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Instruments / Technical Area	Publication Details	Published	Actions
	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
	Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet MgCr2O4 S. Gao , K. Guratinder , U. Stuhr , J. S. White , Martin Mansson , B. Roessli , T. Fennell , V. Tsurkan , A. Loidl , M. Ciomaga Hatnean , G. Balakrishnan , S. Raymond , Laurent Chapon , V. O. Garlea , A. T. Savici , A. Cervellino , A. Bombardi , D. Chernyshov , Ch. Rüegg , J. T. Haraldsen , O. Zaharko Physical Review B 97 DOI: 10.1103/PhysRevB.97.134430 Show Abstract ▼ Abstract: In spinels ACr2O4(A=Mg,Zn), realization of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et al. [Pow. Diff. 17, 230 (2002)], while other features depend on sample or cooling protocol. A complex, partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1=( 1 2 1 2 0),k2=(10 1 2 ) appear. The ordered moment reaches 1.94(3) μB/Cr3+ for k1 and 2.08(3) μB/Cr3+ for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1−10] diagonals and a smaller c component. We use inelastic neutron scattering to investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of clusterlike excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.	Apr 2018
	Understanding the complex phase diagram of uranium: the role of electron-phonon coupling S. Raymond , J. Bouchet , G. H. Lander , M. Le Tacon , G. Garbarino , M. Hoesch , J. P. Rueff , M. Krisch , J. C. Lashley , R. K. Schulze , R. C. Albers Physical Review Letters 107 (13), 136401 DOI: 10.1103/PhysRevLett.107.136401 PMID: 22026877 Show Abstract ▼ Abstract: We report an experimental determination of the dispersion of the soft phonon mode along [100] in uranium as a function of pressure. The energies of these phonons increase rapidly, with conventional behavior found by 20 GPa, as predicted by recent theory. New calculations demonstrate the strong pressure (and momentum) dependence of the electron-phonon coupling, whereas the Fermi-surface nesting is surprisingly independent of pressure. This allows a full understanding of the complex phase diagram of uranium and the interplay between the charge-density wave and superconductivity.	Sep 2011

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