Publications (180)656.51 Total impact

Article: Magnetic structure and crystalfield states of the pyrochlore antiferromagnet Nd 2 Zr 2 O 7
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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 cystalfield 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 longrange allin/allout antiferromagnetic order below TN≈0.4 K with propagation vector k = (0 0 0) and an ordered moment of 1.26(2)μB/Nd at 0.1 K. The ordered moment is much smaller than the estimated moment of 2.65μB/Nd for the local 〈111〉 Ising ground state of Nd3+ (J=9/2) suggesting that the ordering is partially suppressed by quantum fluctuations. The inelastic neutron scattering experiment further confirms the Ising anisotropic ground state of Nd3+ and also reveals its dipolaroctupolar character which possibly induces the quantum fluctuation. The crystalfield level scheme and ground state wave function have been determined. 
Article: Electronphonon coupling and thermal transport in the thermoelectric compound Mo 3 Sb 7 − x Te x
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ABSTRACT: Phonon properties of $\mathrm{Mo_3Sb_{7x}Te_x}$ ($x=0,1.5, 1.7$), a potential hightemperature thermoelectric material, have been studied with inelastic neutron and xray scattering, and with firstprinciples simulations. The substitution of Te for Sb leads to pronounced changes in the electronic structure, local bonding, phonon density of states (DOS), dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electronphonon screening, and a large overall stiffening of interatomic forceconstants. The suppression in electronphonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloydisorder scattering, and resulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes nonuniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. The respective roles of changes in phonon group velocities and phonon lifetimes on the lattice thermal conductivity are quantified. Our results highlight the importance of the electronphonon coupling on phonon meanfreepaths in this compound, and also estimates the contributions from boundary scattering, umklapp scattering, and pointdefect scattering.  [Show abstract] [Hide abstract]
ABSTRACT: Inelastic neutron scattering experiments have been carried out on a powder sample of Cu2PO4OH, which consists of diamondshaped tetramer spin units with S=1/2. We have observed two nearly dispersionless magnetic excitations at E1∼12 and E2∼20 meV whose energy widths are broader than the instrumental resolution. The simplest square tetramer model with one dominant interaction, which predicts two sharp excitation peaks at E1 and E2 (=2E1), does not explain the experimental result. We found that two diagonal intratetramer interactions compete with the main interaction, and weak intertetramer interactions connect the tetramers. The main intratetramer interaction is found to split into two inequivalent ones due to a structural distortion below 160 K. Cu2PO4OH is considered to be a good material to study the S=1/2 Heisenberg tetramer system.  [Show abstract] [Hide abstract]
ABSTRACT: Research in the rapidlydeveloping field of 2Delectronic materials has thus far been focused on metallic and semiconducting materials. However, complementary dielectric materials such as nonlinear dielectrics are needed to enable realistic device architectures. Candidate materials require tunable dielectric properties and pathways for heterostructure assembly. Here we report on a family of cationdeficient transition metal thiophosphates whose unique chemistry makes them a viable prospect for these applications. In these materials, naturally occurring ferrielectric heterostructures composed of centrosymmetric In4/3P2S6 and ferrielectricallyactive CuInP2S6 are realized by controllable chemical phase separation in vanderWaals bonded single crystals. CuInP2S6 by itself is a layered ferroelectric with Tc just over roomtemperature which rapidly decreases with homogenous doping. Surprisingly, in our composite materials, the ferrielectric Tc of the polar CuInP2S6 phase increases. This effect is enabled by unique spinodal decomposition that retains the overall vanderWaals layered morphology of the crystal, but chemically separates CuInP2S6 and In4/3P2S6 within each layer. The average spatial periodicity of the distinct chemical phases can be finely controlled by altering the composition and/or annealing conditions. One intriguing prospect for such layered spinodal alloys is large volume synthesis of 2D inplane heterostructures with periodically alternating polar and nonpolar phases.  [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: 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.  [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: Inelastic neutron scattering experiments have been carried out on a powder sample of Cu2PO4OH, which consists of diamondshaped tetramer spin units with S=1/2. We have observed two nearly dispersionless magnetic excitations at E1~12 and E2~20 meV, whose energy width are broader than the instrumental resolution. The simplest square tetramer model with one dominant interaction, which predicts two sharp excitation peaks at E1 and E2(=2E1), does not explain the experimental result. We found that two diagonal intratetramer interactions compete with the main interaction and weak intertetramer interactions connect the tetramers. The main intratetramer interaction is found to split into two inequivalent ones due to a structural distortion below 160 K. Cu2PO4OH is considered to be a good material to study the S=1/2 Heisenberg tetramer system.  [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: Magnetocaloric (MC) materials present an avenue for chemicalfree, solid state refrigeration through cooling via adiabatic demagnetization. We have used inelastic neutron scattering to measure the lattice dynamics in the MC material Ni45Co5Mn36.6In13.4. Upon heating across the Curie Temperature (TC), the material exhibits an anomalous increase in phonon entropy of 0.22 +/ 0.04 kB/atom, which is ten times larger than expected from conventional thermal expansion. This transition is accompanied by an abrupt softening of the transverse optic phonon. We present firstprinciple calculations showing a strong coupling between lattice distortions and magnetic excitations.  [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 also 

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.  [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.  [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.
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3k  Citations  
656.51  Total Impact Points  
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Institutions

19932015

Oak Ridge National Laboratory
 • 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


19952001

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


19911996

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