Exciton-phonon Dynamics Research Articles

Interplay of coulomb and exciton-phonon coupling controls singlet fission dynamics in two pentacene polymorphs.

Pentacene is an important model organic semiconductor in both the singlet exciton fission (SF) and organic electronics communities. We have investigated the effect of changing crystal structure on the SF process, generating multiple triplet excitons from an initial singlet exciton, and subsequent triplet recombination. Unlike for similar organic semiconductors that have strong SF sensitive to polymorphism, we find almost no quantitative difference between the kinetics of triplet pair (TT) formation in the two dominant polymorphs of pentacene. Both pairwise dimer coupling and momentum-space crystal models predict much faster TT formation from the bright singlet excited state of the Bulk vs ThinFilm polymorph, contrasting with the experiment. GW and Bethe-Salpeter equation calculations, including exciton-phonon coupling, reveal that ultrafast phonon-driven transitions in the ThinFilm polymorph compensate the intrinsically slower purely Coulomb-mediated TT formation channel, rationalizing the similarity in observed rates. Taking into account the influence of subtle structural distinctions on both the direct and phonon-mediated SF pathways reveals a predictive capability to these methods, expected to be applicable to a wide variety of molecular crystals.

Coherent Phonon Assisted Ultrafast Order-Parameter Reversal and Hidden Metallic State in Ta_{2}NiSe_{5}.

The nonequilibrium dynamics during photoinduced insulator-to-metal transition (IMT) in the excitonic insulator (EI) candidate Ta_{2}NiSe_{5} have been investigated, which reproduce the timescale and spectral features of the ultrafast switch and reveal intricate many-body interactions involving multidegrees of freedom. The key role of lattice order parameter (OP) reversal, occurring on a timescale comparable to that of purely electronic processes (<100 fs), is identified. This reversal is enabled by the anharmonic interactions between EI-OP-coupled phonons and the conventional coherent phonons, leading to a modified potential energy landscape and a high-frequency mode up-conversion. The phonon excitation depends on the dynamics of photocarriers distributed around the Fermi level, and thus intertwines with the excitonic quenching and the complete gap collapse. These findings provide a comprehensive understanding of exciton-phonon dynamics in correlated quantum materials.

Indirect Exciton–Phonon Dynamics in MoS2 Revealed by Ultrafast Electron Diffraction

AbstractTransition metal dichalcogenides layered nano‐crystals are emerging as promising candidates for next‐generation optoelectronic and quantum devices. In such systems, the interaction between excitonic states and atomic vibrations is crucial for many fundamental properties, such as carrier mobilities, quantum coherence loss, and heat dissipation. In particular, to fully exploit their valley‐selective excitations, one has to understand the many‐body exciton physics of zone‐edge states. So far, theoretical and experimental studies have mainly focused on the exciton–phonon dynamics in high‐energy direct excitons involving zone‐center phonons. Here, ultrafast electron diffraction and ab initio calculations are used to investigate the many‐body structural dynamics following nearly‐ resonant excitation of low‐energy indirect excitons in MoS2. By exploiting the large momentum carried by scattered electrons, the excitation of in‐plane K‐ and Q‐ phonon modes are identified with 𝑬′ symmetry as key for the stabilization of indirect excitons generated via near‐infrared light at 1.55 eV, and light is shed on the role of phonon anharmonicity and the ensuing structural evolution of the MoS2 crystal lattice. The results highlight the strong selectivity of phononic excitations directly associated with the specific indirect‐ exciton nature of the wavelength‐dependent electronic transitions triggered in the system.

Fully Quantum Modeling of Exciton Diffusion in Mesoscale Light Harvesting Systems.

It has long been a challenge to accurately and efficiently simulate exciton–phonon dynamics in mesoscale photosynthetic systems with a fully quantum mechanical treatment due to extensive computational resources required. In this work, we tackle this seemingly intractable problem by combining the Dirac–Frenkel time-dependent variational method with Davydov trial states and implementing the algorithm in graphic processing units. The phonons are treated on the same footing as the exciton. Tested with toy models, which are nanoarrays of the B850 pigments from the light harvesting 2 complexes of purple bacteria, the methodology is adopted to describe exciton diffusion in huge systems containing more than 1600 molecules. The superradiance enhancement factor extracted from the simulations indicates an exciton delocalization over two to three pigments, in agreement with measurements of fluorescence quantum yield and lifetime in B850 systems. With fractal analysis of the exciton dynamics, it is found that exciton transfer in B850 nanoarrays exhibits a superdiffusion component for about 500 fs. Treating the B850 ring as an aggregate and modeling the inter-ring exciton transfer as incoherent hopping, we also apply the method of classical master equations to estimate exciton diffusion properties in one-dimensional (1D) and two-dimensional (2D) B850 nanoarrays using derived analytical expressions of time-dependent excitation probabilities. For both coherent and incoherent propagation, faster energy transfer is uncovered in 2D nanoarrays than 1D chains, owing to availability of more numerous propagating channels in the 2D arrangement.

Pronounced exciton and coherent phonon dynamics in BiI3.

Trivalent bismuth compounds featuring octahedral halide coordination, such as BiI3, double-perovskite structures and vacancy-ordered perovskite derivatives, have recently attracted considerable attention as potential environmentally benign thin film materials for solar light-harvesting and optoelectronic applications aiming to replace toxic lead-based perovskites. Yet, their photovoltaic performance still lags considerably behind their lead-counterparts. Improving their performance requires a detailed knowledge of the photoinduced carrier dynamics and kinetics in these systems. In the current study, we present a detailed investigation of layered BiI3. The compound exhibits pronounced exciton dynamics which is associated with a characteristic second-derivative type feature in the transient absorption spectra. Accurate time constants are assigned to carrier cooling and recombination processes. Pronounced coherent oscillations are observed in the kinetic traces indicating strong electron-phonon coupling. Analysis of the coherences recovers the steady-state Raman spectrum of BiI3 with a dominant mode of Ag symmetry at 114 cm-1, associated with the zone center C(Γ) phonon. The line position is well reproduced by DFT calculations. In terms of the carrier lifetime, high absorption coefficient and band-gap position, BiI3 is a promising candidate as a photovoltaic material but challenges posed are the difficulty to split the strongly bound excitons which are confined in the 2D layers and the efficient extraction of electrons and holes at n- and p-selective contacts, beside well-known issues regarding high resistivity and the intrinsic softness of this material.

Exciton-phonon dynamics on complex networks: Comparison between a perturbative approach and exact calculations.

A method combining perturbation theory with a simplifying ansatz is used to describe the exciton-phonon dynamics in complex networks. This method, called PT^{*}, is compared to exact calculations based on the numerical diagonalization of the exciton-phonon Hamiltonian for eight small-sized networks. It is shown that the accuracy of PT^{*} depends on the nature of the network, and three different situations were identified. For most graphs, PT^{*} yields a very accurate description of the dynamics. By contrast, for the Wheel graph and the Apollonian network, PT^{*} reproduces the dynamics only when the exciton occupies a specific initial state. Finally, for the complete graph, PT^{*} breaks down. These different behaviors originate in the interplay between the degenerate nature of the excitonic energy spectrum and the strength of the exciton-phonon interaction so that a criterion is established to determine whether or not PT^{*} is relevant. When it succeeds, our study shows the undeniable advantage of PT^{*} in that it allows us to perform very fast simulations when compared to exact calculations that are restricted to small-sized networks.

Coupled fractional nonlinear differential equations and exact Jacobian elliptic solutions for exciton–phonon dynamics

Abstract An improved quantum model for exciton–phonon dynamics in an α-helix is investigated taking into account the interspine coupling and the influence of power-law long-range exciton–exciton interactions. Having constructed the model Hamiltonian, we derive the lattice equations and employ the Fourier transforms to go in continuum space showing that the long-range interactions (LRI) lead to a nonlocal integral term in the equations of motion. Indeed, the non-locality originating from the LRI results in the dynamic equations with space derivatives of fractional order. New theoretical frameworks are derived, such that: fractional generalization of coupled Zakharov equations, coupled nonlinear fractional Schrodinger equations, coupled fractional Ginzburg–Landau equations, coupled Hilbert–Zakharov equations, coupled nonlinear Hilbert–Ginzburg–Landau equations, coupled nonlinear Schrodinger equations and coupled nonlinear Hilbert–Schrodinger equations. Through the F-expansion method, we derive a set of exact Jacobian solutions of coupled nonlinear Schrodinger equations. These solutions include Jacobian periodic solutions as well as bright and dark soliton which are important in the process of energy transport in the molecule. We also discuss of the impact of LRI on the energy transport in the molecule.

Communication: Exciton–phonon information flow in the energy transfer process of photosynthetic complexes

Non-Markovian and nonequilibrium phonon effects are believed to be key ingredients in the energy transfer in photosynthetic complexes, especially in complexes which exhibit a regime of intermediate exciton-phonon coupling. In this work, we utilize a recently developed measure for non-Markovianity to elucidate the exciton-phonon dynamics in terms of the information flow between electronic and vibrational degrees of freedom. We study the measure in the hierarchical equation of motion approach which captures strong coupling effects and nonequilibrium molecular reorganization. We propose an additional trace distance measure for the information flow that could be extended to other master equations. We find that for a model dimer system and for the Fenna-Matthews-Olson complex the non-Markovianity is significant under physiological conditions.

Parametric resonance-induced time-convolutionless master equation breakdown in finitesize exciton–phonon systems

A detailed analysis is performed to show that the second order time-convolutionless masterequation fails to describe the exciton–phonon dynamics in a finite size lattice. To proceed,special attention is paid to characterizing the coherences of the exciton reduced densitymatrix. These specific elements measure the ability of the exciton to developsuperimpositions involving the vacuum and the one-exciton states. It is shown that thecoherences behave as wavefunctions whose dynamics is governed by a time-dependenteffective Hamiltonian defined in terms of the so-called time-dependent relaxationoperator. Due to the confinement, quantum recurrences provide to the relaxationoperator an almost periodic nature, so the master equation reduces to a linearsystem of differential equations with almost periodic coefficients. We show that, inaccordance with the Floquet theory, unstable solutions emerge due to parametricresonances involving specific frequencies of the relaxation operator and specificexcitonic eigenfrequencies. These resonances give rise to an unphysical exponentialgrowth of the coherences, indicating the breakdown of the second order masterequation.

Dynamics of a narrow-band exciton coupled with optical phonons: A time-convolutionless master-equation approach

A time-convolutionless master equation is established for studying the dynamics of a narrow-band exciton coupled with optical phonons. Within the nonadiabatic weak-coupling limit, the diagonal hypothesis works quite well so that the exciton-phonon dynamics is mainly governed by the so-called time-dependent dephasing function. It has been shown that the dephasing function tends to zero by exhibiting damped oscillations that characterize a series of dephasing-rephasing mechanisms. Indeed, the correlation time of the exciton-phonon interaction is defined as the time needed to the exciton to cover a few lattice sites. Therefore this correlation time is sufficiently long so that the system dynamics remains sensitive to the coherent nature of the lattice vibrations. Because the phonon memory recurs periodically, the exciton experiences a series of dephasing-rephasing processes. Although each rephasing does not exactly compensate the previous dephasing, the coherence survives. Consequently, the exciton keeps its wavelike nature and a coherent energy transfer occurs according to an effective hopping constant smaller than the bare hopping constant.

Memory and correlation effects in the exciton–phonon kinetics

AbstractMemory and correlation effects in the interband absorption from quantum wells due to exciton–phonon dynamics are investigated. They are traced back to the frequency dependence and matrix character of the self energy arising in a 2nd Born theory. It is found that interpeak absorption increases with respect to the case in which this memory and correlation effects are neglected. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Exciton–phonon complexes and optical properties in CdSe nanocrystals

A method to calculate the quantum states of exciton–phonon complexes in semiconductornanocrystals is presented. The exciton–phonon complexes are built from a basis set made ofproducts of phonon states and electron–hole pairs, which are coupled through theelectron–phonon Fröhlich interaction, and the electron–hole Coulomb and exchangeinteractions. In CdSe nanocrystals, the conduction band electrons are describedby the effective mass equation, while the holes are represented by the spherical4 × 4 Baldereschi–Lipari Hamiltonian. It is shown that a flexible and complete electron–hole basis, not limitedto the 1s–1S3/2 octet, is essential to obtain converged eigenvalues and the correct polaron shift to theexciton energy. A study of the spectral properties is presented; in particular, the spectralregion which involves the lowest exciton–phonon complex eigenstates is analysed in details.Specifically, the non-adiabatic nature of the exciton–phonon dynamics in thenanocrystals examined is clearly shown by the vibron eigenstates that were obtained.

Resonance Raman Spectra and Time-Resolved Emission Spectra in Exciton-Phonon Systems: Calculation in the Dynamical Coherent Potential Approximation

Light scattering spectra, both stationary response for monochromatic excitation and time-resolved response for pulse excitation, at zero temperature are calculated in the model of a Frenkel exciton coupled to Einstein phonons, by using the dynamical coherent potential approximation. For resonant excitation, calculated spectra are characterized by hot luminescences from a nearly free exciton state and also from vibronic states of a self-trapped exciton. Calculated time-resolved spectra manifest exciton-phonon dynamics involving these states, such as configurational tunneling between the free state and the self-trapped state.

Excitons in polar semiconductors

A brief review of the main influences of the exciton-phonon interaction on exciton properties in polar semiconductors is presented. We make use of the great flexibility of the variational approach to the involved mathematical problem in order to be able to present a number of applications. The effects discussed range from binding energy shifts and corresponding changes of oscillator strengths, over exciton dispersion and phonon side bands to excitons in a magnetic field and quenching of diamagnetic shifts due to phonon- and anisotropy-influences. Finally, a short outlook is presented by discussing some results in nonlinear exciton-phonon dynamics.