1,720,995 research outputs found
Theoretical description of protein field effects on electronic excitations of biological chromophores
Full text linkPhotoinitiated phenomena play a crucial role in many living organisms. Plants, algae, and bacteria absorb sunlight to perform photosynthesis, and convert water and carbon dioxide into molecular oxygen and carbohydrates, thus forming the basis for life on Earth. The vision of vertebrates is accomplished in the eye by a protein called rhodopsin, which upon photon absorption performs an ultrafast isomerisation of the retinal chromophore, triggering the signal cascade. Many other biological functions start with the photoexcitation of a protein-embedded pigment, followed by complex processes comprising, for example, electron or excitation energy transfer in photosynthetic complexes. The optical properties of chromophores in living systems are strongly dependent on the interaction with the surrounding environment (nearby protein residues, membrane, water), and the complexity of such interplay is, in most cases, at the origin of the functional diversity of the photoactive proteins. The specific interactions with the environment often lead to a significant shift of the chromophore excitation energies, compared with their absorption in solution or gas phase. The investigation of the optical response of chromophores is generally not straightforward, from both experimental and theoretical standpoints; this is due to the difficulty in understanding diverse behaviours and effects, occurring at different scales, with a single technique. In particular, the role played by ab initio calculations in assisting and guiding experiments, as well as in understanding the physics of photoactive proteins, is fundamental. At the same time, owing to the large size of the systems, more approximate strategies which take into account the environmental effects on the absorption spectra are also of paramount importance. Here we review the recent advances in the first-principle description of electronic and optical properties of biological chromophores embedded in a protein environment. We show their applications on paradigmatic systems, such as the light-harvesting complexes, rhodopsin and green fluorescent protein, emphasising the theoretical frameworks which are of common use in solid state physics, and emerging as promising tools for biomolecular systems
A monolayer transition-metal dichalcogenide as a topological excitonic insulator
Full text linkTopological insulators have been studied primarily with regard to the behaviour of electrons. A theoretical study now shows that a single layer of a metal dichalcogenide can become a topological insulator for excitons.Monolayer transition-metal dichalcogenides in the T ' phase could enable the realization of the quantum spin Hall effect(1) at room temperature, because they exhibit a prominent spin-orbit gap between inverted bands in the bulk(2,3). Here we show that the binding energy of electron-hole pairs excited through this gap is larger than the gap itself in the paradigmatic case of monolayer T ' MoS2, which we investigate from first principles using many-body perturbation theory(4). This paradoxical result hints at the instability of the T ' phase in the presence of spontaneous generation of excitons, and we predict that it will give rise to a reconstructed 'excitonic insulator' ground state(5-7). Importantly, we show that in this monolayer system, topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers(8-16) where the frustration between the two orders is relieved by breaking time reversal symmetry(13,15,16). The excitonic topological insulator is distinct from the bare topological phase because it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric(17,18) as an effect of electron-electron interaction
Optical properties of lead-free double perovskites by ab initio excited-state methods
Full text linkWe discuss the nature of the optical excitations of Cs2AgBiBr6, the archetypal compound of lead-free double perovskites. Such quaternary material shows an indirect electronic band gap with a broad optical absorption spectrum above 2 eV. By means of ab initio excited-state methods we show that the first absorption peak is due to a bound direct exciton (near the X point of the Brillouin zone), while the photoluminescence spectrum is explained in terms of phonon-assisted radiative recombination of indirect-bound excitons with transferred momenta along the L-X and Gamma-X directions. To address the role of metal and halide atoms on the electronic and optical properties of this materials class, we investigate two additional ternary double perovskites, i.e., Cs2In2X6 (X = F, Br). On the basis of the accurate determination of the absorption coefficients and minimum gaps, we estimate the spectroscopic limited maximum efficiency of solar cells based on such compounds, providing relevant information for their application in photovoltaics
Anomalous screening in narrow-gap carbon nanotubes
Full text linkThe screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes the range and strength of the force that acts on charge carriers and binds electron-hole pairs into excitons. In doped tubes, the effective Coulomb interaction drives the competition between Luttinger liquid and Wigner crystal, whereas in undoped narrow-gap tubes it dictates the Mott or excitonic nature of the correlated insulator observed at low temperature. Here, by computing the dielectric function of selected narrow- and zero-gap tubes from first principles, we show that the standard effective-mass model of screening systematically underestimates the interaction strength at long wavelength, hence missing the binding of low-energy excitons. The reason is that the model critically lacks the full three-dimensional topology of the tube, being adapted from graphene theory. As ab inito calculations are limited to small tubes, we develop a two-band model dielectric function based on the plane-wave expansion of Bloch states and the exact truncated Coulomb cutoff technique. We demonstrate that our - computationally cheap - approach provides the correct screening for narrow-gap tubes of any size and chirality. A striking result is that the screened interaction remains long-ranged even in gapless tubes, as an effect of the microscopic local fields generated by the electrons moving on the curved tube surface. As an application, we show that the effective electron-electron force that is felt at distances relevant to quantum transport experiments is super Coulombic
Publisher's Note: Optical properties of graphene nanoribbons: The role of many-body effects (Physical Review B - Condensed Matter and Materials Physics (2008) 77, (041404))
No full textGoing Beyond the GW Approximation Using the Time-Dependent Hartree-Fock Vertex
No full textThe time-dependent Hartree-Fock (TDHF) vertex of many-body perturbation theory (MBPT) makes it possible to extend TDHF theory to charged excitations. Here we assess its performance by applying it to spherical atoms in their neutral electronic configuration. On a theoretical level, we recast the TDHF vertex as a reducible vertex, highlighting the emergence of a self-energy expansion purely in orders of the bare Coulomb interaction; then, on a numerical level, we present results for polarizabilities, ionization energies (IEs), and photoemission satellites. We confirm the superiority of THDF over simpler methods such as the random phase approximation for the prediction of atomic polarizabilities. We then find that the TDHF vertex reliably provides better IEs than GW and low-order self-energies do in the light-atom, few-electron regime; its performance degrades in heavier, many-electron atoms instead, where an expansion in orders of an unscreened Coulomb interaction becomes less justified. New relevant features are introduced in the satellite spectrum by the TDHF vertex, but the experimental spectra are not fully reproduced due to a missing account of nonlinear effects connected to hole relaxation. We also explore various truncations of the self-energy given by the TDHF vertex, but do not find them to be more convenient than low-order approximations such as GW and second Born (2B), suggesting that vertex corrections should be carried out consistently both in the self-energy and in the polarizability
Role of Quantum-confinement in Anatase nanosheets
No full textDespite most of the applications of anatase nanostructures rely on photoexcited charge processes, yet profound theoretical understanding of fundamental related properties is lacking. Here, by means of ab initio ground and excited-state calculations, we reveal, in an unambiguous way, the role of quantum confinement effect and of the surface orientation, on the electronic and optical properties of anatase nanosheets (NSs). The presence of bound excitons extremely localized along the (001) direction, whose existence has been recently proven also in anatase bulk, explains the different optical behavior found for the two orientations — (001) and (101) — when the NS thickness increases. We suggest also that the almost two-dimensional nature of these excitons can be related to the improved photoconversion efficiency observed when a high percentage of (001) facet is present in anatase nanocrystals
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