Ultrafast electron diffraction (UED) enables direct insight into structural dynamics of solids. Relativistic MeV-scale electron beams yield access to high-momentum scattering and preserve beam coherence, yet their application at high repetition rates for high-sensitivity UED has been limited. We discuss the High Repetition-rate Electron Scattering (HiRES) instrument at Berkeley Lab and its first applications to UED of metallic films and quantum materials. HiRES employs a state-of-the-art photoinjector with RF bunch compression to generate high-brightness, relativistic 0.75 MeV electron pulses with up to 105-106 el./pulse and with highest achievable coherence length of 10 nm. The resulting high momentum range (±10 Å-1) yields access over multiple Brillouin zones. The sub-500 fs electron pulses are provided at 0.1-250 kHz repetition rate, and combined with optical pumping via a 1.03 µm fiber amplifier enable UED of cryogenically cooled materials. We will show examples of first experiments including transient Debye-Waller dynamics in ultrathin metals at kHz repetition rate as well as studies of charge density waves in 2D materials. Work at LBNL was supported by the DOE Office of Basic Energy Sciences.

1. Supported by the Department of Energy, Office of Basic Energy Sciences
Kilohertz-rate MeV ultrafast electron
diffraction for time-resolved studies of
materials
Khalid M. Siddiqui1, Daniel B. Durham2,3, Fuhao Ji4, Andrew M. Minor2,3,
Robert A. Kaindl1 and Daniele Filippetto4
1Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA
2National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA
3Department of Materials Science and Engineering, University of California, Berkeley, CA
4Accelerator Technology and Applied Physics Division, Lawrence Berkeley National Laboratory, Berkeley
APS March Meeting 2020 – Denver, CO, March 2, 2020

2. UED: forefront technique to probe transient atomic structures
Ultrafast Electron Diffraction
• Higher elastic scattering cross-sections & elastic/inelastic ratios
lower sample damage & preferred choice for poorly scattering systems
• Shorter wavelengths
Flatter Ewald sphere and sub-atomic resolutions
• Easier to manipulate
Optics design for microscopy applications
advantages of electrons vs. x-rays
• Non-equillibrium lattice dynamics
• Evolution of order parameter
• Transient molecular structures
Scientific scope

3. UED applied for dynamics in materials
ultrafast energy flow & melting
Ultrafast melting of aluminium
Siwick et al, Science 302 (5649), 1382-1385.
Disentangle degrees of freedom
Ultrafast Laser control of exotic phases in TaS2
A. Zong et al, Sci. Adv. 4, eaau5501 (2018).
Structural phase transitions
Waldecker et al,Nature Mater 14, 991–995 (2015)
Photoinduced phase transformation of Ge2Sb2Te5
• UED has been used to study range of topics in condensed matter physics and material science

4. Current state-of-the-art in UED
Relativistic UED
Non-relativistic UED
SLAC MeV-UED setup (Stanford)
RF based gun technology
UCLA/BNL/Osaka
3-5 MeV
Limited in repetition rate
(Currently 360 Hz)
Compact (DC) & Hybrid (DC+RF) electron sources
McGill/Göttingen/MIT/Berlin
30-150 keV
Lower electron energies
Repetition rates up to 10 kHz
100-200 keV-UED (Hamburg/Toronto)

5. High Repetition-rate Electron Scattering (HiRES) beamline
Phys. B: At. Mol. Opt. Phys. 49, 104003 (2016)
750 keV
single-shot to 187 MHz
VHF RF technology
Relativistic and high repetition-rate
Located inside ALS synchrotron

6. High Repetition-rate Electron Scattering (HiRES) beamline
HiRES is a high-repetition & high brightness electron instrument
High-quality diffraction patterns obtained at HiRES!
Momentum resolution ~ 0.1 Å-1 , s-range = ± 10 Å-1
Bi2Se3 1T-TaS2 Bi-2212
Temporal resolution
~ 800 fs (2.5 fC)
Bi
Temporal resolution
(unbunched)
HiRES delivers
• High transverse coherence length ~ 10 nm
• High average flux ~104-106 electrons/pulse
• 100-1000 fs temporal resolution
• Small focal spots of e- beams for small samples ( 0.1 µm < σ < 100 µm)
F. Ji et al, Communication Physics,,2, 54 (2019)

7. HiRES Laser system
• Commercial fibre laser system (Active Fiber Systems) for
sample excitation and generation of electron bunches
M2 ~ 1.3
Intensity autocorrelation
σ ~ 315 fs (FWHM)
• Optical delay line with up to 1 ns delay range
• Option of an OPCPA for 10 fs (FWHM) 800 nm laser pulses
Laser spectrum
λcentre = 1.03 µm
σ = 7 nm (FWHM)
Parameter Value OPCPA
Photon energy 1.2 eV 1.55
Repetition
Rate
1 Hz -> 250
kHz
1 Hz -> 250
kHz
Pulse energy up to 200 µJ 15.4 µJ
Pulse length ~ 315 fs
(FWHM)
~20 fs
(FWHM)
Beam size @
sample
750 x 750 µm -
Optical delay 0 to 1 ns 0 to 1 ns
Laser beam profile at sample

8. Cryogenic capabilities for UED at HiRES
Courtesy: Nord Andersen, CXRO (LBNL)
Closed-cycle helium cryocooler with low-vibration interface
electron
Thermal shielding
6 slots for samples
1x slot for pinhole
Sample Holder for
Cryo stage
• 10 K reached at the sample
(cooldown time < 3 h)
• 4 DOF translation (x,y and tip & tilt)
Customised thermal shielding and sample holder design allows efficient cooling

9. First HiRES PP results: ultrafast heating of gold film
Motivates UED studies at higher repetition rates
𝐼(𝑡)
𝐼0
= 𝑒[2𝑠ℎ𝑘𝑙
2 𝑢2 𝑇0 − 𝑢2(𝑇 𝑙)
4
Transient Debye-Waller effect
s scattering vector
T0 initial lattice temperature
Tl lattice temperature
u2 mean square atomic displacement
11 nm free standing Au film
system response up
to 10 kHz without
significant damage!
Repetition Rate dependenceTemporal response to 1030 nm
Laser excitation
• Measured transient Debye-
Waller effect in gold for first
demonstration of HiRES
capabilities as UED instrument
Observations in line with previous
results. See e.g. Appl. Phys. Lett. 108,
041909 (2016)
420
Higher repetition will enable higher S/N and
shorter acquisition times

10. Commensurate CDW melting in TaS2 at 10 K
• Extensively studied system using UED and
ULEED and using other ultrafast probes
• Excellent candidate for benchmark studies
and exploration of hidden states
1.2 eV
S. Vogelgesang et al, Nat. Phys. 14, 184–190 (2018)
M. Eichberger, Nature 468, 799–802 (2010)
A. Zong et al, Sci. Adv. 4, eaau5501 (2018).
TaS2 samples: Collaboration with Sef Tongay (Arizona State University)
Strong suppression of CCDW peaks at 10 K observed following 2.3 mJ cm-2 1030 nm pulses
Time-resolved studies are ongoing with focus on CCDW to NCCDW phase transition

11. Ongoing experiments at HiRES
Ultrafast energy flow in ferromagnetic cobalt thin films
hcp fcc
1030 nm
Cobalt samples: Collaboration with Andreas Schmid (Molecular Foundry/LBNL)
Other projects being pursued:
• Charge density wave dynamics in TMDs and strongly-correlated materials (ASU/LBNL/PSU)
• Ultrafast dynamics of quantum nanowires (UCB, Strobe)
• Single-shot UED of irreversible processes, e.g. radiation damage and melting dynamics of
metals (UNR)
• Questions being addressed:
- How does energy flow in the two phases of cobalt metal?
- How do electron, phonon and spin subsystems couple?
- What are the kinetics of energy transfer in two phases?

12. Summary
• HiRES beamline at LBNL provides unique
opportunities for higher repetition rate
(limited only by sample relaxation) and low
temperature experiments
• Capabilities of HiRES open up exploration of new
materials, e.g. conventional and high-Tc
superconductors, and gas phase systems via UED
• Several materials are being studied with HiRES
producing high-quality data with high reliability
• Next phase: commissioning of buncher
for ~100-200 fs electron pulses
Debye-waller effect in Au
1030 nm
Martensite transformation in cobalt
CDW melting dynamics in TMDs
Additional acknowledgments
Frederick Cropp (UCLA)
Johan Daniel Carlstroem (LBNL)
Diego Novoa (UC Berkeley)