Description

Spin-crossover materials switching between low-spin (LS) and high-spin (HS) states are prototypes of bistable molecular solids having serious potential applications as reversible high density memories, actuators, displays or mini-sensors that are sensitive to pressure, temperature and acoustic waves. First-order phase transitions in molecular spin-crossover single crystals manifest themselves in a spectacular way when observed by optical microscopy. The temperature-induced transformation between the LS and the HS states (having different unit cell volumes) is accompanied by the emergence of the unique HS domain, characterized by a highly regular HS/LS interface, propagating at constant velocity. This phenomenon, still escaping to deep understanding, is nicely followed by optical microscopy thanks to the different colors of the HS and LS phases. Similarly to a tsunami, the transformation starts at some point at the border of the crystal (a stochastic process) and propagates in a deterministic way. The objective of this thesis is to conduct deep experimental investigations on the conditions of appearance of the transformation fronts, and to study their physical properties (structure, shape, width, speed, orientation etc.). The investigations will be performed on several families of spin-crossover solids, whose thermodynamic properties are known. Our goal is to demonstrate that the processes involved are universal although some aspects remain sample-dependent, like the presence of crystal defects which certainly influence the nucleation and propagation process. Theoretical developments, based on Monte Carlo simulations on Ising-like models with elastic interactions (including the volume change at the transition), will be conducted on home-made models. These investigations will allow the identification of the fundamental physical mechanisms governing the spatiotemporal dynamics of first-order transitions accompanied with a volume change.
Références:
1. A. Slimani, F. Varret, D. Garrot, H. Oubouchou, S. Kaizaki and K. Boukheddaden, Phys. Rev. Lett. 110 (2013) 087208;
2. A. Slimani, k; Boukheddaden, F. Varret, H. Oubouchou, M. Nishino, S. Miyashita , Phys. Rev. B. 87, 014111 (2013).
3. M. Sy, F. Varret, K. Boukheddaden, J. Marrot, G. Bouchez, S. Kaizaki, Angew. Chem. Int. Edt, 139, 194706 (2014)
4. M. Sy, D. Garrot, A. Slimani, M. Paez-Espejo, F. Varret, K. Boukheddaden, Angew. Chem. Int. Edt. Angew. Chem. Int. Ed., 55, 1755 –1759 (2016),
5. M. Paez-Espejo, M. Sy, F. Varret and K. Boukheddaden, Phys. Rev. B. 89, 024306 (2014).

Skills required

Physicist or physico-chemist, with solid skills in phase transitions and more generally material science/condensed matter. A good background in modeling/simulations and statistical physics will be highly appreciated. A good knowledge in optics is welcome.

Bibliography

A. Slimani Phys. Rev. Lett. 110,087208 (2013);
M. Paez-Espejo et al., Phys. Rev. B, 89, 224303 (2014).
M. Sy et al. Angew. Chem. Int. Edt (2016), 55, 1755 –1759;
M. Paez-Espejo et al., J. Amer. Chem. Soc. (2016)138 (9), pp 3202–3210
M. Sy and K Boukheddaden, J. Phys. Chem. C. 124, 51, 28093–28107 (2020)
K Boukheddaden, M Sy, Advanced Theory and Simulations 1, 1800080 (2018).
Y. Singh et al, Materials Today Physics 27, 100842 (2022).

Keywords

spin transition, optical microscopy, interface propagation, Theory and modelling, first order Phase transitions, Numerical simulations