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ABSTRACT: The high flux and low background of the new backscattering spectrometer at the SNS combine to produce an excellent signal to noise ratio, allowing us to investigate a low lying weak excitation never seen before in the spin ice, Ho2Ti2O7. This non-dispersive excitation has been observed at E = 26.3 µeV below 100 K but is resolution limited only below ∼ 65 K. It is indifferent to magnetic fields below µ0H = 4.5 T, at 1.6 K. These characteristics help us to identify the excitation as due to the nuclear spin system.
J. Phys.: Conf. Ser. 01/2010; 25121949.
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ABSTRACT: A single nondispersive excitation is observed by means of neutron backscattering, at E_{0}=26.3 microeV in the spin ice Ho2Ti2O7 but not in the isotopically enriched 162Dy2Ti2O7 analogue. The intensity of this excitation is rather small, less, similar0.2% of the elastic intensity. It is clearly observed below 80 K but resolution limited only below approximately 65 K. The application of a magnetic field up to micro_{0}H=4.5 T, at 1.6 K, has no measurable effect on the energy or intensity. This nuclear excitation is believed to perturb the electronic, Ising spin system resulting in the persistent spin dynamics observed in spin ice compounds.
Physical Review Letters 02/2009; 102(1):016405. · 7.37 Impact Factor
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ABSTRACT: We present a comparison between theoretical predictions of the generalized Langevin equation for cooperative dynamics (CDGLE) and neutron spin echo data of dynamic structure factors for polyethylene melts. Experiments cover an extended range of length and time scales, providing a compelling test for the theoretical approach. Samples investigated include chains with increasing molecular weights undergoing dynamics across the unentangled to entangled transition. Measured center-of-mass (com) mean-square displacements display a crossover from subdiffusive to diffusive dynamics. The generalized Langevin equation for cooperative dynamics relates this anomalous diffusion to the presence of the interpolymer potential, which correlates the dynamics of a group of slowly diffusing molecules in a dynamically heterogeneous liquid. Theoretical predictions of the subdiffusive behavior, of its crossover to free diffusion, and of the number of macromolecules undergoing cooperative motion are in quantitative agreement with experiments.
The Journal of Physical Chemistry B 01/2009; 112(50):16220-9. · 3.70 Impact Factor
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ABSTRACT: We present a comparison between theoretical predictions of the generalized Langevin equation for cooperative dynamics (CDGLE) and neutron spin echo data of dynamic structure factors for polyethylene melts. Experiments cover an extended range of length and time scales, providing a compelling test for the theoretical approach. Samples investigated include chains with increasing molecular weights undergoing dynamics across the unentangled to entangled transition. Measured center-of-mass (com) mean-square displacements display a crossover from subdiffusive to diffusive dynamics. The generalized Langevin equation for cooperative dynamics relates this anomalous diffusion to the presence of the interpolymer potential, which correlates the dynamics of a group of slowly diffusing molecules in a dynamically heterogeneous liquid. Theoretical predictions of the subdiffusive behavior, of its crossover to free diffusion, and of the number of macromolecules undergoing cooperative motion are in quantitative agreement with experiments.
12/2008;
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G Ehlers,
E Mamontov, M Zamponi,
A Faraone,
Y Qiu,
A L Cornelius,
C H Booth,
K C Kam,
R Le Toquin,
A K Cheetham,
J S Gardner
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ABSTRACT: We have studied the evolution of the structural properties as well as the static and dynamic spin correlations of spin ice Ho(2)Ti(2)O(7), where Ho was partially replaced by non-magnetic La. The crystal structure of diluted samples Ho(2-x)La(x)Ti(2)O(7) was characterized by x-ray and neutron diffraction and by Ho L(III)-edge and Ti K-edge extended x-ray absorption fine structure (EXAFS) measurements. It is found that the pyrochlore structure remains intact until about x = 0.3, but a systematic increase in local disorder with increasing La concentration is observed in the EXAFS data, especially from the Ti K edge. Quasi-elastic neutron scattering and ac susceptibility measurements show that, in x≤0.4 samples at temperatures above macroscopic freezing, the spin-spin correlations are short ranged and dynamic in nature. The main difference with pure spin ice in the dynamics is the appearance of a second, faster, relaxation process.
Journal of Physics Condensed Matter 06/2008; 20(23):235206. · 2.55 Impact Factor
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G Ehlers,
E Mamontov, M Zamponi,
A Faraone,
Y Qiu,
A L Cornelius,
C H Booth,
K C Kam,
R Le Toquin,
A K Cheetham,
J S Gardner
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ABSTRACT: We have studied the evolution of the structural properties as well as the static and dynamic spin correlations of spin ice Ho2Ti2O7, where Ho was partially replaced by non-magnetic La. The crystal structure of diluted samples Ho2−xLaxTi2O7 was characterized by x-ray and neutron diffraction and by Ho LIII-edge and Ti K-edge extended x-ray absorption fine structure (EXAFS) measurements. It is found that the pyrochlore structure remains intact until about x = 0.3, but a systematic increase in local disorder with increasing La concentration is observed in the EXAFS data, especially from the Ti K edge. Quasi-elastic neutron scattering and ac susceptibility measurements show that, in x≤0.4 samples at temperatures above macroscopic freezing, the spin–spin correlations are short ranged and dynamic in nature. The main difference with pure spin ice in the dynamics is the appearance of a second, faster, relaxation process.
Journal of Physics Condensed Matter 04/2008; 20(23):235206. · 2.55 Impact Factor
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ABSTRACT: The DMP · CLA and DMP · CLA-d2 crystals below 100 K are monoclinic, space group P21/c with four molecules in the unit cells. Infinite chains of hydrogen bonded counterparts are formed with O⋯N distances equal to 2.767(2) and 2.639(2) for non-deuterated DMP · CLA. Deuteration leads to the Ubbelohlde effect particularly well manifested in the second short O–H⋯N bridge (elongation by ca. 0.02 Å). In the IR spectra a Hadži’s trio of the broad absorption is observed characteristic of strong hydrogen bonds. In the INS spectra the vibrational density of states and methyl rotational tunnel splittings were determined. The temperature dependence of tunneling bands enabled to make mode assignments and to determine the methyl rotational potentials. Comparison of the results to the pure electron donor DMP was made and the difference found can be almost completely assigned to the steric changes of the environment. A weak isotope effect with deuteration of the OH⋯N bridges of the DMP · CLA complex is assigned to a charge transfer of δe/e = 0.006.
Chemical Physics. 05/2007; 332(1):1-9.
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ABSTRACT: Understanding the elusive catalytic role of titanium-based additives on the reversible hydrogenation of complex hydrides is an essential step toward developing hydrogen storage materials for the transport sector. Improved bulk diffusion of hydrogen is one of the proposed effects of doping sodium alanate with TiCl3, and here we study hydrogen dynamics in doped and undoped Na3AlH6 using a combination of density functional theory calculations and quasielastic neutron scattering. The hydrogen dynamics is found to be vacancy mediated and dominated by localized jump events, whereas long-range bulk diffusion requires significant activation. The fraction of mobile hydrogen is found to be small for both undoped and doped Na3AlH6, even at 350 K, and improved hydrogen diffusion as a result of bulk-substituted titanium is found to be unlikely. We also propose that previously detected low-temperature point defect motion in sodium alanate could result from vacancy-mediated sodium diffusion.
The Journal of Physical Chemistry B 05/2007; 111(15):3886-92. · 3.70 Impact Factor
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ABSTRACT: Understanding the catalytic role of titanium-based additives on the reversible hydrogenation of complex metal hydrides is an essential step towards developing hydrogen storage materials for the transport sector. Improved bulk diffusion of hydrogen is one of the proposed catalytic effects, and here we study hydrogen dynamics in undoped and TiCl 3 -doped samples of NaAlH 4 and Na 3 AlH 6 using a combination of density functional theory calculations and quasielastic neutron scattering. Hydrogen dynamics is found to be limited and mediated by hydrogen vacancies in both alanate phases, requiring an activation in excess of 0.3 eV. Even at 390 K, less than 1% of the hydrogen in NaAlH 4 performs long range diffusion, and only localized hydrogen dynamics is observed in Na 3 AlH 6 . The effect of the TiCl 3 dopant on hydrogen bulk diffusion is found to be negligible.
Journal of Alloys and Compounds 01/2007; 446447:469-473. · 2.29 Impact Factor
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ABSTRACT: The x-ray diffraction studies of the title complex were carried out at room temperature and 14 K for H/D (in hydrogen bridge) isotopomers. At 82 K a phase transition takes place leading to a doubling of unit cells and alternation of the hydrogen bond lengths linking tetramethylpyrazine (TMP) and chloranilic acid molecules. A marked H/D isotope effect on these lengths was found at room temperature. The elongation is much smaller at 14 K. The infrared isotopic ratio for O-H(D)...N bands equals to 1.33. The four tunnel splittings of methyl librational ground states of the protonated complex required by the structure are determined at a temperature T=4.2 K up to pressures P=4.7 kbars by high resolution neutron spectroscopy. The tunnel mode at 20.6 microeV at ambient pressure shifts smoothly to 12.2 microeV at P=3.4 kbars. This is attributed to an increase of the strength of the rotational potential proportional to r(-5.6). The three other tunnel peaks show no or weak shifts only. The increasing interaction with diminishing intermolecular distances is assumed to be compensated by a charge transfer between the constituents of deltae/e approximately 0.02 kbar(-1). The phase transition observed between 3.4 and 4.7 kbars leads to increased symmetry with only two more intense tunneling bands. In the isotopomer with deuterated hydrogen bonds and P=1 bar all tunnel intensities become equal in consistency with the low temperature crystal structure. The effect of charge transfer is confirmed by a weakening of rotational potentials for those methyl groups whose tunnel splittings were independent of pressure. Density functional theory calculations for the model TMP.(HF)2 complex and fully ionized molecule TMP+ point out that the intramolecular rotational potential of methyl groups is weaker in the charged species. They do not allow for the unequivocal conclusions about the role of the intermolecular charge transfer effect on the torsional frequencies.
The Journal of Chemical Physics 12/2006; 125(19):194525. · 3.33 Impact Factor
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ABSTRACT: The dynamics of binary polymer blends of few labeled long chains in successively shorter matrix chains has been investigated by neutron spin echo (NSE) spectroscopy. For the first time the effect of constraint release on the chain relaxation has been directly observed on a microscopic scale. Decreasing the matrix chain length reduces the topological confinement until unconfined Rouse motion is observed, when the matrix chains are too short to confine the long chain in a tube. Whereas an analytical description of the effect is not yet available, a new simulation based on the slip-link model shows perfect agreement with the NSE data over the full range of matrix molecular weights.
Physical Review Letters 07/2006; 96(23):238302. · 7.37 Impact Factor
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ABSTRACT: In entangled polymer melts, contour length fluctuations are supposed to lift the chain confinement starting from the chain ends. Using neutron spin echo spectroscopy on polyethylene chains of intermediate length, where only the center part is labeled, allows us to separate the contribution of contour length fluctuations to the dynamic structure factor and thereby explicitly prove the effect on a molecular level. While a corresponding fully labeled chain shows the relaxation by contour length fluctuations additional to the primary relaxation, the data of the center-labeled chain, where the visibility of the fluctuations is switched off, is in excellent agreement with results for a nearly infinite long chain, where contour length fluctuations are known to be negligible.
http://dx.doi.org/10.1209/epl/i2005-10338-1.
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ABSTRACT: The close comparison of linear rheology data of some long-chain polymer melts with the reptation model indicates the existence of additional degrees of freedom limiting the topological chain confinement. As one candidate contour length fluctuations (CLF) were proposed and indeed account for the observed power law in the dynamic loss modulus G″(ω). In order to study these mechanisms on a microscopic scale we have performed neutron spin echo investigations of the single-chain dynamic structure factor from polyethylene (PE) melts over a large range of chain lengths. A systematic loosening of the confinement with decreasing chain length is found and can be described quantitatively by the effect of CLF on the confining tube establishing this mechanism on a molecular level in space and time.
Physica B: Condensed Matter.
