58 research outputs found
Electromechanical coupling in free-standing AlGaN/GaN planar structures
The strain and electric fields present in free-standing AlGaN/GaN slabs are
examined theoretically within the framework of fully-coupled continuum elastic
and dielectric models. Simultaneous solutions for the electric field and strain
components are obtained by minimizing the electric enthalpy. We apply
constraints appropriate to pseudomorphic semiconductor epitaxial layers and
obtain closed-form analytic expressions that take into account the wurtzite
crystal anisotropy. It is shown that in the absence of free charges, the
calculated strain and electric fields are substantially differently from those
obtained using the standard model without electromechanical coupling. It is
also shown, however, that when a two-dimensional electron gas is present at the
AlGaN/GaN interface, a condition that is the basis for heterojunction
field-effect transistors, the electromechanical coupling is screened and the
decoupled model is once again a good approximation. Specific cases of these
calculations corresponding to transistor and superlattice structures are
discussed.Comment: revte
Energy levels in polarization superlattices: a comparison of continuum strain models
A theoretical model for the energy levels in polarization superlattices is
presented. The model includes the effect of strain on the local
polarization-induced electric fields and the subsequent effect on the energy
levels. Two continuum strain models are contrasted. One is the standard strain
model derived from Hooke's law that is typically used to calculate energy
levels in polarization superlattices and quantum wells. The other is a
fully-coupled strain model derived from the thermodynamic equation of state for
piezoelectric materials. The latter is more complete and applicable to strongly
piezoelectric materials where corrections to the standard model are
significant. The underlying theory has been applied to AlGaN/GaN superlattices
and quantum wells. It is found that the fully-coupled strain model yields very
different electric fields from the standard model. The calculated intersubband
transition energies are shifted by approximately 5 -- 19 meV, depending on the
structure. Thus from a device standpoint, the effect of applying the
fully-coupled model produces a very measurable shift in the peak wavelength.
This result has implications for the design of AlGaN/GaN optical switches.Comment: Revtex
Where have all the beetles gone? Longâterm study reveals carabid species decline in a nature reserve in Northern Germany
1. The drastic insect decline has received increasing attention in scientific as well as in public media. Long-term studies of insect diversity trends are still rare, even though such studies are highly important to assess extent, drivers and potential consequences of insect loss in ecosystems.
2. To gain insights into carabid diversity trends of ancient and sustainably managed woodlands, we analysed data of carabid beetles from a trapping study that has been run for 24 years in an old nature reserve of Northern Germany, the Luneburg Heath. We examined temporal changes in several diversity measures ⏠(e.g. biomass, species richness, functional diversity and phylogenetic diversity) and tested diverse species traits as predictor variables for species occurrence.
3. In contrast to recently published long-term studies of insect diversity, we did not observe a decline in biomass, but in species richness and phylogenetic diversity in carabids at our study site. Additionally, hibernation stage predicted the occurrence probability of carabids: Species hibernating as imagines or both imagines and larvae and breeding in spring showed strongest declines.
4. We assume the detected trends to be the result of external effects such as climate change and the application of pesticides in the surrounding. Our results suggest that the drivers for the insect decline and the responses are multifaceted. This highlights the importance of long-term studies with identification of the catches to, at best, species level to support the understanding of mechanisms driving changes in insect diversity and abundance
Wafer-Scale Epitaxial Modulation of Quantum Dot Density
Precise control of the properties of semiconductor quantum dots (QDs) is
vital for creating novel devices for quantum photonics and advanced
opto-electronics. Suitable low QD-density for single QD devices and experiments
are challenging to control during epitaxy and are typically found only in
limited regions of the wafer. Here, we demonstrate how conventional molecular
beam epitaxy (MBE) can be used to modulate the density of optically active QDs
in one- and two- dimensional patterns, while still retaining excellent quality.
We find that material thickness gradients during layer-by-layer growth result
in surface roughness modulations across the whole wafer. Growth on such
templates strongly influences the QD nucleation probability. We obtain density
modulations between 1 and 10 QDs/ and periods ranging from several
millimeters down to at least a few hundred microns. This novel method is
universal and expected to be applicable to a wide variety of different
semiconductor material systems. We apply the method to enable growth of
ultra-low noise QDs across an entire 3-inch semiconductor wafer
High Purcell factor generation of indistinguishable on-chip single photons
On-chip single-photon sources are key components for integrated photonic quantum technologies. Semiconductor quantum dots can exhibit near-ideal single-photon emission, but this can be significantly degraded in on-chip geometries owing to nearby etched surfaces. A long-proposed solution to improve the indistinguishablility is to use the Purcell effect to reduce the radiative lifetime. However, until now only modest Purcell enhancements have been observed. Here we use pulsed resonant excitation to eliminate slow relaxation paths, revealing a highly Purcell-shortened radiative lifetime (22.7âps) in a waveguide-coupled quantum dotâphotonic crystal cavity system. This leads to near-lifetime-limited single-photon emission that retains high indistinguishablility (93.9%) on a timescale in which 20 photons may be emitted. Nearly background-free pulsed resonance fluorescence is achieved under Ï-pulse excitation, enabling demonstration of an on-chip, on-demand single-photon source with very high potential repetition rates
Photography-based taxonomy is inadequate, unnecessary, and potentially harmful for biological sciences
The question whether taxonomic descriptions naming new animal species without type specimen(s) deposited in collections should be accepted for publication by scientific journals and allowed by the Code has already been discussed in Zootaxa (Dubois & NemĂ©sio 2007; Donegan 2008, 2009; NemĂ©sio 2009aâb; Dubois 2009; Gentile & Snell 2009; Minelli 2009; Cianferoni & Bartolozzi 2016; Amorim et al. 2016). This question was again raised in a letter supported
by 35 signatories published in the journal Nature (Pape et al. 2016) on 15 September 2016. On 25 September 2016, the following rebuttal (strictly limited to 300 words as per the editorial rules of Nature) was submitted to Nature, which on
18 October 2016 refused to publish it. As we think this problem is a very important one for zoological taxonomy, this text is published here exactly as submitted to Nature, followed by the list of the 493 taxonomists and collection-based
researchers who signed it in the short time span from 20 September to 6 October 2016
A NEW FERRITE MATERIAL FOR VIDEO RECORDING HEADS
Une nouvelle technique modifiĂ©e de pressage Ă chaud permet la fabrication de ferrites de Mn-Zn prĂ©sentant une structure granulaire homogĂšne, un diamĂštre moyen de grain ajustable entre 0,1 et 1 mm et une porositĂ© infĂ©rieure Ă 0,1 %. La mĂȘme composition prĂ©parĂ©e par la technique ordinaire de pressage Ă chaud fournit un diamĂštre de grain de 10 Ă 20 ”m seulement. Le matĂ©riau Ă gros grain, qui possĂšde des joints de grain d'une forme sinueuse particuliĂšre, prĂ©sente une permĂ©abilitĂ© aux basses frĂ©quences ”'' plus faible mais montre des pertes moins Ă©levĂ©es et une permĂ©abilitĂ© plus grande dans une gamme de frĂ©quences atteignant 10 MHz. Les avantages de ce matĂ©riau pour les applications Ă l'enregistrement vidĂ©o sont discutĂ©s.A new modified hot-pressing technique enables the fabrication of Mn-Zn ferrite materials with a homogenous grain structure, an adjusted average grain size of 0.1 to 1 mm and a porosity lower than 0.1 percent. The same composition prepared by a normal HP technique shows a grain-size of only 10 to 20 ”m. The large grain material, which possesses a peculiar snaky form of grain-boundaries, has a lower permeability ”'' at low frequencies but shows lower losses and a higher permeability at frequencies in the range up to 10 MHz. The suitability of this material for video-recorder application is discussed
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