G. Ehlers

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

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Publications (176)461.23 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Spin dynamics in the intermediate ordered phases (between 4 and 9 K) in [Formula: see text] have been studied with inelastic neutron scattering. It is found that the spin waves are very diffuse, indicative of short lived correlations and the coexistence of paramagnetic moments with the long-range ordered state.
    Journal of Physics Condensed Matter 06/2015; 27(25):256003. DOI:10.1088/0953-8984/27/25/256003 · 2.22 Impact Factor
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    ABSTRACT: Protein low-frequency vibrational modes are an important portion of a proteins' dynamical repertoire. Yet, it is notoriously difficult to isolate specific vibrational features in the spectra of proteins. Given an appropriately chosen model peptide, and using different experimental conditions, we can simplify the system and gain useful insights into the protein vibrational properties. Combining neutron scattering, depolarized light scattering, and molecular dynamics simulations, we analyse the low frequency vibrations of biological molecules, comparing the results from a small globular protein, lysozyme, and an amphiphilic peptide, NALMA, both in solution and in powder states. Lysozyme and NALMA present similar spectral features in the frequency range between 1 and 10 THz. With the aid of MD simulations, we assign the spectral features to methyl groups' librations (1–5 THz) and hindered torsions (5–10 THz) in NALMA. Our data also show that, while proteins display boson peak vibrations in both powder and solution forms, NALMA exhibits boson peak vibrations in powder form only. This provides insight into the nature of this feature, suggesting a connection of BP collective motions to a characteristic length scale of heterogeneities present in the system. These results provide context for the use of model peptide systems to study protein dynamics; demonstrating both their utility, and the great care that has to be used in extrapolating results observed in powder to solutions.
    Physical Chemistry Chemical Physics 06/2015; 17(17). DOI:10.1039/c4cp05388e · 4.20 Impact Factor
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    ABSTRACT: Poly-L-glutamic acid (PGA) is a widely used biomaterial, with applications ranging from drug delivery and biological glues to food products and as a tissue engineering scaffold. A biodegradable material with flexible conjugation functional groups, tunable secondary structure and mechanical properties, PGA has potential as a tunable matrix material in mechanobiology. Recent studies in proteins connecting dynamics, nanometer length scale rigidity, and secondary structure suggest a new point of view from which to analyze and develop this promising material. We have characterized the structure, topology, and rigidity properties of PGA prepared with different molecular weights and secondary structures through various techniques including SEM, FT-IR, light and neutron scattering spectroscopy. On the length scale of a few nanometers rigidity is determined by hydrogen bonding interactions in presence of neutral species and by electrostatic interactions when the polypeptide is negatively charged. When probed over hundreds of nanometers, the rigidity of these materials is modified by long range intermolecular interactions that are introduced by the supramolecular structure. This article is protected by copyright. All rights reserved. Copyright © 2015 Wiley Periodicals, Inc., A Wiley Company.
    Journal of Biomedical Materials Research Part A 02/2015; DOI:10.1002/jbm.a.35427 · 2.83 Impact Factor
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    ABSTRACT: Instrument backgrounds at neutron scattering facilities directly affect the quality and the efficiency of the scientific measurements that users perform. Part of the background at pulsed spallation neutron sources is caused by, and time-correlated with, the emission of high energy particles when the proton beam strikes the spallation target. This prompt pulse ultimately produces a signal, which can be highly problematic for a subset of instruments and measurements due to the time-correlated properties, and different to that from reactor sources. Measurements of this background have been made at both SNS (ORNL, Oak Ridge, TN, USA) and SINQ (PSI, Villigen, Switzerland). The background levels were generally found to be low compared to natural background. However, very low intensities of high-energy particles have been found to be detrimental to instrument performance in some conditions. Given that instrument performance is typically characterised by S/N, improvements in backgrounds can both improve instrument performance whilst at the same time delivering significant cost savings. A systematic holistic approach is suggested in this contribution to increase the effectiveness of this. Instrument performance should subsequently benefit.
  • The European Physical Journal Conferences 01/2015; 83:03004. DOI:10.1051/epjconf/20158303004
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    ABSTRACT: The structure and lattice dynamics of rock-salt thermoelectric materials SnTe and PbTe are investigated with single-crystal and powder neutron diffraction, inelastic neutron scattering (INS), and first-principles simulations. Our first-principles calculations of the radial distribution function in both SnTe and PbTe show a clear asymmetry in the first nearest-neighbor (1NN) peak, which increases with temperature, in agreement with recent experimental reports. We show that this peak asymmetry for the 1NN Sn-Te or Pb-Te bond results from large-amplitude anharmonic vibrations (phonons). No atomic off centering is found in our simulations. In addition, the atomic mean-square 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 first-principles calculations. These results provide clear evidence for large-amplitude 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
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    ABSTRACT: Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experiments were performed on a single crystal of the heavy fermion compound Ce(Ni$_{0.935}$Pd$_{0.065}$)$_2$Ge$_2$ in order to study the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for $T \leq$ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP ($\rho(T) \sim T^{3/2}$ and $\gamma(T) \sim \gamma_0 - b T^{1/2}$). However, for 2 $\leq T \leq$ 10 K, the susceptibility and specific heat vary as $log T$ and the resistivity varies linearly with temperature. Furthermore, despite the fact that the resistivity and specific heat exhibit the non-Fermi liquid behavior expected at a QCP, the correlation length, correlation time, and staggered susceptibility of the spin fluctuations remain finite at low temperature. We suggest that these deviations from the divergent behavior expected for a QCP may result from alloy disorder.
    Journal of Physics Condensed Matter 11/2014; 27:015602. DOI:10.1088/0953-8984/27/1/015602 · 2.22 Impact Factor
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    ABSTRACT: Bulk magnetization, transport and neutron scattering measurements were performed to investigate the electronic and magnetic properties of a polycrystalline sample of the newly discovered ferromagnetic superconductor, CeO$_{0.3}$F$_{0.7}$BiS$_{2}$. Ferromagnetism develops below T$_{FM}$ = 6.54(8) K and superconductivity is found to coexist with the ferromagnetic state below T$_{SC}$ ~ 4.5 K. Inelastic neutron scattering measurements reveal a very weakly dispersive magnetic excitation at 1.8 meV that can be explained by an Ising-like spin Hamiltonian. Under application of an external magnetic field, the direction of the magnetic moment changes from the c-axis to the ab-plane and the 1.8 meV excitation splits into two modes. A possible mechanism for the unusual magnetism and its relation to superconductivity is discussed.
    Physical Review B 11/2014; 90(22). DOI:10.1103/PhysRevB.90.224410 · 3.66 Impact Factor
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    ABSTRACT: The phonon dispersions and scattering rates of the thermoelectric material Ag1-xSb1+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 x-ray 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 short-range ordering of the Ag and Sb cations on their sublattice. Single-crystal 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: We have used high-resolution neutron spectroscopy experiments to determine the complete spin wave spectrum of the heavy fermion antiferromagnet CeRhIn$_5$. The spin wave dispersion can be quantitatively reproduced with a simple $J_1$-$J_2$ model that also naturally explains the magnetic spin-spiral ground state of CeRhIn$_5$ and yields a dominant in-plane nearest-neighbor magnetic exchange constant $J_0$ = 0.74 meV. Our results pave the way to a quantitative understanding of the rich low-temperature phase diagram of the prominent Ce$T$In$_5$ ($T$ = Co, Rh, Ir) class of heavy fermion materials.
    Physical Review Letters 08/2014; 113:246403. DOI:10.1103/PhysRevLett.113.246403 · 7.73 Impact Factor
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    ABSTRACT: Dynamics of water confined in ~5 Å diameter channels of beryl and cordierite single crystals were studied by using inelastic (INS) and quasielastic (QENS) neutron scattering. The INS spectra for both samples were similar and showed that there are no hydrogen bonds acting on water molecule, which experiences strong anisotropic potential, steep along the channels and very soft perpendicular to it. The high resolution (3.4 mkeV) QENS data revealed gradual freezing out of the water molecule dynamics for both minerals at temperatures below about 80 K when the scattering momentum transfer was parallel to the channels, but not when it was perpendicular to the channels. The QENS study with medium energy resolution (0.25 meV) of the beryl with the scattering momentum transfer along the channels showed gradual freezing out of water molecule dynamics at temperatures below about 200 K, while at higher temperatures the data could be described as 2-fold rotational jumps about the axis coinciding with the direction of the dipole moment (that is, perpendicular to the channels), with a residence time of 5.5 ps at 225 K. The energy resolution dependence of the apparent dynamics freezing temperature suggests gradual slowing down of the rotational jumps as the temperature is decreased, until the associated QENS broadening can no longer be detected, rather than actual freezing.
    The Journal of Physical Chemistry B 08/2014; 118(47). DOI:10.1021/jp505355b · 3.38 Impact Factor
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    ABSTRACT: Neutron diffraction measurements were carried out on single crystals and powders of Yb2Pt2Pb, where Yb moments form planes of orthogonal dimers in the frustrated Shastry-Sutherland Lattice (SSL). Yb2Pt2Pb orders antiferromagnetically at TN=2.07 K, and the magnetic structure determined from these measurements features the interleaving of two orthogonal sublattices into a 5*5*1 magnetic supercell that is based on stripes with moments perpendicular to the dimer bonds, which are along (110) and (-110). Magnetic fields applied along (110) or (-110) suppress the antiferromagnetic peaks from an individual sublattice, but leave the orthogonal sublattice unaffected, evidence for the Ising character of the Yb moments in Yb2Pt2Pb. Specific heat, magnetic susceptibility, and electrical resistivity measurements concur with neutron elastic scattering results that the longitudinal critical fluctuations are gapped with E about 0.07 meV.
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    ABSTRACT: Modern spallation neutron sources are driven by proton beams ~ GeV energies. Whereas low energy particle background shielding is well understood for reactors sources of neutrons (~20 MeV), for high energies (100s MeV to multiple GeV) there is potential to improve shielding solutions and reduce instrument backgrounds significantly. We present initial measured data on high energy particle backgrounds, which illustrate the results of particle showers caused by high energy particles from spallation neutron sources. We use detailed physics models of different materials to identify new shielding solutions for such neutron sources, including laminated layers of multiple materials. In addition to the steel and concrete, which are used traditionally, we introduce some other options that are new to the neutron scattering community, among which there are copper alloys as used in hadronic calorimeters in high energy physics laboratories. These concepts have very attractive energy absorption characteristics, and simulations predict that the background suppression could be improved by one or two orders of magnitude. These solutions are expected to be great benefit to the European Spallation Source, where the majority of instruments are potentially affected by high energy backgrounds, as well as to existing spallation sources.
    Journal of Physics Conference Series 07/2014; 528(1):012013. DOI:10.1088/1742-6596/528/1/012013
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    ABSTRACT: Complementary neutron- and light-scattering results on nine proteins and amino acids reveal the role of rigidity and secondary structure in determining the time- and lengthscales of low-frequency collective vibrational dynamics in proteins. These dynamics manifest in a spectral feature, known as the boson peak (BP), which is common to all disordered materials. We demonstrate that BP position scales systematically with structural motifs, reflecting local rigidity: disordered proteins appear softer than α-helical proteins; which are softer than β-sheet proteins. Our analysis also reveals a universal spectral shape of the BP in proteins and amino acid mixtures; superimposable on the shape observed in typical glasses. Uniformity in the underlying physical mechanism, independent of the specific chemical composition, connects the BP vibrations to nanometer-scale heterogeneities, providing an experimental benchmark for coarse-grained simulations, structure/rigidity relationships, and engineering of proteins for novel applications.
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    ABSTRACT: The emergence of intrinsically disordered proteins (IDP) as a recognized structural class has forced the community to confront a new paradigm of structure, dynamics, and mechanical properties for proteins. We present novel data on the similarities and differences in the dynamics and nanomechanical properties of IDPs and other biomacromolecules on the picosecond timescale. An IDP, β-casein (CAS), has been studied in a calcium bound and unbound state using neutron and light scattering techniques. We show that CAS partially folds and stiffens upon calcium binding, but in the unfolded state it is softer than folded proteins such as green fluorescence protein (GFP). We also see that some localized diffusive motions in CAS have larger amplitude than in GFP at this timescale, but are still smaller than those observed in tRNA. In spite of these differences, CAS dynamics are consistent with the classes of motions seen in folded protein on this time scale.
    The Journal of Physical Chemistry B 06/2014; 118(26). DOI:10.1021/jp503788r · 3.38 Impact Factor
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    ABSTRACT: We present new magnetic heat capacity and neutron scattering results for two magnetically frustrated molybdate pyrochlores: $S=1$ oxide Lu$_2$Mo$_2$O$_7$ and $S={\frac{1}{2}}$ oxynitride Lu$_2$Mo$_2$O$_5$N$_2$. Lu$_2$Mo$_2$O$_7$ undergoes a transition to an unconventional spin glass ground state at $T_f {\sim} 16$ K. However, the preparation of the corresponding oxynitride tunes the nature of the ground state from spin glass to quantum spin liquid. The comparison of the static and dynamic spin correlations within the oxide and oxynitride phases presented here reveals the crucial role played by quantum fluctuations in the selection of a ground state. Furthermore, we estimate an upper limit for a gap in the spin excitation spectrum of the quantum spin liquid state of the oxynitride of ${\Delta} {\sim} 0.05$ meV or ${\frac{\Delta}{|\theta|}}\sim0.004$, in units of its antiferromagnetic Weiss constant ${\theta} {\sim}-121$ K.
    Physical Review Letters 05/2014; 113(11). DOI:10.1103/PhysRevLett.113.117201 · 7.73 Impact Factor
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    ABSTRACT: We present new magnetic heat capacity and neutron scattering results for two magnetically frustrated molybdate pyrochlores: $S=1$ oxide Lu$_2$Mo$_2$O$_7$ and $S={\frac{1}{2}}$ oxynitride Lu$_2$Mo$_2$O$_5$N$_2$. Lu$_2$Mo$_2$O$_7$ undergoes a transition to an unconventional spin glass ground state at $T_f {\sim} 16$ K. However, the preparation of the corresponding oxynitride tunes the nature of the ground state from spin glass to quantum spin liquid. The comparison of the static and dynamic spin correlations within the oxide and oxynitride phases presented here reveals the crucial role played by quantum fluctuations in the selection of a ground state. Furthermore, we estimate an upper limit for a gap in the spin excitation spectrum of the quantum spin liquid state of the oxynitride of ${\Delta} {\sim} 0.05$ meV or ${\frac{\Delta}{|\theta|}}\sim0.004$, in units of its antiferromagnetic Weiss constant ${\theta} {\sim}-121$ K.
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    ABSTRACT: The Spallation Neutron Source at Oak Ridge National Laboratory now hosts four direct geometry time-of-flight chopper spectrometers. These instruments cover a range of wave-vector 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):045113-045113-13. DOI:10.1063/1.4870050 · 1.58 Impact Factor
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    Physical Review B 02/2014; 89(9). DOI:10.1103/PhysRevB.89.099901 · 3.66 Impact Factor
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    ABSTRACT: Polyoligosilsesquioxanes (POSS) are a large family of Si−O cage molecules that have diameters of 1−2 nm and can be viewed as perfectly monodisperse silica nanoparticles. POSS can be synthesized with a wide variety of functional ligands attached to their surfaces. Here we report the results of a comprehensive study of the crystal structure and ligand dynamics of one of the simplest POSS nanoparticles, octamethyl-POSS or Si8O12(CH3)8, where the central Si8O12 cage is surrounded by eight methyl ligands. Neutron powder diffraction data highlight the presence of strongly temperature-dependent methyl group rotational dynamics. Vibrational spectra were measured using Raman and inelastic neutron scattering techniques, and the results of the measurements were compared with the predictions of density functional theory calculations. In particular, the inelastic neutron scattering spectra show the fundamental and first overtone transitions of the methyl torsional vibrations; these transitions are forbidden in both Raman and infrared spectroscopy for the molecule with its ideal octahedral symmetry. The energies of these transitions are used to determine the height of the torsional energy barrier. Direct measurements of the methyl group dynamics using quasielastic incoherent neutron scattering provide the hydrogen atom jump distance and the activation energy for rotation of the methyl groups. Together these results provide a detailed picture of the structure and ligand dynamics of this POSS molecule.
    The Journal of Physical Chemistry C 02/2014; 118:5579−5592. DOI:10.1021/jp412228r · 4.84 Impact Factor

Publication Stats

1k Citations
461.23 Total Impact Points

Institutions

  • 2004–2015
    • Oak Ridge National Laboratory
      • • Neutron Scattering Science Division
      • • Spallation Neutron Source
      Oak Ridge, Florida, United States
  • 2004–2011
    • National Institute of Standards and Technology
      • NIST Center for Neutron Research
      GAI, Maryland, United States
  • 1999–2008
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 2007
    • University of Maryland, College Park
      • Department of Materials Science and Engineering
      Maryland, United States
  • 2001
    • Los Alamos National Laboratory
      Лос-Аламос, California, United States
  • 2000
    • Christian-Albrechts-Universität zu Kiel
      Kiel, Schleswig-Holstein, Germany