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We present a theoretical study of dispersion of states that form the bulk band-gap edges in the three-dimensional topological insulator Bi 2Te3. Within density functional theory, we analyze the effect of atomic positions varied within the error range of the available experimental data and approximation chosen for the exchange-correlation functional on the bulk band gap and k-space location of valence- and conduction-band extrema. For each set of the positions with different exchange-correlation functionals, we show how many-body corrections calculated within a one-shot GW approach affect the mentioned characteristics of electronic structure of Bi2Te3. We thus also illustrate to what degree the one-shot GW results are sensitive to the reference one-particle band structure in the case of bismuth telluride. We found that for this topological insulator the GW corrections enlarge the fundamental band gap and for certain atomic positions and reference band structure bring its value in close agreement with experiment. © 2013 American Physical Society. ; We also acknowledge partial support from the Basque Country Government, Departamento de Educación, Universidades e Investigación (Grant No. IT-366-07), and the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02- 00), and the Ministry of Education and Science of Russian Federation (Grant No. 2.8575.2013). ; Peer Reviewed

Motivated by a recent experiment that reported the successful synthesis of hexagonal (h) AlN [Tsipas, Appl. Phys. Lett. 103, 251605 (2013)APPLAB0003-695110.1063/1.4851239], we investigate structural, electronic, and vibrational properties of bulk, bilayer, and monolayer structures of h-AlN by using first-principles calculations. We show that the hexagonal phase of the bulk h-AlN is a stable direct-band-gap semiconductor. The calculated phonon spectrum displays a rigid-layer shear mode at 274 cm-1 and an Eg mode at 703 cm-1, which are observable by Raman measurements. In addition, single-layer h-AlN is an indirect-band-gap semiconductor with a nonmagnetic ground state. For the bilayer structure, AA′-type stacking is found to be the most favorable one, and interlayer interaction is strong. While N-layered h-AlN is an indirect-band-gap semiconductor for N=1-9, we predict that thicker structures (N≥10) have a direct band gap at the Γ point. The number-of-layer-dependent band-gap transitions in h-AlN is interesting in that it is significantly different from the indirect-to-direct crossover obtained in the transition-metal dichalcogenides. ; Flemish Science Foundation (FWO-Vl); Methusalem foundation of the Flemish government; TUBITAK Project (114F397); FWO Pegasus Long Marie Curie Fellowship

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs2AgBiBr6, shows attractive optical and electronic features, making it promising for high-efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal-engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs(2)AgBiBr(6)under ambient conditions. The band-gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first-principles calculations indicate that enhanced Ag-Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band-gap narrowing effect. This work provides new insights for achieving lead-free double perovskites with suitable band gaps for optoelectronic applications. ; Funding Agencies|Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation; Swedish Energy AgencySwedish Energy Agency [2018-004357]; VR Starting Grant [2019-05279]; Carl Tryggers Stiftelse; Olle Engkvist Byggmastare Stiftelse; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish Energy AgencySwedish Energy Agency; SSFSwedish Foundation for Strategic Research; China Scholarship Council (CSC)China Scholarship Council; Ministry of Science and High Education of the Russian Federation [075-15-2019-872 (14.Y26.31.0027/074-02-2018-327)]; Swedish Research Council (VR)Swedish Research Council [2019-05551]; Swedish Research CouncilSwedish Research Council [2016-07213]

Department of Chemistry, Durgapur Government College, J. N. Avenue, Durgapur-713 214, West Bengal, India E-mail: bankimghosh@yahoo.co.in Manuscript received online 27 April 2019, accepted 24 May 2019 A method of construction of graphs for extended graphene sheets of the formula C6n2H6n, n = 1, 2, 3,., maintaining six-fold symmetry has been developed. It has been shown that their adjacency matrices, which are of size 6n2×6n2 , can be reduced to n2×n2 matrices by exploring rotational symmetry and the reduced matrix can be easily written down as the adjacency matrix of a directed graph with n2 vertices (called 'reduced graph' in this article). A scheme for drawing the reduced graph is devised and from this the graph eigenvalues can be easily determined. It has been shown that the HOMO-LUMO energy gap of the 𝝅-MOs decreases as n increases, and an almost gapless graphene sheet (ΔE = ELUMO – EHOMO = 4.5×10–10 β, where β = resonance integral between two adjacent sp2 -C atoms) begins to form at 6×202 carbon atoms arranged in a 2-dimensional hexagonal lattice.

AgIn7S11 single crystals are herein grown by the vertical Bridgman method. The composition of the obtained single crystals is determined by X-ray microprobe analysis as well as the crystal structure – by X-ray diffraction analysis. It is shown that the obtained single crystals are crystallized in the cubic spinel structure. Using transmission spectra in the tem- perature range 10–320 K we determined the band gap of these single crystals and plotted its temperature dependence. This dependence is similar to that of the majority of semiconductor materials, namely, Eg increases with decreasing the tempera- ture. We showed the agreement of the calculated and experimental values.

The electronic properties, carrier mobility, and strain response of TiS3 nanoribbons (TiS3 NRs) are investigated by first-principles calculations. We found that the electronic properties of TiS3 NRs strongly depend on the edge type (a or b). All a-TiS3 NRs are metallic with a magnetic ground state, while b-TiS3 NRs are direct band gap semiconductors. Interestingly, the size of the band gap and the band edge position are almost independent of the ribbon width. This feature promises a constant band gap in a b-TiS3 NR with rough edges, where the ribbon width differs in different regions. The maximum carrier mobility of b-TiS3 NRs is calculated by using the deformation potential theory combined with the effective mass approximation and is found to be of the order 103cm2V-1s-1. The hole mobility of the b-TiS3 NRs is one order of magnitude lower, but it is enhanced compared to the monolayer case due to the reduction in hole effective mass. The band gap and the band edge position of b-TiS3 NRs are quite sensitive to applied strain. In addition we investigate the termination of ribbon edges by hydrogen atoms. Upon edge passivation, the metallic and magnetic features of a-TiS3 NRs remain unchanged, while the band gap of b-TiS3 NRs is increased significantly. The robust metallic and ferromagnetic nature of a-TiS3 NRs is an essential feature for spintronic device applications. The direct, width-independent, and strain-tunable band gap, as well as the high carrier mobility, of b-TiS3 NRs is of potential importance in many fields of nanoelectronics, such as field-effect devices, optoelectronic applications, and strain sensors. ; Flemish Science Foundation (FWO-Vl); Methusalem foundation of the Flemish government; Hercules Foundation; FWO Pegasus-Long Marie Curie Fellowship; FWO Pegasus-Short Marie Curie Fellowship; TUBITAK (114F397)

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al. ; Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO2. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO2 band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred. ; R. E. M., C. L. M., and R. F. H. were supported by the National Science Foundation (Grant No. DMR-1207507). A. R., P. C., and L. X. acknowledge support by the European Research Council Advanced Grant DYNamo (Grant No. ERC-2010-AdG-267374) and Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-578-13). A. R., P. C., L. X., M. W., and J. S. received support from the European Commission project CRONOS (Grant No. 280879-2) and M. G. from a Marie Curie FP7 Integration Grant within the 7th European Union Framework Programme. ; Peer Reviewed

By means of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations, the electronic band structure of the topological insulator PbBi4Te4Se3 for both five-layer and seven-layer surface terminations is investigated. The measured and calculated band structure features are in good agreement and indicate two well-resolved topological surface states with distinct spatial localizations within bulk band gap of about 0.3 eV. ; This work is supported by the Russian Science Foundation (Grants No. 18-12-00169 in part of the density functional calculations and No. 18-12-00062 in part of the photoemission measurements) and Saint Petersburg State University (Grant ID 40990069). The support from the Academic D.I. Mendeleev Fund Program of Tomsk State University (Project No. 8.1.01.2018), the Russian Foundation for Basic Researches (Grant No. 18-52-06009), the Science Development Foundation under the President of the Republic of Azerbaijan (Grant No. EIF/MQM/Elm-Tehsil-1-2016-1(26)-71/01/4-M33), the Basque Country Government, Departamento de Educación, Universidades e Investigación (Grants No. IT-756-13 and No. IT1301-19) and the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2016-75862-P) are acknowledged. J.S.-B. gratefully acknowledges financial support from the Impuls-und Vernetzungsfonds der Helmholtz-Gemeinschaft under Grant No. HRSF-0067 (Helmholtz-Russia Joint Research Group). Calculations were partly performed using computational resources provided by Resource Center "Computer Center of SPbU" (http://cc.spbu.ru) and the SKIFCyberia sup

M. Miniaci acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement no. 658483. NMP is supported by the European Commission H2020 under the Graphene Flagship Core 1 No. 696656 (WP14 "227 Polymer composites") and FET Proactive "Neurofibres" grant No. 732344. FB is supported by H2020 FET Proactive "Neurofibres" grant No. 732344. FB and NK have been funded by Progetto d'Ateneo/Fondazione San Paolo "Metapp", n. CSTO160004.

[EN] Recent years have witnessed the boom of cavity optomechanics, which exploits the confinement and coupling of optical and mechanical waves at the nanoscale. Among their physical implementations, optomechanical (OM) crystals built on semiconductor slabs enable the integration and manipulation of multiple OM elements in a single chip and provide gigahertz phonons suitable for coherent phonon manipulation. Different demonstrations of coupling of infrared photons and gigahertz phonons in cavities created by inserting defects on OM crystals have been performed. However, the considered structures do not show a complete phononic bandgap, which should enable longer lifetimes, as acoustic leakage is minimized. Here we demonstrate the excitation of acoustic modes in a one-dimensional OM crystal properly designed to display a full phononic bandgap for acoustic modes at 4 GHz. The modes inside the complete bandgap are designed to have high-mechanical Q-factors, limit clamping losses and be invariant to fabrication imperfections. ; This work was supported by the European Commission Seventh Framework Programs (FP7) under the FET-Open project TAILPHOX No 233883. J.G.-B., D.N.-U., E.C., F.A. and C.M.S.-T. acknowledge financial support from the Spanish projects ACPHIN (ref. FIS2009-10150) and TAPHOR (MAT2012-31392). J.G.-B. and D.P. acknowledges funding from the Spanish government through the Juan de la Cierva programme, D. N.-U. acknowledges funding from the Catalan government through the Beatriu de Pinos programme. We thank Juan Sierra for his valuable technical advice. We thank the ICN2's electron microscopy division and M. Sledzinska for the assistance with the SEM images. ; Gomis Bresco, J.; Navarro Urríos, D.; Oudich, M.; El-Jallal, S.; Griol Barres, A.; Puerto Garcia, D.; Chavez, E. (2014). A one-dimensional optomechanical crystal with a complete phononic band gap. Nature Communications. 5(4452):1-6. https://doi.org/10.1038/ncomms5452 ; S ; 1 ; 6 ; 5 ; 4452 ; Kippenberg, T. J. & Vahala, K. J. Cavity optomechanics: ...

arXiv:1401.1691v2.-- et al. ; Recent years have witnessed the boom of cavity optomechanics, which exploits the confinement and coupling of optical and mechanical waves at the nanoscale. Among their physical implementations, optomechanical (OM) crystals built on semiconductor slabs enable the integration and manipulation of multiple OM elements in a single chip and provide gigahertz phonons suitable for coherent phonon manipulation. Different demonstrations of coupling of infrared photons and gigahertz phonons in cavities created by inserting defects on OM crystals have been performed. However, the considered structures do not show a complete phononic bandgap, which should enable longer lifetimes, as acoustic leakage is minimized. Here we demonstrate the excitation of acoustic modes in a one-dimensional OM crystal properly designed to display a full phononic bandgap for acoustic modes at 4 GHz. The modes inside the complete bandgap are designed to have high-mechanical Q-factors, limit clamping losses and be invariant to fabrication imperfections. ; This work was supported by the European Commission Seventh Framework Programs (FP7) under the FET-Open project TAILPHOX N 233883. J.G.-B., D.N.-U., E.C., F.A. and C.M.S.-T. acknowledge financial support from the Spanish projects ACPHIN (ref. FIS2009-10150) and TAPHOR (MAT2012-31392). J.G.-B. and D.P. acknowledges funding from the Spanish government through the Juan de la Cierva programme, D.N.- U. acknowledges funding from the Catalan government through the Beatriu de Pinos programme. ; Peer Reviewed

Topological insulators (TIs) with an inverted bulk band and a strong spin-orbit coupling exhibit gapless topological surface states (TSSs) protected by time-reversal symmetry. Helical spin textures driven by spin-momentum locking offer the opportunity to generate spin-polarized currents and therefore TIs are expected to be used for future spintronic applications. For practical applications TIs are urgently required that are operable at room temperature due to a wide bulk band gap as well as a distinct topological surface state that is robust to atmospheric exposure. Here we show two distinguishable TSSs originating from different terminations on PbBi4Te4S3 by using spin- and angle-resolved photoemission spectroscopy. We find that one TSS is persistently observed, while the other becomes invisible upon intentional oxygen exposure. The result signifies the presence of a protected TSS buried under the topmost surface. Our finding paves the way for realizing a topological spintronics device under atmospheric conditions. ; This work was partly supported by the bilateral collaboration program between RFBR (Russia; No. 15-52-50017) and JSPS (Japan) and also by KAKENHI Grants No. 17H06138 and No. 18H03683. K.S. was financially supported by a Grant-in-Aid for JSPS Fellows (No. 16J03874). The study has also been supported by the Russian Science Foundation (Grant No. 17-12-01047) and RFBR (Grant No. 17-08-00955) for growing the single crystals and characterizing the samples. I.V.S. acknowledges financial support from the Russian Science Foundation (Grant No. 18-12-00169) for band structure calculations and the Ministry of Education and Science of the Russian Federation within governmental program Megagrants (State Task No. 3.9003.2017/9.10) for charge density calculations. This work has been partly performed in the framework of the nanoscience foundry and fine analysis (NFFA-MIUR Italy, Progetti Internazionali) facility. E.V.C. acknowledges financial support from the Saint Petersburg State University Project No. 15.61.202.2015 and the Spanish Ministry of Science and Innovation Grant No. FIS2016-75862-P. ; Peer reviewed

Si nanocrystals were formed in synthetic opals by Si-ion implantation and their optical properties studied using microphotoluminescence and reflection techniques. The properties of areas with high crystalline quality are compared with those of disordered regions of samples. The photoluminescencespectrum from Si nanocrystals embedded in silica spheres is narrowed by the inhibition of emission at wavelengths corresponding to the opalphotonic pseudoband gap (∼690 nm). Measurements of photoluminescencespectra from individual implanted silica spheres is also demonstrated and the number of emitting Si nanocrystals in single brightly emitting spheres is estimated to be of the order of one thousand. ; This work was supported by GACR (202/03/0789), NATO (PST.CLG.978100), and by the Royal Swedish Academy of Sciences. One of the authors ~J.V.! appreciates financial support from the French government (program Echange).

Si nanocrystals were formed in synthetic opals by Si-ion implantation and their optical properties studied using microphotoluminescence and reflection techniques. The properties of areas with high crystalline quality are compared with those of disordered regions of samples. The photoluminescencespectrum from Si nanocrystals embedded in silica spheres is narrowed by the inhibition of emission at wavelengths corresponding to the opalphotonic pseudoband gap (∼690 nm). Measurements of photoluminescencespectra from individual implanted silica spheres is also demonstrated and the number of emitting Si nanocrystals in single brightly emitting spheres is estimated to be of the order of one thousand. ; This work was supported by GACR (202/03/0789), NATO (PST.CLG.978100), and by the Royal Swedish Academy of Sciences. One of the authors ~J.V.! appreciates financial support from the French government (program Echange).