Publications (144)566.95 Total impact

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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. 
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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. 

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 
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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 
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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 
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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 
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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 
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ABSTRACT: The insulatortometal transition continues to be a challenging subject, especially when electronic correlations are strong. In layered compounds, such as La2xSrxNiO4 and La2xBaxCuO4, the doped charge carriers can segregate into periodically spaced charge stripes separating narrow domains of antiferromagnetic order. Although there have been theoretical proposals of dynamically fluctuating stripes, direct spectroscopic evidence of chargestripe fluctuations has been lacking. Here we report the detection of critical lattice fluctuations, driven by chargestripe correlations, in La2xSrxNiO4 using inelastic neutron scattering. This scattering is detected at large momentum transfers where the magnetic form factor suppresses the spin fluctuation signal. The lattice fluctuations associated with the dynamic charge stripes are narrow in q and broad in energy. They are strongest near the chargestripe melting temperature. Our results open the way towards the quantitative theory of dynamic stripes and for directly detecting dynamical charge stripes in other strongly correlated systems, including hightemperature superconductors such as La2xSrxCuO4.Nature Communications 07/2014; 5:3467. DOI:10.1038/ncomms4467 · 10.74 Impact Factor 
Article: Crystallography and physical properties of BaCo2As2, Ba0.94K0.06Co2As2, and Ba0.78K0.22Co2As2
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ABSTRACT: The crystallographic and physical properties of polycrystalline and single crystal samples of BaCo2As2 and Kdoped Ba{1x}K{x}Co2As2 (x = 0.06, 0.22) are investigated by xray and neutron powder diffraction, magnetic susceptibility chi, magnetization, heat capacity Cp, {75}As NMR and electrical resistivity rho measurements versus temperature T. The crystals were grown using both Sn flux and CoAs selfflux, where the Sngrown crystals contain 1.62.0 mol% Sn. All samples crystallize in the tetragonal ThCr2Si2type structure (space group I4/mmm). For BaCo2As2, powder neutron diffraction data show that the caxis lattice parameter exhibits anomalous negative thermal expansion from 10 to 300 K, whereas the aaxis lattice parameter and the unit cell volume show normal positive thermal expansion over this T range. No transitions in BaCo2As2 were found in this T range from any of the measurements. Below 4050 K, we find rho ~ T^2 indicating a Fermi liquid ground state. A large density of states at the Fermi energy D(EF) ~ 18 states/(eV f.u.) for both spin directions is found from lowT Cp(T) measurements, whereas the band structure calculations give D(EF) = 8.23 states/(eV f.u.). The {75}As NMR shift data versus T have the same T dependence as the chi(T) data, demonstrating that the derived chi(T) data are intrinsic. The observed {75}As nuclear spin dynamics are consistent with the presence of ferromagnetic and/or stripetype antiferromagnetic spin fluctuations. The crystals of Ba{0.78}K{0.22}Co2As2 were grown in Sn flux and show properties very similar to those of undoped BaCo2As2. On the other hand, the crystals from two batches of Ba{0.94}K{0.06}Co2As2 grown in CoAs selfflux show evidence of weak ferromagnetism at T < 10 K with small ordered moments at 1.8 K of 0.007 and 0.03 muB per formula unit, respectively.Physical Review B 06/2014; 90(6). DOI:10.1103/PhysRevB.90.064517 · 3.66 Impact Factor 
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ABSTRACT: We use neutron scattering to study the spin excitations associated with the stripe antiferromagnetic order in semiconducting K_{0.85}Fe_{1.54}Se_{2} (T_{N}=280 K). We show that the spinwave spectra can be accurately described by an effective Heisenberg Hamiltonian with highly anisotropic inplane couplings at T=5 K. At high temperature (T=300 K) above T_{N}, shortrange magnetic correlation with anisotropic correlation lengths are observed. Our results suggest that, despite the dramatic difference in the Fermi surface topology, the inplane anisotropic magnetic couplings are a fundamental property of the ironbased compounds; this implies that their antiferromagnetism may originate from local strong correlation effects rather than weak coupling Fermi surface nesting.Physical Review Letters 05/2014; 112(17):177002. · 7.73 Impact Factor 
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ABSTRACT: Relaxor ferroelectrics exemplify a class of functional materials where interplay between disorder and phase instability results in inhomogeneous nanoregions. Although known for about 30 years, there is no definitive explanation for polar nanoregions (PNRs). Here we show that ferroelectric phonon localization drives PNRs in relaxor ferroelectric PMN30%PT using neutron scattering. At the frequency of a preexisting resonance mode, nanoregions of standing ferroelectric phonons develop with a coherence length equal to one wavelength and the PNR size. Anderson localization of ferroelectric phonons by resonance modes explains our observations and, with nonlinear slowing, the PNRs and relaxor properties. Phonon localization at additional resonances near the zone edges explains competing antiferroelectric distortions known to occur at the zone edges. Our results indicate the size and shape of PNRs that are not dictated by complex structural details, as commonly assumed, but by phonon resonance wave vectors. This discovery could guide the design of next generation relaxor ferroelectrics.Nature Communications 04/2014; 5:3683. DOI:10.1038/ncomms4683 · 10.74 Impact Factor 
Article: A comparison of four direct geometry timeofflight spectrometers at the Spallation Neutron Source
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ABSTRACT: The Spallation Neutron Source at Oak Ridge National Laboratory now hosts four direct geometry timeofflight chopper spectrometers. These instruments cover a range of wavevector and energy transfer space with varying degrees of neutron flux and resolution. The regions of reciprocal and energy space available to measure at these instruments are not exclusive and overlap significantly. We present a direct comparison of the capabilities of this instrumentation, conducted by data mining the instrument usage histories, and specific scanning regimes. In addition, one of the common science missions for these instruments is the study of magnetic excitations in condensed matter systems. We have measured the powder averaged spin wave spectra in one particular sample using each of these instruments, and use these data in our comparisons.Review of Scientific Instruments 04/2014; 85(4):04511304511313. DOI:10.1063/1.4870050 · 1.58 Impact Factor 
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ABSTRACT: Recently an extended series of equally spaced vibrational modes was observed in uranium nitride (UN) by performing neutron spectroscopy measurements using the ARCS and SEQUOIA timeofflight chopper spectrometers [A. A. Aczel et al., Nat. Commun. 3, 1124 (2012), 10.1038/ncomms2117]. These modes are well described by threedimensional isotropic quantum harmonic oscillator (QHO) behavior of the nitrogen atoms, but there are additional contributions to the scattering that complicate the measured response. In an effort to better characterize the observed neutron scattering spectrum of UN, we have performed Monte Carlo ray tracing simulations of the ARCS and SEQUOIA experiments with various sample kernels, accounting for nitrogen QHO scattering, contributions that arise from the acoustic portion of the partial phonon density of states, and multiple scattering. These simulations demonstrate that the U and N motions can be treated independently, and show that multiple scattering contributes an approximate Qindependent background to the spectrum at the oscillator mode positions. Temperaturedependent studies of the lowest few oscillator modes have also been made with SEQUOIA, and our simulations indicate that the T dependence of the scattering from these modes is strongly influenced by the uranium lattice.Physical Review B 03/2014; 89(14). DOI:10.1103/PhysRevB.89.144302 · 3.66 Impact Factor 
Article: Commensurate antiferromagnetic fluctuations as a signature of the pseudogap in a model cuprate
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ABSTRACT: The superconductivity in the cuprates might be mediated by fluctuations of a distinct order parameter, such as antiferromagnetism [1], q=0 order [2] or chargespin stripes [3].The two most salient and purportedly universal features of the antiferromagnetic (AF) excitation spectrum are the resonance mode, a property of the superconducting (SC) state[46], and the Xshaped hourglass response, whose downward dispersion has been associated with either stripe correlations[3,4] or the resonance[5,6]. Here we use neutron scattering on HgBa$_2$CuO$_{4+{\delta}}$ (Hg1201; moderately doped, transition temperature T$_c = 71$ K), which exhibits a simple tetragonal structure and the highest optimal T$_c$ of all singleCuO2layer cuprates[7], to uncover a gapped Yshaped spectrum in both the SC and the normal state. The response lacks an incommensurate downward dispersion and a sharp resonance mode, thus obviating the conventional wisdom concerning magnetism in the cuprates. Instead, the spectrum is reminiscent of the normalstate response of doublelayer YBa$_2$Cu$_3$O${_6+y}$ (YBCO)[8,9] and constitutes a distinct signature of the pseudogap state. We demonstrate for both Hg1201 and YBCO (moderatelydoped, T$_c = 61$ K) that substantial AF fluctuations first appear along with q = 0 magnetism below the pseudogap temperature. This points to the distinct possibility that the appearance of q=0 magnetism is aided by commensurate AF fluctuations, setting the stage for chargedensitywave order[1012] and superconductivity at lower temperature. 
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ABSTRACT: Inelastic neutron scattering measurements on silver oxide (Ag$_2$O) with the cuprite structure were performed at temperatures from 40 to 400\,K, and Fourier transform farinfrared spectra were measured from 100 to 300\,K. The measured phonon densities of states and the infrared spectra showed unusually large energy shifts with temperature, and large linewidth broadenings. First principles molecular dynamics (MD) calculations were performed at various temperatures, successfully accounting for the negative thermal expansion (NTE) and local dynamics. Using the Fouriertransformed velocity autocorrelation method, the MD calculations reproduced the large anharmonic effects of Ag$_2$O, and were in excellent agreement with the neutron scattering data. The quasiharmonic approximation (QHA) was less successful in accounting for much of the phonon behavior. The QHA could account for some of the NTE below 250 K, although not at higher temperatures. Strong anharmonic effects were found for both phonons and for the NTE. The lifetime broadenings of Ag$_2$O were explained by anharmonic perturbation theory, which showed rich interactions between the Agdominated modes and the Odominated modes in both up and downconversion processes.Physical Review B 02/2014; 89:054306. DOI:10.1103/PhysRevB.89.054306 · 3.66 Impact Factor 
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ABSTRACT: The ferropnictide superconductors exhibit a sensitive interplay between the lattice and magnetic degrees of freedom, including a number of phonon modes that are much softer than predicted by nonmagnetic calculations using density functional theory (DFT). However, it is not known what effect, if any, the longrange magnetic order has on phonon frequencies above 23 meV, where several phonon branches are very closely spaced in energy and it is challenging to isolate them from each other. We measured these phonons using inelastic timeofflight neutron scattering in ~40 Brillouin zones, and developed a technique to determine their frequencies. We find this method capable of determining phonon energies to ~0.1 meV accuracy, and that the DFT calculations using the experimental structure yield qualitatively correct energies and eigenvectors. We do not find any effect of the magnetic transition on these phonons.Physical Review B 02/2014; 89(6). DOI:10.1103/PhysRevB.89.064310 · 3.66 Impact Factor
Publication Stats
2k  Citations  
566.95  Total Impact Points  
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Institutions

1992–2015

Oak Ridge National Laboratory
 • Quantum Condensed Matter Division
 • Materials Science and Technology Division
 • Neutron Scattering Science Division
 • 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
