Controlling ELECTROnic phase transitions in molecular materials by non-linear PHONonics – ELECTROPHONE

For this first period, we focused our attention on 3 essential aspects of the project, focusing on the following points:
1 development of a femtosecond infrared pump / optical probe experiment.
2 studies and characterization of vibration modes by IR and Raman spectroscopy at equilibrium.
3 theoretical approach and calculation of vibration modes.
To perform the nonlinear phononic experiments, it is important to understand the nature of the vibration modes and the coupling between the IR modes observed experimentally and the Raman modes driving the change of spin state. The link with the theoretical approach is therefore essential.

Developing nonlinear phononic experiments requires a perfect knowledge of the vibration modes of the materials studied, in their different states: nature of the vibration mode, symmetry, frequency. Various Raman and IR spectroscopy measurement campaigns were carried out at IMN (P2) in collaboration with IPR. An experimental development carried out within the framework of this ANR now makes it possible to carry out IR spectroscopy measurements on single crystal at low temperature. The studies carried out on single crystal of Fe (phen) 2 (NCS) 2 give all the information on the symmetries and frequencies of the modes in the two electronic states. New measurement campaigns will be carried out on other materials and will involve in particular L. Chaban, recruited as a post-doctoral fellow at IMN.
The measurements carried out on Fe (phen) 2 (NCS) 2 have also made it possible to demonstrate a dynamic photoinduced effect at low temperature, where the lifetime of the photoinduced state makes it possible to observe it with Raman measurements, without have resort to lightning-fast experiments.
The CPhT (P3) performed the first solid state vibration mode calculations for a Fe (phen) 2 (NCS) 2 crystal. This calculation is very difficult because there are many atoms in the cell and therefore many modes, which requires many hours of calculation. The first results show a very good agreement between the calculated and observed modes, which can be discriminated in symmetry and frequency. In particular, the high frequency IR modes, essential for nonlinear phononics, have been clearly observed experimentally (N-CS vibration mode) and correspond very well to theoretical calculations. The next step is to calculate the couplings between the modes to understand which IR modes will be the most efficient to drive nonlinear phononics in this material.

For the coming period we will therefore:
- Perform the IR probe / femtosecond optical pump experiments on the Fe (phen) 2 (NCS) 2 compound
- Develop theoretical calculations on this system
- Develop this same type of approach on other families of promising materials for nonlinear phononics.

1. G. Azzolina, R. Bertoni, C. Ecolivet, H. Tokoro, S. Ohkoshi, and E.Collet* Landau theory for non-symmetry-breaking electronic instability coupled to symmetry-breaking order parameter applied to Prussian blue analog Physical Review B 102, 134104 (2020)
2. M. Cammarata,* S. Zerdane, L. Balducci, G. Azzolina, S. Mazerat, C. Exertier, M. Trabuco, M. Levantino, R. Alonso-Mori, J. M. Glownia, S. Song, L. Catala, T. Mallah , S. F. Matar, E. Collet* Charge-transfer driven by ultrafast spin-transition in a CoFe Prussian blue analogue Nature Chem 13, 10-14 (2021)
3. Giovanni Azzolina, Roman Bertoni, and Eric Collet* General Landau theory of non-symmetry-breaking and symmetry-breaking spin transition materials Journal of Applied Physics 129, 085106 (2021)
4. Eric Collet and Giovanni Azzolina Coupling and decoupling of spin crossover and ferroelastic distortion: Unsymmetric hysteresis loop, phase diagram, and sequence of phases Physical Review Materials 5, 044401 (2021)
5. D. Babich et al. “Local lattice contraction observed after resistive switching evidences an out of equilibrium Mott transition in a vanadium oxide” submitted Nature Comm.

Photo-induced phase transitions, driven by an intense optical pulse, allow for ultrafast control of the physical properties of materials by light (2 eV range). However, heat dissipation and temperature rise limit the control of coherent atomic motions and functions, therefore other means to drive materials with lower photon energy are required. In addition, the direct activation by light of soft lattice modes that drive phase transitions through lattice instability is not always possible, because of the optically inaccessible frequency range and/or because of the symmetry of the modes precluding optical transitions.

Here we propose to explore the fascinating possibilities offered by Non-Linear Phononics (NLP) to control functional molecular materials. NLP takes advantage of strong infrared excitation (0.2 eV range) for driving a large amplitude high-frequency polar mode QIR, which can couple through nonlinear (anharmonic) terms and activate those "soft modes" able to drive phase transition. The <QIR2> time average creates an “effective” dynamic potential, rectifying the phonon field and adiabatically directing a slow mode, which may significantly change the average atomic positions to create a new phase of different structural and electronic orders. This process occurs abruptly, on the timescale of a phonon period. Ultimately, it appears possible to drive a symmetry breaking towards a more ordered state, allowing to revisit the old adage “structure makes function”. Up to now, this new opportunity is only just emerging, and has essentially been employed only on a few inorganic materials.

In view of tantalising theoretical predictions, experimental opportunity, and the available technology suiting the challenge, we propose to develop nonlinear phononics for controlling electronic phase transitions in molecular materials. Importantly, the latter are rich resources of different functionalities. They present unique instabilities of molecular electronic states (charge, spin, …) that are strongly coupled to structural distortions of both the soft molecules and the soft lattice, and as such they are fitting test bed candidates for exploring NLP concepts in condensed matter.

Our approach that consists in mixing experimental and theoretical expertise in material science seems an effective and attractive strategy in view of different types of coupling and different physical processes behind NLP driven phase transitions. ELECTROPHONE will benefit from the expertise of the different partners, as developing this challenging project will require detailed knowledge of crystalline structure, phonons and symmetry, theoretical calculations of intra- and inter-molecular modes, description of their couplings, as well as time-resolved experiments on the ultrafast time-scale. The ultimate goal of this project consists in recasting a new physical picture of Non-Linear Phononics in electronic phase transition materials by networking experimentalists and theorists.