122 research outputs found
Structural and electronic properties of Pb1-xCdxTe and Pb1-xMnxTe ternary alloys
A systematic theoretical study of two PbTe-based ternary alloys, Pb1-xCdxTe
and Pb1-xMnxTe, is reported. First, using ab initio methods we study the
stability of the crystal structure of CdTe - PbTe solid solutions, to predict
the composition for which rock-salt structure of PbTe changes into zinc-blende
structure of CdTe. The dependence of the lattice parameter on Cd (Mn) content x
in the mixed crystals is studied by the same methods. The obtained decrease of
the lattice constant with x agrees with what is observed in both alloys. The
band structures of PbTe-based ternary compounds are calculated within a
tight-binding approach. To describe correctly the constituent materials new
tight-binding parameterizations for PbTe and MnTe bulk crystals as well as a
tight-binding description of rock-salt CdTe are proposed. For both studied
ternary alloys, the calculated band gap in the L point increases with x, in
qualitative agreement with photoluminescence measurements in the infrared. The
results show also that in p-type Pb1-xCdxTe and Pb1-xMnxTe mixed crystals an
enhancement of thermoelectrical power can be expected.Comment: 10 pages, 13 figures, submitted to Physical Review
Structure and energetics of the Si-SiO_2 interface
Silicon has long been synonymous with semiconductor technology. This unique
role is due largely to the remarkable properties of the Si-SiO_2 interface,
especially the (001)-oriented interface used in most devices. Although Si is
crystalline and the oxide is amorphous, the interface is essentially perfect,
with an extremely low density of dangling bonds or other electrically active
defects. With the continual decrease of device size, the nanoscale structure of
the silicon/oxide interface becomes more and more important. Yet despite its
essential role, the atomic structure of this interface is still unclear. Using
a novel Monte Carlo approach, we identify low-energy structures for the
interface. The optimal structure found consists of Si-O-Si "bridges" ordered in
a stripe pattern, with very low energy. This structure explains several
puzzling experimental observations.Comment: LaTex file with 4 figures in GIF forma
Topological crystalline insulator states in Pb(1-x)Sn(x)Se
Topological insulators are a novel class of quantum materials in which
time-reversal symmetry, relativistic (spin-orbit) effects and an inverted band
structure result in electronic metallic states on the surfaces of bulk
crystals. These helical states exhibit a Dirac-like energy dispersion across
the bulk bandgap, and they are topologically protected. Recent theoretical
proposals have suggested the existence of topological crystalline insulators, a
novel class of topological insulators in which crystalline symmetry replaces
the role of time-reversal symmetry in topological protection [1,2]. In this
study, we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a
topological crystalline insulator for x=0.23. Temperature-dependent
magnetotransport measurements and angle-resolved photoelectron spectroscopy
demonstrate that the material undergoes a temperature-driven topological phase
transition from a trivial insulator to a topological crystalline insulator.
These experimental findings add a new class to the family of topological
insulators. We expect these results to be the beginning of both a considerable
body of additional research on topological crystalline insulators as well as
detailed studies of topological phase transitions.Comment: v2: published revised manuscript (6 pages, 3 figures) and
supplementary information (5 pages, 8 figures
Ecosystem shift of a mountain lake under climate and human pressure : A move out from the safe operating space
A multiproxy approach including chironomid, diatom, pollen and geochemical analyses was applied on short gravitational cores retrieved from an alpine lake (Lacul Balea) in the Southern Carpathians (Romania) to unveil how this lake responded to natural and anthropogenic forcing over the past 500 years.On the basis of chironomid and diatom assemblage changes, and supported by sediment chemical data and historical information, we distinguished two main phases in lake evolution. Before 1926 the lake was dominated by chironomids belonging to Micropsectra insignilobus-type and benthic diatoms suggesting well-oxygenated oligotrophic environment with only small-scale disturbance. We considered this state as the lake's safe operational space. After 1926 significant changes occurred: Tanytarsus lugens-type and T. mendax-type chironomids took over dominance and collector filterers increased until 1970 pointing to an increase in available nutrients. The diatom community showed the most pronounced change between 1950 and 1992 when planktonic diatoms increased. The highest trophic level was reconstructed between 1970 and 1992, while the indicator species of increasing nutrient availability, Asterionella formosa spread from 1982 and decreased rapidly at 1992. Statistical analyses evidenced that the main driver of the diatom community change was atmospheric reactive nitrogen (Nr) fertilization that drastically moved the community towards planktonic diatom dominance from 1950. The transformation of the chironomid community was primarily driven by summer mean temperature increase that also changed the dominant feeding guild from collector gatherers to collector falterers. Our results overall suggest that the speed of ecosystem reorganisation showed an unprecedented increase over the last 100 years; biological systems in many cases underwent threshold type changes, while several system components displayed non-hysteretic change between alternating community composition. We conclude that Lake Balea is outside of its safe operating space today. The main trigger of changes since 1926 was climate change and human impact acting synergically. (C) 2020 The Authors. Published by Elsevier B.V.Peer reviewe
Isolated oxygen defects in 3C- and 4H-SiC: A theoretical study
Ab initio calculations in the local-density approximation have been carried out in SiC to determine the possible configurations of the isolated oxygen impurity. Equilibrium geometry and occupation levels were calculated. Substitutional oxygen in 3C-SiC is a relatively shallow effective mass like double donor on the carbon site (O-C) and a hyperdeep double donor on the Si site (O-Si). In 4H-SiC O-C is still a double donor but with a more localized electron state. In 3C-SiC O-C is substantially more stable under any condition than O-Si or interstitial oxygen (O-i). In 4H-SiC O-C is also the most stable one except for heavy n-type doping. We propose that O-C is at the core of the electrically active oxygen-related defect family found by deep level transient spectroscopy in 4H-SiC. The consequences of the site preference of oxygen on the SiC/SiO2 interface are discussed
Inhibition of platelet aggregation by carbon monoxide-releasing molecules (CO-RMs): comparison with NO donors
Carbon monoxide (CO) and CO-releasing molecules (CO-RMs) inhibit platelet aggregation in vitro. Herein, we compare the anti-platelet action of CORM-3, which releases CO rapidly (t½ 1 min), and CORM-A1, which slowly releases CO (t½ = 21 min). The anti-platelet effects of NO donors with various kinetics of NO release were studied for comparison. The effects of CO-RMs and NO donors were analyzed in washed human platelets (WP), platelets rich plasma (PRP), or whole blood (WB) using aggregometry technique. CORM-3 and CORM-A1 inhibited platelet aggregation in human PRP, WP, or WB, in a concentration-dependent manner. In all three preparations, CORM-A1 was more potent than CORM-3. Inhibition of platelets aggregation by CORM-A1 was not significantly affected by a guanylate cyclase inhibitor (ODQ) and a phosphodiesterase-5 inhibitor, sildenafil. In contrast, inhibition of platelet aggregation by NO donors was more potent with a fast NO releaser (DEA-NO, t½ = 2 min) than slow NO releasers such as PAPA-NO (t½ = 15 min) or other slow NO donors. Predictably, the anti-platelet effect of DEA-NO and other NO donors was reversed by ODQ while potentiated by sildenafil. In contrast to NO donors which inhibit platelets proportionally to the kinetics of NO released via activation of soluble guanylate cyclase (sGC), the slow CO-releaser CORM-A1 is a superior anti-platelet agent as compared to CORM-3 which releases CO instantly. The anti-platelet action of CO-RMs does not involve sGC activation. Importantly, CORM-A1 or its derivatives representing the class of slow CO releasers display promising pharmacological profile as anti-platelet agents
Spectroscopy and piezospectroscopy of the Lyman transitions and Fano resonances of indium in silicon
Unbound states in quantum heterostructures
We report in this review on the electronic continuum states of semiconductor Quantum Wells and Quantum Dots and highlight the decisive part played by the virtual bound states in the optical properties of these structures. The two particles continuum states of Quantum Dots control the decoherence of the excited electron – hole states. The part played by Auger scattering in Quantum Dots is also discussed
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