Exciton dynamics in functional materials: new theoretical frontiers

I will present spectroscopic studies in a new material platform based on layered organic metal-chalcogenides, more robust and easily accessible than 2D perovskites, reminiscent of 2D transition metal dichalcogenides. The material tunable excitonic properties allows to access different energy scales and testing correlated many-body phenomena. Our recent experimental work supported by GW-BSE calculations [1] and a study lead by Shuck group at Columbia University [2] demonstrated that layered metal organic chalcogenides host two-dimensional, room temperature stable anisotropic excitons. Moreover, I will show that in the [AgSePh] ∞ system, exciton self-trapping causes a complete and highly effective material de-excitation within a few picoseconds. While fast trapping is often considered detrimental in the context of energy conversion, where charge carriers need to be collected within their lifetime, it opens new possibilities in technologically compelling field of ultrafast photonic switches. In this talk I present a fully abinitio approach to model the generation of non-equilibrium coherent excitonic states with ultra-short laser pulses. The modelling is achieved via the real-time propagation of the density matrix projected in the Kohn-Sham basis set, within the time–dependent Hartree plus Screened EXchange (TD-HSEX) approximation [1]. I show how the generated density matrix can be used to model transient spectroscopy signals in the presence of strongly bound excitons. Using LiF as a prototype material, I show that the scheme is able to capture the exciton signature both in time-resolved angle-resolved photoemission spectroscopy and transient absorption experiments [2, 3]. The approach is completely general and can become the reference scheme for modeling pump and probe experiment in a wide range of materials. Finally, I discuss how the generated coherent excitonic states can be related to non-coherent states with a thermal distribution of excitons, and how to compute their lifetimes [4, 5]. Abstract: Moiré heterostructures of transition metal dichalcogenide (TMD) exhibit novel optical phenomena, such as the emergence of interlayer excitons. The nature of interlayer and intralayer excitons depends on the twist angle, allowing for the tunability of their decay mechanisms and lifetimes. However, to date, most theoretical calculations of exciton properties in twisted TMD heterostructures are based on model moiré potentials. An ab-initio understanding of the excitons in these systems is crucial to not only interpret experiments, but also design heterostructures with desired properties. In this work, we study interlayer and intralayer excitons in a twisted MoS 2 /MoSe 2 heterostructure using many-body perturbation theory. We present a new approach to unfold moiré excitons from the moiré Brillouin zone to the Brillouin zones of the pristine layers composing the heterostructures. Applying this method to the twisted MoS 2 /MoSe 2 heterostructure, we find that the optically forbidden momentum-indirect electron-hole transitions become allowed in the heterostructure. Our findings suggest a pathway to control and brighten dark excitons in TMDs by tuning the twist angle. Developing a predictive first principles framework to accurately describe phonon mediated (or limited) exciton diffusion in complex materials remains an open challenge. In organic semiconductors - a class of promising photovoltaic materials with strong light-matter interactions and near endless chemical tunability - understanding exciton transport is further complicated by the fact that exciton bandwidths and exciton-phonon coupling strengths are similar in magnitude. In turn it is unclear a priori whether exciton diffusion is best described by phonon-limited Boltzmann-like or thermally activated hopping-like theories. Several computational approaches have been put forward to understand exciton dynamics in the hopping or band-like regime separately; however, to date, few approaches exist which are general enough to be applied to both regimes. In this talk, using state-of-the-art density functional perturbation theory and ab initio Bethe-Salpeter equation approach, we put forward a self-contained framework for computing exciton diffusion coefficients in both the band-like and hopping exciton-polaron regimes. Our reciprocal-space based, linear response method explicitly takes into account the entire crystalline environment and can seamlessly be applied to study both spin-singlet and -triplet excitations. We apply our method to a select set of acene crystals shedding additional light on microscopic origins of exciton diffusion in these classic materials. We utilize first-principles theory to investigate the optoelectronic properties of a series of monolayer materials, emphasizing the role of electron-phonon interactions, a phenomenon that can dominate the properties of low dimensional systems due to their reduced screening. We utilize first-principles density functional theory (DFT) and many-body perturbation theory (MBPT) to describe excited state transitions and the special displacement method recently developed by Zacharias and Giustino to describe the role of phonons. For monolayer GeSe, a promising monochalcogenide material, our calculations predict that the optical absorption spectrum is red-shifted by ~ 0.1 eV and that the Wannier exciton wavefunction is distorted due to electron-phonon interactions, with optical phonons at ~100 cm -1 coupling most strongly to the excitonic state. To better understand the role of exciton-phonon interactions in low dimensions, we study the band gap renormalization for a series of 2D materials and show that the strength of electron-phonon interactions is highly dependent on the bonding structure. Overall, this framework allows for a systematic theoretical exploration of exciton-phonon coupling. [1] Huang, Lewis, F. S. J. Phys. Chem. Lett. , 12 , 3802 (2021) The creation, transfer, and stabilization of localized excitations are studied in a donor−acceptor Frenkel exciton model in an atomistic treatment of reduced-size double quantum dots (QDs) of various sizes [1]. The explicit time-dependent dynamics simulations carried out by hybrid time-dependent density functional theory/configuration interaction [2] show that laser-controlled hole trapping in stacked, coupled germanium/silicon quantum dots can be achieved by a UV/IR pump−dump pulse sequence. The first UV excitation creates an exciton localized on the topmost QD and after some coherent transfer time, an IR pulse dumps and localizes an exciton in the bottom QD. While hole trapping is observed in each excitation step, we show that the stability of the localized electron depends on its multiexcitonic character. We present how size and geometry variations of three Ge/Si nanocrystals influence transfer times and thus the efficiency of laser-driven populations of the electron−hole pair states. Krause, Bande, Excitonic insulators represent an exotic state of matter which arise from the spontaneous formation of excitons [1] and are expected to give rise to properties such as high temperature superconductivity. The experimental realization of such states is still a challenge. In the first part of the talk, we propose heterostructures of TMDC monolayers and organic molecules as a platform to realize excitonic insulators exploiting spatially indirect interlayer excitons. We calculate for the exemplary excitonic insulating candidate WS2- F6TCNNQ the phase diagram as function of the experimentally relevant parameters: temperature, gap energy, and dielectric environment. The distinct excitonic phases show unique signatures in far-infrared to THz spectroscopy. In the second part of the talk, we will focus on the formation of non-equilibrium steady states in monolayers of TMDCs during continuous wave excitation experiments. While cw experiments are extensively performed, the underlying exciton distributions which form upon continuous wave excitation have not been studied so far. Here, we apply a microscopic theory for the exciton dynamics [2], to investigate the interplay of intervalley Coulomb exchange coupling, exciton phonon scattering [3] and exciton recombination to the formation of nonequilibrium steady states after continuous wave pumping. We clarify, how the formation of the resulting stationary states contributes to the radiative emission of TMDC excitons. Interestingly, our calculations reveal sharp features in the degree of polarization of the emitted light as a function of excess energy originating from optical phonon transitions which dominate the exchange coupling. Engineering material properties via the strong light-matter coupling that can be achieved by embedding the material in an optical cavity is an attractive avenue in material design. Here, I present first principles-based results on the formation of exciton-light hybrid states in 2D crystals embedded in a cavity and illustrate the control over their energetics and brightness. I show how the strong light-matter coupling allows for the design of a three-way exciton-phonon- photon quasiparticle which shows unique features in optical response. Finally, I introduce the concept of a photo-ground state by demonstrating that the vacuum fluctuations of light can induce a change of the collective phase from paraelectric to ferroelectric in the ground state of SrTiO3, which has thus far only been achieved in out-of equilibrium strongly excited conditions. These findings demonstrate the potential of cavity material engineering as a new paradigm for material manipulation. In this talk we address the non-equilibrium properties of the exciton superfluid generated when a semiconductor is illuminated by laser pulses resonant with a bright excitonic energy. This transient state is characterized by coherent oscillations of the complex superfluid condensate that induce a subgap excitonic sideband in the time-resolved ARPES spectrum [