time-resolved diffuse scattering: phonon spectoscopy with ... · david a. reis pulse institute,...
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Time-resolved Diffuse Scattering:phonon spectoscopy with ultrafast x rays
David A. ReisPULSE Institute, Departments of Photon Science and
Applied Physics, Stanford UniversitySLAC National Accelerator Laboratory
Science at the Hard X-ray Diffraction Limit: Ultrafast Science… Cornell, June 20–21, 2011
phonons play defining role in materials properties
Superconductivity
materials with coupled degrees of freedom
Acoustic phonon “engineering”
Ferroelectricity
Electrical and thermal conductivity
2Ultrafast Tickle and Probe
3
Why do physics in the time domain?
Spectra alone are sometimes inadequate
Based on slide from Phil Bucksbaum
01234567
294 330 349 392 440
beat
s
frequency [Hz]
Ultrafast Tickle and Probe
Separation of time-scales
4Ultrafast Tickle and Probe
Sheu et al. unpublished Trigo et al. unpublished
Murray et al. PRB 72, 060301 (R) 2005.
a a
a
Coherent A1g Mode is strongly softened and chirped.
Peierls Distorted
Example: Photoexcited bismuth (all optical experiments)
5Ultrafast Tickle and Probe
Low excitation (tickle regime)Pump-probe vs. Spontaneous Raman
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Collaboration with Roberto Merlin, UM
Jian Chen, Jinjing Li, unpublished
Subtle difference between the two…
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0 0( )0 0
tF e tF tt
( ) 0 02 2 2 2
cos( )sin[ ( )]( ) ( )( ) ( )
ttt t F e F e ttQ t e F d
Jian Chen, Jinjing Li, unpublished
Electronic Softening in Bi by femtosecond X-ray Diffraction
Johnson et al. PRL 2009.
Reis Physics Viewpoint 2009
D. M. Fritz et al. Science 315, 2007.
?
OpticalModes}
}1% e-
0% e-
AcousticModes
Murray et al. PRB 75 2007.
DFPT calculation
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Momentum and Time-resolved Phonon Spectroscopies virtually nonexistent
9
Unobserved!
phonon-phonon and electron-phonon coupling, interatomic forces.
n(q,t) (q,t)
Ultrafast Tickle and Probe
x-ray diffuse scattering: measure deviations from average structure
d crystal + phonons
Example, diffuse scattering bythermal phononsVery weak, we need bright x-ray
pulses!
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(hkl)
Ghkl
(000)k0
ks
ks – k0 = Ghkl
Bragg scattering
Ultrafast Tickle and Probe 11
q
(000)
Ghkl
k0
ks
(hkl)Large detector
Diffuse scattering
Ultrafast Tickle and Probe 12
2d x-rayJoynson, Phys. Rev. 94, 851 (1954)……M. Holt et al., PRL 83, 1999.
Phonon Dispersion from TDS and limitations
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TDS: Limited to simple cases (# fit parameters low) and have a constraint (assumes Bose-Einstein distribution)
?
Simulation of InP impulse softening of TA by 20%
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Hillyard, Reis and Gaffney PRB 77, 195213 (2008).
Fourier transform of I(q,t) yields phonon dispersion (excited state)
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Alternatively, can use multiple coherent pulses
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2d
If you want to go really crazy, multiple colors, incidence angle, stimulated x-ray (electronic)Raman selected w & q…
Can we do x-ray 4-wave mixing, ala K. Nelson or S. Mukamel?
Ultrafast Tickle and Probe
100ps 1ns 5ns
10ns 18ns 25ns
50ns 75ns 100ns
Laser-melt and epitaxial regrowth of Bi on Sapphire
TDS
samplek0
ks
laser
Synchrotron data limited by time-resolution
17Trigo et al., unpublished
BioCARS beamline at APS~1% of LCLS photons/pulse but 100ps
Average of 5 shots
Advanced Photon Source
15 keV x-rays
~ 100 single x-ray Pulses
Equivalent to a single LCLS shot!
InP, 300K
Trigo et al. Phys. Rev. B, 82(23):235205, 2010.
Time-resolved x-ray diffuse scatteringwith 100ps resolution
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bright spots due to acoustic phonons near zone center
p-Polarization forbids scattering at 90o
High-symmetry directions, where acoustic branches are “soft”.
Beam stop shadow
“Bragg Rods” from surface scattering
Primarily TDS
19Ultrafast Tickle and Probe Trigo et al. Phys. Rev. B, 82(23):235205, 2010.
0.01
-0.005
0.005
0
[ I(400ps) −I(100ps) ] / I(off)
If processes were only thermal,
But, more than heating
20Ultrafast Tickle and Probe Trigo et al. Phys. Rev. B, 82(23):235205, 2010.
Similar to equilibrium
image
Sharp raise + exponential decay
Positive and negative
differential scattering
Delayed
Complex dynamics in the phonon populations due to the anharmoniccoupling between modes
time delay [ns]
U SVT
Interpretation by Singular Value Decomposition
Ultrafast Tickle and Probe 21Trigo et al. Phys. Rev. B, 82(23):235205, 2010.
LA TA
Brillouin zones
Contribution from acoustic phonon branches
Ultrafast Tickle and Probe 22Trigo et al. Phys. Rev. B, 82(23):235205, 2010.
Decay channel: LO TO + LA
k‐k
LO
TO
LA1.26 THz
9.24 THz
10.5 THz
LA
LO
TO
LA
Other phonons
Stimulated decay and emission of
TO + LAChen, Khurgin, Merlin, APL 80 2901, 2002.
Debernardi
Can we modify optical phonon lifetime?
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Will study anharmonic decay at LCLS this fall!
Acknowldegements
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Marino Trigo, Jian Chen, Mason Jian, Matthias Fuchs, Mike Kozina, Shambhu Ghimire, Vinayak Vishwanath, PULSE, SLAC, Stanford
Stanford PULSE Institute, SLAC National Accelerator LaboratoryYu-Miin Sheu,
Los AlamosTim Graber, Robert Henning,
CARS, U. Chicago
Supported by the U.S. Department of Energy, Office of Basic Energy Science
Lattice instabilities
25Lindenberg et al., Science 308, 2005;Gaffney et al., PRL 95, 2005; Hillyard et al., PRL 98, 2007.
Diffraction data consistent with complete softening
DFPT predicts instability first develops at X point
Model assuming uniform softeningGives similar results to inertial dynamics
formation of a nonequilibrium liquid
LCLS beamtime this fall to study Ultrafast diffuse scattering will measure evolution of interatomic forces and look for instability
Ultrafast Tickle and Probe