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}
}
2020
E; Marini Perfetto, A; Stefanucci
Self-consistent screening enhances the stability of the nonequilibrium excitonic insulator phase Journal Article
In: Physical Review B, vol. 102, iss. 8, no. 085203, 2020, ISSN: 2469-9950; eISSN 2469-9969.
Abstract | Links | BibTeX | Tags: Condensed matter, General Materials Science, Multidisciplinary, Nanoscience
@article{nokey,
title = {Self-consistent screening enhances the stability of the nonequilibrium excitonic insulator phase},
author = {Perfetto, E; Marini, A; Stefanucci, G},
editor = {AMER PHYSICAL SOC},
doi = {10.1103/PhysRevB.102.085203},
issn = {2469-9950; eISSN 2469-9969},
year = {2020},
date = {2020-08-24},
urldate = {2020-08-24},
journal = {Physical Review B},
volume = {102},
number = {085203},
issue = {8},
abstract = {The nonequilibrium excitonic insulator (NEQ-EI) is an excited state of matter characterized by a finite density of coherent excitons and a time-dependent macroscopic polarization. The stability of this exciton superfluid as the density grows is jeopardized by the increased screening efficiency of the looser excitons. In this work we put forward a Hartree plus screened exchange scheme to predict the critical density at which the transition toward a free electron-hole plasma occurs. The dielectric function is calculated self-consistently using the NEQ-EI polarization and found to vanish in the long-wavelength limit. This property makes the exciton superfluid stable up to relatively high densities. Numerical results for the MoS2 monolayers indicate that the NEQ-EI phase survives up to densities of the order of 10(12) cm(-)(2).},
keywords = {Condensed matter, General Materials Science, Multidisciplinary, Nanoscience},
pubstate = {published},
tppubtype = {article}
}
The nonequilibrium excitonic insulator (NEQ-EI) is an excited state of matter characterized by a finite density of coherent excitons and a time-dependent macroscopic polarization. The stability of this exciton superfluid as the density grows is jeopardized by the increased screening efficiency of the looser excitons. In this work we put forward a Hartree plus screened exchange scheme to predict the critical density at which the transition toward a free electron-hole plasma occurs. The dielectric function is calculated self-consistently using the NEQ-EI polarization and found to vanish in the long-wavelength limit. This property makes the exciton superfluid stable up to relatively high densities. Numerical results for the MoS2 monolayers indicate that the NEQ-EI phase survives up to densities of the order of 10(12) cm(-)(2).
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.
