Publications (172)624.95 Total impact
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ABSTRACT: We use inelastic neutron scattering to study the temperature and energy dependence of the spin excitation anisotropy in uniaxialstrained electrondoped iron pnictide BaFe$_{1.9}$Ni$_{0.1}$As$_2$ near optimal superconductivity ($T_c=20$ K). Our work has been motivated by the observation of inplane resistivity anisotropy in the paramagnetic tetragonal phase of electronunderdoped iron pnictides under uniaxial pressure, which has been attributed to a spindriven Isingnematic state or orbital ordering. Here we show that the spin excitation anisotropy, a signature of the spindriven Isingnematic phase, exists for energies below $\sim$60 meV in uniaxialstrained BaFe$_{1.9}$Ni$_{0.1}$As$_2$. Since this energy scale is considerably larger than the energy splitting of the $d_{xz}$ and $d_{yz}$ bands of uniaxialstrained Ba(Fe$_{1x}$Co$_x$)$_2$As$_2$ near optimal superconductivity, spin Isingnematic correlations is likely the driving force for the resistivity anisotropy and associated electronic nematic correlations.Physical Review B 11/2015; 92(18). DOI:10.1103/PhysRevB.92.180504 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present synchrotron xray diffraction, neutron powder diffraction and timeofflight inelastic neutron scattering measurements on the rare earth pyrochlore oxide Nd2Zr2O7 to study the ordered state magnetic structure and cystal field states. The structural characterization by highresolution synchrotron xray diffraction confirms that the pyrochlore structure has no detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals long range allin/allout antiferromagnetic order below T_N ~ 0.4 K with propagation vector k = (0 0 0) and an ordered moment of 1.26(2) \mu_B/Nd at 0.1 K. The ordered moment is much smaller than the estimated moment of 2.65 \mu_B/Nd for the local <111> Ising ground state of Nd3+ (J=9/2) suggesting that the ordering is partially suppressed by quantum fluctuations. The strong Ising anisotropy is further confirmed by the inelastic neutron scattering data which reveals a wellisolated dipolaroctupolar type Kramers doublet ground state. The crystal field level scheme and ground state wavefunction have been determined.  [Show abstract] [Hide abstract]
ABSTRACT: Magnetism in transitionmetal compounds (TMCs) has traditionally been associated with spin degrees of freedom, because the orbital magnetic moments are typically largely quenched. On the other hand, magnetic order in 4f and 5delectron systems arises from spin and orbital moments that are rigidly tied together by the large intraatomic spinorbit coupling (SOC). Using inelastic neutron scattering on the archetypal 4delectron Mott insulator Ca$_2$RuO$_4$, we report a novel form of excitonic magnetism in the intermediatestrength regime of the SOC. The magnetic order is characterized by ``soft'' magnetic moments with large amplitude fluctuations manifested by an intense, lowenergy excitonic mode analogous to the Higgs mode in particle physics. This mode heralds a proximate quantum critical point separating the soft magnetic order driven by the superexchange interaction from a quantumparamagnetic state driven by the SOC. We further show that this quantum critical point can be tuned by lattice distortions, and hence may be accessible in epitaxial thinfilm structures. The unconventional spinorbitallattice dynamics in Ca$_2$RuO$_4$ identifies the SOC as a novel source of quantum criticality in TMCs.  [Show abstract] [Hide abstract]
ABSTRACT: We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting Ba(Fe${_{0.957}}$Cu${_{0.043}}$)${_2}$As${_2}$, a composition close to a quantum critical point between AFM ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low Cu composition as well as the parent compound BaFe$_2$As$_2$ and superconducting Ba(Fe$_{1x}$Ni$_x$)$_2$As$_2$ compounds, we demonstrate that paramagnonlike spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe$_{0.957}$Cu$_{0.043}$)$_2$As$_2$, which is distinct from the AFMlike spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. We also show that the spinspin correlation length, ${\xi(T)}$, increases rapidly as the temperature is lowered and find ${\omega/T}$ scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.  [Show abstract] [Hide abstract]
ABSTRACT: We present timeoffight neutronscattering measurements on single crystals of $La_{2x}Ba_{x}CuO_{4}$ (LBCO) with 0 $\leq$ x $\leq$ 0.095 and $La_{2x}Sr_{x}CuO_{4}$ (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional (3D) commensurate long range antiferromagnetic order, for x $\leq$ 0.02, to two dimensional (2D) incommensurate antiferromagnetism coexisting with superconductivity for x $\geq$ 0.05. Previous work on lightly doped LBCO with x = 0.035 showed a clear resonant enhancement of the inelastic scattering coincident with the low energy crossings of the highly dispersive spin excitations and quasi2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore we show that the low temperature, low energy magnetic spectral weight is substantially larger for samples with nonsuperconducting ground states relative to any of the samples with superconducting ground states. Spin gaps, suppression of low energy magnetic spectral weight as a function of decreasing temperature, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO.  [Show abstract] [Hide abstract]
ABSTRACT: Complex lowtemperature ordered states in chiral magnets are typically governed by a competition between multiple magnetic interactions. The chirallattice multiferroic Cu2OSeO3 became the first insulating helimagnetic material in which a longrange order of topologically stable spin vortices known as skyrmions was established. Here we employ stateoftheart inelastic neutron scattering (INS) to comprehend the full threedimensional spin excitation spectrum of Cu2OSeO3 over a broad range of energies. Distinct types of high and lowenergy dispersive magnon modes separated by an extensive energy gap are observed in excellent agreement with the previously suggested microscopic theory based on a model of entangled Cu4 tetrahedra. The comparison of our INS data with model spindynamical calculations based on these theoretical proposals enables an accurate quantitative verification of the fundamental magnetic interactions in Cu2OSeO3 that are essential for understanding its abundant lowtemperature magnetically ordered phases.  [Show abstract] [Hide abstract]
ABSTRACT: High energy vibrational scattering in the binary systems UC and US is measured using timeofflight inelastic neutron scattering. A clear set of welldefined peaks equally separated in energy is observed in UC, corresponding to harmonic oscillations of the light C atoms in a cage of heavy U atoms. The scattering is much weaker in US and only a few oscillator peaks are visible. We show how the difference between the materials can be understood by considering the neutron scattering lengths and masses of the lighter atoms. Monte Carlo ray tracing is used to simulate the scattering, with near quantitative agreement with the data in UC, and some differences with US. The possibility of observing anharmonicity and anisotropy in the potentials of the light atoms is investigated in UC. Overall the observed data is well accounted for by considering each light atom as a single atom isotropic quantum harmonic oscillator.  [Show abstract] [Hide abstract]
ABSTRACT: A central issue in material science is to obtain understanding of the electronic correlations that control complex materials. Such electronic correlations frequently arise because of the competition of localized and itinerant electronic degrees of freedom. Although the respective limits of welllocalized or entirely itinerant ground states are well understood, the intermediate regime that controls the functional properties of complex materials continues to challenge theoretical understanding. We have used neutron spectroscopy to investigate plutonium, which is a prototypical material at the brink between bonding and nonbonding configurations. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum mechanical superposition of localized and itinerant electronic configurations as recently predicted by dynamical mean field theory. Our results not only resolve the longstanding controversy between experiment and theory on plutonium’s magnetism but alsoScience Advances 07/2015; 1:1500188. DOI:10.1126/sciadv.1500188 

Dataset: PhysRevB.89.054306
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ABSTRACT: A variety of crystals contain quasionedimensional substructures, which yield distinctive electronic, spintronic, optical and thermoelectric properties. There is a lack of understanding of the lattice dynamics that influences the properties of such complex crystals. Here we employ inelastic neutron scatting measurements and density functional theory calculations to show that numerous lowenergy optical vibrational modes exist in higher manganese silicides, an example of such crystals. These optical modes, including unusually lowfrequency twisting motions of the Si ladders inside the Mn chimneys, provide a large phase space for scattering acoustic phonons. A hybrid phonon and diffuson model is proposed to explain the low and anisotropic thermal conductivity of higher manganese silicides and to evaluate nanostructuring as an approach to further suppress the thermal conductivity and enhance the thermoelectric energy conversion efficiency. This discovery offers new insights into the structureproperty relationships of a broad class of materials with quasionedimensional substructures for various applications.Nature Communications 04/2015; 6:6723. DOI:10.1038/ncomms7723 · 11.47 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: MCViNE (MonteCarlo VIrtual Neutron Experiment) is a versatile Monte Carlo (MC) neutron raytracing program that provides researchers with tools for performing computer modeling and simulations that mirror real neutron scattering experiments. By adopting modern software engineering practices such as using composite and visitor design patterns for representing and accessing neutron scatterers, and using recursive algorithms for multiple scattering, MCViNE is flexible enough to handle sophisticated neutron scattering problems including, for example, neutron detection by complex detector systems, and single and multiple scattering events in a variety of samples and sample environments. In addition, MCViNE can take advantage of simulation components in linearchainbased MC ray tracing packages widely used in instrument design and optimization, as well as NumPybased components that make prototypes useful and easy to develop. These developments have enabled us to carry out detailed simulations of neutron scattering experiments with nontrivial samples in timeofflight inelastic instruments at the Spallation Neutron Source. Examples of such simulations for powder and singlecrystal samples with various scattering kernels, including kernels for phonon and magnon scattering, are presented. With simulations that closely reproduce experimental results, scattering mechanisms can be turned on and off to determine how they contribute to the measured scattering intensities, improving our understanding of the underlying physics.  [Show abstract] [Hide abstract]
ABSTRACT: Inelastic neutron scattering measurements on monoclinic zirconia $({\mathrm{ZrO}}_{\text{2}})$ and 8 mol% yttriumstabilized zirconia were performed at temperatures from 300 to $1373w\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Temperaturedependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. Firstprinciples lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Gr\"uneisen parameters. These mode Gr\"uneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttriumstabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.Physical Review B 04/2015; 91(14). DOI:10.1103/PhysRevB.91.144302 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The vibrational behavior of heavy substitutional impurities $(M=\phantom{\rule{4.pt}{0ex}}\text{Ir},\phantom{\rule{4.pt}{0ex}}\text{Os})$ in ${\text{Fe}}_{1$${}x}{M}_{x}\text{Si}\phantom{\rule{4.pt}{0ex}}(x=0,0.02,0.04,0.1)$ was investigated with a combination of inelastic neutron scattering (INS), transport measurements, and firstprinciples simulations. Our INS measurements on single crystals mapped the fourdimensional dynamical structure factor, $S(\mathbf{Q},E)$, for several compositions and temperatures. Our results show that both Ir and Os impurities lead to the formation of a weakly dispersive resonance vibrational mode, in the energy range of the acoustic phonon dispersions of the FeSi host. We also show that Ir doping, which introduces free carriers, leads to softened interatomic force constants compared to doping with Os, which is isoelectronic to Fe. We analyze the phonon $S(\mathbf{Q},E)$ from INS through a Green'sfunction model incorporating the phonon selfenergy based on firstprinciples density functional theory simulations, and we study the disorderinduced lifetimes on large supercells. Calculations of the quasiparticle spectral functions in the doped system reveal the hybridization between the resonance and the acoustic phonon modes. Our results demonstrate a strong interaction of the host acoustic dispersions with the resonance mode, likely leading to the large observed suppression in lattice thermal conductivity.Physical Review B 03/2015; 91(9). DOI:10.1103/PhysRevB.91.094307 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Although the rutile structure of TiO$_2$ is stable at high temperatures, the harmonic approximation predicts that several acoustic phonons decrease anomalously to zero frequency with thermal expansion, incorrectly predicting a structural collapse at temperatures well below 1000 K. Inelastic neutron scattering was used to measure the temperature dependence of the phonon density of states (DOS) of rutile TiO$_2$ from 300 to 1373 K. Surprisingly, these anomalous acoustic phonons were found to increase in frequency with temperature. Firstprinciples calculations showed that with lattice expansion, the potentials for the anomalous acoustic phonons transform from quadratic to quartic, stabilizing the rutile phase at high temperatures. In these modes, the vibrational displacements of adjacent Ti and O atoms cause variations in hybridization of $3d$ electrons of Ti and $2p$ electrons of O atoms. With thermal expansion, the energy variation in this "phonontracked hybridization" becomes less sensitive to displacement, flattening the bottom of the interatomic potential and inducing the phonon quarticity.Physical Review B 02/2015; 92(5). DOI:10.1103/PhysRevB.92.054304 · 3.74 Impact Factor 
Article: The ARCS radial collimator
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ABSTRACT: We have designed, installed, and commissioned a scattered beam radial collimator for use at the ARCS Wide Angular Range Chopper Spectrometer at the Spallation Neutron Source. The collimator has been designed to work effectively for thermal and epithermal neutrons and with a range of sample environments. Other design considerations include the accommodation of working within a high vacuum environment and having the ability to quickly install and remove the collimator from the scattered beam. We present here characterization of the collimator's performance and methodologies for its effective use.The European Physical Journal Conferences 01/2015; 83:03014. DOI:10.1051/epjconf/20158303014  [Show abstract] [Hide abstract]
ABSTRACT: The intensity distributions of beam monitors in directgeometry timeofflight neutron spectrometers provide important information about the instrument resolution. For shortpulse spallation neutron sources in particular, the asymmetry of the source pulse may be extracted and compared to Monte Carlo source simulations. An explicit formula using a Gaussianconvolved IkedaCarpenter distribution is given and compared to data from the ARCS instrument at the Spallation Neutron Source.The European Physical Journal Conferences 01/2015; 83:03001. DOI:10.1051/epjconf/20158303001  [Show abstract] [Hide abstract]
ABSTRACT: Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were $$\langle${}$\Delta${}{$\varepsilon${}}_{i}/{$\varepsilon${}}_{i}$\Delta${}T$\rangle${}=$${}0.07$, giving a mean isobaric Gr\"uneisen parameter of $+6.95\ifmmode\pm\else\textpm\fi{}0.67$, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes $0.15{k}_{\mathrm{B}}$/atom to the vibrational entropy, compared to $0.03{k}_{\mathrm{B}}$/atom from quasiharmonicity. Excellent agreement was found between the entropy from phonon DOS measurements and the reference NISTJANAF thermodynamic entropy from calorimetric measurements.Physical Review B 01/2015; 91(1):014307. DOI:10.1103/PhysRevB.91.014307 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have used timeofflight inelastic neutron scattering to measure the spin wave spectrum of the canonical halfdoped manganite Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$, in its magnetic and orbitally ordered phase. Comparison of the data, which cover multiple Brillouin zones and the entire energy range of the excitations, with several different models shows that only the CEtype ordered state provides an adequate description of the magnetic ground state, provided interactions beyond nearest neighbor are included. We are able to rule out a ground state in which there exist pairs of dimerized spins which interact only with their nearest neighbors. The Zener polaron ground state, which comprises strongly bound magnetic dimers, can be ruled out on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of subtle discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. interpolating back to the CE model. These results are consistent with theoretical analysis of the degenerate double exchange model for halfdoping.  [Show abstract] [Hide abstract]
ABSTRACT: The structure and lattice dynamics of rocksalt thermoelectric materials SnTe and PbTe are investigated with singlecrystal and powder neutron diffraction, inelastic neutron scattering (INS), and firstprinciples simulations. Our firstprinciples calculations of the radial distribution function in both SnTe and PbTe show a clear asymmetry in the first nearestneighbor (1NN) peak, which increases with temperature, in agreement with recent experimental reports. We show that this peak asymmetry for the 1NN SnTe or PbTe bond results from largeamplitude anharmonic vibrations (phonons). No atomic off centering is found in our simulations. In addition, the atomic meansquare displacements derived from our diffraction data reveal stiffer bonding at the anion site, in good agreement with the partial phonon densities of states from INS and firstprinciples calculations. These results provide clear evidence for largeamplitude anharmonic phonons associated with the resonant bonding leading to the ferroelectric instability.Physical Review B 12/2014; 90(21):214303. DOI:10.1103/PhysRevB.90.214303 · 3.74 Impact Factor
Publication Stats
2k  Citations  
624.95  Total Impact Points  
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Institutions

19932015

Oak Ridge National Laboratory
 • Quantum Condensed Matter Division
 • Materials Science and Technology Division
 • Neutron Scattering Science Division
 • Spallation Neutron Source
 • Solid State Division
Oak Ridge, Florida, United States


2001

Argonne National Laboratory
 Division of Materials Science
Лимонт, Illinois, United States


19952000

European Synchrotron Radiation Facility
 Division of Experiments
Grenoble, RhôneAlpes, France


1996

University of Illinois, UrbanaChampaign
Urbana, Illinois, United States


19911993

Massachusetts Institute of Technology
 Department of Physics
Cambridge, Massachusetts, United States
