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In this issue:
SULI and STAR Interns Shine
Science Today: New Mechanism for Charge Density Wave Formations
Traces of Dark Matter?
SLAC Kids Day Tomorrow
Sand Hill Road Closure

SLAC Today

Thursday - August 14, 2008

Susan Schultz presents Adam Zok with the Ernest Coleman award. (Photo by Martin Sanchez. Click for larger image.)

SULI and STAR Interns Shine

Mentors and interns from SLAC’s Science Undergraduate Laboratory Internship and Science Teacher and Researcher programs celebrated a summer of research and learning with a barbeque lunch in the Sector 6 picnic area yesterday. Participants shared stories from the summer. Director Persis Drell was on hand to offer congratulations and talk about the two programs.  

The SULI and STAR programs pair college students and recent college graduates with SLAC mentors, who guide the interns through an eight- or nine-week intensive research program. SULI, which celebrates its 40th anniversary this year, gives undergraduates the opportunity to conduct research on topics ranging from accelerator physics to science writing.

(Daily Column - Science Today)

New Mechanism for Charge Density Wave Formations

Using beamline 5-4 at the Stanford Synchrotron Radiation Laboratory, researchers from SSRL and Fudan University in Shanghai have worked out the mechanism behind the formation of charge density waves in 2H-structured transition metal dichalcogenides, or 2H-TMDs. The results were published in the November 21, 2007 edition of Physical Review Letters.

Charge density waves, or CDWs, are quantum mechanical ordering phenomena akin to superconductivity. In a normal conductive metal, electrons persist in a "sea" wherein they are evenly distributed and the ions form a perfectly periodic lattice. A charge density wave occurs under certain circumstances—for example, in low-dimensional materials—that cause the electrons and ions to couple together to lower their energy. A modulation of the ion positions and "waves" in the electron sea are observed, creating new periodicities that hinder the conducting electrons, thus lowering the material's conductivity.

Understanding CDW formation is an important step in characterizing the fundamental electronic properties of matter. The mechanism behind CDW formation is typically ascribed to Fermi surface nesting, a phenomenon wherein a material's electrons correlate with each other on the basis of their momentum and energy. However, CDW formation in 2H-TMDs appears to arise through a different mechanism. Using angle resolved photoemission spectroscopy, the Fudan and SSRL researchers have for the first time characterized this new mechanism.

Read more in the full scientific highlight.

Traces of Dark Matter?

Yesterday, Nature's online edition announced that a European satellite team has glimpsed what could be the tracks of dark matter in the Milky Way. "If it's true, it's a major discovery," Fermilab theorist Dan Hooper told Nature. The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics mission, or PAMELA, spotted an excess of high-energy positrons consistent with dark matter predictions of supersymmetry theory. The results were announced at this month's International Conference on High Energy Physics in Philadelphia.

In 2006, SLAC astrophysicist Maruša Bradač and her colleagues made the first observation of dark matter in isolation.

Read about the new finding in Nature online.

SLAC Kids Day Tomorrow

(Photo - SLAC Kid's Day 2007)
SLAC Kids Day 2007.
(Click for larger image.)

Hundreds of children ages 9–16 will arrive at SLAC tomorrow morning at 8:00 a.m. for a day of science workshops, talks, lunch and treats as part of SLAC Kids Day 2008. Scores of SLAC volunteers will teach and lead the kids in projects, including constructions of soda-bottle water rockets. Thanks to everyone at SLAC who has provided empty two-Liter bottles for the kids!

Sand Hill Road Closure

If you're planning to come to the SLAC campus via Interstate 280 this weekend, you may need to take a detour. Signs posted along 280 announce: the Sand Hill exit going east toward SLAC will be closed for construction 9:00 p.m. to 9:00 a.m., Friday and Saturday, August 15–16.

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