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
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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
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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
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Separation of time-scales
4Ultrafast Tickle and Probe
Sheu et al. unpublished Trigo et al. unpublished
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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
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Low excitation (tickle regime)Pump-probe vs. Spontaneous Raman
6Ultrafast Tickle and Probe
Collaboration with Roberto Merlin, UM
Jian Chen, Jinjing Li, unpublished
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Subtle difference between the two…
7Ultrafast Tickle and Probe
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Jian Chen, Jinjing Li, unpublished
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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
8Ultrafast Tickle and Probe
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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
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x-ray diffuse scattering: measure deviations from average structure
d crystal + phonons
Example, diffuse scattering bythermal phononsVery weak, we need bright x-ray
pulses!
10Ultrafast Tickle and Probe
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(hkl)
Ghkl
(000)k0
ks
ks – k0 = Ghkl
Bragg scattering
Ultrafast Tickle and Probe 11
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q
(000)
Ghkl
k0
ks
(hkl)Large detector
Diffuse scattering
Ultrafast Tickle and Probe 12
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2d x-rayJoynson, Phys. Rev. 94, 851 (1954)……M. Holt et al., PRL 83, 1999.
Phonon Dispersion from TDS and limitations
13Ultrafast Tickle and Probe
TDS: Limited to simple cases (# fit parameters low) and have a constraint (assumes Bose-Einstein distribution)
?
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Simulation of InP impulse softening of TA by 20%
14Ultrafast Tickle and Probe
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Hillyard, Reis and Gaffney PRB 77, 195213 (2008).
Fourier transform of I(q,t) yields phonon dispersion (excited state)
15Ultrafast Tickle and Probe
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Alternatively, can use multiple coherent pulses
16Ultrafast Tickle and Probe
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?
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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
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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
18Ultrafast Tickle and Probe
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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.
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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.
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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.
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LA TA
Brillouin zones
Contribution from acoustic phonon branches
Ultrafast Tickle and Probe 22Trigo et al. Phys. Rev. B, 82(23):235205, 2010.
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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?
23Ultrafast Tickle and Probe
Will study anharmonic decay at LCLS this fall!
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Acknowldegements
24Ultrafast Tickle and Probe
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
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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