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Instruments / Technical Area	Publication Details	Published
	Spin fluctuations associated with the collapse of the pseudogap in a cuprate superconductor M. Zhu , D. J. Voneshen , S. Raymond , O. J. Lipscombe , C. C. Tam , S. M. Hayden Nature Physics 19, 99 - 105 DOI: 10.1038/s41567-022-01825-3 Show Abstract ▼ Abstract: Theories of the origin of superconductivity in cuprates depend on an understanding of their normal state, which exhibits various competing orders. Transport and thermodynamic measurements on La2 − xSrxCuO4 show signatures of a quantum critical point and the associated fluctuations, including a peak in the electronic specific heat versus doping, near the doping p* where the pseudogap collapses. The fundamental nature of these quantum fluctuations is unclear. Here we use inelastic neutron scattering to show that, close to the superconducting critical temperature and near p, there are very-low-energy collective spin excitations with characteristic energies of ~5 meV. Cooling and applying a magnetic field creates a mixed state with a stronger magnetic response below 10 meV. We conclude that the low-energy spin fluctuations are due to the collapse of the pseudogap combined with an underlying tendency to magnetic order. We show that the large specific heat near p can be understood in terms of collective spin fluctuations. The spin fluctuations we measure exist across the superconducting phase diagram and may be related to the strange metal behaviour observed in overdoped cuprates.	Jan 2023
	Role of defects in determining the magnetic ground state of ytterbium titanate D. F. Bowman , E. Cemal , T. Lehner , A. R. Wildes , L. Mangin-Thro , G. J. Nilsen , M. J. Gutmann , D. J. Voneshen , D. Prabhakaran , A. T. Boothroyd , D. G. Porter , C. Castelnovo , K. Refson , J. P. Goff Nature Communications 10 DOI: 10.1038/s41467-019-08598-z Open Access Show Abstract ▼ Abstract: Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.	Feb 2019
	Diffusion mechanism in the sodium-ion battery material sodium cobaltate T. J. Willis , D. G. Porter , D. J. Voneshen , S. Uthayakumar , F. Demmel , M. J. Gutmann , M. Roger , K. Refson , J. P. Goff Scientific Reports 8 DOI: 10.1038/s41598-018-21354-5 Open Access Show Abstract ▼ Abstract: High performance batteries based on the movement of Li ions in Li x CoO2 have made possible a revolution in mobile electronic technology, from laptops to mobile phones. However, the scarcity of Li and the demand for energy storage for renewables has led to intense interest in Na-ion batteries, including structurally-related Na x CoO2. Here we have determined the diffusion mechanism for Na0.8CoO2 using diffuse x-ray scattering, quasi-elastic neutron scattering and ab-initio molecular dynamics simulations, and we find that the sodium ordering provides diffusion pathways and governs the diffusion rate. Above T ~ 290 K the so-called partially disordered stripe superstructure provides channels for quasi-1D diffusion, and melting of the sodium ordering leads to 2D superionic diffusion above T ~ 370 K. We obtain quantitative agreement between our microscopic study of the hopping mechanism and bulk self-diffusion measurements. Our approach can be applied widely to other Na- or Li-ion battery materials.	Feb 2018
I16-Materials and Magnetism	Two-dimensional Cs-vacancy superstructure in iron-based superconductor Daniel Porter , Eron Cemal , David Voneshen , Keith Refson , Matthias Gutmann , Alessandro Bombardi , Andrew Boothroyd , A. Krzton-Maziopa , E. Pomjakushina , K. Conder , Jon Goff Physical Review B 91 (14) DOI: 10.1103/PhysRevB.91.144114 Diamond Proposal Number(s): [8686, 10377] Show Abstract ▼ Abstract: Single crystal neutron diffraction is combined with synchrotron x-ray scattering to identify the different superlattice phases present in Cs 0.8 Fe 1.6 Se 2 . A combination of single crystal refinements and first principles modeling are used to provide structural solutions for the √ 5 × √ 5 and √ 2 × √ 2 superlattice phases. The √ 5 × √ 5 superlattice structure is predominantly composed of ordered Fe vacancies and Fe distortions, whereas the √ 2 × √ 2 superlattice is composed of ordered Cs vacancies. The Cs vacancies only order within the plane, causing Bragg rods in reciprocal space. By mapping x-ray diffraction measurements with narrow spatial resolution over the surface of the sample, the structural domain pattern was determined, consistent with the notion of a majority antiferromagnetic √ 5 × √ 5 phase and a superconducting √ 2 × √ 2 phase.	Apr 2015
	Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate D. J, Voneshen , K. Refson , E. Borissenko , M. Krisch , A. Bosak , A. Piovano , E. Cemal , M. Enderle , M. J. Gutmann , M. Hoesch , M. Roger , L. Gannon , A. T. Boothroyd , S. Uthayakumar , D. G. Porter , J. P. Goff Nature Materials 12, 1028–1032 DOI: 10.1038/nmat3739 Show Abstract ▼ Abstract: The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures1. Rattling of ions in cages results in low thermal conductivity2, 3, 4, 5, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells6. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na0.8CoO2, which has a large-period superstructure7. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.	Aug 2013

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