Andrei Savici

Mantid—Data analysis and visualization package for neutron scattering and μ SR experiments

The Mantid framework is a software solution developed for the analysis and visualization of neutron scattering and muon spin measurements. The framework is jointly developed by software engineers and scientists at the ISIS Neutron and Muon Facility and the Oak Ridge National Laboratory. The objectives, functionality and novel design aspects of Mantid are described.

Effect of Covalent Bonding on Magnetism and the Missing Neutron Intensity in Copper Oxide Compounds

Covalent bonding and magnetism in cuprates Andrew C. Walters, Toby G. Perring, Jean-Sébastien Caux, Andrei T. Savici, Genda D. Gu, Chi-Cheng Lee, Wei Ku, and Igor A. Zaliznyak 1 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK Department of Physics, University College London, Gower Street, London WC1E 6BT 3 London Centre for Nanotechnology, 17-19 Gordon Street, London WC1H 0AJ, UK 4 Institute for Theoretical Physics, University of Amsterdam, 1018 XE Amsterdam, The NetherlandsA study of a one-dimensional system may have finally resolved the long-standing discrepancy between the expected and measured inelastic neutron scattering intensities in the high-temperature cuprate superconductors.

Excitations in the field-induced quantum spin liquid state of α-RuCl3

The celebrated Kitaev quantum spin liquid (QSL) is the paradigmatic example of a topological magnet with emergent excitations in the form of Majorana Fermions and gauge fluxes. Upon breaking of time-reversal symmetry, for example in an external magnetic field, these fractionalized quasiparticles acquire non-Abelian exchange statistics, an important ingredient for topologically protected quantum computing. Consequently, there has been enormous interest in exploring possible material realizations of Kitaev physics and several candidate materials have been put forward, recently including α-RuCl3. In the absence of a magnetic field this material orders at a finite temperature and exhibits low-energy spin wave excitations. However, at moderate energies, the spectrum is unconventional and the response shows evidence for fractional excitations. Here we use time-of-flight inelastic neutron scattering to show that the application of a sufficiently large magnetic field in the honeycomb plane suppresses the magnetic order and the spin waves, leaving a gapped continuum spectrum of magnetic excitations. Our comparisons of the scattering to the available calculations for a Kitaev QSL show that they are consistent with the magnetic field induced QSL phase.Condensed Matter Physics: magnetic field drives spins to a liquidA sufficiently large magnetic field suppresses long-range magnetic order in α-RuCl3, leaving a disordered state with a gapped continuum spectrum of magnetic excitations, similar to that expected for the famous Kitaev quantum spin liquid. An international team led by Stephen E. Nagler from Oak Ridge National Laboratory in the USA performed time-of-flight neutron scattering to study low energy magnetic excitations of α-RuCl3. They observed that the application of a sufficiently large magnetic field to this material suppressed spin waves associated with the long-range order, and drove it to an unusual excited state. By comparison with calculations, these results are consistent with the Kitaev quantum spin liquid state in a magnetic field. The results provide important information of a possible route to producing gapped excitations related to magnetic Majorana Fermions towards topologically protected quantum computation.

Liquidlike Correlations in Single-Crystalline Y2Mo2O7: An Unconventional Spin Glass

The spin glass behavior of Y2Mo2O7 has puzzled physicists for nearly three decades. Free of bulk disorder within the resolution of powder diffraction methods, it is thought that this material is a rare realization of a spin glass resulting from weak disorder such as bond disorder or local lattice distortions. Here, we report on the single crystal growth of Y2Mo2O7. Using neutron scattering, we present unique isotropic magnetic diffuse scattering arising beneath the spin glass transition despite having a well-ordered structure at the bulk level. Despite our attempts to model the diffuse scattering using a computationally exhaustive search of a class of simple spin Hamiltonians, we were unable to replicate the experimentally observed energy-integrated (diffuse) neutron scattering. A T^2-temperature dependence in the heat capacity and density functional theory calculations hint at significant frozen degeneracy in both the spin and orbital degrees of freedom resulting from spin-orbital coupling (Kugel-Khomskii type) and random fluctuations in the Mo environment at the local level.

Friedel-Like Oscillations from Interstitial Iron in Superconducting Fe1+yTe0.62Se0.38

Using polarized and unpolarized neutron scattering, we show that interstitial Fe in superconducting Fe(1+y)Te(1-x)Se(x) induces a magnetic Friedel-like oscillation that diffracts at Q⊥=(1/2 0) and involves >50 neighboring Fe sites. The interstitial >2μ(B) moment is surrounded by compensating ferromagnetic four-spin clusters that may seed double stripe ordering in Fe(1+y)Te. A semimetallic five-band model with (1/2 1/2) Fermi surface nesting and fourfold symmetric superexchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction.

High-energy continuum of magnetic excitations in the two-dimensional quantum antiferromagnet Sr2CuO2Cl2

We have measured the magnetic excitation spectrum of the model square-lattice spin-1/2 antiferromagnet

Spin pseudogap in Ni-doped SrCuO2

rm Sr_2CuO_2Cl_2

Stripeless incommensurate magnetism in strongly correlated oxide La 1.5 Sr 0.5 CoO 4

over a broad range of energy and momentum using high-resolution inelastic neutron scattering (INS). The magnon dispersion along the zone boundary was accurately measured to be a 43 meV between (1/2,0) and (3/4,1/4) indicating the importance of coupling beyond nearest neighbors in the spin Hamiltonian. We observe a strong momentum dependent damping of the zone-boundary magnons at (1/2,0) revealing a high energy continuum of magnetic excitations. A direct comparison between our INS measurements and resonant inelastic X-ray scattering (RIXS) measurements shows that the RIXS spectrum contains significant contributions from higher energy excitations not previously considered. Our observations demonstrate that this high-energy continuum of magnetic fluctuations is a ubiquitous feature of insulating monolayer cuprates, apparent in both inelastic neutron and light scattering measurements.

Expanding Lorentz and spectrum corrections to large volumes of reciprocal space for single-crystal time-of-flight neutron diffraction

The S=1/2 spin chain material SrCuO2 doped with 1% S=1 Ni impurities is studied by inelastic neutron scattering. At low temperatures, the spectrum shows a pseudogap Δ≈8u2009u2009meV, absent in the parent compound, and not related to any structural phase transition. The pseudogap is shown to be a generic feature of quantum spin chains with dilute defects. A simple model based on this idea quantitatively accounts for the experimental data measured in the temperature range from 2 to 300 K, and allows us to represent the momentum-integrated dynamic structure factor in a universal scaling form.

Neutron scattering study of spin ordering and stripe pinning in superconducting La1.93Sr0.07CuO4

We studied the nano-scale structure of the short-range incommensurate magnetic order in La1.5Sr0.5CoO4 by elastic neutron scattering. We find that magnetic diffuse scattering is isotropic in the a-b plane, in contrast with the naive expectation based on the popular stripe model. Indeed, charge segregation into lines favoring certain lattice direction(s) would facilitate linear stacking faults in an otherwise robust antiferromagnetism of un-doped material, leading to anisotropic disorder, with a characteristic symmetry pattern present in the neutron scattering data.