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## Quantum Dynamics in Molecular and Nano-Materials: Mechanisms and Functionality

#### Tel Aviv University, Israel

#### Organisers

The description of the generation of photocurrent in molecular/nano-junctions is an extremely difficult task. In fact, one may consider this to be the “nightmare” for theorists. This is because transport is a nonequilibrium problem which requires the description of the dynamics of electrons, holes, excitons, coupled to phonons and electromagnetic fields, in an open system. In the language of quantum mechanics, this is a strongly interacting vibronic system in which the Born-Oppenheimer approximation breaks down while at the same time it is driven away from equilibrium by the applications of electromagnetic fields and bias potentials. An exact solution is only available for simple, non-interacting, model systems. Otherwise, approximate techniques and simulation techniques are required.

The main idea of this workshop is to bring together experts in quantum dynamics and quantum transport from different communities and to explore different methodologies developed to tackle this challenging problem. We will be focusing on theoretical approaches ranging from explicit quantum dynamics for wave functions [1] and density operators to Green's function techniques, [2] time dependent density functional approaches, [3] master equations, [4] and mixed quantum-classical strategies. [5]

Both nuclear and electronic dynamical processes are involved in these systems and in both types of processes, mean-field treatments are not adequate. Thus, correlations play a key role, both with regard to electronic interactions, electron-phonon and phonon-phonon couplings. In the context of the nano-structured systems at hand, this poses a formidable challenge to practically all of the methods outlined above. Much activity in the field is thus, directed at pushing the limits of the existing techniques. The proposed workshop will provide a snapshot of this very active field and aims to stimulate the development of new directions.

Current advances in materials design and fabrication of electronic devices continue to press towards the molecular level. The engineering of such devices is predicated upon a deep understanding of their fundamental, and often quantum mechanical, properties that give rise to a number of unique effects. Consequently, simultaneous strides have been made in the theoretical understanding and prediction of the properties of these new materials as well as in the experimental detection and characterization of these properties. Thus, seemingly disjoint types of systems like organic semiconducting polymers and nano-crystalline materials are found to exhibit similarities in the elementary photo-physical pathways that eventually determine their function. Likewise, the processes at single molecule junctions and interfaces (heterojunctions) are sensitive to the intrinsic molecular properties of their molecular constituents.

By the fact that a quantum description of the electronic structure and electronic and nuclear dynamics is of key importance for the understanding of the elementary processes in these systems, the basic theoretical tools are closely related to techniques that have been developed in the context of atomic and molecular physics, theoretical chemistry, system-bath theory, quantum transport theory, scattering theory, etc. However, the novelty and complexity of the systems under study requires re-thinking and adapting existing methods. The recent literature -- both experimental and theoretical -- reflects this process in several key areas:

• Quantum transport in the specific context of electronic conduction through single-molecule junctions and single nano-junctions. [6]

• System-environment interactions adapted to transport phenomena, involving bosonic and fermionic reservoirs and including particle exchange between system and reservoirs. [7]

• Exciton dissociation dynamics and multiexciton generation (singlet fission) in organic materials and semiconductor nanocrystals. [8]

• Excitation transfer and migration, trapping and dissociation in nano-structured molecular arrays. [9]

To tackle these systems, a considerable range of theoretical methods are applied, including explicit quantum dynamics, various mixed quantum-classical techniques, multidimensional vibronic coupling models, on-the-fly non-Born-Oppenheimer dynamics, density functional theory (DFT) and its time-dependent extension (TD-DFT), non-equilibrium many-body Green's function techniques, path integral approaches, master equations, to name but a few approaches. To obtain a quantitative insight, it is now recognized that all of these methods need to be adapted so as to include electron-nuclear couplings, as well as electronic, nuclear, and electronic-nuclear correlations. Furthermore, the theoretical treatment is often complicated by the fact that various dynamical effects compete and interfere (for example, exciton migration can typically interfere with phonon-assisted exciton dissociation yielding polaron pairs).

In a vast majority of cases, the relevant dynamical regimes are such that standard perturbative assumptions like weak coupling, adiabatic separability of nuclear vs. electronic motions, separation of time scales between system vs. bath degrees of freedom (Markovian limit), and rapid dephasing of quantum coherence, are not valid. This is highlighted by a number of recent time-resolved experiments which demonstrate the importance of vibronic coupling effects and the role of vibrational and electronic coherence which is often found to be surprisingly long-lived despite the presence of the environment.

While the overall scenario poses a formidable challenge to the existing theoretical techniques, the advances in ultrafast time-resolved and multi-dimensional experiments provide a unique opportunity to guide and test new theory developments. Against this background, the present workshop aims to give an overview of current state-of-the-art methodology and various new approaches, while offering direct exchange with leading experimentalists. .

## References

**Israel**

Jacqueline Gorsky (Tel-Aviv University) - Organiser

Eran Rabani (Tel Aviv University and University of California, Berkeley) - Organiser