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Controlling Photoinduced Transitions with Strong Light Pulses in Condensed Matter.

Periodic Reporting for period 1 - StrongLights (Controlling Photoinduced Transitions with Strong Light Pulses in Condensed Matter.)

Reporting period: 2018-11-01 to 2020-10-31

Since the discovery of the quantum nature of light and matter, many researcher have focus their efforts to understand their interaction. There is no doubt that light is crucial for our current understanding of life since its interactions with biological matter enabled the conversion of the solar energy to the formation of chemical bonds, for example during the photosynthesis.
Scientist investigated this phenomena in order to take advantage of the photoinduced phenomena to rational control desired properties in materials. The development of new theories and the rational understanding of the light-matter interactions enabled the invention of new technologies currently used broadly in our day-to-day life as for example the LEDS (light emitter diodes), light harvesting devices, or pieces of our electronic devices.
This project focused in the rational and theoretical understanding on the effects of shooting a quantum material with ultrashort pulses at the near-infrared (IR) and ultra-violet (UV-VIS) photon laser. These class of materials present a very strong interactions between the electrons and the molecular/lattice vibration leading a multiple phase competition, i.e. photo-induced phase transition (PIPT).
Tuning and controlling the opto-electronic properties of this kind funcional organic materials can lead many commercial applications such as in organic LEDs or organic photodetector industry.
The overall objective of the StrongLights project is understand the key factors that govern the PIPT and how to control them using ultra-strong light pulses.
Three scientific objectives has been stablished for the accomplishment of this project:
1) Theoretical and computational modelling of the low and high temperature phase of the (MeBr-dcnqi)2Cu molecular crystal using density functional theory (DFT).
2) Identify the key factors that govern the initial steps of the PIPT: exciton transfer, charge carrier diffusion and lattice modes dynamics.
3) To investigate the possibility to stabilise a desired phase by applying an IR strong pulse after the PIPT.
"For the success of the StrongLight project we worked mainly in two complementary research lines:
1) Electronic description of the Low Temperature and Hight Temperature phases of the (MeBr-dcnqi)2Cu molecular crystal:
Using a state-of-the-art level of theory such as the use of periodic-DFT at PBE including van der Waals interactions, for the proper description of the pi-system, and Hubbard-U method to include strong-correlations effects due to the on-site Coulomb repulsion, we determined the optimized geometry and the band structure for both phases. We have shown the need of a surprisingly high Hubbard-U value for the π-system in order to reproduce the experimentally observed charge separated phase at low temperature. Besides we determined the possible vibrational changes involved in the initial steps of the PIPT, being the most relevant the change in the copper coordination environment. A scientific paper is under preparation for its publication.
2) Scientific code and methodology developing:
We developed a new method based on a combination of the a local density analysis and a real-time propagation density functional theory (p-TDDFT) at the linear response regime that enable to determine the electronic excitation properties such as the transition dipole moments and transition densities. Besides, we proofed that this method is useful to compute the exciton coupling in complex systems. This method and its implementation in the OCTOPUS code have been published in peer-review journal. In addition, we have worked (and still going on) on a new way to determine molecular excitation spectra using machine learning techniques such as Kernel Ridge Regression and Deep Neural Networks.

Besides within the frame of another collaboration with Berkeley University, we have provide a mid-temperature synthesis of backbone-substituted polycylic aromatic hydrocarbons (PAH). Our DFT calculations demonstrated the plausible reaction mechanisms proving an energy barrier of less than 0.6 eV, in good agreement with the experimental values. The results of this grateful collaboration will soon published in a high impact peer-review journal.

During the reported period of the project, the Researcher has exploited its result by the following ways:
Papers:
Four peer-reviewed scientific publications: 2 published and 2 submitted. Besides, 3 more research papers are in preparation and we plan to submit them during the 2020 year.
Contributions in two conferences:
Oral contribution to the ""Theoretical Chemistry and Computational Modeling: 20 years promoting Excellence in Science (20TCCM)"" , May 30, 2019 to June 01, 2019, Donostia-San Sebastián (SPAIN). Talk title: ""Transition Density Formulation from TDDFT Enables Exciton Coupling Calculations in Large Systems”.
Poster contribution to ""HANDS-ON DFT AND BEYOND: HIGH-THROUGHPUT SCREENING AND BIG-DATA ANALYTICS, TOWARDS EXASCALE COMPUTATIONAL MATERIALS SCIENCE” August 26th to September 6th, 2019, Barcelona, Spain. Poster title: ""Predicting molecular conformers using Machine Learning""
Codes:
All codes developed by the Researcher are under the GNU General Public License, and are publicly available at:
- Octopus Code : https://gitlab.com/octopus-code/octopus
- QuantumLight - The Game :https://gitlab.com/qjornet/quantumlight-game (still in preparation)
Besides, all calculations performed during the reported period have been or will be uploaded to the ab-initio calculation repository of NOMAD (NOvel MAterial Discovery Lab)."
Although the project has finished before the expected date, we went beyond the state-of-the art of this field by determining for the first time the charge separated phase (low temperature) at atomic level.
This will also help the collaborator experimentalist group which could not resolve the X-ray structure because of the instabilities issues with the sample and radiation.
Besides, we have shown the crucial role the on-site electron repulsion takes on the charge separation and hence on the electronic behaviour as insulator/conductor.
The new formulation for the computation of electronic transition properties from real-time TDDFT calculation and its implementation will enable future research to determine excitonic coupling effects on photo-sensible materials, not only in extended systems but also for biological molecular complex.
The project is in a good progress and it will require the characterization of the electron - vibrational coupling and the analysis of the excitation dynamics to fully characterize the PIPT process and determine its control factors.
Graphical representation of the methodology developed during the project period