Inelastic Neutron Scattering B. Fultz, J. Lin, O. Delaire, M. Kresch Caltech

Science interestsWhere is the field going?Tasks and goals of the DANSE subprojectLooking for new scientific opportunities

Excitations in Solids:

An Optical Illusion ofSpace-Time Correlations

ARCS software is going well

ARCS reduction software demo…

WBS 10: Science Scope for Inelastic Scattering

1) spin correlations in magnets, superconductors, and materials close to metal-insulator transitions

2) vibrational excitations in solids and their relationship to phase diagrams and equations of state of materials

3) tunneling and reorientation transitions of small molecules on surfaces

4) vibrational spectra of molecules or individual atoms such as hydrogen

5) diffusional motions and relaxation processes studied by quasielastic scattering

Tasks for Inelastic Scattering WBS 10

10.1.1.1 Data reduction for CNCS 08/31/06 02/15/07 10.1.1.2 Data reduction for SEQUOIA 03/01/08 08/16/0810.1.2.1 Data reduction for HYSPEC 05/06/10 07/06/1010.1.2.2 Data reduction for Backscatter 02/07/07 03/16/07

10.2.1.1 Gilat-Raubenheimer method 01/05/08 03/08/08 10.2.1.2 Pack and Monkhorst method 03/11/10 05/08/1010.2.2.1 Polycrystal 07/31/07 01/06/08 10.2.2.2 Single Crystal 11/13/08 06/14/0910.2.2.3 Dispersion Algorithms 08/12/08 11/14/0810.2.3 Separation of Nuclear and Spin Scattering 11/23/09 03/16/1010.2.4.1 CLIMAX 04/21/07 08/11/07 10.2.5 Chemical Spectroscopy Preprocessing 03/01/10 02/28/11

10.3.1.1 Coherent Inelastic Scattering Kernel 03/11/07 04/21/07 10.3.1.2 Incoherent Inelastic Scattering Kernel 04/06/07 04/21/0710.3.1.3 G(r,t) from individual neutron scatterings 03/25/08 11/09/0810.3.1.4 G(r,t) from dynamics simulation 07/03/10 09/18/1010.3.2.1 Multiple scattering correction 04/21/09 12/06/0910.3.3 Disordered Spin Dynamics 04/13/09 02/28/11

Science Activity for Physical Chemistry: Dynamics of Excited Materials and Molecules

• Density-functional methods have revolutionized the calculation of electronic structure• They are not justified for excited states, however.• New effort to study excited states of molecules by pump-probe Raman spectrometry

Science Activity for Physical Chemistry: Hydrogen Molecule Dynamics in Physisorption

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QuickTime™ and aTIFF (Uncompressed) decompressor

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Rotational Librational

Ground

Excited

Entropy

W is the number of

equivalent ways of

finding a system with the

same macrostate.

Ways of “what”?

Classical Limit

1 2

p1…p3N–3

q1…q3N–3

S = kB lnV1

V2( )

~1/

Phonon DOS of V with 3d-series solutes

• increasing stiffening along 3d series

• Ti: softening,

• Cr-Ni: stiffening

• impurity masses ~ host

• trend opposite to mass increase

electronic / size effect

Entropy of Alloying

Use Case

Inelastic Scattering with Fermi Chopper Instruments

• Approach:

- Reduce NeXus file to “S(Q,E)”

- S(Q,E) to phonon DOS, g(E)

-

reduce<include>NeXUSfile

g(E)Z(T)User<include>BvK

• Goal: Obtain phonon partition function, Z(T).

Today – Three Problems

• /M differs between atoms (neutron weighting)

• Q information will be rich, but neglected in analysis

• Errors in Z are unknown - reliability of thermodynamic quantities?

Computation will Solve These

Today (Neutron De-weighting Solution)NeXusreductiongnw(E), d2/dΩdE B v K{Φ1 ’, ...}xx/M |QU (exp iQr) |2,Compare

Alter(Φ1 , ..}xx

( )g E .phononthermo py, , Z F SLaptop Linux Clustergnw( )E

Near Future: Fits with Q information

New CapabilityZ, Pmm, displ.PHONONVASPE(Q)Linux ClusterLaptopARCS, parampyre-McStasNeXus{Φ1 , ...}xx, M params

Error Bars in Thermodynamic Quantitiesreduce<include>VASP

E(Q)g(E)<include>BvKZex(T)User<include>SimulNeXus<include>Zth(T)

Error Bars in Thermodynamic QuantitiesB v Kg(E)phononthermo.pyZ, F, SError in Freductiongnw(E), d2/dΩdE B v K{Φ1xx’, ...}/M |QU (exp iQr) |2,Compare

Alter(Φ1 , ..}xx

( )g E .phononthermo py, , Z F SLaptop Linux Clustergnw( )E, , .Z Pmm displPHONONVASP(E Q) Linux ClusterLaptop, ARCS param-pyre McStasNeXus{Φ1 , ...}xx, M params

Test Case – Use Known Force Constants from NiB v Kg(E)phononthermo.pyZ, F, SError in Freductiongnw(E), d2/dΩdE B v K{Φ1xx’, ...}/M |QU (exp iQr) |2,Compare

Alter(Φ1 , ..}xx

( )g E .phononthermo py, , Z F SLaptop Linux Clustergnw( )E, , .Z Pmm displPHONONVASP(E Q) Linux ClusterLaptop, ARCS param-pyre McStasNeXus{Φ1 , ...}xx, M params

Resolution of a Fermi Chopper Spectrometer (e.g., Windsor)

~4.2 for ARCS

ARCS Has Moveable Fermi Choppers

Typical ARCS Resolution (Energy Dependent)

Multiple Energies Give Higher Resolution

Summary of WBS 10: Inelastic Scattering

1) Scope for:- physical chemistry of small molecules- phonon excitations in solids- magnetic excitations in solids

2) Progress with ARCS and other inelastic instruments has been crucial for establishing the science scope

3) Experience in materials theory is essential too

end of presentation