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Exploring Magnetic Excitations for Condensed Matter Research

Dive into the fascinating world of magnetic excitations, including examples from crystals like KCuF3, and learn about phonon scattering from a scientific and project perspective.

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Exploring Magnetic Excitations for Condensed Matter Research

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  1. Baseline ReviewThe Path of ARCS from Science to a Project Brent Fultz California Institute of Technology

  2. Magnetic Excitations • Energy of the excitations can be large, often beyond • the spectrum from reactor sources. • d- and f-electron form factors are large in r, small in k • For small Q and high Ei, forward detector coverage • must be good.

  3. Examples of Magnetic Excitations • KCuF3 – a 1D • Heisenberg antiferromagnet • quantized excitations in • spin chain • calculated by field theory: • sharp dispersive modes • continuum from • free spinons (not FD or BE)

  4. Examples of Magnetic Excitations KCuF3 is a linear crystal, aligned along qi It is a special case, but 2D crystals can also be accommodated 3D crystals require goniometer manipulation

  5. Phonon Scattering Thermodynamics of materials T = 0 all internal coordinates in ground state. T > 0 creates excitations. Degeneracy of excitations gives entropy S = k lnW. F = E – TS favors different structures of materials. (Parallel thermodynamics for magnetic scattering.)

  6. Phonon Scattering Ei < 70 meV, light elements higher ds/dW~Q2, prefer higher Q (until multiphonon problems) Q-dependence needed for data analysis from coherent scattering and separation of magnetic scattering

  7. Everybody wants More Flux! • No inelastic neutron scattering experiment has ever • suffered from excessive flux • SNS source and p steradian detector coverage • will give ARCS new capabilities in practice: • 1. Parametric studies • Present -- compare A vs. B. Future -- A(T,H) vs. B(T,H) • 2. Single crystals • 3. Small quantities of new materials • 4. Sample environments

  8. History 1. HELIOS (Mason, Broholm, Fultz) Abernathy too 2. VERTEX (McQueeney, Fultz) 3. SNS EFAC selection of two (Abernathy) 4. ARCS proposal, June 2001 (held the U.S. community together)

  9. Pit Area Beamstop Sample E0 Detectors T0 History • SNS EFAC • selection of two: • high flux • high resolution

  10. Proposed ARCS Reviewer Comments: 1. Do not use 2 flightpaths (not universal opinion) 2. Software is a big job

  11. Bifurcation of ARCS • Full instrument had no contingency funds • Canadian CFI program prompted interest in a • second high-energy chopper instrument • International class facility should have a general • purpose and a magnetism instrument Presently CNCS (Ei<50 meV) 2% resolution 140o at 3 m SNACS 1% resolution 45o at 5.5 m ARCS 2% resolution 140o at 3 m

  12. The Present Concept for ARCS Angle-Range Chopper Spectrometer A High-Resolution Direct-Geometry Chopper Spectrometer A Medium-Resolution Chopper Spectrometer

  13. The Present Concept for ARCS

  14. ARCS -- The Project -- Hardware d2A/dt2 < 0

  15. ARCS -- The Project -- Budget d2$/dt2 > 0

  16. ARCS -- Software Roadmap

  17. Road 1 – S(Q,E) from TOF Data • Bare minimum for users to take home • General – model independent • Non-trivial for single crystals, especially when real-time decisions on 3D sample orientation are required • Must accommodate different visualization needs of different users, and packages such as Matlab and IDL Outside the scope: data mining — e.g., recognition of dispersions

  18. Road 2 – Fits and Inversions of S(Q,E) • Analytical results from the theory of thermal neutron scattering by condensed matter • Monte-Carlo inversions of measured data to obtain, for example, force constants or exchange energies

  19. Road 3 – Full Experiment Simulations

  20. ARCS -- The Project -- Software

  21. ARCS -- The Project -- Scope ARCS will be finished and working at the end of the project. • No missing detectors • Software for data acquisition and analysis • including a menu of working Python scripts • Some sample environment

  22. ARCS -- The Project -- Some Stakeholder Issues Stakeholder Expectations: ARCS will be finished and working on time and under budget. • DOE BES reporting and reviewing requirements • U.S. user community / ARCS IDT • communications, engage in software, sample environment • SNS • interface, MOU with Caltech • ARCS staff • hiring postdoctoral fellows, designers, funds between Caltech and ANL • Caltech and ANL • surprisingly quiet

  23. ARCS -- The Project -- “Quality Policy” Fact: ARCS will be the fundamental condensed matter science instrument at the $ 1,411,000,000 SNS. Policy: ARCS must be a full system solution: reliable, maintainable, and scientifically productive • best engineering practice • few risks • emphasize quality over quantity • (e.g., completed detector coverage even if • some sacrifice in resolution)

  24. ARCS -- The Project -- Risks Technical: operation of detectors in vacuum (test facility underway) sample environment for single crystals (user community still undecided) Cost and Schedule: Infrastructure for installation (we might follow other instruments, but not by much) Budget authority and float in schedule Memorandum of Understanding (awaiting action by SNS, 3He reduces detector contingency) Taxes: Tennessee state, Caltech overhead (switch title?)

  25. ARCS -- The Project -- Key People and Institutions Brent Fultz -- Caltech (project leader, coordinator, “aligner of personnel”) Doug Abernathy -- Argonne, Caltech (Oak Ridge too) (hardware project manager, Visiting Associate in Materials Science) Michael Aivazis -- Caltech (software project manager) Hardware Engineering -- Argonne Hardware Construction -- Oak Ridge Software -- Caltech Science -- centered at Caltech

  26. ARCS -- The Project -- Funding

  27. ARCS -- The Project -- Baseline Review 1. Check the instrument concept High intensity Decent resolution 2+ % Completed hardware Significant software 2. Examine the details of the project plan Some missing, but can you see the picture clearly enough to tell if the cost and schedule are realistic? 3. Emphasis Are we missing something?

  28. ARCS -- The Project -- Management

  29. ARCS -- The Project -- Software

  30. ARCS -- The Project -- Management

  31. Future Directions -- Developments in Concepts • 1. Software Enabled • coherent scattering from polycrystals • disordered solids • 2. Hardware and Software • coherent scattering from 3D single crystals

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