D. L. Abernathy

Oak Ridge National Laboratory, Oak Ridge, Florida, United States

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Publications (180)656.51 Total impact

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    ABSTRACT: We present synchrotron x-ray diffraction, neutron powder diffraction, and time-of-flight 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 high-resolution synchrotron x-ray diffraction confirms that the pyrochlore structure has no detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals long-range all-in/all-out 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 dipolar-octupolar character which possibly induces the quantum fluctuation. The crystal-field level scheme and ground state wave function have been determined.
    No preview · Article · Dec 2015 · Physical Review B
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    ABSTRACT: Phonon properties of $\mathrm{Mo_3Sb_{7-x}Te_x}$ ($x=0,1.5, 1.7$), a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles 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 electron-phonon screening, and a large overall stiffening of interatomic force-constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloy-disorder scattering, and resulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes non-uniformly 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 electron-phonon coupling on phonon mean-free-paths in this compound, and also estimates the contributions from boundary scattering, umklapp scattering, and point-defect scattering.
    No preview · Article · Dec 2015 · Physical Review B
  • M. Matsuda · S. E. Dissanayake · D. L. Abernathy · K. Totsuka · A. A. Belik
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    ABSTRACT: Inelastic neutron scattering experiments have been carried out on a powder sample of Cu2PO4OH, which consists of diamond-shaped 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.
    No preview · Article · Nov 2015 · Physical Review B
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    ABSTRACT: Research in the rapidly-developing field of 2D-electronic materials has thus far been focused on metallic and semiconducting materials. However, complementary dielectric materials such as non-linear 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 cation-deficient 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 ferrielectrically-active CuInP2S6 are realized by controllable chemical phase separation in van-der-Waals bonded single crystals. CuInP2S6 by itself is a layered ferroelectric with Tc just over room-temperature 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 van-der-Waals 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 in-plane heterostructures with periodically alternating polar and non-polar phases.
    Full-text · Article · Nov 2015 · ACS Nano
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    ABSTRACT: We present synchrotron x-ray diffraction, neutron powder diffraction and time-of-flight 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 high-resolution synchrotron x-ray diffraction confirms that the pyrochlore structure has no detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals long range all-in/all-out 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 well-isolated dipolar-octupolar type Kramers doublet ground state. The crystal field level scheme and ground state wavefunction have been determined.
    Full-text · Article · Nov 2015
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    ABSTRACT: We use inelastic neutron scattering to study the temperature and energy dependence of the spin excitation anisotropy in uniaxial-strained electron-doped 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 in-plane resistivity anisotropy in the paramagnetic tetragonal phase of electron-underdoped iron pnictides under uniaxial pressure, which has been attributed to a spin-driven Ising-nematic state or orbital ordering. Here we show that the spin excitation anisotropy, a signature of the spin-driven Ising-nematic phase, exists for energies below $\sim$60 meV in uniaxial-strained BaFe$_{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 uniaxial-strained Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ near optimal superconductivity, spin Ising-nematic correlations is likely the driving force for the resistivity anisotropy and associated electronic nematic correlations.
    Preview · Article · Nov 2015 · Physical Review B
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    ABSTRACT: Magnetism in transition-metal 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 5d-electron systems arises from spin and orbital moments that are rigidly tied together by the large intra-atomic spin-orbit coupling (SOC). Using inelastic neutron scattering on the archetypal 4d-electron Mott insulator Ca$_2$RuO$_4$, we report a novel form of excitonic magnetism in the intermediate-strength regime of the SOC. The magnetic order is characterized by ``soft'' magnetic moments with large amplitude fluctuations manifested by an intense, low-energy 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 quantum-paramagnetic 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 thin-film structures. The unconventional spin-orbital-lattice dynamics in Ca$_2$RuO$_4$ identifies the SOC as a novel source of quantum criticality in TMCs.
    Preview · Article · Oct 2015
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    ABSTRACT: We present the results of elastic and inelastic neutron scattering measurements on non-superconducting 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$_{1-x}$Ni$_x$)$_2$As$_2$ compounds, we demonstrate that paramagnon-like 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 AFM-like 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 spin-spin 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.
    Preview · Article · Oct 2015 · Physical Review B
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    ABSTRACT: We present time-of-fight neutron-scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO) with 0 $\leq$ x $\leq$ 0.095 and $La_{2-x}Sr_{x}CuO_{4}$ (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 co-existing 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 quasi-2D 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 non-superconducting 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.
    No preview · Article · Sep 2015
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    M. Matsuda · S. E. Dissanayake · D. L. Abernathy · K. Totsuka · A. A. Belik
    [Show abstract] [Hide abstract]
    ABSTRACT: Inelastic neutron scattering experiments have been carried out on a powder sample of Cu2PO4OH, which consists of diamond-shaped 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.
    Full-text · Article · Sep 2015
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    ABSTRACT: Complex low-temperature ordered states in chiral magnets are typically governed by a competition between multiple magnetic interactions. The chiral-lattice multiferroic Cu2OSeO3 became the first insulating helimagnetic material in which a long-range order of topologically stable spin vortices known as skyrmions was established. Here we employ state-of-the-art inelastic neutron scattering (INS) to comprehend the full three-dimensional spin excitation spectrum of Cu2OSeO3 over a broad range of energies. Distinct types of high- and low-energy 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 spin-dynamical 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 low-temperature magnetically ordered phases.
    Full-text · Article · Sep 2015
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    ABSTRACT: High energy vibrational scattering in the binary systems UC and US is measured using time-of-flight inelastic neutron scattering. A clear set of well-defined 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.
    Full-text · Article · Aug 2015
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    ABSTRACT: Magnetocaloric (MC) materials present an avenue for chemical-free, 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 first-principle calculations showing a strong coupling between lattice distortions and magnetic excitations.
    Full-text · Article · Jul 2015 · Physical Review B
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    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 well-localized 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 long-standing controversy between experiment and theory on plutonium’s magnetism but also
    Full-text · Article · Jul 2015 · Science Advances
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    Full-text · Dataset · May 2015
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    Full-text · Dataset · May 2015
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    ABSTRACT: A variety of crystals contain quasi-one-dimensional 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 low-energy optical vibrational modes exist in higher manganese silicides, an example of such crystals. These optical modes, including unusually low-frequency 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 structure-property relationships of a broad class of materials with quasi-one-dimensional substructures for various applications.
    No preview · Article · Apr 2015 · Nature Communications
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    ABSTRACT: MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo (MC) neutron ray-tracing 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 linear-chain-based MC ray tracing packages widely used in instrument design and optimization, as well as NumPy-based components that make prototypes useful and easy to develop. These developments have enabled us to carry out detailed simulations of neutron scattering experiments with non-trivial samples in time-of-flight inelastic instruments at the Spallation Neutron Source. Examples of such simulations for powder and single-crystal 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.
    Full-text · Article · Apr 2015
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    ABSTRACT: Inelastic neutron scattering measurements on monoclinic zirconia $({\mathrm{ZrO}}_{\text{2}})$ and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to $1373w\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Temperature-dependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. First-principles 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. Yttrium-stabilized 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.
    Full-text · Article · Apr 2015 · Physical Review B
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    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 first-principles simulations. Our INS measurements on single crystals mapped the four-dimensional 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's-function model incorporating the phonon self-energy based on first-principles density functional theory simulations, and we study the disorder-induced 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.
    No preview · Article · Mar 2015 · Physical Review B

Publication Stats

3k Citations
656.51 Total Impact Points

Institutions

  • 1993-2015
    • 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
  • 1995-2001
    • European Synchrotron Radiation Facility
      • Division of Experiments
      Grenoble, Rhône-Alpes, France
  • 1991-1996
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States