Publications (163)623.01 Total impact


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 · 10.74 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 · 3.66 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 · 3.66 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.  [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: 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 · 3.66 Impact Factor  The European Physical Journal Conferences 01/2015; 83:03001. DOI:10.1051/epjconf/20158303001

Article: The ARCS radial collimator
The European Physical Journal Conferences 01/2015; 83:03014. DOI:10.1051/epjconf/20158303014  [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 · 3.66 Impact Factor 
Conference Paper: Direct determination of changes in lattice dynamics across the metaltoinsulator transition in VO 2
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ABSTRACT: The relatively simple binary oxide, VO 2 , has served for decades as a prototypical material challenging the ability of scientists to understand how a hightemperature, metallic conductor emerges from a lowtemperature band (Peierls) or stronglycorrelated (Mott) insulator. A predictive microscopic description remains elusive and controversial. The firstorder metalinsulator transition (MIT) in VO 2 occurs just above room temperature (T c ~340 K), where the conductivity changes by four orders of magnitude, and concurrently, the lattice structure changes from hightemperature tetragonal (rutile) to lowtemperature monoclinic (M1). A fundamental gap in our knowledge is the lack of an accurate description of changes in lattice dynamics associated with the MIT in VO 2 . Phonon dispersion curves normally obtained using singlecrystal inelastic neutron scattering (INS) measurements are not available due to the large incoherent vanadium cross section. We have now determined the changes in lattice dynamics and vibrational entropy associated with the MIT. We used inelastic neutron scattering at the SNS/ARCS spectrometer to obtain the Qintegrated phonon density of states, xray scattering at APS/33BM to obtain 3dimensional maps of energyintegrated thermal diffuse scattering, inelastic xray scattering at APS/HERIX to directly measure phonon dispersion along symmetry axes, and ab initio calculations to elucidate the atomic mechanisms driving the MIT. Our results show that the transition entropy change driving the MIT is dominated by vibrational rather than electronic contributions. Moreover, we show that proposals of an " Rpoint soft mode " phase transition are incorrect, and instead, we identify strongly anharmonic lattice dynamics as the underlying mechanism stabilizing the metallic phase.Materials Research Society, Fall Meeting, Boston, MA; 11/2014  [Show abstract] [Hide abstract]
ABSTRACT: There are two renowned theories of superfluidity in liquid ^{4}He, quite different and each with specific domains of application. In the first, the Landau theory, superflow follows from the existence of a welldefined collective mode supported by dense liquid ^{4}He, the phononroton mode. In the second, superflow is a manifestation of BoseEinstein condensation (BEC) and phase coherence in the liquid. We present combined measurements of superfluidity, BEC and phononroton (PR) modes in liquid ^{4}He confined in the porous medium MCM41. The results integrate the two theories by showing that welldefined PR modes exist where there is BEC. The two are common properties of a Bose condensed liquid and either can be used as a basis of a theory of superfluidity. In addition, the confinement and disorder suppresses the critical temperature for superfluidity, T_{c}, below that for BEC creating a localized BEC "phase" consisting of islands of BEC and PR modes. This phase is much like the pseudogap phase in the cuprate superconductors.Physical Review Letters 11/2014; 113(21):215302. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Phase competition underlies many remarkable and technologically important phenomena in transition metal oxides. Vanadium dioxide (VO2) exhibits a firstorder metalinsulator transition (MIT) near room temperature, where conductivity is suppressed and the lattice changes from tetragonal to monoclinic on cooling. Ongoing attempts to explain this coupled structural and electronic transition begin with two alternative starting points: a Peierls MIT driven by instabilities in electronlattice dynamics and a Mott MIT where strong electronelectron correlations drive charge localization. A key missing piece of the VO2 puzzle is the role of lattice vibrations. Moreover, a comprehensive thermodynamic treatment must integrate both entropic and energetic aspects of the transition. Here we report that the entropy driving the MIT in VO2 is dominated by strongly anharmonic phonons rather than electronic contributions, and provide a direct determination of phonon dispersions. Our ab initio calculations identify softer bonding in the tetragonal phase, relative to the monoclinic phase, as the origin of the large vibrational entropy stabilizing the metallic rutile phase. They further reveal how a balance between higher entropy in the metal and orbitaldriven lower energy in the insulator fully describes the thermodynamic forces controlling the MIT. Our study illustrates the critical role of anharmonic lattice dynamics in metal oxide phase competition, and provides guidance for the predictive design of new materials.Nature 11/2014; DOI:10.1038/nature13865 · 42.35 Impact Factor 
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ABSTRACT: The phonon dispersions and scattering rates of the thermoelectric material Ag1xSb1+xTe2+x (x = 0,0.1,0.2) were measured with inelastic neutron scattering, as function of both temperature T and off stoichiometry x. In addition, detailed measurements of diffuse scattering were performed with both neutron and synchrotron xray diffraction. The results show that phonon scattering rates are large and weakly dependent on T or x, and the lattice thermal conductivity calculated from these scattering rates and group velocities is in good agreement with bulk transport measurements. We also find that the scattering rates and their temperature dependence cannot be accounted for with common models of phonon scattering by anharmonicity or point defects. The diffuse scattering measurements show a pervasive, complex signal, with several distinct components. In particular, broad superstructure reflections indicate a shortrange ordering of the Ag and Sb cations on their sublattice. Singlecrystal Bragg peak intensities also reveal large static atomic displacements, compatible with results from Rietveld refinement of neutron powder diffraction data. Our results indicate that a complex nanostructure, arising from multiple variants of nanoscale anisotropic superstructures of cations, and large atomic displacements, is likely responsible for the strong phonon scattering.Physical Review B 10/2014; 90(13):134303. DOI:10.1103/PhysRevB.90.134303 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: True inorganic spinPeierls materials are extremely rare, but ${\mathrm{NaTiSi}}_{2}{\mathrm{O}}_{6}$ was at one time considered to be an ideal candidate owing to its well separated chains of edgesharing ${\mathrm{TiO}}_{6}$ octahedra. At low temperatures, this material undergoes a phase transition from $C2/c$ to $P\overline{1}$ symmetry, where ${\mathrm{Ti}}^{3+}\text{$${}}{\mathrm{Ti}}^{3+}$ dimers begin to form within the chains. However, it was quickly realized with magnetic susceptibility that simple spin fluctuations do not progress to the point of enabling such a transition. Since then, considerable experimental and theoretical endeavors have been undertaken to find the true ground state of this system and explain how it manifests. Here, we employ the use of xray diffraction, neutron spectroscopy, and magnetic susceptibility to directly and simultaneously measure the symmetry loss, spin singlettriplet gap, and phonon modes. A gap of 53(3) meV was observed, fit to the magnetic susceptibility, and compared to previous theoretical models to unambiguously assign ${\mathrm{NaTiSi}}_{2}{\mathrm{O}}_{6}$ as having an orbitalassisted Peierls ground state.Physical Review B 10/2014; 90(14):140402. DOI:10.1103/PhysRevB.90.140402 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Neutron scattering is used to probe magnetic interactions as superconductivity develops in optimally doped Fe 1+δ Se x Te 1−x. Applying the first moment sum rule to comprehensive neutron scattering data, we extract the change in magnetic exchange energy [J R−R S R · S R ] in the superconducting state referenced to the normal state. Oscillatory changes are observed for FeFe displacements R < ξ, where ξ = 1.3(1) nm is the superconducting coherence length. Dominated by a large reduction in the second nearest neighbor exchange energy [−1.2(2) meV/Fe], the overall reduction in magnetic interaction energy is H mag = −0.31(9) meV/Fe. Comparison to the superconducting condensation energy E SC = −0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe 1+δ Se x Te 1−x .Physical Review B 09/2014; 90(10):100501(R). DOI:10.1103/PhysRevB.90.100501 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have engineered and installed a radial collimator for use in the scattered beam of a neutron timeofflight spectrometer at a spallation neutron source. The radial collimator may be used with both thermal and epithermal neutrons, reducing the detected scattering intensity due to material outside of the sample region substantially. The collimator is located inside of the sample chamber of the instrument, which routinely cycles between atmospheric conditions and cryogenic vacuum. The oscillation and support mechanism of the collimator allow it to be removed from use without breaking vacuum. We describe here the design and characterization of this radial collimator.Review of Scientific Instruments 08/2014; 85(8):0851010851019. DOI:10.1063/1.4891302 · 1.58 Impact Factor
Publication Stats
2k  Citations  
623.01  Total Impact Points  
Top Journals
Institutions

1992–2015

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


1995–2001

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


1991–1998

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


1996

University of Illinois, UrbanaChampaign
Urbana, Illinois, United States
