Ab initio variational approach for evaluating lattice thermal conductivity

We present a first-principles theoretical approach for evaluating the lattice thermal conductivity based on the exact solution of the Boltzmann transport equation. We use the variational principle and the conjugate gradient scheme, which provide us with an algorithm faster than the one previously used in literature and able to always converge to the exact solution. Three-phonon normal and umklapp collision, isotope scattering and border effects are rigorously treated in the calculation. Good agreement with experimental data for diamond is found. Moreover we show that by growing more enriched diamond samples it is possible to achieve values of thermal conductivity up to three times larger than the commonly observed in isotopically enriched diamond samples with 99.93% C12 and 0.07 C13

Excitons in van der Waals materials : From monolayer to bulk hexagonal boron nitride

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.Peer reviewe

Exciton band structure of molybdenum disulfide: from monolayer to bulk

Exciton band structures analysis provides a powerful tool to identify the exciton character of materials, from bulk to isolated systems, and goes beyond the mere analysis of the optical spectra. In this work, we focus on the exciton properties of molybdenum sisulfide (MoS 2 ) by solving the ab initio many-body Bethe–Salpeter equation, as a function of momentum, to obtain the excitation spectra of both monolayer and bulk MoS 2 . We analyse the spectrum and the exciton dispersion on the basis of a model excitonic Hamiltonian capable of providing an efficient description of the excitations in the bulk crystal, starting from the knowledge of the excitons of a single layer. In this way, we obtain a general characterization of both bright and darks excitons in terms of the interplay between the electronic band dispersion (i.e. interlayer hopping) and the electron–hole exchange interaction. We identify for both the 2D and the 3D limiting cases the character of the lowest-energy excitons in MoS 2 , we explain the effects and relative weights of both band dispersion and electron–hole exchange interaction and finally we interpret the differences observed when changing the dimensionality of the system

Exciton energy-momentum map of hexagonal boron nitride

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 $\mathbf{q}$, as previously reported, but also at large $\mathbf{q}$. 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.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.Peer reviewe

Excitons in van der Waals Heterostructures: from Monolayer to Bulk Hexagonal Boron Nitride

International audienc

Understanding and tailoring unique electronic and phononic hBN properties

International audienceAfter having served for years as a perfect second on the heels of the bright star graphene, hexagonal boron nitride (hBN) has now demonstrated to be able to shine with its own light, offering a unique combination of physical properties that enable its use for a broad range of applications, from quantum technologies to deep UV optoelectronics, mid-infrared nano-photonics and thermal management applications [1]. hBN is indeed a natural hyperbolic material in the mid-IR range. It exhibits strong, thickness-dependent second-order non-linearities. By engineering its defects it can become a room-temperature, single-photon emitter and, finally, even being indirect, its wide bandgap offers very high internal quantum efficiency for deep UV emitters and detectors. All these features stem from its highly anisotropic crystal structure and its polar chemical bond which lead to peculiar electronic band structure, phonon dispersion, strong electron-phonon coupling, and huge excitonic effects [2,3]. Starting then from the description of its constituent elements I will outline the reasons for its exclusive properties and I will highlight recent discoveries and possible routes for further exploiting its huge potential [4,5].[1]B Gil, G Cassabois, R Cusco, G Fugallo, L Artus Nanophotonics 9: 3483–3504 (2020)[2]G. Fugallo, M. Aramini, J. Koskelo, K. Watanabe, T. Taniguchi, M. Hakala, S. Huotari, M. Gatti, and F. Sottile, Phys. Rev. B 92, 165122 (2015).[3]Cepellotti, A., G. Fugallo, L. Paulatto, M. Lazzeri, F. Mauri, and N. Marzari, Nature Comm. 6, 6400. (2015)[4]C. Elias, G. Fugallo, P. Valvin, C. L’Henoret, J. Li, J. H. Edgar, F. Sottile, M. Lazzeri, A. Ouerghi, B. Gil, and G. Cassabois Phys. Rev. Lett. 127, 137401 (2021).[5] G. Cassabois, G. Fugallo, C. Elias, P. Valvin, A. Rousseau, B. Gil, A. Summerfield, C. J. Mellor, T. S. Cheng, L. Eaves, C. T. Foxon, P. H. Beton, M. Lazzeri, A. Segura, and S. Novikov, Phys Rev X 12, 011057 (2022

Understanding and tailoring unique electronic and phononic hBN properties

International audienceAfter having served for years as a perfect second on the heels of the bright star graphene, hexagonal boron nitride (hBN) has now demonstrated to be able to shine with its own light, offering a unique combination of physical properties that enable its use for a broad range of applications, from quantum technologies to deep UV optoelectronics, mid-infrared nano-photonics and thermal management applications [1]. hBN is indeed a natural hyperbolic material in the mid-IR range. It exhibits strong, thickness-dependent second-order non-linearities. By engineering its defects it can become a room-temperature, single-photon emitter and, finally, even being indirect, its wide bandgap offers very high internal quantum efficiency for deep UV emitters and detectors. All these features stem from its highly anisotropic crystal structure and its polar chemical bond which lead to peculiar electronic band structure, phonon dispersion, strong electron-phonon coupling, and huge excitonic effects [2,3]. Starting then from the description of its constituent elements I will outline the reasons for its exclusive properties and I will highlight recent discoveries and possible routes for further exploiting its huge potential [4,5].[1]B Gil, G Cassabois, R Cusco, G Fugallo, L Artus Nanophotonics 9: 3483–3504 (2020)[2]G. Fugallo, M. Aramini, J. Koskelo, K. Watanabe, T. Taniguchi, M. Hakala, S. Huotari, M. Gatti, and F. Sottile, Phys. Rev. B 92, 165122 (2015).[3]Cepellotti, A., G. Fugallo, L. Paulatto, M. Lazzeri, F. Mauri, and N. Marzari, Nature Comm. 6, 6400. (2015)[4]C. Elias, G. Fugallo, P. Valvin, C. L’Henoret, J. Li, J. H. Edgar, F. Sottile, M. Lazzeri, A. Ouerghi, B. Gil, and G. Cassabois Phys. Rev. Lett. 127, 137401 (2021).[5] G. Cassabois, G. Fugallo, C. Elias, P. Valvin, A. Rousseau, B. Gil, A. Summerfield, C. J. Mellor, T. S. Cheng, L. Eaves, C. T. Foxon, P. H. Beton, M. Lazzeri, A. Segura, and S. Novikov, Phys Rev X 12, 011057 (2022