Cold Neutron Chopper Spectrometer
CNCS | BL-5 | SNS
The scientific communities that use the Cold Neutron Chopper Spectrometer (CNCS) at the Spallation Neutron Source (SNS) include condensed matter physics, physical chemistry, materials chemistry, and physical biology. Due to its versatility, the instrument can be employed for a variety of experiments, but it is particularly well suited for measuring collective excitations in single crystals (spin waves and phonons) with the crystal rotation method. Optimized ancillary sample environment is available to provide a complete control of thermodynamic variables such as temperature, magnetic field, and pressure.
Typical experiments conducted over the past years at the CNCS are listed below.
- CNCS is especially powerful at mapping the spin wave excitation spectrum in magnetic single crystals. In such measurements multiple datasets are acquired for different orientations of the crystal, and are combined in software to map the four-dimensional scattering function, S(Q,E), as a function of energy transfer E and wave-vector transfer Q. Thus it allows one build up a 4-dimensional dataset that, in principle, covers energy and Q in all directions. Typically, for spin 1/2 systems, one day is required to cover ~90 degrees of sample rotation with 1 gram of sample. For a recent example that shows a perfectly resolved Magnetic excitation spectrum in the geometric frustrated multiferroic CuCrO2, see: M. Frontzek, J. T. Haraldsen, A. Podlesnyak, M. Matsuda, A. D. Christianson, R. S. Fishman, A. S. Sefat, Y. Qiu, J. R. D. Copley, S. Barilo, S. V. Shiryaev, and G. Ehlers, Magnetic excitations in the geometric frustrated multiferroic CuCrO2, Phys. Rev. B 84, 094448 (2011). (3 days of beamtime used)
- The phonon spectrum of a single crystal can be determined with the crystal rotation method. In such measurements multiple datasets are acquired for different orientations of the crystal, and are combined in software to map the four-dimensional scattering function, S(Q,E), as a function of energy transfer E and wave-vector transfer Q. In the best case (several grams of sample), 2-3 full scans (~90 degrees of sample rotation) can be measured in one day. A recent example is a study of the phonon spectra in PbTe, one of the leading thermoelectric materials. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, a strong anharmonic coupling between the ferroelectric transverse optic (TO) mode and the longitudinal acoustic (LA) modes was identified, see: O. Delaire, J. Ma, K. Marty, A. F. May, M. A. McGuire, M. H. Du, D. J. Singh, A. Podlesnyak, G. Ehlers, M. D. Lumsden and B. C. Sales, Giant anharmonic phonon scattering in PbTe, Nat. Mater. 10(8) p. 614-619 (2011). (4 days of beamtime used).
- An important application of quasi-elastic neutron scattering (QENS) is in studies of the spatial characteristics (Q dependence) of diffusion processes in soft matter. CNCS has great potential for such detailed diffusion studies for the following reasons: (i) high neutron flux, (ii) large dynamic range (ratio of the accessible energy transfer to energy resolution), (iii) excellent instrument stability. A recent example shows an investigation of the diffusion process in water on the surface of TiO2 oxide nanoparticles: X. Chu, G. Ehlers, E. Mamontov, A. Podlesnyak, W. Wang, and D. J. Wesolowski, Diffusion processes in water on oxide surfaces: Quasielastic neutron scattering study of hydration water in rutile nanopowder, Phys. Rev. E 84, 031505 (2011) (5 days of beamtime used).
- CNCS can be used to measure a detailed diffraction map S(Q,E=0) in search for incommensurate and superstructure reflections. The large range of accessible scattering angles (up to 2Θ=135), the flexibility in the setting of the incident neutron energy, and the high flux enable to see Bragg peaks barely visible in powder and four-circle diffraction experiments. A full diffraction map (360 degrees sample rotation) can be measured in 1-2 hours. Recent example: M. Frontzek, G. Ehlers, A. Podlesnyak, H. Cao, M. Matsuda, O. Zaharko, N. Aliouane, S. Barilo, and S. V. Shiryaev, Magnetic structure of CuCrO2: a single crystal neutron diffraction study, J. Phys.: Cond. Matter 24, 016004, (2012). (2 days of beamtime used).
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