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Abstract: The microscopic origins of emergent behaviours in condensed matter systems are encoded in their excitations. In ordinary magnetic materials, single spin-flips give rise to collective dipolar magnetic excitations called magnons. Likewise, multiple spin-flips can give rise to multipolar magnetic excitations in magnetic materials with spin S ≥ 1. Unfortunately, since most experimental probes are governed by dipolar selection rules, collective multipolar excitations have generally remained elusive. For instance, only dipolar magnetic excitations have been observed in isotropic S = 1 Haldane spin systems. Here, we unveil a hidden quadrupolar constituent of the spin dynamics in antiferromagnetic S = 1 Haldane chain material Y2BaNiO5 using Ni L3-edge resonant inelastic x-ray scattering. Our results demonstrate that pure quadrupolar magnetic excitations can be probed without direct interactions with dipolar excitations or anisotropic perturbations. Originating from on-site double spin-flip processes, the quadrupolar magnetic excitations in Y2BaNiO5 show a remarkable dual nature of collective dispersion. While one component propagates as non-interacting entities, the other behaves as a bound quadrupolar magnetic wave. This result highlights the rich and largely unexplored physics of higher-order magnetic excitations.

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Abstract: While electron-phonon coupling (EPC) is crucial for Cooper pairing in conventional superconductors, its role in high- T c superconducting cuprates is debated. Using resonant inelastic x-ray scattering at the oxygen K edge, we study the EPC in Bi 2 Sr 2 Ca Cu 2 O 8 + δ (Bi2212) and Nd 1 + x Ba 2 − x Cu 3 O 7 − δ (NBCO) at different doping levels ranging from heavily underdoped ( p = 0.07 ) to overdoped ( p = 0.21 ). We analyze the data with a localized Lang-Firsov model that allows for the coherent excitations of two phonon modes. While electronic band dispersion effects are non-negligible, we are able to perform a study of the relative values of EPC matrix elements in these cuprate families. In the case of NBCO, the choice of the excitation energy allows us to disentangle modes related to the CuO chains and the Cu O 2 planes. Combining the results from the two families, we find the EPC strength decreases with doping at q ∥ = ( − 0.25 , 0 ) r.l.u., but has a nonmonotonic trend as a function of doping at smaller momenta. This behavior is attributed to the screening effect of charge carriers. We also find that the phonon intensity is enhanced in the vicinity of the charge-density-wave excitations while the extracted EPC strength appears to be less sensitive to their proximity. By performing a comparative study of two cuprate families, we are able to identify general trends in the EPC for the cuprates and provide experimental input to theories invoking a synergistic role for this interaction in d -wave pairing.

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Abstract: Interactions between electrons and lattice vibrations are responsible for a wide range of material properties and applications. Recently, there has been considerable interest in the development of resonant inelastic x-ray scattering (RIXS) as a tool for measuring electron-phonon ( e -ph) interactions. Here, we demonstrate the ability of RIXS to probe the interaction between phonons and specific electronic states both near to, and away from, the Fermi level. We perform carbon K -edge RIXS measurements on graphite, tuning the incident x-ray energy to separately probe the interactions of the π ∗ and σ ∗ electronic states. Our high-resolution data reveal detailed structure in the multiphonon RIXS features that directly encodes the momentum dependence of the e -ph interaction strength. We develop a Green’s-function method to model this structure, which naturally accounts for the phonon and interaction-strength dispersions, as well as the mixing of phonon momenta in the intermediate state. This model shows that the differences between the spectra can be fully explained by contrasting trends of the e -ph interaction through the Brillouin zone, being concentrated at the Γ and K points for the π ∗ states while being significant at all momenta for the σ ∗ states. Our results advance the interpretation of phonon excitations in RIXS and extend its applicability as a probe of e -ph interactions to a new range of out-of-equilibrium situations.

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Abstract: Oxidation and reduction of the oxide ions in the bulk of cathode materials is a potential route towards increasing the energy density of Li-ion batteries. Here, we present neutron PDF data which demonstrates the presence of short 1.2 Å O–O distances in a charged O-redox cathode material, corresponding to the bond length of molecular O2. This was achieved by collecting our data close to absolute zero (2 K), suppressing thermal motion which may have obscured detection by PDF previously. This direct detection of molecular O2 trapped in the material by diffraction complements the X-ray spectroscopy studies by e.g. RIXS while avoiding issues of possible beam damage as well as being a bulk average technique.

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Abstract: Lcation keywords: Li-ion Batteries; Cathode Materials; Resonant Inelastic X-ray Scattering ithium-rich cathode materials exhibit a pronounced drop in voltage between the charging and discharging processes (known as ‘voltage hysteresis’) that limits the available energy density. This needs to be overcome for them to be used in next-generation battery applications. In lithium-rich materials, the charging and discharging processes are accompanied by chemical changes to the oxide ions that form part of the cathode’s crystal structure. The reversibility of these chemical processes is critical in determining how well a cathode material functions. The voltage hysteresis seen in the lithium-rich materials implies the oxide ions undergo chemical changes that are not fully reversible, but the mechanism behind this was not well understood. Researchers used the Resonant Inelastic X-ray Scattering (RIXS) beamline (I21) at Diamond Light Source to study the chemical changes to the oxide ions at different stages of the battery charge-discharge cycle using RIXS. The measurements acquired from this technique offer a direct probe of the electronic structure on oxygen. Using high-resolution RIXS, they detected underlying vibrational fine structure that revealed the identity of the oxidised oxide ions as molecular oxygen. These oxygen molecules are trapped within the charged cathode material, but they can be reduced back to oxide ions on discharge. However, this process takes place at a lower voltage, giving rise to the voltage hysteresis. Researchers can now devise strategies to suppress the release of oxygen, or prevent its formation, in next-generation lithium-ion (Li-ion) batteries with higher energy density. Higher energy density batteries will extend the driving range of electric vehicles and increase the battery life of portable electronics between charges.