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Excitons play an essential role in the optical response of two-dimensional materials. These are bound states showing up in the band gaps of many-body systems and are conceived as quasiparticles formed by an electron and a hole. By performing real-time simulations in hBN, we show that an ultrashort (few-fs) UV pulse can produce a coherent superposition of excitonic states that induces an oscillatory motion of electrons and holes between different valleys in reciprocal space, leading to a sizeable exciton migration in real space. We also show that an ultrafast spectroscopy scheme based on the absorption of an attosecond pulse in combination with the UV pulse can be used to read out the laser-induced coherences, hence to extract the characteristic time for exciton migration. This work opens the door towards ultrafast electronics and valleytronics adding time as a control knob and exploiting electron coherence at the early times of excitation. Charge migration is one of the key applications of attosecond science mainly studied in molecular systems. Here the authors propose the extension of this process to two-dimensional materials by exploiting the excitonic interactions typical on these systems, performing numerical simulations to demonstrate the possibility to produce and read exciton migration in monolayer hBN. [ABSTRACT FROM AUTHOR]

To investigate the effects of polymer donors with isomeric alkyl chains on the interfacial charge-transfer (CT) processes, the rates of exciton dissociation (kED) and charge recombination (kCR) have been calculated for complexes PM6/Y6 and PM6i/Y6, where isomer PM6i was designed by shifting the side chains in the electron-withdrawing units to the thiophene-bridges based on PM6. For complex PM6/Y6, kED of PM6 and Y6 exciton are calculated to be 1010–1012 s−1 and 1012–1013 s−1, and kCR is 1011–1012 s−1 for recombination of the lowest CT (CT0) excitons into the lowest triplet states of Y6, which are consistent with the experimental results. When going to complex PM6i/Y6, kED of PM6i exciton is higher than that of PM6 exciton, and of Y6 exciton is decreased to 1011–1012 s−1, but kCR is decreased to 1010–1011 s−1. The calculated results reveal that isomeric alkyl chains of polymer donors can enhance the polymer exciton dissociation and suppress the charge-recombination processes, underlining the important role of isomeric alkyl chains in design of polymer donors for high performance non-fullerene organic photovoltaics. [ABSTRACT FROM AUTHOR]

The first-order phase transition between the liquid and gaseous phases ends at a critical point. Critical opalescence occurs at this singularity. Discovered in 1822, it is known to be driven by diverging fluctuations in the density. During the past two decades, boundaries between the gas-like and liquid-like regimes have been theoretically proposed and experimentally explored. Here, we show that fast cooling of near-critical sulfur hexafluoride (SF6), in presence of Earth's gravity, favors dark opalescence, where visible photons are not merely scattered, but also absorbed. When the isochore fluid is quenched across the critical point, its optical transmittance drops by more than three orders of magnitude in the whole visible range, a feature which does not occur during slow cooling. We show that transmittance shows a dip at 2eV near the critical point, and the system can host excitons with binding energies ranging from 0.5 to 4 eV. The spinodal decomposition of the liquid-gas mixture, by inducing a periodical modulation of the fluid density, can provide a scenario to explain the emergence of this platform for coupling between light and matter. The possible formation of excitons and polaritons points to the irruption of quantum effects in a quintessentially classical context. The first-order phase boundary between the liquid and gaseous phases ends at a critical point where the fluid, kept at thermodynamic equilibrium, displays a turbidity known as 'critical opalescence'. The authors quench a fluid across its critical point, find blackness instead of turbidity, and argue that, out of equilibrium, photons can be absorbed, not merely scattered. [ABSTRACT FROM AUTHOR]