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In this issue:
From the Director of LCLS: Take a Moment, Pause and Celebrate
Topological Insulators Take Two Steps Forward
Word of the Week: Coherence

SLAC Today

Friday - August 6, 2010

From the Director of LCLS:
Take a Moment, Pause and Celebrate

(Photo - Joachim Stöhr)

Amidst the daily hustle it is sometimes good to take a break and exhale. Two such opportunities are coming up in the next two weeks for the entire lab. The first is an opportunity to "Pause," to look at our progress in working safely, remind ourselves not to let our guard down and slip back into bad habits, and take the time to think how we can do even better. The second is a SLAC milestone event on Monday, August 16: the dedication of the Linac Coherent Light Source. We will take a moment to reflect on a lab-wide accomplishment and celebrate the beginning of a new era in the proud history of SLAC. We will share this event with prominent guests, among them the Secretary of Energy and other leaders of the Department of Energy, the Stanford President, local political representatives and prominent scientists from around the world.  Read more...

(Image - gap)
This small gap indicates a big change in the properties of a topological insulator. (Image courtesy Yulin Chen.)

Topological Insulators Take Two Steps Forward

A team of researchers from the Stanford Institute of Materials and Energy Science, a joint institute of the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, and their international collaborators have pushed research into topological insulators not just one, but two steps forward.

Taken together, both steps bring the unique material closer to use in the nascent technology of spintronics, with the potential to result in, among other advances, smaller, more efficient transistors and memory devices. Both steps could also give physicists from fields as varied as condensed matter physics, cosmology, and particle physics glimpses into such exotic phenomena as magnetic monopoles, the fractional quantum Hall effect and axion fields, for starters.

As explained in a paper appearing online today in Science, in step 1, Chen and his collaborators took the marvelous property of a three-dimensional topological insulator—in which electrons flow inexorably along the surface in the same direction, slaloming around defects instead of scattering off them in all directions—and broke it.


(Image - coherent and non-coherent light)

Word of the Week: Coherence

Coherence is a defining characteristic of laser light—one that separates lasers from other light sources such as light bulbs or the sun. Coherence occurs when the crests and troughs of each light wave line up with one another, traveling forward in perfect step. This alignment causes constructive interference, so that the intensity of a laser beam is equal to the sum of all the individual light waves. Coherent laser light will travel in a straight line for very long distances, provided nothing gets in its way.

Incoherent light, by contrast, consists of light waves with different wavelengths traveling in many directions.

Generally speaking, coherence describes the correlation between the physical properties of a wave, but there are also specific types of coherence. Imagine stopping a wave in time, taking a slice of that wave and putting it at a different position along the wave. If the peaks and valleys match up, the wave is said to be spatially coherent. Now take that same slice, let the wave continue traveling for a certain amount of time, and put the slice back at the same point in space. If the shape still matches, the wave is known as temporally coherent.

The Linac Coherent Light Source at SLAC has high spatial coherence.


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