Theory of phonon-assisted luminescence in solids: Application to hexagonal boron nitride

International audienceIn this manuscript we study luminescence of hexagonal boron nitride (hBN) by means of non-equilibrium Green's functions plus time-dependent perturbation theory. We derive a formula for light emission in solids in the limit of a weak excitation that includes perturbatively the contribution of electron-phonon coupling at the first order. This formula is applied to study luminescence in bulk hBN. This material has attracted interest due to its strong luminescence in the ultraviolet [Watanabe et al., Nature Mat. 3, 404(2004)]. The origin of this luminescence has been widely discussed, but only recently has a clear signature of phonon mediated light emission emerged in the experiments [Cassabois et al., Nature Phot. 10, 262(2016)]. By means of our new theoretical approach we provide a clear and full explanation of light emission in hBN

Exploring approximations to the GW self-energy ionic gradients

The accuracy of the many-body perturbation theory GW formalism to calculate electron-phonon coupling matrix elements has been recently demonstrated in the case of a few important systems. However, the related computational costs are high and thus represent strong limitations to its widespread application. In the present study, we explore two less demanding alternatives for the calculation of electron-phonon coupling matrix elements on the many-body perturbation theory level. Namely, we test the accuracy of the static Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the constant screening approach, where variations of the screened Coulomb potential W upon small changes of the atomic positions along the vibrational eigenmodes are neglected. We find this latter approximation to be the most reliable, whereas the static COHSEX ansatz leads to substantial errors. Our conclusions are validated in a few paradigmatic cases: diamond, graphene and the C60 fullerene. These findings open the way for combining the present many-body perturbation approach with efficient linear-response theories

Optical properties of flavin mononucleotide: A QM/MM study of protein environment effects

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Excitonic effects in third harmonic generation: the case of carbon nanotubes and nanoribbons

International audienceLinear and nonlinear optical properties of low-dimensional nanostructures have attracted great interest from the scientific community as tools to probe the strong confinement of electrons and for possible applications in optoelectronic devices. In particular it has been shown that the linear optical response of carbon nanotubes [F. Wang et al., Science 308, 838 (2005)] and graphene nanoribbons [Nat. Commun. 5 4253 (2014)] is dominated by bounded electron-hole pairs, excitons. The role of excitons in linear response has been widely studied, but still, little is known about their effect on nonlinear susceptibilities. Using a recently developed methodology [Phys. Rev. B 88, 235113 (2013)] based on well-established ab initio many-body perturbation theory approaches, we find that quasiparticle shifts and excitonic effects significantly modify the third-harmonic generation in carbon nanotubes and graphene nanoribbons. For both systems the net effect of many-body effects is to reduce the intensity of the main peak in the independent-particle spectrum and redistribute the spectral weight among several excitonic resonances

Zero point motion effect on the electronic properties of diamond, trans-polyacetylene and polyethylene

It has been recently shown, using ab-initio  methods, that bulk diamond is characterized by a large band-gap renormalization (~0.6 eV) induced by the electron-phonon interaction. In this work we show that in polymers, compared to bulk materials, the larger amplitude of the atomic vibrations makes the real excitations of the system be composed by entangled electron-phonon states. We prove that these states carry only a fraction of the electronic charge, thus leading, inevitably, to the failure of the electronic picture. The present results cast doubts on the accuracy of purely electronic calculations. They also lead to a critical revision of the state-of-the-art description of carbon-based nanostructures, opening a wealth of potential implications

Strong second harmonic generation in SiC, ZnO, GaN two-dimensional hexagonal crystals from first-principles many-body calculations

International audienceThe second harmonic generation (SHG) intensity spectrum of SiC, ZnO, GaN two-dimensional hexagonal crystals is calculated by using a real-time first-principles approach based on Green's function theory [Attaccalite et al.,Phys Rev B 2013 88, 235113]. This approach allows one to go beyond the independent particle description used in standard first-principles nonlinear optics calculations by including quasiparticle corrections (by means of the GW approximation), crystal local field effects and excitonic effects. Our results show that the SHG spectra obtained by the latter approach differ significantly from their independent particle counterparts. In particular they show strong excitonic resonances at which the SHG intensity is about two times stronger than within the independent particle approximation. All the systems studied (whose stability have been predicted theoretically) are transparent and at the same time exhibit a remarkable SHG intensity in the range of frequencies at which Ti:Sapphire and Nd:YAG lasers operate thus they can be of interest for nanoscale nonlinear frequency conversion devices. Specifically the SHG intensity at 800 nm (1.55 eV) ranges from about 40-80 pm/V in ZnO and GaN to 0.6 nm/V in SiC. The latter value in particular is 1 order of magnitude larger than values in standard nonlinear crystals

Excitonic effects in third-harmonic generation: The case of carbon nanotubes and nanoribbons

Linear and nonlinear optical properties of low-dimensional nanostructures have attracted great interest from the scientific community as tools to probe the strong confinement of electrons and for possible applications in optoelectronic devices. In particular it has been shown that the linear optical response of carbon nanotubes [F. Wang, Science 308, 838 (2005)SCIEAS0036-807510.1126/science.1110265] and graphene nanoribbons [Nat. Commun. 5 4253 (2014)2041-172310.1038/ncomms5253] is dominated by bounded electron-hole pairs, excitons. The role of excitons in linear response has been widely studied, but still, little is known about their effect on nonlinear susceptibilities. Using a recently developed methodology [Phys. Rev. B 88, 235113 (2013)PRBMDO1098-012110.1103/PhysRevB.88.235113] based on well-established ab initio many-body perturbation theory approaches, we find that quasiparticle shifts and excitonic effects significantly modify the third-harmonic generation in carbon nanotubes and graphene nanoribbons. For both systems the net effect of many-body effects is to reduce the intensity of the main peak in the independent-particle spectrum and redistribute the spectral weight among several excitonic resonances