Browsing by Author "de Souza, NR"

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    Boson peak in ultrathin alumina layers investigated with neutron spectroscopy
    (American Physical Society, 2020-06-11) Cortie, DL; Cyster, MJ; Ablott, TA; Richardson, C; Smith, JS; Iles, GN; Wang, XL; Mitchell, DRG; Mole, RA; de Souza, NR; Yu, DH; Cole, JH
    Bulk glasses exhibit extra vibrational modes at low energies, collectively known as the boson peak. The vibrational dynamics in nanoscale alumina glasses have an impact on the performance of qubits and other superconducting devices; however, the frequency of the boson peak has not been previously measured. Here we report neutron spectroscopy experiments on Al/Al2O3 nanoparticles consisting of spherical metallic cores with a radii from 20 to 1000 nm surrounded by a 3.5-nm-thick alumina glass. A low-energy peak is observed at ωBP = 2.8 ± 0.6 meV for highly oxidized particles, indicating an excess in the density of states. The intensity of the peak scales inversely with particle size and oxide fraction, indicating a surface origin, and is redshifted by 3 meV with respect to the van Hove singularity of γ -phase Al2O3 nanocrystals. Molecular-dynamics simulations of α-Al2O3, γ -Al2O3 and α-Al2O3 show that the observed boson peak is a signature of the ultrathin glass surface and the characteristic frequency is reduced compared to the peak in the bulk glass. © 2020 The Authors. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license.
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    Design and first year-operation results from EMU, the high-resolution backscattering spectrometer at ANSTO
    (International Conference on Neutron Scattering, 2017-07-12) de Souza, NR
    ANSTO recently commissioned a high-resolution neutron spectrometer, EMU, at its OPAL research reactor. The spectrometer is based on crystal backscattering, implemented through spherical focusing between a Si(111) crystal monochromator and Si(111) analyser arrays, and as first performed on the IN16 spectrometer at ILL. The incident neutron energies are modulated by fast oscillation of the monochromator, yielding a net energy transfer range of ±31 ?eV. The most unique feature of EMU is to maintain its high, 1.2 ?eV energy transfer resolution from 1.95 to 0.1 Å-1momentum transfers. Neutron events are resolved by two 3He LPSD arrays. Data analysis is carried out with the Mantid software package. Results from the first year of operation are presented to highlight performance. EMU is especially suited to characterizing diffusion processes occurring in macromolecular and functional materials e.g. polymers, proteins, lipid membranes, solid-state conductors, or molecular liquids and crystals. Most scientific user experiments to date were carried out over the accessible temperature range of 30 mK up to 800 K. Other sample environments such as controlled gas delivery, pressure, electric and magnetic fields, etc. are available or under testing. EMU presently offers a typical 1000:1 signal-to-noise ratio. Measures aimed at increasing signal-to-noise, as well as energy transfer range, are being considered to further spectrometer performance.
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    Does the boson peak survive in an ultrathin oxide glass?
    (arXiv.org, 2019-07-29) Cortie, DL; Cyster, MJ; Smith, JS; Iles, GN; Wang, XL; Mitchell, DRG; Mole, RA; de Souza, NR; Yu, DH; Cole, JH
    Bulk glasses exhibit extra vibrational modes at low energies, known as the boson peak. The microscopic dynamics in nanoscale alumina impact the performance of qubits and other superconducting devices, however the existence of the boson peak in these glasses has not been previously measured. Here we report neutron spectroscopy on Al/Al2O3−x nanoparticles consisting of spherical metallic cores from 20 to 1000 nm surrounded by a 3.5 nm thick alumina glass. An intense low-energy peak is observed at ωBP = 2.8 ± 0.6 meV for highly oxidised particles, concurrent with an excess in the density of states. The intensity of the peak scales inversely with particle size and oxide fraction indicating a surface origin, and is red-shifted by 3 meV with respect to the van-Hove singularity of γ-phase Al2O3−x nanocrystals. Molecular dynamics simulations of α-Al2O3−x, γ-Al2O3−x and a-Al2O3−x show that the observed boson peak is a signature of the ultrathin glass surface, and the frequency is softened compared to that of the hypothetical bulk glass.
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    EMU - high-resolution neutron backscattering spectroscopy at ANSTO
    (Australian Institute of Physics, 2019-02-06) de Souza, NR
    EMU, the high-resolution neutron spectrometer installed at the OPAL reactor, ANSTO delivers 1 𝜇eV FWHM energy transfer resolution for an accessible ±31 𝜇eV energy transfer range. The spectral resolution is achieved by neutron backscattering from Si (111) on the primary and second flight paths, which also determines the accessible 0.35 to 1.95 Å-1 momentum transfer range. Two years of user operation document strong demand for QENS characterization of microscopic diffusion processes in energy materials such as solid-state electrolytes, and increasingly in biorelated soft materials. Over the same time frame most experiments were carried out with standard cryo-furnaces (2 to 800 K temperature range). Spectrometer beam-time access is meritbased, thus welcoming experiments beyond the first two-year 'sample', and including experiments that may require other ancillary equipment such as (existing) controlled-gas delivery, pressure, applied fields, etc. Examples of the spectrometer capabilities will be shown, with an emphasis on QENS line shape and mean-square displacements analyses. Scientific support is presently focused on enabling data analysis of the collected data, and on the instrumental side reaching the design 0.1 Å-1 minimum momentum transfer range and growing signal-to-noise ratio beyond its current ∼1650:1 value.
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    EMU - the high-resolution backscattering spectrometer at ANSTO
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2018-11-19) de Souza, NR; Klapproth, A
    EMU, the high-resolution neutron spectrometer installed at the OPAL reactor, ANSTO [1] delivers 1 μeV FWHM energy transfer resolution for an accessible ± 31 μeV energy transfer range. The spectral resolution is achieved by neutron backscattering from Si (111) on the primary and second flight paths, which also determines the accessible 0.35 to 1.95 Å^-1 momentum transfer range. Two years of user operation document strong demand for QENS characterization of microscopic diffusion processes in energy materials such as solid-state electrolytes, and increasingly in bio-related soft materials [2,3]. Over the same time frame most experiments were carried out with standard cryo-furnaces (2 to 800 K temperature range). Spectrometer beam-time access is merit-based, thus welcoming experiments beyond the first two-year ‘sample’, and including experiments that may require other ancillary equipment such as (existing) controlled-gas delivery, pressure, applied fields, Page 29 ANBUG-AINSE Neutron Scattering Symposium, AANSS 2018 / Book of Abstracts etc. Examples of the spectrometer capabilities will be shown, with an emphasis on QENS line shape and mean-square displacements analyses. Scientific support is presently focused on enabling data analysis of the collected data, and on the instrumental side reaching the design 0.1 Å^-1 minimum momentum transfer range and growing signal-to-noise ratio beyond its current ~ 1650:1 value. © The Authors.
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    EMU, the backscattering spectrometer at the Australian Centre for Neutron Scattering
    (International Conference on Neutron Scattering, 2017-07-12) Iles, GN; de Souza, NR; Klapproth, A
    The cold-neutron backscattering spectrometer, EMU, one of the four spectrometers at ANSTO, received its operating licence in 2016. First spectra were obtained from measurements on laboratory standards such as polyethylene, m-Xylene and ammonia perchlorate [1]. The high energy resolution of EMU allows dynamics in the nanosecond timeframe to be observed. This high resolution is due to backscattering from the Si (111) crystal monochromator and analyser arrays, delivering a spectrometer FWHM energy resolution in the order of 1.2µeV. EMU also features a linear Doppler drive modulating incident neutron energies over ± 31 µeV. Scattered, analysed neutrons are counted in 3He LPSD arrays. By setting the Doppler-driven backscattering monochromator to zero motion, elastic fixed window scans (EFW) can be performed. Changes in intensity of the analysed neutrons,with changing temperature, for example, correspond to changing dynamics in the system. Alternatively, when the incident energy is modulated, quasi-elastic neutron scattering (QENS) can be used to observe changes in the profile shape of the elastic peak. Finally, EMU can be used to observe purely inelastic scattering, such as observed in samples exhibiting rotational tunnelling. Future work will involve developing MANTID software for data treatment and analysis, and continuing to improve the signal-to-noise ratio.
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    EMU, the cold-neutron backscattering spectrometer at the Bragg Institute, ANSTO
    (Australian Institute of Physics, 2015-02-03) de Souza, NR; Klapproth, A; Iles, GN
    The Bragg Institute is currently in the final installation stage of a cold-neutron backscattering spectrometer in the ANSTO OPAL research reactor neutron guide hall. This spectrometer, called EMU, is based on Si (111) crystal backscattering and extracts neutrons from a cold neutron guide via a double HOPG (002) crystal premonochromator setup. Backscattering occurs through implementation of spherical focusing between the Si (111) crystal monochromator and analyser arrays, aiming to deliver a spectrometer FWHM energy resolution in the order of 1.2 μeV. EMU also features a 7-metre long focusing guide located between the two premonochromators, a so-called graphite chopper alternating beam delivery to the backscattering crystal monochromator and then into the secondary spectrometer, and a linear Doppler drive modulating incident neutron energies over ± 31 μeV. Scattered, analysed neutrons are counted in 3He LPSD arrays. EMU is provisioned for future extensions of its dynamic range via higher-resolution, undeformed Si (111) crystal analyser arrays, and variable HOPG (002) crystal premonochromator reflection angles. Access to the EMU spectrometer will be via beam-time requests to the OPAL neutron-beam user facility. EMU is ideally suited for measuring relaxation times from a few 10 ps to over 1 ns, for momentum transfers up to 2 Å-1, and readily from cryogenic temperatures up to 700 K.
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    EMU, the high resolution backscattering spectrometer at ANSTO
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) de Souza, NR; Klapproth, A; Iles, GN
    The energy range and resolution of backscattering spectrometers are well suited to characterizing relaxations on an atomic and molecular scale, such as diffusion processes occurring in e.g. polymer chains, membranes, proteins, molecular crystals, between interstitial crystal lattice sites. The EMU spectrometer can be used to study the dynamics of water molecules in the confined space of a host structure or ionic diffusion in conductor materials. In addition, quantum rotational tunnelling of functional groups (e.g. -CH3, -NH4) and hyperfine splitting of nuclear energy levels can be investigated. Relaxation times from a few 10 ps to over 1 ns are accessible. We will present the first -CH3 tunneling and diffusional motion spectra, obtained during the instrument commission, as an example of EMU’s present capabilities. The experiments have been performed in a temperature range from 3 – 650K, using top- and bottom-loading cryo furnaces. Other sample environments such as pressure, magnetic fields, controlled gas delivery systems, sub-K cryostats etc. are also available or currently under testing. EMU entered user service in 2016 and we welcome proposals in a wide range of scientific disciplines. The EMU instrument has the highest energy resolution of the neutron spectrometers at ANSTO and provides a momentum transfer range from as low as 0.1 Å-1 up to 1.95 Å-1. The high energy resolution is obtained by neutron backscattering, which occurs twice, through spherical focusing onto the sample, located between the Si (111) crystal monochromator and the analyser arrays [1]. A linear Doppler drive modulates the incident neutron energies over an energy range of ± 31 µeV. The inelastic scattered neutrons are counted in two 3He linear-position sensitive detector arrays.
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    First spectrum measured on EMU, the cold-neutron backscattering spectrometer at the Bragg Institute, ANSTO
    (Australian Institute of Physics, 2016-02-04) de Souza, NR; Klapproth, A; Iles, GN
    The cold-neutron backscattering spectrometer, EMU, one of the four spectrometers at ANSTO received its commissioning licence in 2015. This allowed opening the neutron beam onto the instrument and after measuring nominal background radiation we made our first measurements with the instrument. EMU is based on Si(111) crystal backscattering and extracts neutrons from a cold neutron guide via a double HOPG (002) crystal premonochromator setup. Backscattering occurs through implementation of spherical focusing between the Si (111) crystal monochromators and analyser arrays, aiming to deliver a spectrometer FWHM energy resolution in the order of 1.2 μeV. EMU also features a 7-metre long focusing guide located between the two premonochromators, a so-called graphite chopper alternating beam delivery to the backscattering crystal monochromator and then into the secondary spectrometer [1] and a linear Doppler drive modulating incident neutron energies over ± 31 μeV. Scattered, analysed neutrons are counted in 3He LPSD arrays. We measured two samples, one a vanadium sample can and secondly a polyethylene sheet. Using event counting obtained from two temporarily placed 3He detector tubes, we were able to obtain a backscattered spectrum. It is critical to ensure that detected neutrons have been backscattered. Backscattered neutrons travel a further distance than those that scatter immediately at the sample and therefore the timing signal must be known accurately, to distinguish between spurious and actual data. Future work will involve developing MANTID software for data treatment and analysis.
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    First users on EMU, the cold-neutron backscattering spectrometer at the Australian Centre for Neutron Scattering
    (Australian Institute of Physics, 2017-01-31) Iles, GN; de Souza, NR; Klapproth, A
    The cold-neutron backscattering spectrometer, EMU, one of the four spectrometers at ANSTO, received its operating licence in 2016. First spectra were obtained from measurements on laboratory standards such as polyethylene, m-Xylene and ammonia perchlorate [1]. The high energy resolution of EMU, allows dynamics in the nanosecond timeframe to be observed. This high resolution is due to backscattering from the Si (111) crystal monochromator and analyser arrays, delivering a spectrometer FWHM energy resolution in the order of 1.2 geV. EMU also features a linear Doppler drive modulating incident neutron energies over ± 31 geV. Scattered, analysed neutrons are counted in 3He LPSD arrays. By setting the Doppler driven backscattering monochromator to zero motion, elastic fixed window scans (EFW) can be performed. Changes in intensity of the analysed neutrons, with changing temperature, for example, correspond to changing dynamics in the system. Alternatively, when the incident energy is modulated, quasi-elastic neutron scattering (QENS) can be used to observe changes in the profile shape of the elastic peak. Finally, EMU can be used to observe purely inelastic scattering, such as observed in samples exhibiting rotational tunnelling. The first users have now conducted experiments on EMU in a range of disciplines. We have measured the high temperature dynamics in lead-free ferroelectrics using (QENS) [2], and investigated the long-range oxygen diffusion in an ionic conductor [3]. We have also measured water diffusion in clays using (EFW) [4]. Future work will involve developing MANTID software for data treatment and analysis, and continuing to improve the signal-to noise ratio.
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    Kondo behavior, ferromagnetic correlations, and crystal fields in the heavy-fermion compounds Ce3X (X = In, Sn).
    (American Physical Society, 2010-06-25) Wang, CH; Lawrence, JM; Christianson, AD; Goremychkin, EA; Fanelli, VR; Gofryk, K; Bauer, ED; Ronning, F; Thompson, JD; de Souza, NR; Kolesnikov, AI; Littrell, KC
    We report measurements of inelastic neutron scattering, magnetic susceptibility, magnetization, and the magnetic field dependence of the specific heat for the heavy Fermion compounds Ce3In and Ce3Sn. The neutron scattering results show that the excited crystal field levels have energies E1=13.2 meV, E2=44.8 meV for Ce3In and E1=18.5 meV, E2=36.1 meV for Ce3Sn. The Kondo temperature deduced from the quasielastic linewidth is 17 K for Ce3In and 40 K for Ce3Sn. The low-temperature behavior of the specific heat, magnetization, and susceptibility cannot be well described by J=1/2 Kondo physics alone, but require calculations that include contributions from the Kondo effect, broadened crystal fields, and ferromagnetic correlations, all of which are known to be important in these compounds. We find that in Ce3In the ferromagnetic fluctuation makes a 10%–15% contribution to the ground state doublet entropy and magnetization. The large specific heat coefficient γ in this heavy fermion system thus arises more from the ferromagnetic correlations than from the Kondo behavior. © 2010, American Physical Society
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    The recent progress of polarized neutron scattering techniques at SIKA
    (Australian Nuclear Science and Technology Organisation, 2021-11-26) Yano, SC; Deng, GH; Rule, KC; de Souza, NR; Manning, AG; Peng, H; Wu, CM
    SIKA, the cold-neutron triple-axis spectrometer is on the CG4 beam port at the OPAL reactor, ACNS, ANSTO. We have reported the capabilities and status of SIKA in the last several user's meetings. In this meeting, we discuss the recent development of polarized neutron scattering experiments on SIKA. A 3He polarization analysis system is available for SIKA. We have performed several user experiments and commissioning experiments in the last two years. We would like to present some results by introducing the techniques we are trying to implement. In addition, we discuss our plan for the polarized neutron scattering experiment on the SIKA. © The Authors
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    Solid ionic conductors for energy applications: developing a complete picture from structure and dynamics
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2018-11-19) Cheung, E; de Souza, NR; Sharma, N
    There has been renewed interest in solid state sodium-ion batteries (SIBs) as a safe, sustainable and cost-effective alternative system for large scale energy storage applications.[1] This, in turn, has motivated many studies on the development of materials that facilitate high ionic conductivity over Page 3 ANBUG-AINSE Neutron Scattering Symposium, AANSS 2018 / Book of Abstracts multiple charge-discharge cycles. Layered sodium manganates and the NASICON family of compounds are promising candidate sodium electrode and solid-state electrolyte materials respectively. In both cases, it has been shown that the overall performance of these materials for their respective functions is significantly improved through structural modifications, including by hydration or chemical doping.[2-8] However, the characterisation of these materials are typically limited to techniques which only offer a macroscopic picture, such as electrochemical impedance spectroscopy. As such, direct links between conductivity and structure, particularly with reference to the effect of chemical doping on the microscopic properties of materials are rarely investigated. We have selected candidate materials which have been shown to be amongst the best performing for their purpose and use high resolution diffraction data to solve their average structure. In parallel, we use quasielastic neutron scattering spectroscopy to gain insight into the diffusion mechanisms at an atomic level. We consequently aim to form a fuller picture of the effects that structural modifications have on the ionic conductivity and hence overall performance of these materials. © The Authors.
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    Structure and dynamics in Mg2+-stabilized γ‑Na3PO4
    (American Chemical Society, 2021-10-20) Cheung, EA; Nguyen, H; Tang, H; Stampfl, APJ; Avdeev, M; Meng, YS; Sharma, N; de Souza, NR
    In parallel with advances in the synthesis of solid-state ionic conductors, there is a need to understand the underlying mechanisms behind their improved ionic conductivities. This can be achieved by obtaining an atomic level picture of the interplay between the structure of materials and the resultant ionic diffusion processes. To this end, the structure and dynamics of Mg2+-stabilized rotor phase material γ-Na3PO4, characterized by neutron scattering, are detailed in this work. The Mg2+-stabilized rotor phase is found to be thermally stable from 4 to 650 K. However, signatures of orientational disorder of the phosphate anions are also evident in the average structure. Long-range Na+ self-diffusion was probed by quasi-elastic neutron scattering and subsequently modeled via a jump diffusion matrix with consideration of the phosphate anion rotations. The resultant diffusion model points directly to coupled anion-cation dynamics. Our approach highlights the importance of considering the whole system when developing an atomic level picture of structure and dynamics, which is critical in the rational design and optimization of energy materials. © American Chemical Society
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    Time-disordered crystal structure of AlPO4-5
    (American Chemical Society, 2017-08-03) Cortie, DL; McBride, BR; Narayanan, N; de Souza, NR; Avdeev, M; Mole, RA; McIntyre, GJ; Kearley, GJ; Withers, R; Yu, DH; Liu, Y
    Modern ab initio calculations have become increasingly accurate for predicting the symmetry of crystal structures; however, the standard methods usually only determine the 0 K static configuration and therefore misrepresent structures where dynamics play a key role. In this work, we demonstrate a clear experimental example of this phenomenon in the AlPO4-5 molecular sieve where the average high-symmetry P6cc structure emerges from local dynamics involving corner-sharing tetrahedra. Quasielastic and inelastic neutron scattering experiments were conducted to clarify the thermally activated motions between 1.5 and 300 K. Through comparison with ab initio molecular dynamics, we explain why the theoretically predicted structure is not observed in diffraction experiments. Instead, the results indicate this material is an inorganic analogue of a plastic crystal where thermal dynamics dictate the average symmetry. © 2017 American Chemical Society