented. The maximum free-carrier concentration (n) obtained in GaAs(631):Sn was 2 × 1019 cm−3, an order of magnitude higher than previously reported for GaAs(631):Si, and within the same order of magnitude for the growth of GaAs(100):Si. The electron mobility was suitable for many optoelectronic applications. Raman spectroscopy showed low lattice disorder in (631) oriented samples, compared with singular (100) samples. The photoluminescence characterization of the samples revealed the blueshift of the optical transitions close to E0 associated with the Moss–Burstein effect for Sn doping. Photoreflectance spectroscopy was used to study the doping properties at the critical points E1 and E1 + Δ1, where the major affectation with n was perceived in the broadening parameter Γ. [ABSTRACT FROM AUTHOR]

This paper examines the feasibility of cloaking an obstacle using Plate-type Acoustic Metamaterials (PAMs). We present two distinct strategies to cloak this obstacle, using either the near-zero-density regime of a periodic arrangement of plates or the acoustic doping phenomenon to achieve simultaneous zero-phase propagation and impedance matching. The strong limitations induced by viscothermal and viscoelastic losses that cannot be avoided in such a system are studied. A hiding zone is reported analytically, numerically, and experimentally. In contrast to cloaking, where zero-phase propagation must be accompanied by total transmission and zero reflection, the hiding configuration requires that the scattering properties of the hiding device must not be affected by the presence of the obstacle embedded in it. Contrary to cloaking, the hiding phenomenon is achievable even with a realistic PAM possessing unavoidable losses. [ABSTRACT FROM AUTHOR]

The strain distribution and the electronic band structure of InAs quantum dots (QDs) embedded in asymmetrical (Al)GaAs barriers were studied by numerical analysis based on the finite element method. The outlines of the structures were designed considering experimental outcomes such as QDs morphology, wetting layer thickness, and the composition of the materials observed for the molecular beam epitaxial growth and capping of InAs/(Al)GaAs samples. The Al content in the AlGaAs alloy encapsulating material prompted variations on the island's shape, so regular and truncated pyramidal QDs were simulated. According to the simulations, higher values of positive biaxial strain tensor εxx were obtained above the apex zone in pyramidal QDs as compared to truncated ones. The heavy hole and light hole bands intercalated relative positions along the internal QDs profile, a consequence of the compressive and tensile strain distribution inside the pyramidal QDs. The biaxial strain and the elastic energy analyzed above the apex zone and below the islands are important for the vertical correlation probability, and we found dependence on the shape of the nanostructure and the distance from the top of the islands to the surface spacer. Finally, those nanoislands for which the capping procedure did not change the geometry, showed a higher number of confined eigenstates, which is required for many optoelectronic applications. [ABSTRACT FROM AUTHOR]

Stealthy hyperuniform point patterns are characterized by a vanishing spatial Fourier transform around the origin of the reciprocal vector space. The long-range point density fluctuations are suppressed as well in materials consisting of such distribution of scatterers, opening up opportunities to control waves. Beside wave transport in such structured materials are driven by several elements, such as the acoustic properties of the host material, the scatterer characteristics, i.e., dimensions or resonant features, and the scatterer distribution patterns. The effects of these three basic elements on the wave transport properties are usually hard to discriminate. In this work, we analyze the transport properties of acoustic waves in one-dimensional phononic materials constituted of either non-resonant or resonant scatterers distributed along stealthy hyperuniform patterns in air. The pattern is controlled by the stealthiness, allowing us to continuously vary from random phononic materials to phononic crystals. The properties of the scatterers are controlled by their size and/or the resonant frequencies. The properties of the host material are controlled by the viscothermal losses. Transport properties of stealthy hyperuniform materials are found to be robust to both the scatterer dimensions and inherent viscothermal losses, while strongly affected by the scatterer resonances, which introduce sharp dips in the transmission coefficient. [ABSTRACT FROM AUTHOR]

Sound diffusion refers to the ability of a surface to evenly scatter sound energy in both time and space. However, omni-directional radiation of sound, or perfect diffusion, can be impractical or difficult to reach under traditional means. This is due to the considerable size required, and the lack of tunability, of typical quarter-wavelength scattering strategies necessary for producing the required complexity of the surface acoustic impedance. As such, it can be a challenge to design sound diffusing structures that can display near perfect diffusion performance within slim dimensions. In this work, we propose a method for obtaining quasi-perfect and broadband sound diffusion coefficients using deep-subwavelength acoustic diffusers, i.e., metadiffusers. The relation among the geometry of the metasurface, the bandwidth, and the diffusion performance is analytically and numerically studied. For moderate bandwidths, around 1/3 of an octave, the method results in nearly perfect sound diffusion, while for a bandwidth of 2.5 octaves, a normalized diffusion coefficient of 0.8 was obtained using panels 1/30th thinner than traditional phase-grating designs. The ratio between the wavelength and the size of the unit cell was identified as a limitation of the performance. This work demonstrates the versatility and effectiveness of metadiffusers to generate diffuse reflections outperforming those of classical sound diffusers. [ABSTRACT FROM AUTHOR]

A theoretical and experimental study of the propagation of sound beams in- and behind three-dimensional sonic crystals at frequencies close to the band edges is presented. An efficient collimation of the beam behind the crystal is predicted and experimentally demonstrated. This effect could allow the design of sources of high spatial quality sound beams. [ABSTRACT FROM AUTHOR]

The physical properties of a periodic distribution of absorbent resonators is used in this work to design a tunable wideband bandstop acoustic filter. Analytical and numerical simulations as well as experimental validations show that the control of the resonances and the absorption of the scatterers along with their periodic arrangement in air introduce high technological possibilities to control noise. Sound manipulation is perhaps the most obvious application of the structures presented in this work. We apply this methodology to develop a device as an alternative to the conventional acoustic barriers with several properties from the acoustical point of view but also with additional esthetic and constructive characteristics. [ABSTRACT FROM AUTHOR]

Evanescent modes in complete sonic crystals (SCs) and SC with point defects are reported both theoretically and experimentally in this paper. Plane wave expansion (PWE) and in general, ω(k) methods have been used to calculate band structures showing gaps that have been interpreted as ranges of frequencies where no real k exists. In this work, we extend PWE to solve the complex k(ω) problem applied to SC, introducing the supercell approximation for studying one vacancy. Explicit matrix formulation of the equations is given. This k(ω) method enables the calculation of complex band structures, as well as enabling an analysis of the propagating modes related with real values of the function k(ω), and the evanescent modes related with imaginary values of k(ω). This paper shows theoretical results and experimental evidences of the evanescent behavior of modes inside the SC band gap. Experimental data and numerical results using the finite elements method are in very good agreement with the predictions obtained using the k(ω) method. [ABSTRACT FROM AUTHOR]

We theoretically, numerically, and experimentally analyze the Density-Near-Zero (DNZ) regime of a one-dimensional acoustic metamaterial. This acoustic metamaterial is composed of thin elastic plates periodically clamped in an air-filled waveguide, and the effective dynamic zero mass density is obtained from the strong dispersion around the bandgaps associated with the resonances of the plates. We emphasize the importance of the impedance mismatch between the acoustic metamaterial and the surrounding waveguide at the frequency of the zero effective density in addition to the consequences of the inherent losses. As a result, the frequency of the zero phase propagation, i.e., the acoustic propagation with zero phase delay, is not exactly the frequency of the zero density and lies in the frequency bandgap where the effective density is negative. Considering these limitations, the zero phase propagation is still experimentally observed and a subwavelength acoustic dipole is numerically designed, thus demonstrating the possible realistic implementations of DNZ acoustic metamaterials. [ABSTRACT FROM AUTHOR]

An acoustic metadiffuser is a subwavelength locally resonant surface relying on slow sound propagation. Its design consists of rigidly backed slotted panels, with each slit being loaded by an array of Helmholtz resonators. Due to the slow sound properties, the effective thickness of the panel can therefore be dramatically reduced when compared to traditional diffusers made of quarter-wavelength resonators. The aim of this work is to experimentally validate the concept of metadiffusers from the scattering measurements of a specific metadiffuser design, i.e., a quadratic residue metadiffuser. The experimental results reported herein are in close agreement with analytical and numerical predictions, therefore showing the potential of metadiffusers for controlling sound diffusion at very low frequencies. [ABSTRACT FROM AUTHOR]

This work theoretically analyzes the sound absorption properties of a chirped multi-layer porous material including transmission, in particular showing the broadband unidirectional absorption properties of the system. Using the combination of the impedance matching condition and the balance between the leakage and the intrinsic losses, the system is designed to have broadband unidirectional and quasi perfect absorption. The transfer and scattering matrix formalism, together with numerical simulations based on the finite element method are used to demonstrate the results showing excellent agreement between them. The proposed system allows to construct broadband sound absorbers with improved absorption in the low frequency regime using less amount of material than the complete bulk porous layer. [ABSTRACT FROM AUTHOR]

Asymmetric propagation of acoustic waves is theoretically reported in a chirped phononic crystal made of the combination of two different nonlinear solids. The dispersion of the system is spatially dependent and allows the rainbow trapping inside the structure. Nonlinearity is used to activate the self-demodulation effect, which is enhanced due to the particular dispersion characteristics of the system. The performed numerical study reveals an efficient generation of the demodulated wave, up to 15% in terms of the pressure amplitude, as well as strong attenuation for undesired frequency components above the cut-off frequency. The obtained energy rectification ratio is in the order of 104 for the whole range of amplitudes employed in this work, indicating the robustness of the asymmetry and non-reciprocity of the proposed device for a wide operational range. [ABSTRACT FROM AUTHOR]

Using the concepts of slow sound and critical coupling, an ultra-thin acoustic metamaterial panel for perfect and quasi-omnidirectional absorption is theoretically and experimentally conceived in this work. The system is made of a rigid panel with a periodic distribution of thin closed slits, the upper wall of which is loaded by Helmholtz Resonators (HRs). The presence of resonators produces a slow sound propagation shifting the resonance frequency of the slit to the deep subwavelength regime (λ=88). By controlling the geometry of the slit and the HRs, the intrinsic viscothermal losses can be tuned in order to exactly compensate the energy leakage of the system and fulfill the critical coupling condition to create the perfect absorption of sound in a large range of incidence angles due to the deep subwavelength behavior. [ABSTRACT FROM AUTHOR]

The reflection of sound of frequency below 1 kHz, by a rigid-backed structure that contains subwavelength resonators is studied in this work. In particular, only single mode reflected waves are considered, an approximation which is accurate in this low frequency regime. A method of analysis of absorption that uses the structure of the reflection coefficient in the complex frequency plane is proposed. In the absence of losses, the reflection coefficient supports pairs of poles and zeros that are complex conjugate and which have imaginary parts linked to the energy leakage by radiation. When losses are introduced and balanced to the leakage, the critical coupling condition is satisfied and total absorption is obtained. Examples of a slot resonator and of multiple Helmholtz resonators are analyzed to obtain both narrow and broadband total absorption. [ABSTRACT FROM AUTHOR]

Energetically unstable crystalline surfaces, among their uses, can be templates for the growth of periodic arrays of one-dimensional (1D) nanoscale structures. However, few studies have explored self-assembled faceting on high-index (HI) planes inside the stereographic triangle, and extant studies have not produced any criteria for encouraging the formation of one-dimensional periodic arrays. In this Letter, by analyzing the MBE growth of homoepitaxial facets on (631)A GaAs, a HI plane inside the triangle, we present a criteria to produce highly uniform 1D periodic arrays on unexplored surfaces. These families of planes are those belonging to the lines connecting the energetically stable HI GaAs (11 5 2) plane with any of the (100), (110), and (111) planes at the corners of the stereographic triangle. This novel strategy can lead to new possibilities in self-assembling 1D structures and manipulating physical properties, which in turn may result in new HI- and 1D-based experiments and devices. [ABSTRACT FROM AUTHOR]

We experimentally report perfect acoustic absorption through the interplay of the inherent losses and transparent modes with high Q factor. These modes are generated in a two-port, one-dimensional waveguide, which is side-loaded by isolated resonators of moderate Q factor. In symmetric structures, we show that in the presence of small inherent losses, these modes lead to coherent perfect absorption associated with one-sided absorption slightly larger than 0.5. In asymmetric structures, near perfect one-sided absorption is possible (96%) with a deep sub-wavelength sample (π=28, where π is the wavelength of the sound wave in the air). The control of strong absorption by the proper tuning of the radiation leakage of few resonators with weak losses will open possibilities in various wave-control devices. [ABSTRACT FROM AUTHOR]

We report the nonlinear focusing of ultrasonic waves by an axisymmetric diffraction grating immersed in water. In the linear regime, the system presents high focal gain (32 dB), with a narrow beam-width and intense side lobes as it is common in focusing by Fresnel-like lenses. Activating the nonlinearity of the host medium by using high amplitude incident waves, the focusing properties of the lens dramatically change. Theoretical predictions show that the focal gain of the system extraordinary increases in the strongly nonlinear regime (Mach number of 6.1×10-4). Particularly, the harmonic generation is locally activated at the focal spot, and the second harmonic beam is characterized by strongly reduced side-lobes and an excellent beam profile as experiments show in agreement with theory. The results can motivate applications in medical therapy or second harmonic imaging. [ABSTRACT FROM AUTHOR]

The enhancement of sound inside a two dimensional exponentially chirped crystal during the soft reflections of waves is experimentally and theoretically explored in this work. The control of this enhancement is achieved by a gradual variation of the dispersion in the system by means of a chirp of the lattice constant. The sound enhancement is produced at some planes of the crystal in which the wave is softly reflected due to a progressive slowing down of the sound wave. We find that the character of the sound enhancement depends on the function of the variation of dispersion, i.e., on the function of the chirp. A simple coupled mode theory is proposed to find the analytical solutions of the sound wave enhancement in the exponentially chirped crystal. Harmonic and time domain numerical simulations are performed to interpret the concept of the soft reflections, and to check the analytically calculated field distributions both in good agreement with experiments. Specially we obtain stronger sound enhancement than in linearly chirped crystals. This sound enhancement could motivate applications in energy harvesting, e.g., to increase the efficiency of detectors and absorbers. [ABSTRACT FROM AUTHOR]

We study the propagation of waves in a periodic array of absorbing layers. We report an anomalous increase of wave transmission through the structure related to a decrease of the absorption around the Bragg frequencies. The effect is first discussed in terms of a generic coupled wave model extended to include losses, and its predictions can be applied to different types of waves propagating in media with periodic modulation of the losses at the wavelength scale. The particular case of sound waves in an array of porous layers embedded in air is considered. An experiment designed to test the predictions demonstrates the existence of the enhanced transmission band. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a mechanism of sound wave concentration based on soft reflections in chirped sonic crystals. The reported controlled field enhancement occurs at around particular (bright) planes in the crystal and is related to a progressive slowing down of the sound wave as it propagates along the material. At these bright planes, a substantial concentration of the energy (with a local increase up to 20 times) was obtained for a linear chirp and for frequencies around the first band gap. A simple couple mode theory is proposed that interprets and estimates the observed effects. Wave concentration energy can be applied to increase the efficiency of detectors and absorbers. [ABSTRACT FROM AUTHOR]

The authors report a low-temperature photoluminescence (PL) study of multiple GaAs layers grown between AlAs(0.6 nm)/GaAs(0.6 nm) short-period superlattice barriers (SLBs) simultaneously grown on both GaAs(631)A and (100) substrates. Five GaAs-layers of different nominal thicknesses (LW, ranging from 12 to 2.4 nm) were grown by molecular beam epitaxy. By using (631)A-oriented substrates a self-organized and highly ordered corrugation is obtained in the growth of the GaAs layers, and at the end of the SLB growth, flat surfaces are found. Whereas, for the (100)-oriented sample, flat interfaces are confirmed after the growth of GaAs and SLB layers. By reducing LW below ∼3.6 nm in the (631) sample, strong quantum wire (QWR)-like confinement is achieved as deduced from polarized PL spectroscopy where polarization degrees as large as 0.43 are obtained. The PL emission energy of the (631)-QWRs is redshifted, as compared with the transitions of the (100)-oriented quantum wells, when LW is reduced. The authors explain this energy shift by the widening of the effective thickness of the confinement regions in the GaAs layers. [ABSTRACT FROM AUTHOR]

This work studies the Si-doping of GaAs on (631)-oriented substrates as a function of the As4-beam equivalent pressure (PAs). The electrical properties obtained by Hall effect measurements show that the mobility of the layers grown on (631)-substrates present changes related to carrier compensation processes and the Si-doping changes from p- to n-type when PAs is increased. The optical properties of the samples, as observed by photoluminescence (PL) spectroscopy, modified according to the electrical characteristics. For (631)-samples, when PAs is increased the energy of the maximum intensity PL peak redshifts in the p-type region but, after the threshold of the conduction type conversion, the peak blueshifts in the n-type region. The variation of the PL excitation intensity also shifts the emission energy of the samples as a consequence of the increasing recombination rate for close pairs in donor acceptor pair recombination. Photoluminescence as a function of temperature shows that the activation energy of the PL lines transition is enhanced as PAs is increased. [ABSTRACT FROM AUTHOR]

An exhaustive study has been made into the potential improvement in attenuation and focusing of phononic crystal arrays resulting from the deliberate creation of vacancies. Use is made of a stochastic search algorithm based on evolutionary algorithms called the epsilon variable multi-objective genetic algorithm which, in conjunction with the application of multiple scattering theory, enables the design of devices for effectively controlling sound waves. Several parameters are analyzed, including the symmetries used in the distribution of holes and the optimum number of holes. The validity and utility of the general rules obtained have been confirmed experimentally. [ABSTRACT FROM AUTHOR]

In this work, the photoluminescence (PL) properties of Be-doped Al0.2Ga0.3In0.5P layers grown on GaAs by solid source molecular beam epitaxy were studied as a function of temperature. Three samples were grown varying the P-cell cracker temperature (PCT). The Be-cell temperature was 1015 °C for all samples. The 77 K PL spectra from samples show three main transitions located at 2.182, 2.162, and 2.065 eV, related to band-to-band emission (B,B), Be acceptor levels (A0,X) and oxygen deep levels (O,DL), respectively. The authors observed that for the sample grown at the highest PCT the (O,DL) transition dominates the whole 77 K PL spectra, while for the sample grown at intermediate PCT it appears with lower intensity. This peak is not observed in the PL spectra for the layers grown at PCT equal to 800 °C. The authors monitored the evolution with temperature of (O,DL) and observed that it is located from 87 to 135 meV below the conduction band when the temperature is varied from 15 to 120 K. After 120 K, this transition disappears. The NA-ND carrier concentration measured with an ECV profiler shows a diminution of the active Be doping concentration as PCT increases as a consequence of compensation of Be dopants by donorlike O traps. This phenomena is reflected in the PL properties of the samples as a reduction of the intensity of (A0,X). Also, the 300 K PL spectra for the samples shows that by increasing PCT from 800 to 1000 °C, the PL intensity of the (B,B) transition decreased dramatically due to the introduction of oxygen-related nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

The authors report the molecular beam epitaxy growth and characterization of AlGaAs/GaAs(631) heterostructures grown at different As4 molecular beam equivalent pressures. The reflection high-energy electron diffraction patterns taken along the [-1 2 0] azimuth showed that the twofold reconstruction commonly observed during the GaAs-buffer layer growth is preserved during the AlGaAs deposition. The 10 K photoluminiscence (PL) characterization of the samples showed transitions related to the AlGaAs band edge, the incorporation of impurities, and deep centers. The temperature dependence of the band to band PL transition was fitted with the models developed by Varshni, Pässler, and Viña. The intensity of the PL spectra drastically decreases as the As4 pressure is increased. Photoreflectance (PR) spectroscopy also showed the best crystal quality for the sample grown at low As pressure. The authors obtained the built-in internal electric field strength and the band-gap energy from an analysis of the PR spectra close to the GaAs band edge employing the Franz-Keldysh model. [ABSTRACT FROM AUTHOR]

A phenomenological model that simulates the acoustic attenuation behavior of sonic crystals is developed in this paper. The input of the model is a set of parameters that characterizes each experimental setup, and the output is a simulation of the associated attenuation spectrum. The model consists of a combination of a multiresolution analysis based on wavelet functions and a set of artificial neural networks. An optimized coupling of these tools allows us to drastically reduce the experimental data needed, and to obtain a fast computational model that can be used for technological purposes. [ABSTRACT FROM AUTHOR]

We have studied the GaAs growth on (631) oriented substrates by molecular beam epitaxy (MBE). Different samples were prepared by varying the growth temperature and the III/V equivalent pressure ratio. We observed by atomic force microscopy a high density of hilly like features elongated towards the [-5,9,3] direction formed during the MBE growth. The growth temperature dependence of the hillock length and width follows an Arrhenius-type behavior with activation energies of 1.4 and 0.5 eV, respectively. The hillock formation is discussed in terms of adatom diffusion anisotropy and diffusion barriers. Employing photoreflectance spectroscopy we found a splitting of the GaAs band gap energy transition that increases with the hillock density. [ABSTRACT FROM AUTHOR]

We show both experimentally and theoretically the evanescent behavior of modes in the band gap of finite phononic crystal (PC). Based on experimental and numerical data we obtain the imaginary part of the wave vector in good agreement with the complex band structures obtained by the extended plane wave expansion. The calculated and measured acoustic field of a localized mode out of the point defect inside the PC presents also evanescent behavior. The correct understanding of evanescent modes is fundamental for designing narrow filters and waveguides based on PCs with defects. [ABSTRACT FROM AUTHOR]

An improvement in the attenuation capabilities of acoustic metamaterials by means of the creation of defects is considered here as a multiobjective optimization problem. From this point of view, it is possible to define the optimum strategy in the creation of defects to achieve an important increase in acoustic attenuation in a predetermined range of frequencies. A powerful multiobjective optimization algorithm called evMOGA has been used to solve this problem. The study has been restricted to the case of a two-dimensional sonic crystal formed by rigid cylinders in air, the defects being vacancies in the initial structure. [ABSTRACT FROM AUTHOR]

Sonic crystals are periodic structures that have acoustic band gaps centred at frequencies depending on the lattice constant of the array and on the direction of the incident acoustic wave. To eliminate this dependence, this work presents designed mixed structures constructed with rigid scatterers and resonators embedded in air. Specifically, balloons filled with a blend of air and helium were used as resonators, showing experimental evidence about the resonant behavior of an array formed with these balloons. As a result, the authors obtain full band gaps in a predetermined range of frequencies desired. [ABSTRACT FROM AUTHOR]

It is well known that sonic crystals are periodic structures that present acoustic band gaps attenuation centered at frequencies related with the lattice constant of the structure. We present an approach based on genetic algorithms to create band gaps in a predetermined range of frequencies. The mechanism used by genetic algorithms to achieve this objective is the creation of vacancies in a starting sonic crystal, built with air-embedded rigid scatterers. As a result of the optimization process, we obtain quasiordered structures on which acoustic properties are based in the multiple scattering theory. [ABSTRACT FROM AUTHOR]

The symmetry matching between the source and the lens results in fundamental interest for lensing applications. In this work, we have modeled an axisymmetric gradient index (GRIN) lens made of rigid toroidal scatterers embedded in air considering this symmetry matching with radially symmetric sources. The sound amplification obtained in the focal spot of the reported lens (8.24 dB experimentally) shows the efficiency of the axisymmetric lenses with respect to the previous Cartesian acoustic GRIN lenses. The axisymmetric design opens new possibilities in lensing applications in different branches of science and technology. [ABSTRACT FROM AUTHOR]

We report on the self-assembly of large-order-correlated nanoscale faceting on GaAs(631)A substrates grown by molecular beam epitaxy. The surface morphology of the grown samples as a function of the growth temperature and the As-beam equivalent pressure was studied using atomic force microscopy. A two-dimensional autocorrelation function analysis was performed in order to quantitatively determine the uniformity of the surface corrugation. By optimizing the growth conditions, correlated faceted areas as large as 1.7 × 1.7 μm2 are obtained. The highly ordered surface corrugation discussed here provides useful insights to prepare highly ordered facet planes for the self organized growth of quantum wires. [ABSTRACT FROM AUTHOR]

The authors report the photoluminescence and photoreflectance characteristics of molecular beam epitaxy grown InAs nanostructures on GaAs (631)-oriented substrates. Prior to the InAs growth, self-assembled nanochannels on the GaAs buffer layer were formed, which later were used as templates for nanostructures formation. Different samples were prepared by varying the amount of InAs from 0.75 to 2 ML (monolayer), 50 nm of GaAs was grown on top as a capping layer. Low temperature photoluminescence spectroscopy showed intense optical transitions in the spectra, their energy position directly depends on the quantity of InAs deposited. The self-assembling of InAs quantum wires (QWRs) at the early stages of growth is suggested. Anisotropy effects in the InAs nanostructures were corroborated by polarized photoluminescence supporting the proposal of formation of QWRs. Photoreflectance spectroscopy at room temperature was also employed to characterize the samples. It is found that in addition to the band-gap energy transition, features associated with quantum confinement in the wetting layer were observed even for very low quantities of InAs deposited. [ABSTRACT FROM AUTHOR]

The authors have investigated the optical properties of AlxGa1-xAs/GaAs modulation-doped heterostructures grown under different experimental conditions, mainly by changing the molecular species of the As beam stemming from a cracker cell at different temperatures. By low-temperature photoluminescence (PL) spectroscopy, they observed that using As2 instead of As4, the crystal quality of the ternary alloy improves significantly since narrower lines in the near-band-gap energy of AlGaAs were obtained when the temperature of the As-cracker zone increased. The experimental position of the energy of the ternary band gap transition as a function of temperature fits the Varshni and Viña functional forms satisfactorily, while it is difficult to assess the data for those samples grown employing As4. It was found that the As2 molecular beam reduces the creation of donor acceptor levels in AlGaAs, as compared with As4. The PL spectra line shape in the GaAs band gap region shows lines associated with band-to-band transition, exciton-bound to neutral-point defects (d,X), and conduction-band to neutral carbon-acceptor (C,A0). The intensity of these transitions was found to be independent of the As molecular-beam species; nevertheless, the photoreflectance spectra of the samples show line shape changes close to the GaAs band gap energy. This region has been related to the electron mobility of two-dimensional electron-gas systems. [ABSTRACT FROM AUTHOR]

Food webs-networks of predators and prey-have long been known to exhibit "intervality": species can generally be ordered along a single axis in such a way that the prey of any given predator tend to lie on unbroken compact intervals. Although the meaning of this axis-usually identified with a "niche" dimension-has remained a mystery, it is assumed to lie at the basis of the highly non-trivial structure of food webs. With this in mind, most trophic network modelling has for decades been based on assigning species a niche value by hand. However, we argue here that intervality should not be considered the cause but rather a consequence of food-web structure. First, analysing a set of 46 empirical food webs, we find that they also exhibit predator intervality: the predators of any given species are as likely to be contiguous as the prey are, but in a different ordering. Furthermore, this property is not exclusive of trophic networks: several networks of genes, neurons, metabolites, cellular machines, airports, and words are found to be approximately as interval as food webs. We go on to show that a simple model of food-web assembly which does not make use of a niche axis can nevertheless generate significant intervality. Therefore, the niche dimension (in the sense used for food-web modelling) could in fact be the consequence of other, more fundamental structural traits. We conclude that a new approach to food-web modelling is required for a deeper understanding of ecosystem assembly, structure, and function, and propose that certain topological features thought to be specific of food webs are in fact common to many complex networks.