Sujet de thèse en cours : Exploring photoswitching pathways in photomagnetic materials with ultrafast optical and X-ray spectroscopies [Soutenance 04.10.2017]

supervisors Éric Collet and Marco Cammarata (department of materials and light)sous la direction de Éric Collet et de Marco Cammarata dans le département Matériaux et Lumièr

Sonder la structure et la dynamique de matériaux fonctionnels à l'échelle femtoseconde avec les X-FEL

National audienc

Formation of local spin-state concentration waves during the relaxation from photoinduced state in a spin-crossover polymer

International audienceThe complex relaxation from the photoinduced high-spin phase (PIHS) to the low-spin phase of the bimetallic two-dimensional coordination spin-crossover polymer [Fe[(Hg(SCN))](4,4'-bipy)] is reported. During the thermal relaxation, commensurate and incommensurate spin-state concentration waves (SSCWs) form. However, contrary to the steps forming at thermal equilibrium, associated with long-range SSCW order, the SSCWs forming during the relaxation from the PIHS phase correspond to short-range order, revealed by diffuse X-ray scattering. This is interpreted as resulting from the competition between the two types of SSCW order and another structural symmetry breaking, due to ligand ordering, occurring at low temperature and precluding long-range SSCW order

Decoupling with X-FEL electronic and structural dynamics for understanding the emergence of functions in photomagnetic materials

International audienc

New insights into correlated materials in the time domain—combining far-infrared excitation with x-ray probes at cryogenic temperatures

Modern techniques for the investigation of correlated materials in the time domain combine selective excitation in the THz frequency range with selective probing of coupled structural, electronic and magnetic degrees of freedom using x-ray scattering techniques. Cryogenic sample temperatures are commonly required to prevent thermal occupation of the low energy modes and to access relevant material ground states. Here, we present a chamber optimized for high-field THz excitation and (resonant) x-ray diffraction at sample temperatures between 5 and 500 K. Directly connected to the beamline vacuum and featuring both a Beryllium window and an in-vacuum detector, the chamber covers the full (2–12.7) keV energy range of the femtosecond x-ray pulses available at the Bernina endstation of the SwissFEL free electron laser. Successful commissioning experiments made use of the energy tunability to selectively track the dynamics of the structural, magnetic and orbital order of Ca2RuO4 and Tb2Ti2O7 at the Ru (2.96 keV) and Tb (7.55 keV) L-edges, respectively. THz field amplitudes up to 1.12 MV cm−1 peak field were demonstrated and used to excite the samples at temperatures as low as 5 K.ISSN:0953-8984ISSN:1361-648

Photoinduced charge-transfer in Prussian blue analogues: from local self-trapping to macroscopic phase transition

International audienc

Electronic and structural dynamics during the switching of the photomagnetic complex [Fe(L222N5)(CN)2]

International audienceThe [Fe(L N )(CN) ] compound, where L N refers to the macrocyclic Schiff-base ligand, 2,13-dimethyl-3,6,9,-12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,- 16-pentaene, is a photomagnetic Fe based coordination compound, which undergoes light-induced excited spin-state trapping (LIESST). The low spin state is hexacoordinated and the high spin state heptacoordinated. This system also serves as complex for the design of trinuclear or one-dimensional compounds made of other types of bricks with diverse coordinated metals. Here its ultrafast spin-state photoswitching dynamics are studied, by combining femtosecond optical spectroscopy and femtosecond X-ray absorption measurements at the XPP station of the X-ray free-electron laser LCLS. DFT and TD-DFT calculations are used to interpret experimental findings. These studies, performed in the solution phase, show that LIESST in [Fe(L N )(CN) ] occurs on the 100 fs timescale under different types of photoexcitation. In addition, coherent oscillations were observed, resulting from the structural dynamics accompanying LIESST, which were recently evidenced in more conventional octahedral Fe N systems

Photoinduced charge-transfer in Prussian blue analogues: from local self-trapping to macroscopic phase transition

International audienc

Photoinduced charge-transfer in Prussian blue analogues: from local self-trapping to macroscopic phase transition

International audienc