COOPERATIVE ELASTIC SWITCHING IN VOLUME-CHANGING MAGNETIC MATERIALS TRIGGERED BY FEMTOSECOND MOLECULAR PHOTOSWITCHING.

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1 COOPERATIVE ELASTIC SWITCHING IN VOLUME-CHANGING MAGNETIC MATERIALS TRIGGERED BY FEMTOSECOND MOLECULAR PHOTOSWITCHING. E. Collet, R. Bertoni, H. Cailleau, M. Buron, M. Lorenc, M. Cammarata S. Zerdane, A. Marino, E. Trzop Institut de Physique de Rennes, F Rennes, France. M.L. Boillot A. Tissot ICMMO Orsay, F J.F. Létard S. Matar G. Chastanet ICMCB Bordeaux, F J.A. Real ICMol Valencia, E C. Enachescu Department of Physics, Iasi, RO I. Ciofini, L. Wilbraham ENSCParis F H. Lemke LCLS X-FEL Stanford USA

2 Photoinduced phase transition Triggering cooperativity in materials K. Nasu by a single laser pulse

3 Triggering cooperative transformations with a laser pulse Dt= -2 ns Dt= +1 ns E. Collet, et al Science (2003) Å Å Å Å? M. Chollet et al Science (2005) order parameter h reaction coordinate Q

4 Triggering cooperative transformations with a laser pulse Dt= -2 ns Dt= +1 ns Å Å E. Collet, et al Science (2003) Å Å Uemura et al, PRL 2010 Displacive nature of the process: molecules move from one equilibrium position to new one M. Chollet et al Science (2005) Servol et al, PRB 2015

5 X HS Photoinduced spin-state switching in spin-crossover materials LIESST and reverse LIESST Effect : Selective control of electronic redistribution among d orbitals Structural trapping with molecular structure change P Gütlich, A Hauser, H Spiering Angewandte Chemie (1994) S. Decurtins et al Chem. Phys. Lett. (1984). Low Spin S=0 High Spin S=2 Reversible photomagnetic and photochromic 532nm 830nm S. Bonhommeau, et al Angew. Chem. IE (2005) Time (min) M. Duriska et al, Angew Chem (2009) N. Brefuel et al Chem Eur J. (2010)

6 Single laser shot switching: A single crystal study E. Collet et al, Curr. Inorg. Chem. 6, 61 (2016) video can be downloaded provided as supp. material

7 Multiscale: from microscopic to macroscopic Molecule lattice crystal HS Low Spin S=0 High Spin S=2 LS Fe-L What is the speed and efficiency of the process? What is the driving force: Photoinduced, Laser heating? E. Collet et al, Curr. Inorg. Chem. 6, 61 (2016)

8 Physical processes & time The answer is in the time domain Decoupling in time steps Reaching themodynamics equilibrium is not monotonic: takes place in stages. The description on one scale uses information from other scales: Ex in the time scale typical for atomic motions: Nasu K. and al, J. phys : Condens. Matter (2001) -electrons are faster. Act by their quantum/statistical average -other ones, such as volume, are slower and then frozen Cailleau, Acta Cryst A 66 (2010)

9 LIESST mechanism proposed in 1986 revisited with femtosecond pump-probe spectroscopy A. Hauser, Chem. Phys. Let LIESST in FeII SCO M. Cammarata, Phys. Rev. Lett (2014) R. Bertoni, Acc. Chem. Res (2015) Reverse LIESST in FeII SCO A. Marino et al, Angew. Chem. I.E. (2014)

10 Revisiting LIESST mechanism Femtosecond XANES study of Fe(bpy) 3 LCLS X-ray Free Electron Laser with 25 fs resolution Henrik T. Lemke, Eric Collet, Marco Cammarata et al arxiv:

11 Experimental set-up

12 Energy transfer to the lattice Lattice modes activated after few ps

13 Multi-step dynamics in SCO solids probed by ultrafast techniques Step 1.Photo-switching : non-thermal molecular transformation (fs-ps), under constant volume. Step 2. Elastic: larger molecules and lattice heating drive volume expansion (1ns-100ns) Step 3. Thermal population of HS state transient thermal equilibrium (1-10µs) E. Collet et al, Phys. Chem Chem Phys (2012) Fe(phen) 2 (NCS) 2 HS fraction change Volume change T (K) R. Bertoni et al CrystEngComm (2016)

14 Lattice effect: Because of expansion: size matters [Fe III (3-MeO-SalEen) 2 ]PF 6 Single crystal 10*100µm Nano-crystal nm As size decreases: Elastic step occurs faster Heat dissipates to the matrix HS fraction (optical spectroscopy) Lattice parameter change (time-resolved x-ray diffraction) Nanocrystals Single crystal R. Bertoni et al, Nature Materials 15, 606 (2016)

15 Switching on elastic cooperativity Photoresponse of nano-crystals to different excitation densities The number of photo-switched molecules increases linearly with excitation density The number of HS molecules on the elastic step shows -a threshold effect / non-linear response -Self-amplification R. Bertoni et al, Nature Materials 15, 606 (2016)

16 The elastic field k DT DT Elastic interactions arise from lattice distortions due to different molecular sizes Mechanoelastic model: Ball-and-spring p i Local pressure p i The volume change of a switched molecule exerts an instantaneous elastic force on its neighbouring springs molecule-to-molecule interactions: every molecular switch modifies the volume and the shape of the whole crystal. R. Bertoni et al, Nature Materials 15, 606 (2016)

17 kmechanoelastic model DT DT Ball-and-spring: elastic interactions arise from lattice distortions due to different molecular sizes The volume change of a switched molecule exerts an instantaneous elastic force on its neighbouring springs molecule-to-molecule interactions: every molecular switch modifies the volume and the shape of the whole crystal. R. Bertoni et al, Nature Materials 15, 606 (2016)

18 kthe elastic-field physical picture DT Competing: pushing forces on HS molecules and pulling forces on LS molecules The qualifying term "molecular materials" unfolds its genuine meaning: the material response to light stimuli far exceeds the sum of the individual responses of constituting molecules. Soft materials, reminiscent of a feedback mechanism intrinsic to active media: the more volume expands, the more molecules switch, the more volume expands,

19 Symmetry HS-LS order Spin State Concentration Waves

20 Long-range ordering of Bistable molecules with different electronic states / structure Single step and cooperative LS to HS switching Molecule in LS state Molecule in HS state Stepped LS to HS switching H. Watanabe et al, Phys Rev. B (2016)

21 HS fraction What are the order parameters? Symmetric OP: High spin fraction g HS same as density for liquid/gas is related to -the magnetic susceptibility -the intramolecular structure: <Fe-N> <Fe-N> length (Å) average of <Fe-N> <Fe-N>length HS fraction Temperature(K) g HS Symmetry breaking OP: h: ordering between sites Formation of Spin-State Concentration Wave Change of any density Between previously equivalent sites h = <g HS 2i g HS 2i+1 > g HS (r)=g HS +h cos(q.r) E. Collet et al, Phys. Rev. Lett (2012)

22 kelastic frustration responsible for stepwise transitions described with anisotropic next-nearest neighbor Ising model J J J J J c J A J HS LS A H. Watanabe Phys Rev.B (2016) Such frustration can generate stepwise transitions with one or several intermediates phases HS + LS and ultimately generate a Devil s staircase E. Trzop Angew.Chem. Int. Ed. 55,8675 (2016)

23 kelastic frustration Antiferro-elastic interaction responsible for stepwise transitions Different sequences of HS-LS order form on the steps The change of periodicity generates new Bragg peaks E. Trzop et al Angew.Chem. Int.Ed. 55,8675 (2016) g HS (r)=g HS +h cos(q.r)

24 Response of Spin-State Concentration Wave to femtosecond light excitation I(h k 2n+1) = h 2 x[f HS F LS ] 2 I h 2 The spin-state concentration wave disappears 1ms after laser excitation: when thermal equilibrium is reached and HS population equilibrates on each sites initial Photoexcitation equilibrium Photo Excitation Of LS Thermal HS population A. Marino et al, Faraday discussion 177 (2015)

25 Unfolding the genuine meaning of "molecular materials the material response to light stimuli far exceeds the sum of the individual responses of constituting molecules LIESST occurs within 200 fs ultrafast HS structural trapping induces lattice expansion playing on coherence Self-amplification occurs The SCO crystal is an active medium elastic deformation wave up to 10 molecules switched / photon R. Bertoni et al, Nat. Mat 15, 606 (2016)