Exciton dispersion from first principles

Gatti, Matteo; Sottile, Francesco

doi:10.1103/physrevb.88.155113

Cited by 54 publications

(75 citation statements)

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“…[19]. Moreover, despite the accurate BSE results in LiF [17], a recent study based on a simplified exciton kinetic kernel model [20] raised doubts about the capability of the ab initio BSE in general to address the issue of the exciton band structure. Therefore new questions arise: Is LiF only a fortunate case for BSE?…”

Section: Introductionmentioning

confidence: 99%

“…where in the Tamm-Dancoff approximation (TDA) [21] the sum is over valence-conduction (v-c) transitions t, the oscillator strengths areρ t (q) = φ vk−q r |e −iqr |φ ck , k and q r are in the first Brillouin zone, and q = q r + G is the measured momentum transfer with a reciprocal-lattice vector G. The BSE can be cast into an effective two-particle Schrödinger equation [8,17]:…”

Section: Theorymentioning

confidence: 99%

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Exciton energy-momentum map of hexagonal boron nitride

et al. 2015

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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.

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Section: Introductionmentioning

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Section: Theorymentioning

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Exciton energy-momentum map of hexagonal boron nitride

et al. 2015

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“…In the present work we solve the BSE at finite-momentum transfer [12][13][14] to investigate two prototypical isoelectronic molecular crystals: picene and pentacene (see Fig. 1).…”

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Frenkel versus charge-transfer exciton dispersion in molecular crystals

et al. 2013

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By solving the many-body Bethe-Salpeter equation at finite momentum transfer, we characterize the exciton dispersion in two prototypical molecular crystals, picene and pentacene, in which localized Frenkel excitons compete with delocalized charge-transfer excitons. We explain the exciton dispersion on the basis of the interplay between electron and hole hopping and electron-hole exchange interaction, unraveling a simple microscopic description to distinguish Frenkel and charge-transfer excitons. This analysis is general and can be applied to other systems in which the electron wave functions are strongly localized, as in strongly correlated insulators. Excitons are neutral electronic excitations that dominate the low-energy part of the optical spectra in insulators and semiconductors. They consist of bound electron-hole (e-h) pairs that can be excited in several ways: by absorption of light and by relaxation of free electrons and holes after optical or electrical pumping. They play an essential role in many semiconductor applications (e.g., for light-emitting diodes, lasers, and photovoltaic cells) and give rise to the rich field of Bose-Einstein exciton condensates. [1][2][3] In all these cases it is fundamental to understand the decay rate and the propagation of the excitons. The latter is directly related to their energy dispersion as a function of momentum transfer. Recent advances in loss spectroscopies make it possible to map out the full momentum-energy exciton dispersion. 4-7On the other hand, the interpretation of these experimental spectra requires first-principles theoretical approaches able to describe and analyze excitons at finite momentum transfer. The Bethe-Salpeter equation (BSE) from many-body perturbation theory has become the most accurate framework to describe excitonic effects in the optical spectra of many materials. 8,9 However optical spectroscopy probes the zero-momentum-transfer limit only. Therefore first-principles analysis of the exciton dispersion is still an important goal to reach.Molecular crystals represent a textbook case 10,11 that clearly illustrates the need for advanced theoretical tools to understand the exciton dispersion. Typically, the lowest-energy excited states in these materials are strongly localized Frenkel (FR) excitons, where the interacting e-h pairs are localized on the same molecular unit. Charge-transfer (CT) excitons, in which e-h pairs are delocalized on different units, usually appear at higher energies in the spectra. However, when the molecular units are large enough, the effective interactions for e-h pairs localized on the same site or on two different sites become comparable and either CT or FR excitons can occur. Under these conditions many-body effects become crucial to set the character of the excitons and an ab initio treatment of the e-h interactions is thus required.In the present work we solve the BSE at finite-momentum transfer 12-14 to investigate two prototypical isoelectronic molecular crystals: picene and pentacene (see Fig. 1). By switc...

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“…The BSE can be mapped onto an effective two-particle equation, written in a basis of electron-hole transitions (nm), according to [25,26] …”

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“…11 For a detailed treatment of the space indexes at finite momentum see Ref. [26]; for the treatment of occupations factors, see Ref. [58]).…”

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Optical properties of periodic systems within the current-current response framework: Pitfalls and remedies

et al. 2017

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(2017). Optical properties of periodic systems within the current-current response framework: pitfalls and remedies. Physical Review B (Condensed Matter), 95(15), [155203]. DOI: 10.1103/PhysRevB.95.155203 Published in: Physical Review B (Condensed Matter) Document Version: Publisher's PDF, also known as Version of record Queen's University Belfast -Research Portal: Link to publication record in Queen's University Belfast Research Portal Publisher rights © 2017 American Physical Society. This work is made available online in accordance with the publisher's policies. Please refer to any applicable terms of use of the publisher. General rightsCopyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights.Take down policy The Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made to ensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in the Research Portal that you believe breaches copyright or violates any law, please contact openaccess@qub.ac.uk. We compare the optical absorption of extended systems using the density-density and current-current linear response functions calculated within many-body perturbation theory. The two approaches are formally equivalent for a finite momentum q of the external perturbation. At q = 0, however, the equivalence is maintained only if a small q expansion of the density-density response function is used. Moreover, in practical calculations, this equivalence can be lost if one naively extends the strategies usually employed in the density-based approach to the current-based approach. Specifically, we discuss the use of a smearing parameter or of the quasiparticle lifetimes to describe the finite width of the spectral peaks and the inclusion of electron-hole interaction. In those instances, we show that the incorrect definition of the velocity operator and the violation of the conductivity sum rule introduce unphysical features in the optical absorption spectra of three paradigmatic systems: silicon (semiconductor), copper (metal) and lithium fluoride (insulator). We then demonstrate how to correctly introduce lifetime effects and electron-hole interactions within the current-based approach.

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Exciton dispersion from first principles

Cited by 54 publications

References 48 publications

Exciton energy-momentum map of hexagonal boron nitride

Exciton energy-momentum map of hexagonal boron nitride

Frenkel versus charge-transfer exciton dispersion in molecular crystals

Optical properties of periodic systems within the current-current response framework: Pitfalls and remedies

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