Diffusion inélastique résonante de rayons X

La diffusion inélastique résonante de rayons X (en anglais : Resonant Inelastic X-ray Scattering/RIXS) est une des techniques de diffusion des rayons X qui permet d'étudier la configuration électronique des molécules et des matériaux. vignette|300x300px|Processus du RIXS direct. Les rayons X entrants excitent un électron situé dans un niveau de cœur vers la bande de valence non occupée. L'état de cœur non occupé est ensuite rempli par un électron des états occupés avec l'émission de rayon X. Ce processus de RIXS crée une valence d'excitation avec une quantité de mouvement k′ − k et une énergie ħω − ħω′. La diffusion inélastique des rayons X est une technique expérimentale en développement rapide. C'est une spectroscopie photon-in / photon-out où l'on peut mesurer en même temps l'énergie et le changement de quantité de mouvement pour les photons diffusés. L'énergie et la quantité de mouvement perdues par photon sont transférées aux excitations intrinsèques des matériaux étudiés. Donc

Resonant Inelastic X-ray Scattering Studies of Low-Dimensional and Frustrated 3d Transition-Metal Oxides

Yi Tseng

3d transition-metal oxide materials with strong electron correlations and low dimensionality give rise to emerging exotic phases. Resonant inelastic X-ray scattering (RIXS) has developed as a powerful spectroscopic tool for probing the collective excitations in such correlated materials. The RIXS technique provides an ideal approach for assessing electronic instabilities in condensed matter. This thesis comprises the experimental RIXS response of the spin chain-ladder cuprates Sr14-xCax(Cu,Co)24O41 and frustrated honeycomb nickelate Na2Ni2TeO6. The spin and charge excitations of Sr14-xCaxCu24O41 (x=0 & 12.2) are studied by Cu L3-edge RIXS. With increasing Ca content, a crossover from collective two-triplon excitations (x=0) to a damped incoherent magnetic spectrum ~280 meV (x=12.2) in the ladders is uncovered from the RIXS spectra, dominated by spin-flip ÎS=1 scattering. With model calculations, the localized broad magnetic mode at x=12.2 is shown to be a consequence of enhanced elecrtronic localization. This is supported by polarization-dependent measurements, where the evaluated non-spin-flip ÎS=0 scattering is showing weight depletion below 1 eV. The RIXS spectral evolution suggests that a suppressed carrier mobility dominates the low-energy physics in Ca-rich phases. The low-energy excitations of Sr14Cu24O41 are also studied by O K-edge RIXS at the upper Hubbard band. The RIXS spectra show a sharp dispersing peak of similar energies to the ÎS=1 two-triplon excitations, and a broad non-dispersive mode at higher-energy. By comparing the experimental results to the existing theories of doped ladders, the observed spectral modes are attributed to interacting holon-spinon quasiparticles, with additional spectral contributions from the ÎS=0 multi-triplon bound states and continuum. These observations highlight the RIXS capability for resolving composite spin-charge quasiparticles and long-lived ÎS=0 magnetic excitations, which are difficult to detect by other experiments. Charge order and phonons for the two-leg ladders of Sr14(Cu,Co)24O41 are studied. At the ladder hole peak in O K-edge XAS signal, a concomitant elastic enhancement and phonon-softening is observed across all Co doping levels. The in-plane diffraction and phonon-softening response are found to enhance with increasing Co doping. Morevoer, the observed diffraction signal is further characterized by energy- and temperature-dependent measurements. Lastly, given the strong electron-phonon coupling (EPC) observed in O K-edge RIXS spectra, the EPC strength can be analyzed from the RIXS intensity decay of the phonon overtones . The observed in-plane resonant diffraction and inelastic phonon response strongly suggest an enhanced stripe order via magnetic impurities, reminiscent of the reports in CuO4 plaquettes. Electronic structure and EPC of the honeycomb nickelate Na2Ni2TeO6 are studied. Compared with current literature, the localized excitations (1-4 eV) in the Ni L3-edge RIXS spectra are dominated by the crystal-field splitting of a Ni2+ ion in octahedral coordination. At the O K-edge, the large EPC ~285 meV with 5 recognizable overtones (frequency of ~80 meV) are revealed in the RIXS spectra. The observed modes provide a systematic characterization of charge, orbital and lattice dynamics for a newly proposed frustrated antiferromagnet. This thesis has been carried out in a collaboration between the Paul Scherrer Institut and the Laboratory for Quantum Magnetism at EPFL.

EPFL2021

Femtosecond Dynamics of Momentum-Dependent Magnetic Excitations from Resonant Inelastic X-Ray Scattering in CaCu2O3

Thorsten Schmitt

Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic electronic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them, and determine their respective time scales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are, to a very good approximation, independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence. © 2014 American Physical Society.

American Physical Society2014

Resonant Inelastic X-Ray Scattering Study of Electron-Exciton Coupling in High-T-c Cuprates

Francesco Barantani, Christophe Berthod, Fabrizio Carbone, Ivan Madan, Thorsten Schmitt, Yi Tseng, Dirk Van der Marel

Explaining the mechanism of superconductivity in the high-T-c cuprates requires an understanding of what causes electrons to form Cooper pairs. Pairing can be mediated by phonons, the screened Coulomb force, spin or charge fluctuations, excitons, or by a combination of these. An excitonic pairing mechanism has been postulated, but experimental evidence for coupling between conduction electrons and excitons in the cuprates is sporadic. Here we use resonant inelastic x-ray scattering to monitor the temperature dependence of the dd exciton spectrum of Bi2Sr2CaCu2O8-x crystals with different charge carrier concentrations. We observe a significant change of the dd exciton spectra when the materials pass from the normal state into the superconductor state. Our observations show that the dd excitons start to shift up (down) in the overdoped (underdoped) sample when the material enters the superconducting phase. We attribute the superconductivity-induced effect and its sign reversal from underdoped to overdoped to the exchange coupling of the site of the dd exciton to the surrounding copper spins.

AMER PHYSICAL SOC2022

Resonant Inelastic X-ray Scattering Studies of Low-Dimensional and Frustrated 3d Transition-Metal Oxides

Yi Tseng

EPFL2021