The Neutron Resonance Spin Echo V2/FLEXX at BER II

Transcription

1 The Neutron Resonance Spin Echo V2/FLEXX at BER II Klaus Habicht Helmholtz-Zentrum Berlin für Materialien und Energie

2 Key Question Is an NRSE option better than a dedicated instrument? V2/FLEXX, the cold-neutron host spectrometer Features of the NRSE option at V2/FLEXX 2

3 Acknowledgements FLEX-upgrade project Duc Manh Le now STFC, ISIS, UK Markos Skoulatos now MLZ, Germany Kirrily Rule now ANSTO, Australia NRSE Felix Groitl now at EPFL and PSI, Thomas Keller, MPI Stuttgart, Germany Diana Lucía Quintero-Castro, now Univ. Stavanger, Norway Rasmus Toft-Petersen, now DTU, Denmark Zhilun Lu, HZB, Germany Zita Hüsges, HZB, Germany Siqin Meng, HZB, Germany Thomas Krist, HZB, Germany 3

4 Probing Material Structures: Scattering Techniques structure dynamics Intensity (arbitrary units) Energy transfer (mev) Distance of lattice planes d (Å) Wavevector q [h h 0] (r.l.u.) Neutron scattering techniques probe static or dynamic correlations 4

5 Probing Material Structures: Scattering Techniques structure dynamics Intensity (arbitrary units) data model Distance of lattice planes d (Å) Energy transfer (mev) Wavevector [h h 0] (r.l.u.) Neutron scattering techniques probe static or dynamic correlations 5

6 NRSE Motivation: Quasiparticle Linewidth 2.0 Energy transfer (mev) EE Wavevector q [h h 0] (r.l.u.) Dispersion relates quasiparticle energy to quasiparticle momentum Energy width in the dispersion encodes quasiparticle lifetime 6

7 Quasiparticle Linewidth and Lifetime energy domain time domain Scattering Signal signal (a.u.) EEexp 2Γ Scattering signal (a.u.) ee ττ NNNNNN TT DD = ee Γττ NNNNNN ħ Energy transfer (mev) Correlation time τ NSE (ps) Energy linewidth ΓΓ inversely proportional to lifetime TT DD : ΓΓ = ħ/tt DD 7

8 The Upgraded Cold Neutron TAS FLEXX new primary spectrometer optimized for high flux and low background and optional polarized neutron capabilities Double focusing monochromator PG and Heusler analyzer Velocity selector Polarizer Optional collimators Elliptical guide Virtual source 3 He detector gain factor 2-10 for flux at sample M. D. Le, et al., Nucl. Inst. Meth. A 729, (2013) 8

9 FLEXX Options FLEXX standard TAS mode MultiFLEXX backend XYZ polarization analysis Neutron Resonance Spin Echo Option 9

10 FLEXX Polarizer: S-Bender beam cross section: 60 mm x 125 mm 400 wafers 0.15 x 125 x 120 mm 3 Fe-Si multilayer with m = 3 anti-reflecting Gd-layer/Si/Gd-layer magnetization field > 300 G 10

11 FLEXX S-Bender Transmission spin up transmission [%] spin up transmission [%] device transmission rocking angle [deg] assuming m eff = horizontal translation [mm] spin up transmission [%] FLEXX transmission as measured with monitor at sample position k I [Å -1 ] polarizer transmission is confirmed at FLEXX transmission is entirely due to device transmission 11

12 FLEXX S-Bender Transmission polarizer rocking scans k I =2.56 Å -1 (measurements by Mirrotron) spin up transmission [%] polarizer rocking scan at FLEXX k I =2.66 Å -1 as measured with monitor at sample position FLEXX Mirrotron x=10 mm Mirrotron x=20 mm Mirrotron x=30 mm rocking angle [deg] angular acceptance of polarizer is confirmed at FLEXX 12

13 FLEXX Primary Spectrometer Guide Field M. Skoulatos, K. Habicht, NIM A (2011) 13

14 Guide Field Design Checks Guide Field Calculations 1/4 Calculations using Radia code from ESRF in Mathematica Magnetic field from Radia used to calculate beam depolarization using a depolarization formalism by Rosman and Rekveldt [1] McStas simulations Guide Shielding Collimator Monochromator [1] Rosman and Rekveldt, Z. Phys. B Cond.Mat. 79, (1990) Duc M. Le 14

15 Guide Field Design Checks Guide Field Calculations 1/4 Calculations using Radia code from ESRF in Mathematica Magnetic field from Radia used to calculate beam depolarization using a depolarization formalism by Rosman and Rekveldt [1] McStas simulations [1] Rosman and Rekveldt, Z. Phys. B Cond.Mat. 79, (1990) Duc M. Le 15

16 FLEXX Guide Field Realization 16

17 FLEXX Monochromator Helmholtz Coils 17

18 FLEXX Polarisation Flipping Ratio k I (Å -1 ) Polarization Experiment: 2mm Virtual Source Experiment: 20mm Virtual Source MC Simulation: 5 x 5 x 5 mm k I (Å -1 ) experimental polarization decreases towards larger wavelengths as expected from MC simulation does depend on virtual source width and monochromator curvature 18

19 FLEXX Heusler Analyzer Performance 3 rows, 15 crystals each Cu 2 MnAl (111) Bragg peak 0.42 mosaic (individual crystals) fixed vertical, variable horizontal curvature vertical 0.17 T magnetization field beam cross section: 60 mm x 125 mm horizontal focussing gain ~

20 NRSE Option at V2/FLEXX 20

21 Upgrade of NRSE Option at V2/FLEXX New bootstrap coils for the NRSE option at FLEX (in collaboration with MPI Stuttgart / FRM II) increase in accepted beam width access to steeper dispersion by larger coil tilt angles access to larger scattering angles for Larmor diffraction improved magnetic shielding in the NRSE arms courtesy: Max Planck-Institut für Festkörperphysik Stuttgart F. Groitl et al., Rev. Sci. Instrum. 86, (2015) 21

22 Upgrade of NRSE Option at V2/FLEXX New bootstrap coils for the NRSE option at FLEX (in collaboration with MPI Stuttgart / FRM II) increase in accepted beam width access to steeper dispersion by larger coil tilt angles access to larger scattering angles for Larmor diffraction improved magnetic shielding in the NRSE arms courtesy: Max Planck-Institut für Festkörperphysik Stuttgart F. Groitl et al., Rev. Sci. Instrum. 86, (2015) 22

23 NRSE Coupling Coils F. Groitl et al., Rev. Sci. Instrum. 86, (2015) 23

24 NRSE Option at V2/FLEXX F. Groitl et al., Rev. Sci. Instrum. 86, (2015) 24

25 Direct Beam Calibration Measurements k I =k f =1.57 Å P0 correction 4 PI P0 correction 8 PI 0.6 k I =k f =1.40 Å -1 P tau [ps] 25

26 NRSE Science at V2/FLEXX and TRISP! thermal transport in thermoelectric SrTiO 3 Goal: benchmark first-principles DFT / MD simulations with interatomic force constants including anharmonic lattice dynamics SrTiO 3 phonon dispersion calculated mode Grüneisen parameters L. Feng, T. Shiga, J. Shiomi, Appl. Phys. Express 8, (2015) needs experimental access to phonon lifetimes at low spin echo times 26

27 Thermoelectric SrTiO 3 SrTiO 3 phonon dispersion experimental line broadening energy [mev] TA data Stirling Born von Karman model FLEXX TAS Γ-R direction (0 0 0) ζ [r.l.u.] ( ) Line broadening [µev] K Γ-R direction Gaussian FWHM FLEXX Γ HWHM TRISP NRSE ζ [r.l.u.] 27

28 Instrumental Resolution for NRSE Polarization beam divergence 1 iφ ( k i, k f ) 3 P = S(, ) T (, ) e d k d k c. c. N Q ω TAS k i k f i f + use Gaussian approximation of TAS transmission probability T TAS (k i,k f ) expand total Larmor phase φ(k i,k f ) to second order expand energy conservation to second order integrate by matrix technique SM -1 SS +1 SA +1 [ ] SM -1 SS +1 SA -1 [ ] SM -1 SS -1 SA -1 [ ] SM -1 SS -1 SA +1 [ ] 1 3 at very large spin echo times instrumental resolution has to be considered instrumental limit upper limit typical phonon measurements τ [ns] 28

29 1 Development of Resolution Theory sample mosaic Ε ΔEE Polarization 0.1 raw data resolution function (mosaic + curvature) η curvature of the dispersion surface Ε q reciprocal lattice vector G ΔEE τ NSE [ps] q Development of analytical resolution function for NRSE spectroscopy Data correction: no convolution divide data by calculated resolution function K. Habicht et al., J. Appl. Cryst. 36, 1307 (2003) K. Habicht et al., Physica B 350 E803-E806 (2004) 29

30 Neutron Spin-Echo: Semi-Classical Model v 1 v 2 v phonon Dispersive excitations require tilted magnetic field regions 30