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The industrial production of citrus plants for ornamental use (ornacitrus) began in Italy at the end of the 1960s due to the need for many citrus nurseries to adapt their activities in a time of crisis for citriculture. Nowadays, the ornamental citrus nursery sector is a well-established industry in many European countries such as Portugal, Spain, Greece, and southern Italy. In Italy, nursery production of ornamental citrus plants has become prominent due to the gradual shutdown of many commercial citrus orchards. Currently, Italy maintains its leadership with more than 5.5 million ornacitrus plants produced annually. Ornamental citrus production regards mainly different cultivars of Citrus and Fortunella species, with lemon as the lead species. In this paper, the contribution of breeding and cultural techniques to the innovation of the sector is reported and discussed. This review aims to give an updated scientific and technical description of a sector with large competitive potential that remains still largely unexplored, pointing out its strengths and weaknesses. [ABSTRACT FROM AUTHOR]

In the description of the interaction between electrons beyond the classical Hartree picture, bare exchange often yields a leading contribution. Here we discuss its effect on optical spectra of solids, comparing three different frameworks: time-dependent Hartree-Fock, a recently introduced combined density-functional and Green’s function approaches applied to the bare exchange self-energy, and time-dependent exact exchange within time-dependent density-functional theory (TD-EXX). We show that these three approximations give rise to identical excitonic effects in solids; these effects are drastically overestimated for semiconductors. They are partially compensated by the usual overestimation of the quasiparticle band gap within Hartree-Fock. The physics that lacks in these approaches can be formulated as screening. We show that the introduction of screening in TD-EXX indeed leads to a formulation that is equivalent to previously proposed functionals derived from many-body perturbation theory. It can be simulated by reducing the long-range part of the Coulomb interaction: this produces absorption spectra of semiconductors in good agreement with experiment. [ABSTRACT FROM AUTHOR]

Correlated materials have been studied extensively using photoemission spectroscopy. Their optical properties are instead much less explored. Here we present calculations of the optical absorption spectrum of vanadium dioxide (VO2) in the framework of the Bethe-Salpeter equation (BSE) of many-body perturbation theory. In order to deal with localized electrons we go beyond the standard BSE implementation and extend it to correlated insulators. We show that it is not enough to describe the spectra on the basis of independent electron-hole pairs, even when the electron and hole are separately well described by state-of-the-art one-body Green's functions. Crystal local-field effects are crucial to explain the experimental findings, even qualitatively, and excitonic effects strongly modify the spectra, especially at their onset. In this context, as higheighted by the analysis of the BSE results, the quasi-one-dimensional nature of the vanadium-dimer chains plays a prominent role. [ABSTRACT FROM AUTHOR]

We present ab initio quasiparticle calculations for electronic excitations and the fundamental band gap of the strongly correlated transition-metal oxide CuO using the G W approximation of many-body perturbation theory. Problems related to the suitability of the method for strongly correlated materials and issues of self-consistency are addressed. We explain why quasiparticle self-consistent GW strongly overestimates the band gap of CuO. Apart from the band gap, electron addition and removal spectra in the quasiparticle approximation including lifetime and matrix-element effects are found to be in excellent agreement with the quasiparticle excitations in direct and inverse photoemission data. [ABSTRACT FROM AUTHOR]

We present a scheme to calculate exciton dispersions in real materials that is based on the first-principles many-body Bethe-Salpeter equation. We assess its high level of accuracy by comparing our results for LiF with recent inelastic x-ray scattering experimental data on a wide range of energy and momentum transfer. We show its great analysis power by investigating the role of the different electron-hole interactions that determine the exciton band structure and the peculiar "exciton revival" at large momentum transfer. Our calculations for solid argon are a prediction and a suggestion for future experiments. These results demonstrate that the first-principles Bethe-Salpeter equation is able to describe the dispersion of localized and delocalized excitons on equal footing and represent a key step for the ab initio study of the exciton mobility. [ABSTRACT FROM AUTHOR]

In this paper we propose a semi-random technique for the generation of a class of eIRA codes (a popular class of irregular LDPC codes that can be encoded in linear time) suited to partially parallel decoder implementations. The suggested technique tries to avoid both low-weight codewords and nearcodewords, which limit the performance of the belief propagation decoder. Its effectiveness is verified by comparison with literature results. The obtained codes are versatile, in terms of code-rate and block length, and they are characterized by a low error floor. [ABSTRACT FROM AUTHOR]

Classical Hartree effects contribute substantially to the success of time‐dependent density functional theory, especially in finite systems. Moreover, exchange‐correlation contributions have an asymptotic Coulomb tail similar to the Hartree term, and turn out to be crucial in describing response properties of solids. In this work, we analyze in detail the role of the long‐range part of the Coulomb potential in the dielectric response of finite and infinite systems, and elucidate its importance in distinguishing between optical and electron energy loss spectra (in the long wavelength limit q → 0). We illustrate numerically and analytically how the imaginary part of the dielectric function and the loss function coincide for finite systems, and how they start to show differences as the distance between objects in an infinite array is decreased (which simulates the formation of a solid). We discuss calculations for the model case of a set of interacting and noninteracting beryllium atoms, as well as for various realistic systems, ranging from molecules to solids, and for complex systems, such as superlattices, nanotubes, nanowires, and nanoclusters. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

Mouse embryonic stem cells (mESCs) are pluripotent and can differentiate into cells belonging to the three germ layers of the embryo. However, mESC pluripotency and genome stability can be compromised in prolonged in vitro culture conditions. Several factors control mESC pluripotency, including Wnt/β-catenin signaling pathway, which is essential for mESC differentiation and proliferation. Here we show that the activity of the Wnt/β-catenin signaling pathway safeguards normal DNA methylation of mESCs. The activity of the pathway is progressively silenced during passages in culture and this results into a loss of the DNA methylation at many imprinting control regions (ICRs), loss of recruitment of chromatin repressors, and activation of retrotransposons, resulting into impaired mESC differentiation. Accordingly, sustained Wnt/β-catenin signaling maintains normal ICR methylation and mESC homeostasis and is a key regulator of genome stability. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation of electronic and optical properties of free-base porphyrins based on density functional theory and many-body perturbation theory. The electronic levels of free-base porphine (H2P) and its phenyl derivative, free-base tetraphenylporphyrin (H2TPP) are calculated using the ab initio GW approximation for the self-energy. The approach is found to yield results that compare favorably with the available photoemission spectra. The excitonic nature of the optical peaks is revealed by solving the Bethe–Salpeter equation, which provides an accurate description of the experimental absorption spectra. The lowest triplet transition energies are in good agreement with the measured values. [ABSTRACT FROM AUTHOR]

Homotypic and heterotypic cell-to-cell fusion are key processes during development and tissue regeneration. Nevertheless, aberrant cell fusion can contribute to tumour initiation and metastasis. Additionally, a form of cell-in-cell structure called entosis has been observed in several human tumours. Here we investigate cell-to-cell interaction between mouse mesenchymal stem cells (MSCs) and embryonic stem cells (ESCs). MSCs represent an important source of adult stem cells since they have great potential for regenerative medicine, even though they are also involved in cancer progression. We report that MSCs can either fuse forming heterokaryons, or be invaded by ESCs through entosis. While entosis-derived hybrids never share their genomes and induce degradation of the target cell, fusion-derived hybrids can convert into synkaryons. Importantly we show that hetero-to-synkaryon transition occurs through cell division and not by nuclear membrane fusion. Additionally, we also observe that the ROCK-actin/myosin pathway is required for both fusion and entosis in ESCs but only for entosis in MSCs. Overall, we show that MSCs can undergo fusion or entosis in culture by generating distinct functional cellular entities. These two processes are profoundly different and their outcomes should be considered given the beneficial or possible detrimental effects of MSC-based therapeutic applications. [ABSTRACT FROM AUTHOR]

We present a screened exact-exchange (SXX) method for the efficient and accurate calculation of the optical properties of solids, where the screening is achieved through the zero-wave-vector limit of the inverse dielectric function. The SXX approach can be viewed as a simplification of the Bethe-Salpeter equation (BSE) or, in the context of time-dependent density-functional theory, as a first step towards a new class of hybrid functionals for the optical properties of solids. SXX performs well for bound excitons and continuum spectra in both small-gap semiconductors and large-gap insulators, with a computational cost much lower than that of the BSE. [ABSTRACT FROM AUTHOR]

New and alternative models for agri-food production and consumption have brought up questions regarding the effects they have on local development processes in terms of the economic exploitation of rural areas as well as environmental, cultural, and social factors. The agri-food system proposed by the Slow Food (SF) Presidia Project, which focuses on farm-to-market systems for local, high-quality, sustainable products, can respond to the new and emerging needs of both rural and urban populaces via several approaches in addition to food production itself. However, evaluating these parameters is challenging. The aim of this study was to develop an indicator-based tool to monitor the sustainability in agri-food systems that considers quality as well as economic, ecological, social, and cultural aspects. We: (i) translated the major SF principles of "good," "clean," and "fair" into five major criteria to evaluate sustainability; (ii) designed multiple indicators to monitor progress toward sustainability for each of those criteria; and (iii) applied the monitoring tools to three case studies as a first attempt at end-use validation. Indicators and criteria were weighted either equally or based on their importance to surveyed stakeholders, i.e., consumers, producers, and scientists/experts. The proposed approach performed well as a tool for a broad sustainability evaluation by effectively combining the indicators with the same feedback. With this approach, we demonstrated that the SF Presidia project increases all dimensions of sustainability and in particular socioeconomic and cultural capital by preserving the environmental and quality aspects of the food products. [ABSTRACT FROM AUTHOR]

Strontium titanate SrTiO3 is an extensively studied material. Of particular interest are its electronic properties. Here we present a theoretical study of its optical spectrum on the basis of state-of-the-art calculations including many-body effects. The latter are evaluated by solving the Bethe-Salpeter equation within the GW approximation for the self-energy. Excitonic effects are strong at the onset and at higher energies up to 10 eV. Agreement between theory and experiment is excellent at the onset, but only qualitative at higher energies, where excitonic effects are stronger in theory than in experiment. The origin of structures in the spectrum, as well as the remaining discrepancy between theory and experiment, are discussed. We also present benchmark results for calculations using more approximate ways to determine the spectra, including simplified time-dependent density-functional theory. [ABSTRACT FROM AUTHOR]

In a recent Rapid Communication (J. A. Berger, L. Reining, and F Sottile, Phys. Rev. B 82,041103(R) (2010)], we presented the effective-energy technique to evaluate, in an accurate and numerically efficient manner, electronic excitations by reformulating spectral sum-over-states expressions such that only occupied states appear. In our approach all the empty states are accounted for by a single effective energy that can be obtained from first principles. In this work we provide further details of the effective-energy technique, in particular, when combined with the GW method, in which a huge summation over empty states appears in the calculation of both the screened Coulomb interaction and the self-energy. We also give further evidence of the numerical accuracy ol the effective-energy technique by applying it to the technological important materials SnO2 and ZnO. Finally, we use this technique to predict the band gap of bulk rubrene, an organic molecular crystal with a 140-atom unit cell. [ABSTRACT FROM AUTHOR]

The experimental valence band photoemission spectrum of semiconductors exhibits multiple satellites that cannot be described by the GW approximation for the self-energy in the framework of many-body perturbation theory. Taking silicon as a prototypical example, we compare experimental high energy photoemission spectra with GW calculations and analyze the origin of the GW failure. We then propose an approximation to the functional differential equation that determines the exact one-body Green's function, whose solution has an exponential form. This yields a calculated spectrum, including cross sections, secondary electrons, and an estimate for extrinsic and interference effects, in excellent agreement with experiment. Our result can be recast as a dynamical vertex correction beyond GW, giving hints for further developments. [ABSTRACT FROM AUTHOR]

One of the big challenges of theoretical condensed-matter physics is the description, understanding, and prediction of the effects of the Coulomb interaction on materials properties. In electronic spectra, the Coulomb interaction causes a renormalization of energies and change of spectral weight. Most importantly, it can lead to new structures, often called satellites. These can be linked to the coupling of excitations, also termed dynamical effects. State-of-the-art methods in the framework of many-body perturbation theory, in particular, the widely used GW approximation, often fail to describe satellite spectra. Instead, approaches based on a picture of electron-boson coupling such as the cumulant expansion are promising for the description of plasmon satellites. In this work, we give a unified derivation of the GW approximation and the cumulant expansion for the one-body Green's function. Using the example of bulk sodium, we compare the resulting spectral functions both in the valence and in the core region, and we discuss the dispersion of quasi-particles and satellites. We show that self-consistency is crucial to obtain meaningful results, in particular, at large binding energies. Very good agreement with experiment is obtained when the intrinsic spectral function is corrected for extrinsic and interference effects. Finally, we sketch how one can approach the problem in the case of the two-body Green's function, and we discuss the cancellation of various dynamical effects that occur in that case. [ABSTRACT FROM AUTHOR]

Almond production generates large amounts of by-products rich in polyphenols. In this study, almond skin was explored as a valuable food ingredient in bread making. To this purpose, almond skin was used to produce functional products modifying a traditional sourdough bread recipe. The doughs were prepared replacing semolina with powdered almond skin (PAS) at 5 and 10 % (w/w). Sourdough inoculum was started with a mix of lactic acid bacteria (LAB) and propagated in semolina until reaching pH 3.7. The pH of PAS added breads was higher than that of control (CTR) breads before and after fermentation. Plate counts showed a similar evolution of LAB and total mesophilic microorganisms, but members of Enterobacteriaceae and coliform were detectable in PAS doughs. Illumina data clearly showed a dominance of lactobacilli in all trials, but PAS doughs displayed the presence of Bacillus. The final bread characteristics were influenced by PAS and its addition percentage; in particular, crust and crumb colour resulted darker, the alveolation decreased and, regarding sensory attributes, odour intensity increased, while bread odour diminished. In presence of PAS, bread emissions were characterized by lower percentages of alcohols and aromatic hydrocarbons and higher percentages of the other volatile compound classes, especially terpenoids like β-pinene, β-myrcene and limonene than CTR trial. After in vitro simulated digestion, the final release of phytochemicals from 10 % PAS bread was almost 100 %. Thus, PAS determined an increase of the antioxidant capacity of the breads. Phytochemicals released from digested PAS-fortified bread can provide antioxidant protection in a complex biological environment such as human intestinal-like cells. Besides the positive functional properties of PAS, this work also evidenced the hygienic issues of almond skin and, in order to avoid potential risks for the human health, highlighted the need to preserve its microbiological characteristics during storage for their reuse in bread production. • Powder almond skin (PAS) did not affect the growth of starter lactic acid bacteria. • PAS slightly slowed down the acidification process. • Lactobacilli were absent in PAS, but detected in all doughs at relative abundances. • PAS phytochemicals provided antioxidant protection to human intestinal-like cells. [ABSTRACT FROM AUTHOR]

The influence of active modified atmosphere packaging MAP (10.0 kPa of O2 and 5 kPa of CO2) on palletized system was evaluated to extend the marketability of fresh plums cv. Angeleno. Fruits were wrapped with a polyethylene film in pallet bag units at 1C and 90% relative humidity for up to 60 days. The gas values of the pallet bags, the fruit weight losses and their qualitative and nutraceutical characteristics were periodically monitored to evaluate the goodness of storage. The results showed as the pallet bag have maintained the market life of Angeleno plums to up to 60 days, preserving the most important qualitative traits of fruits. After 20, 40 and 60 days the MAP treatment maintained the pulp harder than control fruits respectively of 0.87, 0.75 and 0.46 kg/cm2 and the SUAC index (quotient of the sum of sugars and sum of acids) was unaffected by the gas composition. Pratical Applications The storage in pallet under different MAP conditions could be a promising technique for plums and specifically allows different items with different technical requirements to be stored in the same cold-storage room. Furthermore, the storage units could also be conveniently used for the fruit transport along the entire supply chain to reach markets far from production areas. [ABSTRACT FROM AUTHOR]

Italian almond germplasm is characterized by a wide diversity in several growing areas among which Sicily is one of the most important. Analysis with consensus and specific primers and DNA sequencing was performed to investigate S-RNase genetic diversity and to elucidate the homology rate within a genetic pool of 27 Italian accessions. Interestingly, some of the self-compatible cultivars did not show the presence of Sf allele. Amplicons from consensus and allele-specific PCR primers revealed a high level of variability. Sequencing of all the S-RNase amplicons derived from consensus primers allowed the identification of two new S-RNase alleles (S51 and S52). Surprisingly, despite the AA replacement mutation, S51 did not exhibit any change of its S-RNase function. Additionally, several mutations, with no effect on amino acid composition, were detected in the intron and/or in the ORF of four known alleles (Sg, S10, S31 and S35). Genetic variation, regarding point mutations and only detected by sequencing, was revealed among 11 of 27 tested cultivars. The new sources of variability might have an interest for product traceability. [ABSTRACT FROM AUTHOR]

A major obstacle for computing optical spectra of solids is the lack of reliable approximations for capturing excitonic effects within time-dependent density functional theory. We show that the accurate prediction of strongly bound electron-hole pairs within this framework using simple approximations is still a challenge and that available promising results have to be revisited. Deriving a set of analytical formulas we analyze and explain the difficulties. We deduce an alternative approximation from an iterative scheme guided by previously available knowledge, significantly improving the description of exciton binding energies. Finally, we show how one can "read" exciton binding energies from spectra determined in the random phase approximation, without any further calculation. [ABSTRACT FROM AUTHOR]

The photoemission spectrum of graphite is still debated. To help resolve this issue, we present photoemission measurements at high photon energy and analyze the results using a Green's function approach that takes into account the full complexity of the loss spectrum. Our measured data show multiple satellite replicas. We demonstrate that these satellites are of intrinsic origin, enhanced by extrinsic losses. The dominating satellite is due to the π + σ plasmon of graphite, whereas the π plasmon creates a tail on the high-binding energy side of the quasiparticle peak. The interplay between the two plasmons leads to energy shifts, broadening, and additional peaks in the satellite spectrum. We also predict the spectral changes in the transition from graphite towards graphene. [ABSTRACT FROM AUTHOR]

Highlights: [•] Molecular characterization of 113 genotypes intercepting Italian and abroad almond varieties. [•] A total of 9 polymorphic SSR loci were employed detecting 159 alleles. [•] Data represent the first overview of genetic diversity among Italian almond cultivars using molecular markers. [•] STRUCTURE analysis allowed to identify admixture among different gene pools. [ABSTRACT FROM AUTHOR]

Abstract: The suitability of the time-dependent density-functional theory (TDDFT) approach for the theoretical study of the optical properties of biomolecules is demonstrated by several examples. We critically discuss the limitations of available TDDFT implementations to address some of the present open questions in the description of the excited-state dynamics of biological complexes. The key objective of the present work is to address the performance of TDDFT in the linear response regime of the bio-molecular systems to the visible or near UV radiation – measured by, e.g. optical absorption or optical dichroism spectra. Although these spectra are essentially determined by the electronic degrees of freedom of small, optically active regions within the usually large biological systems, they can also be strongly influenced by environment effects (solvent, hosting protein, temperature, etc.). Moreover, many key biological processes consist of photo-induced dynamics (photoisomerisation, etc.), and their description requires a coupled treatment of electronic and nuclear degrees of freedom. We illustrate these aspects with a selection of paradigmatic biomolecular systems: chromophores in fluorescent proteins, porphyrins, DNA basis, the azobenzene dye, etc. To cite this article: A. Castro et al., C. R. Physique 10 (2009). [Copyright &y& Elsevier]

This study explores the processability, mechanical, and thermal properties of biocompostable composites based on poly (butylene adipate-co-terephthalate) (PBAT) as polymer matrix and microcrystalline cellulose (MCC) derived from softwood almond (Prunus dulcis) shells (as-MCC) as filler at two different weight concentration, i.e., 10 wt% and 20 wt%. The materials were processed by melt mixing and a commercial MCC (c-MCC) was used as filler comparison. The fibrillar shape of as-MCC particles was found to change the rheological behavior of PBAT, particularly at the highest concentration. The melt mixing processing allowed obtaining a uniform dispersion of both kinds of fillers, slightly reducing the L/D ratio of as-MCC fibers. The as-MCC particles led to a higher increase of the elastic modulus of PBAT if compared to the c-MCC counterparts. Both the MCC fillers caused a drastic reduction of the elongation at break, although it was higher than 120% also at the highest filler concentrations. DSC analysis revealed that both MCC fillers poorly affected the matrix crystallinity, although as-MCC induced a slight PBAT crystallinity increase from 8.8% up to 10.9% for PBAT/as-MCC 20%. Therefore, this work demonstrates the great potential of MCC particles derived from almond shells as filler for biocompostable composites fabrication. [ABSTRACT FROM AUTHOR]

The chromatin fiber folds into loops, but the mechanisms controlling loop extrusion are still poorly understood. Using super-resolution microscopy, we visualize that loops in intact nuclei are formed by a scaffold of cohesin complexes from which the DNA protrudes. RNA polymerase II decorates the top of the loops and is physically segregated from cohesin. Augmented looping upon increased loading of cohesin on chromosomes causes disruption of Lamin at the nuclear rim and chromatin blending, a homogeneous distribution of chromatin within the nucleus. Altering supercoiling via either transcription or topoisomerase inhibition counteracts chromatin blending, increases chromatin condensation, disrupts loop formation, and leads to altered cohesin distribution and mobility on chromatin. Overall, negative supercoiling generated by transcription is an important regulator of loop formation in vivo. [Display omitted] • Super-resolution imaging reveals the structural organization of chromatin loops • Increased looping results into chromatin blending • Transcription regulates cohesin distribution and mobility on chromatin • Transcription-mediated supercoiling controls loop formation The mechanisms underlying chromatin loop formation by cohesin-mediated extrusion are still partially unknown. Using super-resolution imaging, Neguembor et al. visualize how loops are organized at the nanoscale level in intact nuclei and show that transcriptionally generated supercoiling regulates loop formation in vivo. [ABSTRACT FROM AUTHOR]

Rural development policies today include significant directions towards ecological transition and sustainability. Biodiversity plays a fundamental role, especially in fragile environments. The North African oases, for example, are socio-ecological structures with delicate balances in terms of natural resources, where the activation of participatory conservation approaches appears today to be very useful, aiming at long-lasting results. This type of approach was applied in the oasis of El Hamma, in Tunisia. The socio-ecological analysis was carried out through semi-structured interviews with different stakeholders of the oasis. The results were used to activate focus groups and to identify, in a participatory way, a conservation strategy for the species and the varieties at risk of erosion or disappearing. From this research, a wide spread of non-traditional date palm and vegetables emerged in a very diverse social context. These products were recognized as highly significant in terms of traditional knowledge by all stakeholders. Therefore, a Maison des semences and a public conservation center for perennial species were created, representing the first step of a participatory conservation model. Seeds of 11 traditional annual species, 10 date palm varieties and, in perspective, many other fruit species and vegetable varieties have been introduced into conservation. [ABSTRACT FROM AUTHOR]

• 13 C CPMAS NMR spectroscopy for the estimation of lignin/holocellulose ratio. • Cellulose extraction and purification from almond shells. • Employment of almond waste for packaging applications. Lignocellulosic fibers and microcellulose have been obtained by simple alkaline treatment from softwood almond shells. In particular, the Prunus dulcis Miller (D.A.) Webb. was considered as a agro industrial waste largely available in southern Italy. The materials before and after purification have been characterized by 13C CPMAS NMR spectroscopy methodology. A proper data analysis provided the relative composition of lignin and holocellulose at each purification step and the results were compared with thermogravimetric analysis and FT-IR. To value the possibility of using this material in a circular economy framework, the fibrous cellulosic material was used to manufacture a handmade cardboard. The tensile performances on the prepared cardboard proved its suitability for packaging purposes as a sustainable material. These fibers along with the obtained microcellulose can represent a new use for the almond shells that are mainly used as firewood. [ABSTRACT FROM AUTHOR]

A promising route towards nanodevice applications relies on the association of graphene and transition metal dichalcogenides with hexagonal boron nitride (h-BN). Due to its insulating nature, h-BN has emerged as a natural substrate and gate dielectric for graphene-based electronic devices. However, some fundamental properties of bulk h-BN remain obscure. For example, the band structure and the position of the Fermi level have not been experimentally resolved. Here, we report a direct observation of parabolic dispersions of h-BN crystals using high-resolution angle-resolved photoemission spectroscopy (ARPES). We find that h-BN exfoliation on epitaxial graphene enables overcoming the technical difficulties of using ARPES with insulating materials. We show trigonal warping of the intensity maps at constant energy. The valence-band maxima are located around the K points, 2.5 eV below the Fermi level, thus confirming the residual p-type character of typical h-BN. [ABSTRACT FROM AUTHOR]

We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic Hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyze in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the ab initio solution of the many-body Bethe-Salpeter equation as a function of momentum. In this way, we identify the character of the lowest-energy excitons in hBN, discuss the effects of the interlayer hopping and the electron-hole exchange interaction on the exciton dispersion, and illustrate the relation between exciton and plasmon excitations in layered materials. [ABSTRACT FROM AUTHOR]

Low-dimensional materials differ from their bulk counterparts in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal boron nitride, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene. [ABSTRACT FROM AUTHOR]

Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe-Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum q, as previously reported, but also at large q. We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials. [ABSTRACT FROM AUTHOR]