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Andrea Claudio Myrta Daniele Conor

Research

  • Exciton collapse by Pauli blocking in hexagonal BN
  • A. Marini, work in progress
  • What happens when an ultra-strong laser pulse shine a material with an energy resonant with the excitonic energy? It happens that the excitons collapses as a consequence of the reduction of the phase-space associated to the excitonic state.

    Numerical simulation of the exciton collapse in h-BN under the action of an ultra-strong optical laser pulse
  • Phonon-induced dephasing in electronic systems excited out-of-the-equilibrium by means of intense laser pulses
  • A. Marini, work in progress
  • Ultra-fast optical spectroscopy is a powerful tool for the observation of dynamical processes in several kind of materials. The basic time-resolved optical experiment is the so-called “pump-probe”: a first light pulse, the “pump”, resonantly triggers a photo-induced process. The probe pulse photon energy, spectral width and peak intensity creates a certain density of electron-hole pairs in a more or less localized region of space. The subsequent system evolution can be monitored, for example, by the time-dependent transmission changes of a delayed “probe” pulse. After the creation of the initial carrier density the time evolution of the single-particle and many-particle excitations is now governed by a non-trivial interplay between phase coherence and energy relaxation. Indeed, scattering processes tend to destroy the coherence, leading to a de-phasing of the excitations. The role of the electronic correlations at this stage is to stabilize the ensemble by creating quasi-particles and multi-particle states De-phasing will be driven by different phenomena. One of the most important is the energy transfer to the atomic motion in form of phonon excitations. In this research project I am studying a novel approach based on the merging of Non-Equilibrium Green's function theory and Density Functional Theory to treat the phonon-mediated relaxation following the pump excitation. I am analyzing theoretical and methodological aspects of the basic tools, the Kadanoff-Baym equations (KBE), and doing simulations of the pumped electrons dynamics in paradigmatic materials.

    Numerical simulation of the formation of an high-temperature (∼ 9000 Kelvin) plasma in optically pumped GaAs
  • Real-time approach to the optical properties of solids and nanostructures: Time-dependent Bethe-Salpeter equation
  • C. Attaccalite, M. Gruning, and A. Marini, Phys. Rev. B 84, 245110 (2011).
  • Many-body effects are known to play a crucial role in the electronic and optical properties of solids and nano-structures. Nevertheless the majority of theoretical and numerical approaches able to capture the influence of Coulomb correlations are restricted to the linear response regime. In this work we introduce a novel approach based on a real-time solution of the electronic dynamics. The proposed approach reduces to the well-known Bethe-Salpeter equation in the linear limit regime and it makes possible, at the same time, to investigate correlation effects in nonlinear phenomena. We show the flexibility and numerical stability of the proposed approach by calculating the dielectric constants and the effect of a strong pulse excitation in bulk h-BN.
  • Effect of the Quantum Zero-Point Atomic Motion on the Optical and Electronic Properties of Diamond and Trans-Polyacetylene
  • E. Cannuccia, A. Marini, Phys. Rev. Lett. 107, 255501 (2011)
  • The quantum zero-point motion of the carbon atoms is shown to induce strong effects on the optical and electronic properties of diamond and trans polayacetylene, a conjugated polymer. By using an Ab-Initio approach, we interpret the sub-gap states experimentally observed in diamond in terms of entangled electron-phonon states. These states also appear in trans polayacetylene causing the formation of strong structures in the band-structure that even call into question the accuracy of the band theory. This imposes a critical revision of the results obtained for carbon-based nano-structures by assuming the atoms frozen in their equilibrium positions.
  • Anomalous Aharonov-Bohm Gap Oscillations in Carbon Nanotubes
  • D. Sangalli, A. Marini, Nano Letters 11, 4052 (2011)
  • The gap oscillations caused by a magnetic flux penetrating a carbon nanotube represent one of the most spectacular observations of the AharonovÀBohm effect at the nanoscale. Our understanding of this effect is, however, based on the assumption that the electrons are strictly confined on the tube surface, on trajectories that are not modified by curvature effects. Using an ab initio approach based on density functional theory, we show that this assumption fails at the nanoscale inducing important corrections to the physics of the Aharo- novÀBohm effect. Curvature effects and electronic density that is spilled out of the nanotube surface are shown to break the periodicity of the gap oscillations. We predict the key phenom- enological features of this anomalous AharonovÀBohm effect in semiconductive and metallic tubes and the existence of a large metallic phase in the low flux regime of multiwalled nanotubes, also suggesting possible experiments to validate our results.
  • Exciton-Plasmon States in Nanoscale Materials: Breakdown of the Tamm−Dancoff Approximation.
  • M. Gruning, A. Marini, and X. Gonze, Nano Lett. 9, 2820 (2009)
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  • Within the Tamm−Dancoff approximation, ab initio approaches describe excitons as packets of electron−hole pairs propagating only forward in time. However, we show that in nanoscale materials excitons and plasmons hybridize, creating exciton-plasmon states where the electron−hole pairs oscillate back and forth in time. Then, as exemplified by the trans-azobenzene molecule and the carbon nanotubes, the Tamm−Dancoff approximation yields errors larger than the accuracy claimed in ab initio calculations. Instead, we propose a general and efficient approach that avoids the Tamm−Dancoff approximation, correctly describes excitons, plasmons, and exciton-plasmon states, and provides a good agreement with experimental results.
  • Finite Temperaure Excitons
  • A. Marini , Phys. Rev. Lett. 101, 106405 (2008).
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  • The Ab-Initio description of the excitonic states, obtained by solving the Bethe-Salpeter (BS) equation of Many-Body Perturbation Theory, constitutes a well-established approach to interpret the photoexcited properties of bulk materials, surfaces, nanostructures and organic/bio-molecules. Although absorption and photoluminescence experiments are usually performed at room temperature, in the standard approach the BS equation is solved assuming the atoms frozen in their crystallographic positions, thus neglecting the effect of lattice vibrations. As a consequence excitons turn out to be insensitive to the temperature T and to have an infinite lifetime, in stark contrast with the experimental results. Moreover, in bulk semiconductors, it is a well known fact that the absorption line position, width, and intensity show a clear T dependence. In the frozen-atom BS equation this dependence is not described at all. Even in the T→0 limit, where atoms vibrate to fulfill the uncertainty principle (zero-point vibrations), the calculated absorption spectra is commonly convoluted with some artificial, ad-hoc numerical broadening function chosen to yield the best agreement with the experiment. In this work I have shown how to solve, in a fully Ab-Initio manner, the Bethe-Salpeter equation including the coupling with the lattice vibrations. The picture of the excitons obtained within a frozen-atom approximation turns out to be deeply modified, both at zero and finite temperature. Excitons acquire a non-radiative lifetime, otherwise infinite in the frozen-atom approximation. The thermal properties of the excitonic states are explained in terms of a weak and a strong exciton-phonon coupling. In the weak regime the lattice vibrations affect only the electron-hole substrate of the excitonic states, while in the strong case, they participate actively in the excitons build-up.
  • Optical saturation driven by exciton confinement in molecular-chains: a TDDFT study.
  • D. Varsano, A. Marini, and A. Rubio, Phys. Rev. Lett. 101, 133002 (2008).
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  • Excitonic confinement is identified as the main driving force for the saturation in one-dimensional molecular chains (i.e. polyacetylene and H2) of the chain polarizability as a function of the number of molecular units. This conclusion is based on first principles time-dependent density functional theory calculations using a recently developed exchange-correlation kernel that accounts for excitonic effects. The failure of simple local and semi-local functionals is shown to be linked to the lack of memory effects, spatial ultra-non-locality, and self-interaction corrections. These effects get smaller as the gap reduces, in which case such simple approximations do perform better.