Secrets of Almaden

To construct a corral, says Eigler, "We blast a copper surface with argon atoms, until it's superclean and superthin -- about 30 to 40 atoms thick. Then we supercool the smooth metal sheet inside a vacuum chamber and drop a dust of cobalt atoms on top of it." Eigler and his labmates then use the microscope tip to form the loose cobalt atoms into a quantum corral.

One particularly useful shape for a corral is an ellipse, or slightly elongated circle. Since the beginning of the 19th century, scientists have known that elliptically shaped materials possess a peculiar property. Equidistant from the center of an ellipse are two points -- two focuses, or foci. These foci share a remarkable link.

Imagine that an elliptically shaped pan is filled with water. Drop a pebble at one of the foci. Waves of water will ripple out from the splash and bounce off the walls of the dish, thereby creating a wave pattern that repeatedly doubles back on itself. Due to the nature of the ellipse, the pattern of intersecting waves will soon look as if there were two spots where the pebble dropped -- two foci. This phenomenon occurs because the shape of an ellipse perfectly focuses bouncing waves at two points; it reproduces the physical appearance of the first focal point at the second one.

Waves, of course, can move through many different substances, such as water, or through the air as sound. Modern physics considers an electron to be both a particle and a wave.

Inside an ellipse resonating with a pattern of rippling waves, the walls of the ellipse act as a lens, refocusing the center of the first ripple pattern at a second point. In effect, the ellipse makes a copy of the first focus at the second focus. But it is an attenuated copy, like the harmonic of a musical note.

Eigler wondered if the same effect would play out on an atomic level. What would happen, he asked, if he built an elliptical quantum corral out of cobalt atoms, and then placed a single cobalt atom at one of the foci? Since Eigler prides himself on "doing things that have never been done before," he did exactly that. And what he calls a "phantom copy" of the cobalt atom appeared at the second focus. This was the mirage.

Once the electron waves were set in motion by placing the cobalt atom at the primary focus, the electronic structure surrounding the cobalt atom was projected intact, but slightly weakened, to the opposite focus. A copy -- or harmonic -- of the cobalt atom appeared. Eigler calls it a "surprisingly faithful spectroscopic replica" of the original cobalt atom. It is not a hologram, however. Eigler comments that while there is some analogy to the physics that makes three-dimensional holograms, his two-dimensional mirages are different because, among other reasons, light waves are much, much larger than electron waves.

What Eigler and his colleagues produced is not really comparable to anything else in nature. Information about the cobalt atom placed at the first focus was transported -- or projected -- to the second focus, creating the mirage.

It turns out, though, that something more than just a mirage was projected.

A special magnetic energy scale called the Kondo effect, which was attached to the original cobalt atom, also made the trip to the second focus. This journey suggests some very interesting interpretive questions. Does the presence of the Kondo effect at the second focus indicate that a signal is being transmitted from one focus to the other focus? A signal that signifies the existence of the Kondo effect at the first focal point, that is. Or is the Kondo effect itself transported, sent to a remote spot as something more than a signal?

Eigler and his colleagues speculate that both of these interpretations might be right; and in the universe governed by quantum mechanics it is a quite unremarkable notion that two answers can be correct. But this leads to another provocative question: Is the projected Kondo effect a quasi-independent phenomenon? In other words, does the refocusing of the electronic shape of the original cobalt atom actually change the quantum reality at the second focus? Does the harmonic of the cobalt atom produce its own Kondo effect?

"The phantom aspect," says Eigler, "is that electrons at the second foci act as if there were a cobalt atom there. Since there is no cobalt atom there, we think of a 'phantom' atom as being there. However, there is nothing phantom about the electrons at the second foci. They are really doing their thing."

If not provide answers, Eigler says, further experiments should at least sharpen the questions.

Eigler's team was lauded by the scientific establishment when it unveiled the quantum mirage, not only because of the technical deftness the experiment demonstrated, but also because it showed how a well-known, "classical" phenomenon -- an ellipse focusing waves -- can be applied at the nanospace scale.