ALCF Science 1 - Argonne National Laboratory

DIRECTOR’S DISCRETIONARY
Materials Science
Using Quantum Chemistry to Study Photocathodes
Ultra-thin MgO films on Ag(001) surfaces constitute an example of
how ultra-thin surface layers on metals can be used to control the
emittance properties of photocathodes. In addition to substantially
reducing the work function of the metal surface, the MgO layers also
favorably influence the shape of the surface bands, resulting in the
generation of high-brightness electron-beams. As the number of MgO
surface layers varies from 0 to 3, the emitted electron beam becomes
gradually brighter, reducing its transverse emittance to 0.06 mm-mrad.
Collaborators from Argonne, Northern Illinois University, and the
Illinois Institute of Technology are developing photocathodes with
ultra-low transverse emittance—a prerequisite for the development
of x-ray free-electron lasers and energy-recovery linac x-ray sources.
These devices can be employed to obtain sharper images of single,
large molecules, such as vital proteins in physiological solutions. The
research will contribute to the creation of instruments that will enable
the study of phenomena that are not experimentally accessible today,
including those in the biological and environmental
research sector.
“The Director’s Discretionary
Allocation has helped me to
produce high-profile publications
in the field of laser-plasma
accelerators and photocathode
research so far, and I expect to
continue successful application of
supercomputers in various fields
of materials science, recently also
in the analysis of x-ray scattering
obtained of energy storage
materials. The generous amounts
of supercomputing time I have
received helped me to quickly
explore new ideas in the abovementioned
fields and elaborate
research findings that proved to be
the most promising directions.”
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The research team is using the Quantum Espresso/
PWSCF software package—a general quantummechanics
code for atomic/molecular/solid-state
physics and chemistry—to carry out the calculations of
this work. The researchers are running their calculations
on the Blue Gene/P at the Argonne Leadership
Computing Facility and the Cray XT4 and XT5 at the
National Energy Research Scientific Computing Center.
Researchers plan to conduct the screening of several
other metal/oxide systems with the potential for
low-transverse emittance photo-electrons by similar
means. In addition, the team will study the effect of
external electric field, surface roughness, etc. The best
photocathode candidates will be tested experimentally.
Director’s Discretionary Allocation:
0.5 million hours
Surface bands (left-hand panels) and the band structure (right-hand
panels) of MgO(n MgO ML)/Ag(001)(4ML)/MgO(n MgO ML) systems in the
Brillouin zone for n MgO =0–4. Dark blue spots denote k-space regions
with occupied electrons; otherwise, coloring indicates band height
above E F , the Fermi energy. Only surface bands with the highest
energy occupied crystal orbitals in the center of the Brillouin zone are
shown for each value of n MgO .
Figure reproduced with permission from K. Nemeth et. al,
Phys. Rev. Lett. 104, 046801 (2010).
Contact Karoly Nemeth
Argonne National Laboratory | nemeth@anl.gov