Search results for "gaps"
showing 10 items of 73 documents
Bandgap behavior and singularity of the domain-induced light scattering through the pressure-induced ferroelectric transition in relaxor ferroelectri…
2018
[EN] In this letter, we have investigated the electronic structure of A(x)Ba(1-x)Nb(2)O(6) relaxor ferroelectrics on the basis of optical absorption spectroscopy in unpoled single crystals with A = Sr and Ca under high pressure. The direct character of the fundamental transition could be established by fitting Urbach's rule to the photon energy dependence of the absorption edge yielding bandgaps of 3.44(1) eV and 3.57(1) eV for A = Sr and Ca, respectively. The light scattering by ferroelectric domains in the pre-edge spectral range has been studied as a function of composition and pressure. After confirming with x-ray diffraction the occurrence of the previously observed ferroelectric to pa…
High pressure theoretical and experimental analysis of the bandgap of BaMoO4, PbMoO4, and CdMoO4
2019
We have investigated the origin of the bandgap of BaMoO4, PbMoO4, and CdMoO4 crystals on the basis of optical absorption spectroscopy experiments and ab initio electronic band structure, density of states, and electronic localization function calculations under high pressure. Our study provides an accurate determination of the bandgaps Eg and their pressure derivatives d E g / dP for BaMoO4 (4.43 eV, −4.4 meV/GPa), PbMoO4 (3.45 eV, −53.8 meV/GPa), and CdMoO4 (3.71 eV, −3.3 meV/GPa). The absorption edges were fitted with the Urbach exponential model which we demonstrate to be the most appropriate for thick crystals with direct bandgaps. So far, the narrowing of the bandgap of distinct PbMoO4…
Structural, electronic and energetic effects in heterocyclic fluorene derivatives fused with a fulvene unit
2019
Abstract A set of 36 heterocyclic (B, N and O) fluorene (C) derivatives fused in nine ways with fulvene ring have been analyzed by means of different local aromaticity criteria. Molecular geometry of analyzed compounds were optimized at B3LYP/6-311++G(2d,2p) level of theory. The evaluation of the local aromaticity has been carried out through the use of the geometry-based harmonic oscillator model of aromaticity (HOMA) and the magnetism-based zz‐component of the nucleus independent chemical shifts calculated 1 A above the ring center (NICS1zz) indices as well as one aromaticity index derived from the Quantum Theory Atoms in Molecules (QTAIM), i.e. the para-delocalization index (PDI). Additi…
Systematic analysis of the scientific literature on population surveillance
2020
Abstract Introduction Population surveillance provides data on the health status of the population through continuous scrutiny of different indicators. Identifying risk factors is essential for the quickly detecting and controlling of epidemic outbreaks and reducing the incidence of cross-infections and non-communicable diseases. The objective of the present study is to analyze research on population surveillance, identifying the main topics of interest for investigators in the area. Methodology We included documents indexed in the Web of Science Core Collection in the period from 2000 to 2019 and assigned with the generic Medical Subject Heading (MeSH) “population surveillance” or its rela…
Uzturs ar samazinātu ogļhidrātu saturu (SCD/GAPS) autiskā spektra simptomu mazināšanai bērniem- pilotpētījums
2019
Ar autismu saprot plaši izplatītus smadzeņu attīstības traucējumus, kuru etioloģijā būtiska ietekme ir gan vides, gan ģenētiskajiem faktoriem. Autisma izpausmes katram cilvēkam var būt ar individuālu simptomu kopumu, tāpēc biežāk runā par autiskā spektra traucējumiem (AST), ko raksturo simptomu triāde - problēmas integrēties sabiedrībā, komunikatīvie traucējumi un ierobežota, atkārtota uzvedība. Saskaņā ar CDC (Centers for Disease Control and Prevention) 2014. g. datiem ASV viens no 59 bērniem ir ar AST. Tiek vērtēts, ka Latvijā, tāpat kā Eiropā, ar AST ir katrs simtais bērns. Medicīniskajā literatūrā dominē viedoklis, ka šobrīd nav vienas terapijas, kas spētu mazināt AST pamatsimptomus. To…
Nonlinear pressure dependence of the direct band gap in adamantine ordered-vacancy compounds
2010
A strong nonlinear pressure dependence of the optical absorption edge has been measured in defect chalcopyrites CdGa{sub 2}Se{sub 4} and HgGa{sub 2}Se{sub 4}. The behavior is due to the nonlinear pressure dependence of the direct band-gap energy in these compounds as confirmed by ab initio calculations. Our calculations for CdGa{sub 2}Se{sub 4}, HgGa{sub 2}Se{sub 4} and monoclinic {beta}-Ga{sub 2}Se{sub 3} provide evidence that the nonlinear pressure dependence of the direct band-gap energy is a general feature of adamantine ordered-vacancy compounds irrespective of their composition and crystalline structure. The nonlinear behavior is due to a conduction band anticrossing at the {Gamma} po…
Analysis of Profiles of Family Educational Situations during COVID-19 Lockdown in the Valencian Community (Spain)
2022
Due to the pandemic (COVID-19), the education system in Spain was forced to close for three months, creating an unprecedented situation: improvised distance schooling. Family characteristics and their life situations with Information and Communication Technology use would be aspects to be studied as educational conditioning factors. This paper presents the ways in which a representative sample of families in the Valencian Community (Spain) assumed the education of their children during the lockdown. Mixed methods (quantitative -surveys-/qualitative -focus groups-) are used. Multivariate profiles are studied (k-means cluster) that summarise the life circumstances, represented by composite in…
Photoluminescence from strained InAs monolayers in GaAs under pressure
1994
bulk GaAs. At pressures above the band crossover two emission bands are observed. These bands, characterized by having negative pressure coefBcients, are attributed to the type-I transition between conduction-band X „and heavy-hole states of the InAs monolayer and the type-II transition &om X states in GaAs to InAs heavy-hole states. The results are interpreted in terms of tight-binding band-structure calculations for the strained InAs-monolayer — bulk-GaAs system. I. INTRODUCTION Highly strained InAs jGaAs heterostructures have recently attracted interest due to their unusual electronic and optical properties. ~ 4 Epitaxial isomorphic growth of InAs on GaAs can be achieved only up to a sma…
Effects of Nid-levels on the electronic band structure of NixCd1-xO semiconducting alloys
2017
NixCd1-xO has a ∼3 eV band edge offset and bandgap varying from 2.2 to 3.6 eV, which is potentially important for transparent electronic and photovoltaic applications. We present a systematic study of the electronic band structure of NixCd1-xO alloys across the composition range. Ion irradiation of alloy samples leads to a saturation of the electron concentration associated with pinning of the Fermi level (EF) at the Fermi stabilization energy, the common energy reference located at 4.9 eV below the vacuum level. The composition dependence of the pinned EF allows determination of the conduction band minimum (CBM) energy relative to the vacuum level. The unusually strong deviation of the CBM…
Electron–phonon effects on the direct band gap in semiconductors: LCAO calculations
2002
Abstract Using a perturbative treatment of the electron–phonon interaction, we have studied the effect of phonons on the direct band gap of conventional semiconductors. Our calculations are performed in the framework of the tight-binding linear combination of atomic orbitals (LCAO) approach. Within this scheme we have calculated the temperature and isotopic mass dependence of the lowest direct band gap of several semiconductors with diamond and zincblende structure. Our results reproduce the overall trend of available experimental data for the band gap as a function of temperature, as well as give correctly the mass dependence of the band gap on isotopic. A calculation of conduction band in…