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Instruments / Technical Area	Publication Details	Published
I21-Resonant Inelastic X-ray Scattering (RIXS)	Many-body physics of single and double spin-flip excitations in NiO A. Nag , H. C. Robarts , F. Wenzel , J. Li , H. Elnaggar , R.-p. Wang , A. Walters , M. Garcia-fernandez , F. M. F. De Groot , M. W. Haverkort , K. Zhou Physical Review Letters 124 DOI: 10.1103/PhysRevLett.124.067202 Open Access Show Abstract ▼ Abstract: Understanding many-body physics of elementary excitations has advanced our control over material properties. Here, we study spin-flip excitations in NiO using Ni L3-edge resonant inelastic x-ray scattering (RIXS) and present a strikingly different resonant energy behavior between single and double spin-flip excitations. Comparing our results with single-site full-multiplet ligand field theory calculations we find that the spectral weight of the double-magnon excitations originates primarily from the double spin-flip transition of the quadrupolar RIXS process within a single magnetic site. Quadrupolar spin-flip processes are among the least studied excitations, despite being important for multiferroic or spin-nematic materials due to their difficult detection. We identify intermediate state multiplets and intra-atomic core-valence exchange interactions as the key many-body factors determining the fate of such excitations. RIXS resonant energy dependence can act as a convincing proof of existence of nondipolar higher-ranked magnetic orders in systems for which, only theoretical predictions are available.	Feb 2020
I21-Resonant Inelastic X-ray Scattering (RIXS)	Superstructure control of first-cycle voltage hysteresis in O-redox cathodes Robert A. House , Urmimala Maitra , Miguel A. Pérez-osorio , Juan G. Lozano , Liyu Jin , James W. Somerville , Laurent C. Duda , Abhishek Nag , Andrew Walters , Kejin Zhou , Matthew R. Roberts , Peter G. Bruce Nature DOI: 10.1038/s41586-019-1854-3 Show Abstract ▼ Abstract: Alkali-rich transition metal oxide intercalation cathodes can increase the energy density of Li and Na-ion batteries by storing charge on the oxide and transition metal (TM) ions1–10. The high voltage associated with oxidation of O2- on the first charge is not recovered on discharge, resulting in significantly reduced energy density11. Displacement of TM ions from the TM to alkali metal layers has been proposed to explain the first cycle voltage loss9,12–16. By comparing two closely related intercalation cathodes, Na0.75[Li0.25Mn0.75]O2 and Na0.6[Li0.2Mn0.8]O2, we show that the first cycle voltage hysteresis is determined by the superstructure, specifically the local ordering of Li/TM ions in the TM layers. The honeycomb superstructure of Na0.75[Li0.25Mn0.75]O2, present in almost all O-redox compounds, is lost on charging, driven, in part, by formation of molecular O2 inside the solid. The O2 molecules are cleaved on discharge reforming O2-, but the Mn ions have migrated in the plane changing the coordination around O2- significantly lowering the voltage on discharge (hysteresis). The ribbon superstructure in Na0.6[Li0.2Mn0.8]O2 inhibits Mn disorder and hence O2 formation, suppressing hysteresis and promoting stable electron holes on O2- which are revealed by XAS. The results show that voltage hysteresis can be avoided in O-redox cathodes by forming materials with a superstructure (cation ordering) in the TM layers that suppresses TM migration.	Dec 2019
I21-Resonant Inelastic X-ray Scattering (RIXS)	Anisotropic damping and wave vector dependent susceptibility of the spin fluctuations in La 2 − x Sr x CuO 4 studied by resonant inelastic x-ray scattering H. C. Robarts , M. Barthelemy , K. Kummer , Mirian Garcia-fernandez , J. Li , A. Nag , A. C. Walters , K. Zhou , S. M. Hayden Physical Review B 100 DOI: 10.1103/PhysRevB.100.214510 Diamond Proposal Number(s): [18469, 18512] Show Abstract ▼ Abstract: We report high-resolution resonant inelastic x-ray scattering (RIXS) measurements of the collective spin fluctuations in three compositions of the superconducting cuprate system La 2 − x Sr x CuO 4 . We have mapped out the excitations throughout much of the two-dimensional ( h , k ) Brillouin zone. The spin fluctuations in La 2 − x Sr x CuO 4 are found to be fairly well described by a damped harmonic oscillator model, thus our data allows us to determine the full wave vector dependence of the damping parameter. This parameter increases with doping and is largest along the ( h , h ) line, where it is peaked near ( 0.2 , 0.2 ) . We have used a new procedure to determine the absolute wave vector dependent susceptibility for the doped compositions La 2 − x Sr x CuO 4 ( x = 0.12 , 0.16 ) by normalizing our data to La 2 CuO 4 measurements made with inelastic neutron scattering (INS). We find that the evolution with doping of the intensity of high-energy excitations measured by RIXS and INS is consistent. For the doped compositions, the wave vector dependent susceptibility is much larger at ( 1 4 , 1 4 ) than at ( 1 2 , 0 ) . It increases rapidly along the ( h , h ) line towards the antiferromagnetic wave vector of the parent compound ( 1 2 , 1 2 ) . Thus, the strongest magnetic excitations, and those predicted to favor superconductive pairing, occur towards the ( 1 2 , 1 2 ) position as observed by INS.	Dec 2019
I21-Resonant Inelastic X-ray Scattering (RIXS)	High-resolution resonant inelastic x-ray scattering study of the electron-phonon coupling in honeycomb α − Li 2IrO3 J. G. Vale , C. D. Dashwood , E. Paris , L. S. I. Veiga , Mirian Garcia-fernandez , A. Nag , A. Walters , K. Zhou , I.-m. Pietsch , Anton Jesche , P. Gegenwart , R. Coldea , T. Schmitt , D. F. Mcmorrow Physical Review B 100 DOI: 10.1103/PhysRevB.100.224303 Diamond Proposal Number(s): [20569] Show Abstract ▼ Abstract: The excitations in honeycomb α − Li 2 IrO 3 have been investigated with high-resolution resonant inelastic x-ray scattering (RIXS) at the O K edge. The low-energy response is dominated by a fully resolved ladder of excitations, which we interpret as being due to multiphonon processes in the presence of strong electron-phonon coupling (EPC). At higher energies, the orbital excitations are shown to be dressed by phonons. The high quality of the data permits a quantitative test of the analytical model for the RIXS cross section, which has been proposed to describe EPC in transition-metal oxides (TMOs). We find that the magnitude of the EPC is comparable to that found for a range of 3 d TMOs. This indicates that EPC may be of equal importance in determining the phenomenology displayed by corresponding 5 d -based systems.	Dec 2019
Mechanical Engineering	A new procurement strategy to challenge the supplier constraints created when using a fully developed reference design G. E. Howell , N. Baker , S. Davies , M. Garcia-fernandez , H. C. Huang , S. M. Scott , A. Walters , K. Zhou MEDSI2018 DOI: 10.18429/JACoW-MEDSI2018-THOPMA04 Open Access Show Abstract ▼ Abstract: A common procurement strategy is to produce a fully optimised reference design that makes assumptions about the manufacturing process and supplier capability. This approach can restrict the opportunities for some companies to include their own specialist manufacturing capability to provide a more effective and cost efficient solution. A new approach is suggested following the recent experience at Diamond Light Source. The manufacture of high stiffness welded fabrications up to 13m in length for the I21 RIXS Spectrometer is used as an example. The I21 RIXS Spectrometer design was optimised for stiffness and control of vibration. The use of Finite Element Analysis enabled different design options and compromises to be explored utilising the supplier's capabilities. The final design was tested during manufacture to verify the FEA model. With the I21 RIXS Spectrometer commissioned the data collected shows the final stability performance of the system including detector stability over full experiment durations has met the scientific goals of the design.	Dec 2018
	Absence of superconductivity in fluorine-doped neptunium pnictide NpFeAsO A. C. Walters , H. C. Walker , R. Springell , M. Krisch , A. Bosak , A. H. Hill , C. E. Zvorişte-walters , E. Colineau , J.-c. Griveau , D. Bouëxière , R. Eloirdi , R. Caciuffo , T. Klimczuk Journal Of Physics: Condensed Matter 27 DOI: 10.1088/0953-8984/27/32/325702 PMID: 26214712 Show Abstract ▼ Abstract: X-ray diffraction, specific heat, magnetic susceptibility and inelastic x-ray scattering measurements on the transurarium oxypnictides NpFeAsO and NpFeAsO0.85F0.15 are presented. No superconductivity down to 2 K was observed upon fluorine doping, contrary to the structurally analogous rare-earth pnictides. No modification of the phonon density of states was observed upon doping with fluorine. We discuss our results in light of the latest experimental and theoretical studies on the role of phonons in the superconducting pnictide compounds.	Aug 2015

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