2022
Fulvio Paleari, Andrea Marini
Exciton-phonon interaction calls for a revision of the “exciton” concept Journal Article
In: Physical Review B, vol. 106, iss. 12, pp. 125403, 2022.
Abstract | Links | BibTeX | Tags: and Optics, Atomic and Molecular Physics, General Engineering, General Materials Science, General Physics and Astronomy
@article{,
title = {Exciton-phonon interaction calls for a revision of the “exciton” concept},
author = {Fulvio Paleari, Andrea Marini},
editor = {American Physical Society},
url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.106.125403
},
doi = {https://doi.org/10.1103/PhysRevB.106.125403},
year = {2022},
date = {2022-09-15},
urldate = {2022-09-15},
journal = {Physical Review B},
volume = {106},
issue = {12},
pages = {125403},
abstract = {The concept of optical exciton—a photoexcited bound electron-hole pair within a crystal—is routinely used to interpret and model a wealth of excited-state phenomena in semiconductors. Beside originating subband gap signatures in optical spectra, optical excitons have also been predicted to condensate, diffuse, recombine, and relax. However, all these phenomena are rooted on a theoretical definition of the excitonic state based on the following simple picture: “excitons” are actual particles that both appear as peaks in the linear absorption spectrum and also behave as well-defined quasiparticles. In this paper, we show, instead, that the electron–phonon interaction decomposes the initial optical (i.e., “reducible”) excitons into elemental (i.e., “irreducible”) excitons, the latter being a different kind of bound electron-hole pairs lacking the effect caused by the induced, classical, electric field. This is demonstrated within a real-time, many-body perturbation theory approach starting from the interacting electronic Hamiltonian including both electron-phonon and electron-hole interactions. We then apply the results to two realistic and paradigmatic systems, monolayer MoS2 (where the lowest-bound optical exciton is optically inactive) and monolayer MoSe2 (where it is optically active), using first-principles methods to compute the exciton-phonon coupling matrix elements. Among the consequences of optical-elemental decomposition, we point to a homogeneous broadening of absorption peaks occurring even for the lowest-bound optical exciton , and we demonstrate this by computing exciton-phonon transition rates. More generally, our findings suggest that the optical excitons gradually lose their initial structure and evolve as elemental excitons. These states can be regarded as the real intrinsic excitations of the interacting system, the ones that survive when the external perturbation and the induced electric fields have vanished.},
keywords = {and Optics, Atomic and Molecular Physics, General Engineering, General Materials Science, General Physics and Astronomy},
pubstate = {published},
tppubtype = {article}
}
The concept of optical exciton—a photoexcited bound electron-hole pair within a crystal—is routinely used to interpret and model a wealth of excited-state phenomena in semiconductors. Beside originating subband gap signatures in optical spectra, optical excitons have also been predicted to condensate, diffuse, recombine, and relax. However, all these phenomena are rooted on a theoretical definition of the excitonic state based on the following simple picture: “excitons” are actual particles that both appear as peaks in the linear absorption spectrum and also behave as well-defined quasiparticles. In this paper, we show, instead, that the electron–phonon interaction decomposes the initial optical (i.e., “reducible”) excitons into elemental (i.e., “irreducible”) excitons, the latter being a different kind of bound electron-hole pairs lacking the effect caused by the induced, classical, electric field. This is demonstrated within a real-time, many-body perturbation theory approach starting from the interacting electronic Hamiltonian including both electron-phonon and electron-hole interactions. We then apply the results to two realistic and paradigmatic systems, monolayer MoS2 (where the lowest-bound optical exciton is optically inactive) and monolayer MoSe2 (where it is optically active), using first-principles methods to compute the exciton-phonon coupling matrix elements. Among the consequences of optical-elemental decomposition, we point to a homogeneous broadening of absorption peaks occurring even for the lowest-bound optical exciton , and we demonstrate this by computing exciton-phonon transition rates. More generally, our findings suggest that the optical excitons gradually lose their initial structure and evolve as elemental excitons. These states can be regarded as the real intrinsic excitations of the interacting system, the ones that survive when the external perturbation and the induced electric fields have vanished.
CEP (Villegas Villegas, Cesar E. P. ) ; Leite
Efficient hot-carrier dynamics in near-infrared photocatalytic metals Journal Article
In: Physical Review B, vol. 105, iss. 16, no. 165109, 2022, ISSN: 2469-9950; eISSN 2469-9969.
Abstract | Links | BibTeX | Tags: Applied, Atomic and Molecular Physics, Condensed matter, General Materials Science, General Physics and Astronomy, Multidisciplinary, Physics
@article{nokey,
title = {Efficient hot-carrier dynamics in near-infrared photocatalytic metals},
author = {Villegas, CEP (Villegas, Cesar E. P.) ; Leite, MS (Leite, Marina S.) ; Marini, A (Marini, Andrea) ; Rocha, AR (Rocha, Alexandre R.)},
editor = {AMER PHYSICAL SOC},
url = {https://www.webofscience.com/wos/woscc/full-record/WOS:000807392800005},
doi = {10.1103/PhysRevB.105.165109},
issn = {2469-9950; eISSN 2469-9969},
year = {2022},
date = {2022-04-07},
journal = {Physical Review B},
volume = {105},
number = {165109},
issue = {16},
abstract = {Photoexcited metals can produce highly energetic hot carriers whose controlled generation and extraction is a promising avenue for technological applications. While hot-carrier dynamics in Au-group metals have been widely investigated, a microscopic description of the dynamics of photoexcited carriers in the mid-infrared and near-infrared Pt-group metals range is still scarce. Since these materials are widely used in catalysis and, more recently, in plasmonic catalysis, their microscopic carrier dynamics characterization is crucial. We employ ab initio many-body perturbation theory to investigate the hot-carrier generation, relaxation times, and mean free path in bulk Pd and Pt. We show that the direct optical transitions of photoexcited carriers in these metals are mainly generated in the near-infrared range. We also find that the electron-phonon mass enhancement parameter for Pt is 16% higher than Pd, a result that helps explain several experimental results showing diverse trends. Moreover, we predict that Pd (Pt) hot electrons possess total relaxation times of up to 35 fs (24 fs), taking place at approximately 0.5 eV (1.0 eV) above the Fermi energy. Finally, an efficient hot electron generation and extraction can be achieved in nanofilms of Pd (110) and Pd (100) when subject to excitation energies ranging from 0.4 to 1.6 eV.},
keywords = {Applied, Atomic and Molecular Physics, Condensed matter, General Materials Science, General Physics and Astronomy, Multidisciplinary, Physics},
pubstate = {published},
tppubtype = {article}
}
Photoexcited metals can produce highly energetic hot carriers whose controlled generation and extraction is a promising avenue for technological applications. While hot-carrier dynamics in Au-group metals have been widely investigated, a microscopic description of the dynamics of photoexcited carriers in the mid-infrared and near-infrared Pt-group metals range is still scarce. Since these materials are widely used in catalysis and, more recently, in plasmonic catalysis, their microscopic carrier dynamics characterization is crucial. We employ ab initio many-body perturbation theory to investigate the hot-carrier generation, relaxation times, and mean free path in bulk Pd and Pt. We show that the direct optical transitions of photoexcited carriers in these metals are mainly generated in the near-infrared range. We also find that the electron-phonon mass enhancement parameter for Pt is 16% higher than Pd, a result that helps explain several experimental results showing diverse trends. Moreover, we predict that Pd (Pt) hot electrons possess total relaxation times of up to 35 fs (24 fs), taking place at approximately 0.5 eV (1.0 eV) above the Fermi energy. Finally, an efficient hot electron generation and extraction can be achieved in nanofilms of Pd (110) and Pd (100) when subject to excitation energies ranging from 0.4 to 1.6 eV.
2021
A Molina-Sánchez M Marsili, M Palummo
Spinorial formulation of the -BSE equations and spin properties of excitons in two-dimensional transition metal dichalcogenides Journal Article
In: Physical Review B, vol. 103, iss. 15, pp. 155152, 2021, ISSN: 2469-9969 (online), 2469-9950 (print).
Abstract | Links | BibTeX | Tags: and Optics, Atomic and Molecular Physics, General Engineering, General Materials Science, General Physics and Astronomy
@article{nokey,
title = {Spinorial formulation of the -BSE equations and spin properties of excitons in two-dimensional transition metal dichalcogenides},
author = {M Marsili, A Molina-Sánchez, M Palummo, D Sangalli, A Marini},
editor = {American Physical Society},
url = {https://journals.aps.org/prb/pdf/10.1103/PhysRevB.103.155152},
doi = { https://doi.org/10.1103/PhysRevB.103.155152},
issn = {2469-9969 (online), 2469-9950 (print)},
year = {2021},
date = {2021-04-15},
journal = {Physical Review B},
volume = {103},
issue = {15},
pages = {155152},
abstract = {In many paradigmatic materials, such as transition metal dichalcogenides, the role played by the spin degrees of freedom is as important as the one played by the electron-electron interaction. Thus an accurate treatment of the two effects and of their interaction is necessary for an accurate and predictive study of the optical and electronic properties of these materials. Despite the fact that the GW-BSE approach correctly accounts for electronic correlations, the spin-orbit coupling effect is often neglected or treated perturbatively. Recently, spinorial formulations of GW-BSE have become available in different flavors in material-science codes. However, an accurate validation and comparison of different approaches is still missing. In this work, we go through the derivation of the noncollinear GW-BSE approach. The scheme is applied to transition metal dichalcogenides comparing the perturbative and full spinorial approaches. Our calculations reveal that dark-bright exciton splittings are generally improved when the spin-orbit coupling is included nonperturbatively. The exchange-driven intravalley mixing between the A and B excitons is found to play a role for Mo-based systems, being especially strong in the case of MoSe2. We finally compute the excitonic spin and use it to sharply analyze the spinorial properties of transition metal dichalcogenide excitonic states.},
keywords = {and Optics, Atomic and Molecular Physics, General Engineering, General Materials Science, General Physics and Astronomy},
pubstate = {published},
tppubtype = {article}
}
In many paradigmatic materials, such as transition metal dichalcogenides, the role played by the spin degrees of freedom is as important as the one played by the electron-electron interaction. Thus an accurate treatment of the two effects and of their interaction is necessary for an accurate and predictive study of the optical and electronic properties of these materials. Despite the fact that the GW-BSE approach correctly accounts for electronic correlations, the spin-orbit coupling effect is often neglected or treated perturbatively. Recently, spinorial formulations of GW-BSE have become available in different flavors in material-science codes. However, an accurate validation and comparison of different approaches is still missing. In this work, we go through the derivation of the noncollinear GW-BSE approach. The scheme is applied to transition metal dichalcogenides comparing the perturbative and full spinorial approaches. Our calculations reveal that dark-bright exciton splittings are generally improved when the spin-orbit coupling is included nonperturbatively. The exchange-driven intravalley mixing between the A and B excitons is found to play a role for Mo-based systems, being especially strong in the case of MoSe2. We finally compute the excitonic spin and use it to sharply analyze the spinorial properties of transition metal dichalcogenide excitonic states.
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
R Patrick Xian Maciej Dendzik, Enrico Perfetto
Observation of an Excitonic Mott Transition through Ultrafast Core-cum-Conduction Photoemission Spectroscopy Journal Article
In: Physical review letters, vol. 125, iss. 9, pp. 096401, 2020, ISSN: 1079-7114 (online), 0031-9007 (print).
Abstract | Links | BibTeX | Tags: and Optics, Atomic and Molecular Physics, General Engineering, General Materials Science, General Physics and Astronomy
@article{nokey,
title = {Observation of an Excitonic Mott Transition through Ultrafast Core-cum-Conduction Photoemission Spectroscopy},
author = {Maciej Dendzik, R Patrick Xian, Enrico Perfetto, Davide Sangalli, Dmytro Kutnyakhov, Shuo Dong, Samuel Beaulieu, Tommaso Pincelli, Federico Pressacco, Davide Curcio, Steinn Ymir Agustsson, Michael Heber, Jasper Hauer, Wilfried Wurth, Günter Brenner, Yves Acremann, Philip Hofmann, Martin Wolf, Andrea Marini, Gianluca Stefanucci, Laurenz Rettig, Ralph Ernstorfer},
editor = {American Physical Society},
url = {https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.125.096401},
doi = {https://doi.org/10.1103/PhysRevLett.125.096401},
issn = {1079-7114 (online), 0031-9007 (print)},
year = {2020},
date = {2020-08-28},
urldate = {2020-08-28},
journal = {Physical review letters},
volume = {125},
issue = {9},
pages = {096401},
abstract = {Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe2. We present a many-body approximation for the Green’s function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.},
keywords = {and Optics, Atomic and Molecular Physics, General Engineering, General Materials Science, General Physics and Astronomy},
pubstate = {published},
tppubtype = {article}
}
Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe2. We present a many-body approximation for the Green’s function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.
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).
Trovatello, Chiara; Miranda, Henrique P. C.; Molina-Sánchez, Alejandro; Borrego-Varillas, Rocío; Manzoni, Cristian; Moretti, Luca; Ganzer, Lucia; Maiuri, Margherita; Wang, Junjia; Dumcenco, Dumitru; Kis, Andras; Wirtz, Ludger; Marini, Andrea; Soavi, Giancarlo; Ferrari, Andrea C.; Cerullo, Giulio; Sangalli, Davide; Conte, Stefano Dal
Strongly Coupled Coherent Phonons in Single-Layer MoS_2 Journal Article
In: ACS Nano, vol. 14, no. 5, pp. 5700–5710, 2020, ISSN: 1936-086X.
Links | BibTeX | Tags: General Engineering, General Materials Science, General Physics and Astronomy
@article{Trovatello2020,
title = {Strongly Coupled Coherent Phonons in Single-Layer MoS_2},
author = {Chiara Trovatello and Henrique P. C. Miranda and Alejandro Molina-Sánchez and Rocío Borrego-Varillas and Cristian Manzoni and Luca Moretti and Lucia Ganzer and Margherita Maiuri and Junjia Wang and Dumitru Dumcenco and Andras Kis and Ludger Wirtz and Andrea Marini and Giancarlo Soavi and Andrea C. Ferrari and Giulio Cerullo and Davide Sangalli and Stefano Dal Conte},
doi = {10.1021/acsnano.0c00309},
issn = {1936-086X},
year = {2020},
date = {2020-05-26},
journal = {ACS Nano},
volume = {14},
number = {5},
pages = {5700–5710},
publisher = {American Chemical Society (ACS)},
keywords = {General Engineering, General Materials Science, General Physics and Astronomy},
pubstate = {published},
tppubtype = {article}
}
2019
E (Perfetto Perfetto, E. ) ; Sangalli
Pump-driven normal-to-excitonic insulator transition: Josephson oscillations and signatures of BEC-BCS crossover in time-resolved ARPES Journal Article
In: Physical review materials, vol. 3, iss. 12, no. 124601, 2019, ISSN: 2475-9953.
Abstract | Links | BibTeX | Tags: General Materials Science, Multidisciplinary
@article{nokey,
title = {Pump-driven normal-to-excitonic insulator transition: Josephson oscillations and signatures of BEC-BCS crossover in time-resolved ARPES},
author = {Perfetto, E (Perfetto, E.) ; Sangalli, D (Sangalli, D.) ; Marini, A (Marini, A.) ; Stefanucci, G (Stefanucci, G.)},
editor = {AMER PHYSICAL SOC},
doi = {10.1103/PhysRevMaterials.3.124601},
issn = {2475-9953},
year = {2019},
date = {2019-12-16},
urldate = {2019-12-16},
journal = {Physical review materials},
volume = {3},
number = {124601},
issue = {12},
abstract = {We consider a ground-state band insulator turning into a nonequilibrium excitonic insulator (NEQ-EI) upon visiting properly selected and physically relevant highly excited states. The NEQ-EI phase, characterized by self-sustained oscillations of the complex order parameter, neatly follows from a nonequilibrium Green's function treatment on the Konstantinov-Perel' contour. We present the first ab initio band structure of LiF, a ground-state bulk insulator, in different NEQ-EI states. We also show that NEQ-EI states can be generated by currently available pump pulses. Peculiar fingerprints of the NEQ-EI phase in time-resolved angle-resolved photoemission spectroscopy spectra are highlighted: (i) during the pump-driving, the system goes through a BEC-BCS crossover and (ii) concomitantly the excitonic spectral structure undergoes a convex-to-concave shape transition; (iii) attosecond pulses shone after the pump-driving at different times t(delay) produce a photocurrent that oscillates in t(delay) with a pump-tunable frequency-we relate this phenomenon to the ac response of an exotic Josephson junction.},
keywords = {General Materials Science, Multidisciplinary},
pubstate = {published},
tppubtype = {article}
}
We consider a ground-state band insulator turning into a nonequilibrium excitonic insulator (NEQ-EI) upon visiting properly selected and physically relevant highly excited states. The NEQ-EI phase, characterized by self-sustained oscillations of the complex order parameter, neatly follows from a nonequilibrium Green's function treatment on the Konstantinov-Perel' contour. We present the first ab initio band structure of LiF, a ground-state bulk insulator, in different NEQ-EI states. We also show that NEQ-EI states can be generated by currently available pump pulses. Peculiar fingerprints of the NEQ-EI phase in time-resolved angle-resolved photoemission spectroscopy spectra are highlighted: (i) during the pump-driving, the system goes through a BEC-BCS crossover and (ii) concomitantly the excitonic spectral structure undergoes a convex-to-concave shape transition; (iii) attosecond pulses shone after the pump-driving at different times t(delay) produce a photocurrent that oscillates in t(delay) with a pump-tunable frequency-we relate this phenomenon to the ac response of an exotic Josephson junction.
S (Roth Roth, S. ) ; Crepaldi
Photocarrier-induced band-gap renormalization and ultrafast charge dynamics in black phosphorus Journal Article
In: 2D MATERIALS, vol. 6, iss. 3, no. 031001, 2019, ISSN: 2053-1583.
Abstract | Links | BibTeX | Tags: General Materials Science, Multidisciplinary
@article{nokey,
title = {Photocarrier-induced band-gap renormalization and ultrafast charge dynamics in black phosphorus},
author = {Roth, S (Roth, S.) ; Crepaldi, A (Crepaldi, A.) ; Puppin, M (Puppin, M.) ; Gatti, G (Gatti, G.) ; Bugini, D (Bugini, D.) ; Grimaldi, I (Grimaldi, I) ; Barrilot, TR (Barrilot, T. R.) ; Arrell, CA (Arrell, C. A.) ; Frassetto, F (Frassetto, F.) ; Poletto, L (Poletto, L.) ; Chergui, M (Chergui, M.) ; Marini, A (Marini, A.) ; Grioni, M (Grioni, M.) },
editor = {OP Publishing Ltd},
doi = {10.1088/2053-1583/ab1216},
issn = {2053-1583},
year = {2019},
date = {2019-07-01},
urldate = {2019-07-01},
journal = {2D MATERIALS},
volume = {6},
number = {031001},
issue = {3},
abstract = {With its tunable band-gap and its unique optical and electronic properties black phosphorus (BP) opens exciting opportunities for optoelectronic nanotechnology. The band-gap extends from the visible to the mid-infrared spectral range, as a function of sample thickness and external parameters such as electric field and pressure. This, combined with the saturable absorption and in-plane anisotropic optical properties, makes BP a versatile platform for realizing polarization-sensitive photodetectors and absorbers. Although its near-equilibrium properties have been intensively studied, the development of efficient ultrafast optical devices requires detailed knowledge of the temporal dynamics of the photoexcited hot-carriers. Here we address the electronic response of BP to an ultrafast laser excitation, by means of time-and angle-resolved photoelectron spectroscopy. Following the optical excitation, we directly observe a shift of the valence band (VB) position, indicative of band-gap renormalization (BGR). Our data also show that the hole population in the VB relaxes with a characteristic time tau(VB) = 10.2 +/- 1.0 ps, while the lifetime of the electrons accumulated at the minimum of the conduction band is tau(CB) = 1.9 +/- 0.2 ps. The experimental results are well reproduced by ab initio calculations of the out-of-equilibrium electronic properties. Our study sets the reference for the ultrafast carrier dynamics in BP and demonstrates the material's ultrafast BGR, which is promising for optoelectronic switches.},
keywords = {General Materials Science, Multidisciplinary},
pubstate = {published},
tppubtype = {article}
}
With its tunable band-gap and its unique optical and electronic properties black phosphorus (BP) opens exciting opportunities for optoelectronic nanotechnology. The band-gap extends from the visible to the mid-infrared spectral range, as a function of sample thickness and external parameters such as electric field and pressure. This, combined with the saturable absorption and in-plane anisotropic optical properties, makes BP a versatile platform for realizing polarization-sensitive photodetectors and absorbers. Although its near-equilibrium properties have been intensively studied, the development of efficient ultrafast optical devices requires detailed knowledge of the temporal dynamics of the photoexcited hot-carriers. Here we address the electronic response of BP to an ultrafast laser excitation, by means of time-and angle-resolved photoelectron spectroscopy. Following the optical excitation, we directly observe a shift of the valence band (VB) position, indicative of band-gap renormalization (BGR). Our data also show that the hole population in the VB relaxes with a characteristic time tau(VB) = 10.2 +/- 1.0 ps, while the lifetime of the electrons accumulated at the minimum of the conduction band is tau(CB) = 1.9 +/- 0.2 ps. The experimental results are well reproduced by ab initio calculations of the out-of-equilibrium electronic properties. Our study sets the reference for the ultrafast carrier dynamics in BP and demonstrates the material's ultrafast BGR, which is promising for optoelectronic switches.
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.
A (Seidu Seidu, Azimatu) ; Marini
Dynamical correlation effects in a weakly correlated material: Inelastic x-ray scattering and photoemission spectra of beryllium Bachelor Thesis
2018, ISSN: 2469-9950; eISSN 2469-9969.
Abstract | Links | BibTeX | Tags: Condensed matter, General Materials Science, Multidisciplinary, Physics
@bachelorthesis{nokey,
title = {Dynamical correlation effects in a weakly correlated material: Inelastic x-ray scattering and photoemission spectra of beryllium},
author = {Seidu, A (Seidu, Azimatu) ; Marini, A (Marini, Andrea) ; Gatti, M (Gatti, Matteo)},
editor = {AMER PHYSICAL SOC},
doi = {10.1103/PhysRevB.97.125144},
issn = {2469-9950; eISSN 2469-9969},
year = {2018},
date = {2018-03-26},
urldate = {2018-03-26},
journal = {Physical Review B},
volume = {97},
number = {125144},
issue = {12},
abstract = {Beryllium is a weakly correlated simple metal. Still we find that dynamical correlation effects, beyond the independent-particle picture, are necessary to successfully interpret the electronic spectra measured by inelastic x-ray scattering (IXS) and photoemission spectroscopies (PES). By combining ab initio time-dependent density-functional theory (TDDFT) and many-body Green's function theory in the GW approximation (GWA), we calculate the dynamic structure factor, the quasiparticle (QP) properties and PES spectra of bulk Be. We show that band-structure effects (i.e., due to interaction with the crystal potential) and QP lifetimes (LT) are both needed in order to explain the origin of the measured double-peak features in the IXS spectra. A quantitative agreement with experiment is obtained only when LT are supplemented to the adiabatic local-density approximation (ALDA) of TDDFT. Besides the valence band, PES spectra display a satellite, a signature of dynamical correlation due to the coupling of QPs and plasmons, which we are able to reproduce thanks to the combination of the GWA for the self-energy with the cumulant expansion of the Green's function.},
keywords = {Condensed matter, General Materials Science, Multidisciplinary, Physics},
pubstate = {published},
tppubtype = {bachelorthesis}
}
Beryllium is a weakly correlated simple metal. Still we find that dynamical correlation effects, beyond the independent-particle picture, are necessary to successfully interpret the electronic spectra measured by inelastic x-ray scattering (IXS) and photoemission spectroscopies (PES). By combining ab initio time-dependent density-functional theory (TDDFT) and many-body Green's function theory in the GW approximation (GWA), we calculate the dynamic structure factor, the quasiparticle (QP) properties and PES spectra of bulk Be. We show that band-structure effects (i.e., due to interaction with the crystal potential) and QP lifetimes (LT) are both needed in order to explain the origin of the measured double-peak features in the IXS spectra. A quantitative agreement with experiment is obtained only when LT are supplemented to the adiabatic local-density approximation (ALDA) of TDDFT. Besides the valence band, PES spectra display a satellite, a signature of dynamical correlation due to the coupling of QPs and plasmons, which we are able to reproduce thanks to the combination of the GWA for the self-energy with the cumulant expansion of the Green's function.
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.
2016
Pogna, Eva A. A.; Marsili, Margherita; Fazio, Domenico De; Conte, Stefano Dal; Manzoni, Cristian; Sangalli, Davide; Yoon, Duhee; Lombardo, Antonio; Ferrari, Andrea C.; Marini, Andrea; Cerullo, Giulio; Prezzi, Deborah
Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS_2 Journal Article
In: ACS Nano, vol. 10, no. 1, pp. 1182–1188, 2016, ISSN: 1936-086X.
Links | BibTeX | Tags: General Engineering, General Materials Science, General Physics and Astronomy
@article{Pogna2016,
title = {Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS_2},
author = {Eva A. A. Pogna and Margherita Marsili and Domenico De Fazio and Stefano Dal Conte and Cristian Manzoni and Davide Sangalli and Duhee Yoon and Antonio Lombardo and Andrea C. Ferrari and Andrea Marini and Giulio Cerullo and Deborah Prezzi},
doi = {10.1021/acsnano.5b06488},
issn = {1936-086X},
year = {2016},
date = {2016-01-26},
journal = {ACS Nano},
volume = {10},
number = {1},
pages = {1182–1188},
publisher = {American Chemical Society (ACS)},
keywords = {General Engineering, General Materials Science, General Physics and Astronomy},
pubstate = {published},
tppubtype = {article}
}
