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.

Non-linear phononics:

We characterised the vibrational modes of a spin-transition material by studying their symmetry (P1) and spectroscopy (P2) and comparing these with theoretical calculations (P3) [Privault 2022], and identified IR modes for NLP. The results obtained as part of G. Privault’s PhD thesis demonstrated the excitation of low-frequency modes via the resonant excitation of high-frequency IR modes. The theoretical calculations carried out by P3 show a coupling between the excited modes and the activated modes, thereby validating the existence of NLP in this type of material.

P3 has also carried out theoretical studies demonstrating a transient symmetry breaking in KTa03 and via NLP [Gomez 2022, Gomez 2023].

Resonant excitation of a ferroelectric mode in TTF-CA via THz excitation:

The development of THz pulses paves the way for controlling faster processes to polarise a material or reverse its ferroelectric polarisation. We have developed (G. Huitric) a THz pump–optical probe experiment to manipulate the macroscopic polarisation of TTF-CA. The time evolution of reflectivity upon THz excitation highlights the coherent activation of a ferroelectric mode (P1), which we have also described through theoretical calculations (P3)

Symmetry breaking in spin-transition materials

Some spin-transition materials exhibit phase transitions coupled to symmetry breaking. Our studies have enabled us to understand these phenomena and to develop a model based on Landau’s theory of phase transitions, taking into account the coupling between isosymmetric spin conversion and symmetry breaking. This breakthrough has made it possible to explain the significant thermal hysteresis in the charge-transfer material RbMnFe [Azzolina 2020], multi-step transitions [Jackobsen 2022] and the magnetoelectric effect [Torres 2024], the phase diagram of a prototype compound under pressure [Collet 2023] and the emergence of symmetry breaking at high temperatures, which is of great interest to NPL.

Ultrafast dynamics in heterobimetallic charge-transfer materials.

Our studies have revealed the transformation mechanism of an excited CoIII(S=0)FeII(S=0) material, with a spin transition in 50 fs on the Co, followed by a CT in 200 fs to the CoII(S=3/2)FeIII(S=1/2) state [Cammarata 2021], linked to the coherent activation of a Jahn-Teller mode [Hervé 2023]. The dynamics evolve as a function of the excitation wavelength, as we observed in an MnFe material [Azzolina 2021] and a CoW molecular material [Nakamura 2024]. By studying photo-induced macroscopic non-equilibrium dynamics, we were able to demonstrate how local electron transfer induces polarons—sources of strain capable of driving a macroscopic transition involving a change in symmetry—using optical and IR spectroscopy [Privault 2024], XANES and X-ray diffraction [Hervé 2024].

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.