Neutron Sciences Directorate

SEQUOIA is a direct geometry time-of-flight chopper spectrometer with fine energy transfer (ω) and wave-vector (Q) resolution. The instrument is used to conduct forefront research on dynamical processes in materials. In particular, SEQUOIA is enabling unprecedented high-resolution inelastic neutron scattering studies of magnetic excitations and fluctuations and lattice vibrations. The impact on condensed matter, materials science, and geology spans a cross-section of important research areas. These include unconventional superconductors, quantum magnetism, itinerant magnets, ferroelectrics, thermoelectrics, multiferroics, metal hydrides, and hydrogen dynamics in various materials.

SEQUOIA is also an outstanding tool for the investigation of novel systems and materials that are currently unknown. In general, SEQUOIA is the instrument of choice for experiments that require fine Q and ω resolution and large solid angle at low-to-intermediate scattering angles. References [1-2] provide further information regarding the design and operation of the SEQUOIA spectrometer.

[1] G. E. Granroth, A. I. Kolesnikov, T. E. Sherline, J. P. Clancy, K. A. Ross, J. P. C. Ruff, B. D. Gaulin, S. E. Nagler, "SEQUOIA: a newly operating chopper spectrometer at the SNS", Journal of Physics: Conference Series 251, 12058 (2010).

[2] M. B. Stone, J. L. Niedziela, D. L. Abernathy, L. DeBeer-Schmitt, G. Ehlers, O. Garlea, G. E. Granroth, M. Graves-Brook, A. I. Kolesnikov, A. Podlesnyak, and B. Winn, “A comparison of four direct geometry time-of-flight spectrometers at the Spallation Neutron Source”, Rev. Sci. Instrum. 85, 045113 (2014).

Operating Parameters

Several measurements illustrate some of the capabilities of SEQUOIA

1. Spectroscopy on magnetic powders can be used to identify gaps and modes in the magnetic excitation spectrum. Fermi chopper 2 was used with an Ei = 60 meV in a 2% resolution condition to measure the Unconventional Spin-Peierls system TiOBr. This measurement not only determined the singlet-triplet Energy gap but also identified two triplet excitations. Further details are provided in J. P. Clancy, B. D. Gaulin, C. P. Adams, G. E. Granroth, A. I. Kolesnikov, T. E. Sherline, and F. C. Chou, “Singlet-Triplet Excitations in the Unconventional Spin-Peierls TiOBr Compound”, Phys. Rev. Lett. 106, 117401 (2011).
2. Spectroscopy can be used to refine Hydrogen positions and their environments in many materials. Specifically SEQUOIA was used to identify the H environment in Mica from a pegmatite. These studies help to understand the diversity in natural mica that arises from varied H content. L’ubomír Smrčok, Milan Rieder, Alexander I. Kolesnikov, and Garrett E. Granroth, “Combined inelastic neutron scattering and solid-state density functional theory study of dynamics of hydrogen atoms in muscovite 2M1”, American Mineralogist, 96, 301 (2011).
3. There is a diversity Hydrogen containing systems that can be studied on SEQUOIA. Metal Hydrides is another class of materials that is of interest. The phonon density of states of MgH2 has been measured. The single phonon features were identified and compared to Density Functional Theory calculations to determine the environment around the H. A.I. Kolesnikov, V.E. Antonov, V.S. Efimchenko, G. Granroth, S.N. Klyamkin, A.V. Levchenko, M.K. Sakharov, Y. Ren, J. All. Com., 10 156 (2010).
4. On Low dimensional systems, a single crystal can be oriented with the irrelevant Q direction along the beam and magnetic excitations measured quite effectively. On SEQUOIA, Mn Doped BaFe2As2 was measured with the two dimensional planes perpendicular to the incident beam. This configuration reveals that spin fluctuations are induced at the (1/2,1/2) position in that plane. G. S. Tucker, D. K. Pratt, M. G. Kim, S. Ran, A. Thaler, G. E. Granroth, K. Marty, W. Tian, J. L. Zarestky, M. D. Lumsden, S. L. Bud’ko, P. C. Canfield, A. Kreyssig, A. I. Goldman, and R. J. McQueeney “Competition between stripe and checkerboard magnetic instabilities in Mn-doped BaFe2As2” arXiv:1206.3486v1 (2012).
5. Rotation of a single crystal through a large angle allows a complete mapping of the Q-ω space. A single crystal of 160Gd has been measured in this manner. Specifically the structure factor of the magnetic excitations has been measured in great detail and analysis is underway to determine what role the itinerant moment affects the excitations. G.E. Granroth, A. A. Aczel, J. A. Fernandez-Baca, and S. E. Nagler, in preparation.
6. With careful thought more than a single incident energy can be used on SEQUOIA to measure multiple resolution conditions simultaneously. A Single Crystal of KCuF3 was measured with several incident energies allowing the full bandwidth and the absence of a spin gap to be observed. G. E. Granroth, “Advances in Neutron Spectroscopy and High Magnetic Field Instrumentation for Studies of Correlated Electron Systems”, J. Phys. Soc. Jpn. 80, SB016 (2011).
7. On SEQUOIA neutrons are available from 5 meV up to 4 eV. The upper end of this energy range is illustrated by recoil measurements on water. Many of the interesting behaviors of water, (super cooled, super critical, in or on nano particles, etc.) are measured on SEQUOIA and the resultant data is under analysis. However a measurement of the recoil that shows the upper energy range of SEQUOIA is shown in C. Andreani, “Supercooled water researcher finds Sequoia’s power ‘amazing’”, Notiziario Neutroni e Luce di Sincrotrone 16, 2 (2011).