scholarly journals Directly visualizing the momentum-forbidden dark excitons and their dynamics in atomically thin semiconductors

Science ◽  
2020 ◽  
Vol 370 (6521) ◽  
pp. 1199-1204 ◽  
Author(s):  
Julien Madéo ◽  
Michael K. L. Man ◽  
Chakradhar Sahoo ◽  
Marshall Campbell ◽  
Vivek Pareek ◽  
...  
Keyword(s):  
Excited State ◽  
Degrees Of Freedom ◽  
State Distribution ◽  
Electron Hole ◽  
Tungsten Diselenide ◽  
Momentum State ◽  
Near Degeneracy ◽  
Electronic Technologies ◽  
Surprising Finding ◽  
Coulomb Attraction

Resolving momentum degrees of freedom of excitons, which are electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained an elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum-forbidden dark excitons, which critically affect proposed opto-electronic technologies but are not directly accessible using optical techniques. Here, we probed the momentum state of excitons in a tungsten diselenide monolayer by photoemitting their constituent electrons and resolving them in time, momentum, and energy. We obtained a direct visual of the momentum-forbidden dark excitons and studied their properties, including their near degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominated the excited-state distribution, a surprising finding that highlights their importance in atomically thin semiconductors.

scholarly journals Chemical potential and quantum Hall ferromagnetism in bilayer graphene

Science ◽  
2014 ◽  
Vol 345 (6192) ◽  
pp. 58-61 ◽  
Author(s):  
Kayoung Lee ◽  
Babak Fallahazad ◽  
Jiamin Xue ◽  
David C. Dillen ◽  
Kyounghwan Kim ◽  
...  
Keyword(s):  
Degrees Of Freedom ◽  
Chemical Potential ◽  
Hexagonal Boron Nitride ◽  
Quantum Hall ◽  
Bilayer Graphene ◽  
Zero Magnetic Field ◽  
High Magnetic Fields ◽  
Complex Interplay ◽  
Electron Hole ◽  
Quantum Hall State

Bilayer graphene has a distinctive electronic structure influenced by a complex interplay between various degrees of freedom. We probed its chemical potential using double bilayer graphene heterostructures, separated by a hexagonal boron nitride dielectric. The chemical potential has a nonlinear carrier density dependence and bears signatures of electron-electron interactions. The data allowed a direct measurement of the electric field–induced bandgap at zero magnetic field, the orbital Landau level (LL) energies, and the broken-symmetry quantum Hall state gaps at high magnetic fields. We observe spin-to-valley polarized transitions for all half-filled LLs, as well as emerging phases at filling factors ν = 0 and ν = ±2. Furthermore, the data reveal interaction-driven negative compressibility and electron-hole asymmetry in N = 0, 1 LLs.

scholarly journals TheoDORE: a Toolbox for a Detailed and Automated Analysis of Electronic Excited State Computations

2019 ◽  
Author(s):  
Felix Plasser
Keyword(s):  
Excited State ◽  
Conjugated Polymer ◽  
Predictive Power ◽  
Transition Metal Complex ◽  
Automated Analysis ◽  
Automated Assignment ◽  
Correlation Effects ◽  
Electron Hole ◽  
Repetitive Tasks ◽  
Electronic Structure Methods

<p>The advent of ever more powerful excited-state electronic structure methods has lead to a tremendous increase in the predictive power of computation but it has also rendered the analysis of these computations more and more challenging and time-consuming. TheoDORE tackles this problem through providing tools for post-processing excited-state computations, which automate repetitive tasks and provide rigorous and reproducible descriptors. Interfaces are available for ten different quantum chemistry codes and a range of excited-state methods implemented therein. This article provides an overview of three popular functionalities within TheoDORE, a fragment-based analysis for assigning state character, the computation of exciton sizes for measuring charge transfer, and the natural transition orbitals used not only for visualisation but also for quantifying multiconfigurational character. Using the examples of an organic push-pull chromophore and a transition metal complex, it is shown how these tools can be used for a rigorous and automated assignment of excited-state character. In the case of a conjugated polymer, we venture beyond the limits of the traditional molecular orbital picture to uncover spatial correlation effects using electron-hole correlation plots and conditional densities.</p>

EXCITED STATE PHOTOLUMINESCENCE IN STEPPED InGaAs/AlGaAs QUANTUM WELLS UNDER PICOSECOND EXCITATION

2003 ◽  
Vol 02 (06) ◽  
pp. 427-435
Author(s):  
V. A. SHALYGIN ◽  
L. E. VOROBJEV ◽  
V. Yu. PANEVIN ◽  
D. A. FIRSOV ◽  
S. HANNA ◽  
...  
Keyword(s):  
Excited State ◽  
Quantum Wells ◽  
Special Interest ◽  
Population Inversion ◽  
Auger Recombination ◽  
Stimulated Emission ◽  
Energy Structure ◽  
Electron Hole ◽  
Picosecond Excitation ◽  
Electron Energy Structure

The technique of photoluminescence (PL) studies based on intense picosecond excitation of electron–hole pairs is applied to investigate the electron energy structure including the positions of high-lying excited levels in stepped quantum wells (QWs). The spectra of PL in regimes of spontaneous and stimulated emission are studied under different excitation levels and light polarizations. Of special interest are intense photoluminescence signals from excited subbands. The feasibility of a e3–e2 intersubband population inversion in stepped QWs is demonstrated and the influence of Auger recombination was examined.

Ground and excited state dipole moments of planar vs. twisted p-N,N-(dimethylamino)benzonitrile systems: maximum charge transfer for minimum overlap

1992 ◽  
Vol 70 (7) ◽  
pp. 1932-1938 ◽  
Author(s):  
Hemant K. Sinha ◽  
S. Muralidharan ◽  
Keith Yates
Keyword(s):  
Excited State ◽  
Charge Transfer ◽  
Degrees Of Freedom ◽  
Theoretical Calculations ◽  
Dipole Moments ◽  
Torsional Angle ◽  
Dimethylamino Group ◽  
Maximum Charge ◽  
Donor Acceptor ◽  
Excited State Dipole Moments

Electric field induced change in the absorption spectrum (electrochromism) has been employed to obtain the ground and excited state dipole moments of planar and sterically hindered (twisted) p-N,N-(dimethylamino)benzonitriles in dioxane solution. These studies support the twisted intramolecular charge transfer (TICT) hypothesis and provide additional insight to the TICT concept. The charge transfer nature of the excited state has been found to directly depend on the torsional angle of the N,N-dimethylamino group with respect to the benzonitrile moiety. It is suggested that solvent coupling is essential to initiate twisting by affecting the intramolecular degrees of freedom and the existence of the highly dipolar excited state is a result of such twisting of the donor–acceptor bond. Theoretical calculations have been performed to explain the observed changes in dipole moment values.

scholarly journals Where are the degrees of freedom responsible for black-hole entropy?

2008 ◽  
Vol 86 (4) ◽  
pp. 653-658 ◽  
Author(s):  
S Das ◽  
S Shankaranarayanan ◽  
S Sur
Keyword(s):  
Black Hole ◽  
Excited State ◽  
Power Law ◽  
Ground State ◽  
Degrees Of Freedom ◽  
Black Hole Entropy ◽  
Quantum Field ◽  
Hole Area ◽  
Area Law ◽  
03.65 Ud

Considering the entanglement between quantum field degrees of freedom inside and outside the horizon as a plausible source of black-hole entropy, we address the question: where are the degrees of freedom that give rise to this entropy located? When the field is in ground state, the black-hole area law is obeyed and the degrees of freedom near the horizon contribute most to the entropy. However, for excited state, or a superposition of ground state and excited state, power-law corrections to the area law are obtained, and more significant contributions from the degrees of freedom far from the horizon are shown.PACS Nos.: 04.60.–m, 04.62., 04.70.–s, 03.65.Ud

scholarly journals TheoDORE: a Toolbox for a Detailed and Automated Analysis of Electronic Excited State Computations

2019 ◽  
Author(s):  
Felix Plasser
Keyword(s):  
Excited State ◽  
Conjugated Polymer ◽  
Predictive Power ◽  
Transition Metal Complex ◽  
Automated Analysis ◽  
Automated Assignment ◽  
Correlation Effects ◽  
Electron Hole ◽  
Repetitive Tasks ◽  
Electronic Structure Methods

<p>The advent of ever more powerful excited-state electronic structure methods has lead to a tremendous increase in the predictive power of computation but it has also rendered the analysis of these computations more and more challenging and time-consuming. TheoDORE tackles this problem through providing tools for post-processing excited-state computations, which automate repetitive tasks and provide rigorous and reproducible descriptors. Interfaces are available for ten different quantum chemistry codes and a range of excited-state methods implemented therein. This article provides an overview of three popular functionalities within TheoDORE, a fragment-based analysis for assigning state character, the computation of exciton sizes for measuring charge transfer, and the natural transition orbitals used not only for visualisation but also for quantifying multiconfigurational character. Using the examples of an organic push-pull chromophore and a transition metal complex, it is shown how these tools can be used for a rigorous and automated assignment of excited-state character. In the case of a conjugated polymer, we venture beyond the limits of the traditional molecular orbital picture to uncover spatial correlation effects using electron-hole correlation plots and conditional densities.</p>

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