Neutron Laue and X-ray diffraction study of a new crystallographic superspace phase in n-nonadecane-urea

International audienceAperiodic composite crystals present long-range order without translational symmetry. These materials may be described as the intersection in three dimensions of a crystal which is periodic in a higher-dimensional space. In such materials, symmetry breaking must be described as structural changes within these crystallographic superspaces. The increase in the number of superspace groups with the increase in the dimension of the superspace allows many more structural solutions. This is illustrated in n-nonadecane-urea, revealing a fifth higher-dimensional phase at low temperature

Coherent control of orbital wavefunctions in the quantum spin liquid Tb2Ti2O7Tb_{2}Ti_{2}O_{7}

Resonant driving of electronic transitions with coherent laser sources creates quantum coherent superpositions of the involved electronic states. Most time-resolved studies have focused on gases or isolated subsystems embedded in insulating solids, aiming for applications in quantum information. Here, we demonstrate coherent control of orbital wavefunctions in pyrochlore $Tb_{2}Ti_{2}O_{7}$, which forms an interacting spin liquid ground state. We show that resonant excitation with a strong THz pulse creates a coherent superposition of the lowest energy Tb 4f states before the magnetic interactions eventually dephase them. The coherence manifests itself as a macroscopic oscillating magnetic dipole, which is detected by ultrafast resonant x-ray diffraction. The induced quantum coherence demonstrates coherent control of orbital wave functions, a new tool for the ultrafast manipulation and investigation of quantum materials

A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam

We present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-Å wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation

Exploration des chemins de photo-commutation dans les matériaux photomagnétiques par spectroscopies ultra-rapides : optique et rayons X

This thesis focuses on the study of the femtosecond photoswitching dynamic in the bistable molecular materials, using the pump-probe experiments which are based on the optical and x-ray spectroscopies.  Part of these experiments was performed at synchrotron and X-FEL (X-ray Free Electron Laser). The first part of the thesis, which is devoted to the study of non-octahedral spin transition systems, revealed different pathways of transformation on the potential surface. The second part focuses on the study of the Prussian Blue Analogues (CoFe), where the ultra-fast experiments allowed to follow the dynamics around the two metal ions.Ce travail de thèse porte sur l’étude de la dynamique femtoseconde de photo-commutation de matériaux moléculaires bistables, à l’aide d’expériences pompe-sonde basées sur les spectroscopies optiques et rayons X. Une partie des expériences a été réalisée sur synchrotron et X-FEL (X-ray Free Electron Laser). La première partie de la thèse, qui est consacrée à l’étude de systèmes à transition de spin non-octaédriques, a révélé différents chemins de transformations sur la surface de potentiel, associés à différents mécanismes de changement d’état électronique et modulant la cohérence de la dynamique structurale pilotant le processus. La seconde partie porte sur l’étude d’analogues du bleu de Prusse (CoFe) où les expériences ultra-rapides ont permis de d’étudier les dynamiques de transformation autour des sites de fer et de cobalt

Comparison of structural dynamics and coherence of d–d and MLCT light-induced spin state trapping

International audienceLight-induced excited spin state trapping (LIESST) in Fe spin-crossover systems is a process that involves the switching of molecules from low (LS, = 0) to high spin (HS, = 2) states. The direct LS-to-HS conversion is forbidden by selection rules, and LIESST involves intermediate states such as MLCT or T. The intersystem crossing sequence results in an HS state, structurally trapped by metal-ligand bond elongation through the coherent activation and damping of molecular breathing. The ultrafast dynamics of this process has been investigated in FeN ligand field systems, under MLCT excitation. Herein, we studied LIESST in an FeNO spin-crossover material of lower symmetry, which allowed for quite intense and low-energy shifted d-d bands. By combining DFT and TD-DFT calculations and fs optical absorption measurements, we demonstrated that shorter intermediates enhanced coherent structural dynamics, and d-d excitation induced faster LS-to-HS switching, compared to MLCT

Photoselective MLCT to d-d pathways for light-induced excited spin state trapping

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Out-of-equilibrium dynamics driven by photoinduced charge transfer in CsCoFe Prussian blue analogue nanocrystals

International audienceIn this paper we study the out-of-equilibrium dynamics associated with photoinduced charge-transfer (CT) in cyanide-bridged Co-Fe Prussian blue analogue nanocrystals. In these coordination networks, the structural trapping of the photoinduced CT polaron involves local electronic and structural reorganizations. Femtosecond X-ray and optical absorption spectroscopies show that the local structural trapping process occurs on similar timescale for particles with 11 nm and 70 nm sizes. The local photoinduced spin transition, elongating the Co-N bonds and driving the (CoFeII)-Fe-III -> (CoFeIII)-Fe-II CT, activates coherent lattice torsion modes. The elastic deformation waves, launched by these bond elongations, drive macroscopic volume expansion and breathing of the particles. The timescale of this macroscopic deformation depends strongly on the size of the particle, which is more evidence of the multiscale nature of photoinduced phenomena in molecular materials

Photoswitchable 11 nm CsCoFe Prussian Blue Analogue Nanocrystals with High Relaxation Temperature

International audiencePhotoswitchable 11 nm nanocrystals with the coordination network Cs{Co[Fe(CN)6]} were obtained using a template-free method. The nanocrystals were recovered from the colloidal solutions as solid materials surrounded by cetyltrimethylammonium (CTA) cations or embedded in the organic polymer polyvinylpyrrolidone (PVP). Complementary magnetic, spectroscopic, and structural techniques, including EPR spectroscopy, reveal a majority (∼70%) of the low-spin and photoactive diamagnetic CoIIIFeII pairs located in the core of the nanocrystals and a mixture of CoIIFeII and CoIIFeIII species present mainly within the shell of the objects. While bulk compounds with similar vacancy concentration do not exhibit noticeable photoinduced charge transfer, the observed photoactivity of the nanocrystals is ascribed to their nanometric size. The relaxation temperature of the photoinduced state shifts upward by ∼55 K when PVP is replaced by CTA. This is ascribed to the larger rigidity of the dense CsCoFe_CTA material, whose metastable state is lower than that for CsCoFe_PVP, leading to a larger relaxation energy barrier and, therefore, to a higher relaxation temperature

Charge transfer driven by ultrafast spin transition in a CoFe Prussian blue analogue

Photoinduced charge-transfer is an important process in nature and technology and is responsible for the emergence of exotic functionalities, such as magnetic order for cyanide-bridged bimetallic coordination networks. Despite its broad interest and intensive developments in chemistry and material sciences, the atomic-scale description of the initial photoinduced process, which couples intermetallic charge-transfer and spin transition, has been debated for decades; it has been beyond reach due to its extreme speed. Here we study this process in a prototype cyanide-bridged CoFe system by femtosecond X-ray and optical absorption spectroscopies, enabling the disentanglement of ultrafast electronic and structural dynamics. Our results demonstrate that it is the spin transition that occurs first on the Co site within ~50 fs, and it is this that drives the subsequent Fe-to-Co charge-transfer within ~200 fs. This study represents a step towards understanding and controlling charge-transfer-based functions using light. [Figure not available: see fulltext.