Research News
Each year, numerous exciting research projects and education efforts are undertaken at the SRC. Highlights of selected research projects from the past year are provided. When possible, links have been made to the related researcher's or research group's own Website at their home institution.
Quantum Behavior of Electrons in Ultra Thin Silver Films on Germanium
Tai C. Chiang, U of Illinois at Urbana-Champaign
Ultra thin films are the future for electronics. Atomic layer uniformity, as demonstrated here, is critical for precision measurements and fundamental studies. Such investigations pave the way for applications ranging from nanoscale electronics, supersensitive detectors, and components for quantum computing.
Thin films are basic building blocks for devices. As device size shrinks, quantum effects become important, and traditional thinking in terms of electron flow must be revised. The figure shows electronic structure (dispersion curves) for 8, 8.6, and 9 atomic layers (AL). At 8.6 AL, the dispersion curves split (circled area) because of the presence of both 8 and 9 atomic-layer thicknesses in the film. At 8 AL, the film thickness is perfectly uniform; each dispersion curve shows a kink (squared area) at the band edge of Ge due to electronic coupling to the substrate. This unexpected observation leads to a better understanding and more accurate description of electronics in the quantum regime.
Giant Oscillations in a Nano-Size Soccer Ball
Pavle Juranic & Ralf Wehlitz, SRC
Synchrotron radiation has been used for a long time as an energy-adjustable source for the ionization and study of various gases and elements. Recently, however, it has also been used more and more often to study larger molecules, the most interesting of which is C60 due to its stability, size, and symmetric structure. Unfortunately, this molecule has, until recently, been very hard to manufacture in any 
reasonable quantity and only in the last few years has it begun to be explored. Though previous measurements have been made of its photoionization properties, we are happy to report that we are the first ones to fully map out its double-to-single photoionization ratio from its double ionization threshold of 19 eV to the inner (K) shell energy of 280 eV.
In the course of this experiment, we have observed previously unreported oscillatory behavior in the ratio which could shed new light on our knowledge of molecules that have a structure similar to C60. With future research, we could see whether this newly observed effect is present in those molecules and whether the oscillatory behavior changes in its frequency or amplitude with different sized molecules. Additionally, the phenomenon would also spark the formation of new theories about the origin of the effect which could then predict the expected oscillations in other substances, opening up a new avenue of high-precision measurements of ratios for these molecules. Most importantly, however, the presence of these oscillations enriches the body of knowledge we have about carbon nanotubes, which could have significant impact on future industry, since carbon nanotubes are considered to be one of the strongest materials in the world.
Surface Chemistry and Nanotribology of Ultrananocrystalline Diamond
Robert W. Carpick, University of Wisconsin-Madison
As machines are built at the micro- and nano-levels, wear and tear of the materials used to comprise them becomes increasingly important. Machines of this size must be able to run without lubrication and, until now, most small machines have been comprised of the most popular choice, silicon. But silicon sticks and wears out far too easily, and its surface will either need to be modified in some way or a 
new material will have to take its place. To this end, Dr. Robert Carpick and his group from the University of Wisconsin-Madison and collaborators at Argonne National Laboratories have embarked on experimentation of ultrananocrystalline diamond (UNCD). The Carpick group has done extensive research at the Synchrotron Radiation Center (SRC) into the tribology--the study of friction, lubrication and wear of moving parts--of UNCD in order to better understand its potential for use in making micro- and nanoelectromechanical systems (MEMS and NEMS).
Hydrogen is attached to diamond using hot plasma. UNCD films normally have excellent quality on their topside, but when the substrate underneath the film is removed (a process often needed
in fabricating micromachines), the exposed underside may not be optimal. By doing x-ray absorption experiments at the SRC, Carpick's group proved that the underside of the UNCD film is not optimal. But this can be cured if non-diamond forms of carbon on the underside of the film—along with any contaminants—are removed by the hydrogen plasma. What is left is a pure UNCD surface capped with hydrogen atoms. Adhesion and friction, measured at the nanoscale by atomic force microscopy, are extremely low.
Light as a Research Tool: High School Students Learn the Power of Light and the Wonder of Physics Through SRC Summer Program
Chris Moore, Educational Outreach Coordinator, SRC and Dan Wallace, Engineering, SRC
When compared with their majority counterparts, the percentage of students of color and those who are first-generation students enrolled in college are low. Several studies have demonstrated that enrollment and graduation rates can be increased by pre-college programs that a) encourage students to aspire to opportunities available through higher education, and b) assist students in developing critical academic skills.
A solution developed by the University of Wisconsin–Madison has been the PEOPLE (Pre-College Enrichment Opportunity Program for Learning Excellence) program, which seeks to support and encourage minority middle school and high school students from Wisconsin in order to prepare them for future success as undergraduates.
During the Summer of 2005, Chris Moore and Dan Wallace of SRC administered a course as part of the PEOPLE program titled " Let the Light Shine! Using Light to See the Unknown." The goal was to expose students to the use of light in research and to prepare them for their future study of physics.
Quantum Criticality in Quasi-One Dimensional Li0.9Mo6O17
J.W. Allen, University of Michigan
For many -- indeed most -- physical systems there are characteristic energy scales set by the 
various forces that act. For example, the ferromagnetism of iron disappears if the temperature T is greater than 1043K (1418 °F) because thermal energy then exceeds the characteristic energy of the magnetic forces in iron.
Quantum critical systems are strikingly different in having no energy scale except temperature itself. Quantum 
criticality (QC) is predicted in theories of quasi-one dimensional systems. QC may be important in nano-technology. Studies of systems in nature are just beginning.
Here we use a technique called photoemission spectroscopy to observe QC in the spectra of the energy distribution of the electrons of a quasi-one dimensional chemical compound. The spectra have the QC scaling property, that their shape depends only the ratio of energy to temperature.
Fermi Surface Evolution in NaxCoO2
Hong Ding, Boston College
The knowledge of band structure and Fermi surface topology is important for understanding unconventional superconductors, such as high temperature superconductors, which have great application potential, including non-dissipating energy transport, high speed computing, and new medical devices.
The recent discovery of superconductivity in
NaxCoO2yH2O (Tc ~ 5 K) has generated great 
interest. We report systematic angle-resolved photoemission studies on NaxCoO2 single crystals for a wide range of Na concentrations. As shown in the top figure, we observe a large Fermi surface (FS) centered at the G point, which satisfies Luttinger theorem. However, the small FS pockets predicted by band theory near the K points are not observed. Instead, “sinking islands” with the binding energy of 100 – 200 meV are observed. The disappearance of the small FS pockets are explained well by our calculation that considers large electron correlations, as shown in the bottom figure.
