2024
Marini, Andrea
Nonadiabatic effects lead to the breakdown of the semiclassical phonon picture Journal Article
In: Physical Review B, vol. 110, iss. 2, pp. 024306, 2024, ISSN: 2469-9969 (online), 2469-9950 (print).
Abstract | Links | BibTeX | Tags: Nanotechnology, Physics
@article{nokey,
title = {Nonadiabatic effects lead to the breakdown of the semiclassical phonon picture},
author = {Marini, Andrea},
editor = {American Physical Society},
url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.110.024306},
doi = {https://doi.org/10.1103/PhysRevB.110.024306},
issn = {2469-9969 (online), 2469-9950 (print)},
year = {2024},
date = {2024-07-03},
urldate = {2024-07-03},
journal = {Physical Review B},
volume = {110},
issue = {2},
pages = {024306},
abstract = {Phonon properties of realistic materials are routinely calculated within the density functional perturbation theory (DFPT). This is a semiclassical approach where the atoms are assumed to oscillate along classical trajectories immersed in the electronic Kohn–Sham system. In this paper, we demonstrate that, in metals, nonadiabatic effects induce a deviation of the DFTP phonon frequencies from the quantistic solution of the Dyson equation—a deviation that increases with the phonon energy width, reflecting the breakdown of the semiclassical DFPT description. The final message is that nonadiabatic phonon effects can be described only by using a fully quantistic approach.},
keywords = {Nanotechnology, Physics},
pubstate = {published},
tppubtype = {article}
}
Phonon properties of realistic materials are routinely calculated within the density functional perturbation theory (DFPT). This is a semiclassical approach where the atoms are assumed to oscillate along classical trajectories immersed in the electronic Kohn–Sham system. In this paper, we demonstrate that, in metals, nonadiabatic effects induce a deviation of the DFTP phonon frequencies from the quantistic solution of the Dyson equation—a deviation that increases with the phonon energy width, reflecting the breakdown of the semiclassical DFPT description. The final message is that nonadiabatic phonon effects can be described only by using a fully quantistic approach.
2021
Smejkal, Valerie; Libisch, Florian; Molina-Sanchez, Alejandro; Trovatello, Chiara; Wirtz, Ludger; Marini, Andrea
Time-Dependent Screening Explains the Ultrafast Excitonic Signal Rise in 2D Semiconductors Journal Article
In: ACS Nano, vol. 15, no. 1, pp. 1179–1185, 2021, ISSN: 1936-086X.
Links | BibTeX | Tags: General Engineering, General Materials Science, General Physics and Astronomy, Nanoscience, Nanotechnology
@article{Smejkal2020,
title = {Time-Dependent Screening Explains the Ultrafast Excitonic Signal Rise in 2D Semiconductors},
author = {Valerie Smejkal and Florian Libisch and Alejandro Molina-Sanchez and Chiara Trovatello and Ludger Wirtz and Andrea Marini},
doi = {10.1021/acsnano.0c08173},
issn = {1936-086X},
year = {2021},
date = {2021-01-26},
urldate = {2021-01-26},
journal = {ACS Nano},
volume = {15},
number = {1},
pages = {1179–1185},
publisher = {American Chemical Society (ACS)},
keywords = {General Engineering, General Materials Science, General Physics and Astronomy, Nanoscience, Nanotechnology},
pubstate = {published},
tppubtype = {article}
}
2018
ZL (Wang Wang, Zilong) ; Molina-Sánchez
Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS2 Journal Article
In: NANO LETTERS, vol. 18, iss. 11, pp. 6882-6891, 2018, ISSN: 1530-6984; eISSN 1530-6992.
Abstract | Links | BibTeX | Tags: Chemistry, Condensed matter, General Materials Science, Multidisciplinary, Nanoscience, Nanotechnology, Physics
@article{nokey,
title = {Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS2},
author = {Wang, ZL (Wang, Zilong) ; Molina-Sánchez, A (Molina-Sanchez, Alejandro) ; Altmann, P (Altmann, Patrick) ; Sangalli, D (Sangalli, Davide) ; De Fazio, D (De Fazio, Domenico) ; Soavi, G (Soavi, Giancarlo) ; Sassi, U (Sassi, Ugo) ; Bottegoni, F (Bottegoni, Federico) ; Ciccacci, F (Ciccacci, Franco) ; Finazzi, M (Finazzi, Marco) ; Wirtz, L (Wirtz, Ludger) ; Ferrari, AC (Ferrari, Andrea C.) ; Marini, A (Marini, Andrea) ; Cerullo, G (Cerullo, Giulio) ; Dal Conte, S (Dal Conte, Stefano)},
editor = {AMER CHEMICAL SOC},
doi = {10.1021/acs.nanolett.8b02774},
issn = {1530-6984; eISSN 1530-6992},
year = {2018},
date = {2018-11-01},
urldate = {2018-11-01},
journal = {NANO LETTERS},
volume = {18},
issue = {11},
pages = {6882-6891},
abstract = {In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics in 1L-WS2. This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin-flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs.},
keywords = {Chemistry, Condensed matter, General Materials Science, Multidisciplinary, Nanoscience, Nanotechnology, Physics},
pubstate = {published},
tppubtype = {article}
}
In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics in 1L-WS2. This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin-flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs.
E (Perfetto Perfetto, E. ) ; Sangalli
Ultrafast Charge Migration in XUV Photoexcited Phenylalanine: A First-Principles Study Based on Real-Time Nonequilibrium Green's Functions Journal Article
In: JOURNAL OF PHYSICAL CHEMISTRY LETTERS, vol. 9, iss. 6, pp. 1353-1358, 2018, ISSN: 1948-7185.
Abstract | Links | BibTeX | Tags: Atomic and Molecular Physics, Chemistry, General Materials Science, Nanoscience, Nanotechnology, Physics
@article{nokey,
title = {Ultrafast Charge Migration in XUV Photoexcited Phenylalanine: A First-Principles Study Based on Real-Time Nonequilibrium Green's Functions},
author = {Perfetto, E (Perfetto, E.) ; Sangalli, D (Sangalli, D.) ; Marini, A (Marini, A.) ; Stefanucci, G (Stefanucci, G.)},
editor = {AMER CHEMICAL SOC},
doi = {10.1021/acs.jpclett.8b00025},
issn = {1948-7185},
year = {2018},
date = {2018-03-15},
urldate = {2018-03-15},
journal = {JOURNAL OF PHYSICAL CHEMISTRY LETTERS},
volume = {9},
issue = {6},
pages = {1353-1358},
abstract = {The early-stage density oscillations of the electronic charge in molecules irradiated by an attosecond XUV pulse takes place on femto- or subfemtosecond time scales. This ultrafast charge migration process is a central topic in attoscience because it dictates the relaxation pathways of the molecular structure. A predictive quantum theory of ultrafast charge migration should incorporate the atomistic details of the molecule, electronic correlations, and the multitude of ionization channels activated by the broad-bandwidth XUV pulse. We propose a first-principles nonequilibrium Green's function method fulfilling all three requirements and apply it to a recent experiment on the photoexcited phenylalanine amino acid. Our results show that dynamical correlations are necessary for a quantitative overall agreement with the experimental data. In particular, we are able to capture the transient oscillations at frequencies 0.15 and 0.30 PHz in the hole density of the amine group as well as their suppression and the concomitant development of a new oscillation at frequency 0.25 PHz after similar to 14 fs.},
keywords = {Atomic and Molecular Physics, Chemistry, General Materials Science, Nanoscience, Nanotechnology, Physics},
pubstate = {published},
tppubtype = {article}
}
The early-stage density oscillations of the electronic charge in molecules irradiated by an attosecond XUV pulse takes place on femto- or subfemtosecond time scales. This ultrafast charge migration process is a central topic in attoscience because it dictates the relaxation pathways of the molecular structure. A predictive quantum theory of ultrafast charge migration should incorporate the atomistic details of the molecule, electronic correlations, and the multitude of ionization channels activated by the broad-bandwidth XUV pulse. We propose a first-principles nonequilibrium Green's function method fulfilling all three requirements and apply it to a recent experiment on the photoexcited phenylalanine amino acid. Our results show that dynamical correlations are necessary for a quantitative overall agreement with the experimental data. In particular, we are able to capture the transient oscillations at frequencies 0.15 and 0.30 PHz in the hole density of the amine group as well as their suppression and the concomitant development of a new oscillation at frequency 0.25 PHz after similar to 14 fs.
2017
A (Molina-Sanchez Molina-Sánchez, Alejandro) ; Sangalli
Ab Initio Calculations of Ultrashort Carrier Dynamics in Two-Dimensional Materials: Valley Depolarization in Single-Layer WSe2 Journal Article
In: NANO LETTERS, vol. 17, iss. 8, pp. 4549-4555, 2017, ISSN: 1530-6984; eISSN 1530-6992.
Abstract | Links | BibTeX | Tags: Chemistry, Condensed matter, General Materials Science, Multidisciplinary, Nanoscience, Nanotechnology, Physics
@article{nokey,
title = {Ab Initio Calculations of Ultrashort Carrier Dynamics in Two-Dimensional Materials: Valley Depolarization in Single-Layer WSe2},
author = {Molina-Sánchez, A (Molina-Sanchez, Alejandro) ; Sangalli, D (Sangalli, Davide) ; Wirtz, L (Wirtz, Ludger) ; Marini, A (Marini, Andrea)},
editor = {AMER CHEMICAL SOC},
doi = {10.1021/acs.nanolett.7b00175},
issn = {1530-6984; eISSN 1530-6992},
year = {2017},
date = {2017-08-01},
urldate = {2017-08-01},
journal = {NANO LETTERS},
volume = {17},
issue = {8},
pages = {4549-4555},
abstract = {In single-layer WSe2, a paradigmatic semi-conducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent K-+/- valleys. Such selective valley population corresponds to a pseudospin polarization. This can be used as a degree of freedom in a "valleytronic" device provided that the time scale for its depolarization is sufficiently large. Yet, the mechanism behind the valley depolarization still remains heavily debated. Recent time-dependent Kerr experiments have provided an accurate way to visualize the valley dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized pump pulse. We present here a clear, accurate and parameter-free description of the valley dynamics. By using an atomistic, ab initio approach, we fully, disclose the elemental mechanisms that dictate the depolarization effects. Our results are in excellent agreement with recent time dependent Kerr experiments. We explain the Kerr dynamics and its temperature dependence in terms of electron-phonon-mediated processes that induce spin-flip intervalley transitions.},
keywords = {Chemistry, Condensed matter, General Materials Science, Multidisciplinary, Nanoscience, Nanotechnology, Physics},
pubstate = {published},
tppubtype = {article}
}
In single-layer WSe2, a paradigmatic semi-conducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent K-+/- valleys. Such selective valley population corresponds to a pseudospin polarization. This can be used as a degree of freedom in a "valleytronic" device provided that the time scale for its depolarization is sufficiently large. Yet, the mechanism behind the valley depolarization still remains heavily debated. Recent time-dependent Kerr experiments have provided an accurate way to visualize the valley dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized pump pulse. We present here a clear, accurate and parameter-free description of the valley dynamics. By using an atomistic, ab initio approach, we fully, disclose the elemental mechanisms that dictate the depolarization effects. Our results are in excellent agreement with recent time dependent Kerr experiments. We explain the Kerr dynamics and its temperature dependence in terms of electron-phonon-mediated processes that induce spin-flip intervalley transitions.
