麻豆传媒

SOMETHING bizarre has popped up in the laboratory of physicist Donald Eigler,
and no one is quite sure what to make of it. It鈥檚 the world鈥檚 smallest ghost, a
phantom on an atomic scale. Eigler and colleagues at IBM鈥檚 Almaden Research
Center in San Jose, California, call it a 鈥渜uantum mirage鈥� because it projects
an atom鈥檚 electronic signature to another place while the atom itself stays
put.

The projection spans only a tiny distance: about 10 nanometres, or 10 000
times narrower than a human hair. Still, it offers the tantalising promise of
transferring information within tiny circuits of the future in which wires are
obsolete and the components are single atoms.

The eerie image may also let physicists probe an atom without disturbing it
directly. That鈥檚 an intriguing prospect in a miniworld where even a few photons
of light can alter the states of particles. Eigler and his team think it may
even be possible to forge a chemical bond with the mirage by moving a compatible
atom next to it. That would create a weird hybrid molecule that IBM physicist
Hari Manoharan calls 鈥渉alf real and half ghost鈥�.

The mirage is yet one more curious manifestation of the quantum world, in
which electrons and other subatomic flecks behave neither like particles nor
like waves, but as a mixture of the two. In this case, the researchers confined
a sea of electrons on a copper surface within a 鈥渜uantum corral鈥�, a picket fence
built of several dozen closely spaced cobalt atoms. The corral kept the surface
electrons from flitting about the surface of the metal with their normal
freedom. What鈥檚 more, Eigler鈥檚 corral is in the shape of a near-perfect ellipse.
That special geometry compelled some of the electrons to cluster around two
spots: the left and right foci of the ellipse.

Using the precise motions of a scanning tunnelling microscope (STM), Eigler鈥檚
team placed an atom of cobalt at one focus. In the frigid temperature and
ultra-high vacuum of the STM chamber, the atom stuck there like a refrigerator
magnet. When the physicists probed the cobalt atom with the STM, they saw a
swarm of electrons around it. Then, when they scanned the other focus, they saw
an unmistakable signature. The swarm surrounding the real atom was mirrored at
the empty focus, even though there was no atom there. The reflection wasn鈥檛
complete, as the STM detected the mirage atom at only one-third the intensity of
the real one. Still, it was a startling apparition.

So what is it? Of one thing Eigler is sure: it鈥檚 not just an illusion. 鈥淭here
are real electrons there,鈥� he says. 鈥淚t鈥檚 a physical object.鈥� But when it comes
to the nitty-gritty physics of describing the genesis of the mirage and its
implications, Eigler and his team admit they鈥檙e puzzled. 鈥淲hat exactly are we
doing? What properties are really there?鈥� asks Manoharan.

It all seems rather surreal, and that other-worldliness is heightened by the
manner in which Manoharan, physicist Christopher Lutz and Eigler use exaggerated
vertical relief and garish colours to depict their quantum corral and the mirage
it contains. There it was, on the cover of the 3 February issue of
Nature: a ring of aggressive yellow atomic spikes enclosing a wavy
orange-and-green sea of electrons and two bright violet islands, the atom and
its mirage. One almost could envision the cursive writing of Ren茅
Magritte under the image: 鈥淐eci n鈥檈st pas un atom.鈥�

The quantum mirage is the natural denouement of progress made during the past
decade along three fronts: STM technology, quantum corrals, and recognition of a
peculiar state called a 鈥淜ondo resonance鈥� around a magnetic atom.

The basic concept of the STM hasn鈥檛 changed since IBM physicists Gerd Binnig
and Heinrich Rohrer developed the apparatus in Zurich in 1981. Researchers use a
fine wire鈥攊n Eigler鈥檚 case, made of pure iridium鈥攖hat tapers to a
single atom at the point. When this tip approaches a conducting surface, such as
a metal, the physicists apply a small voltage. This induces electrons to
鈥渢unnel鈥� across the gap between the tip and the metal. By keeping the tunnelling
current constant as the tip is scanned across the surface, the researchers
create a topographic map of its atomic peaks and valleys. An STM can work at
room temperature, but Eigler鈥檚 machine has to operate at just four or five
kelvin to keep the atoms in place.

Eigler鈥檚 group was the first to use an STM to drag atoms into desired spots.
The researchers raised the voltage in the tip, and brought it close enough to
the surface to form brief chemical bonds with single atoms. Eigler and IBM
colleague Erhard Schweizer made an international splash in 1990 by spelling
鈥淚-B-M鈥� with 35 atoms of xenon atop a layer of nickel.

Three years later, Eigler and Lutz teamed up with IBM physicist Michael
Crommie鈥攏ow at the University of California at Berkeley鈥攖o build the
first quantum corrals. Their circular barrier of iron atoms on a copper surface
caused another sensation, for it revealed in dramatic fashion the wavelike
behaviour of the electrons trapped within. In this case, the electron densities
peaked sharply in the centre of the ring. Surrounding that peak were concentric
rings of lower electron densities, looking for all the world like ripples from a
pebble plopped into a pond.

The third advance that paved the way towards quantum mirages came in 1998
when physicists first observed the Kondo resonance. This effect was proposed in
1964 to explain the strange way that metals interact with atom-sized magnetic
impurities, but until the invention of the STM it was impossible to verify. Two
years ago separate teams led by Schneider and Crommie found unambiguous evidence
that the Kondo resonance actually occurs.

The surface of a conducting metal, such as copper, is covered with a sheen of
electrons that swarm freely across it鈥攚hich is why they conduct
electricity so well. Place a single atom of cobalt or another magnetic element
on this surface, and its electrons disrupt the smooth flow of the conduction
electrons. 鈥淵ou can think of the tightly bound cobalt electrons as a little hard
ball,鈥� says Crommie. 鈥淭he copper electrons swimming around are repelled from the
ball.鈥� In addition, a tiny cloud forms, and within this the copper electrons
spin in the opposite direction from those around the cobalt atom. This alignment
effectively screens the disruptive magnetic field of the intruder atom.

At the atomic scale, nothing looks quite like the Kondo resonance. That鈥檚 why
Eigler鈥檚 team chose the cobalt-on-copper system. Project a Kondo resonance to a
remote location and you have the unmistakable signature of a quantum mirage.

Another critical factor that Eigler exploited is the special geometry of an
ellipse. At school, you probably learned to draw an ellipse by sticking two
tacks on a board鈥攖he foci of the ellipse鈥攖hen forming a loop by
tying a length of string around them. A pencil that pulls the loop tight traces
out an ellipse. In the atomic world, this means that if a signal starts at one
focus and bounces off the ellipse wall toward the other focus, it travels the
same distance no matter which direction it goes.

The consequences for electrons trapped within an elliptical corral are
fascinating. 鈥淥ne path length connects the atoms at each focus,鈥� Manoharan says.
鈥淚t changes the two-dimensional problem to a one-dimensional problem. The
entire ellipse acts like a single wire connecting the atoms at the two foci.鈥� An
electronic disturbance at one focus, such as the Kondo resonance, echoes off the
walls of the corral and appears, ghostlike, at the other.

Whispering galleries

For those who can鈥檛 wrap their brains around quantum mechanics, acoustic
analogies abound. 鈥淲hispering galleries鈥� are often elliptical chambers, in which
sounds from a speaker at one focus echo into the ears of someone standing at the
other. Many musical instruments also feature resonant chambers in which waves
vibrate with characteristic modes. 鈥淭o some extent, what happens within our
corral is nothing more than what happens when you have waves inside a resonant
structure,鈥� Eigler says. 鈥淲here it is special is that the elliptical structure
helps to make the effect we are observing very obvious in one special spot.鈥�

But acoustic descriptions don鈥檛 quite illustrate what exists at the mirage
site and what is illusory. For that aspect, Lutz鈥檚 favourite analogy is inspired
by the image of a solar eclipse in a pinhole camera. 鈥淚t鈥檚 not the Sun itself,
but it has some characteristics of the Sun,鈥� he says. 鈥淚t鈥檚 made of photons that
really came from the Sun, and it shows the changing shape of the Sun. You could
even start a fire with it if you intensified the image with a lens.鈥� One can
learn some of the Sun鈥檚 properties, but by no means all, by studying the
projection. The same may be true of the quantum mirage, Lutz believes.

For now, the only properties of the real atom that show up conclusively in
the mirage are the energy states of its electrons. A spectrum taken with the STM
reveals how many electrons exist at different energy levels around the atoms.
That analysis shows that both the real atom and its evanescent twin display the
distinctive Kondo resonance. The team is optimistic that other properties will
also come into view. For instance, the spins of the electrons surrounding the
cobalt atom also form a unique pattern, but the STM isn鈥檛 yet sensitive to those
signatures. Manoharan thinks it may also be possible to place a simple molecule,
such as carbon monoxide, at one focus, and detect the characteristic vibrations
of its atoms at the other.

It鈥檚 one of the golden rules of physics that you can鈥檛 probe the properties
of an atom without simultaneously affecting that atom in some way. Does the
discovery of mirages change all that? Might it be possible to sense some
properties of the real atom by scanning the mirage, leaving the atom itself
unmolested? Sadly, the answer seems to be no. Whenever physicists investigate
the mirage site, about 10 billion electrons flow through the STM tip into the
corral each second, flooding the ellipse and influencing the real atom.
鈥淎nything you do at one focus is felt by the other,鈥� observes Paul McEuen of the
University of California, Berkeley. 鈥淵ou鈥檙e not getting a free lunch.鈥� Still,
remote sensing disturbs the real atom much less than placing the STM tip
directly above it.

For IBM, Eigler鈥檚 work has important implications in a different direction.
The company has its sights set on one day building electronic circuit out of
assemblies of atomic structures rather than wires, and for this the quantum
corrals show obvious promise. As Eigler points out, the corral transmits
information. That could be very useful when it comes to achieving the
electronics industry鈥檚 goal of building components measuring a mere 100
angstroms (10 nanometres) across. This, Eigler says, is still 鈥渕any generations鈥�
away.

鈥淗ow do I build something that adds two numbers together, has input and
output channels, and fits in a 100-angstrom package?鈥� Eigler asks. Eliminating
wires would be a great start, as IBM has not been slow to point out. Its news
release on the Nature paper touted the discovery of a 鈥渘anotech
communication method鈥�. That鈥檚 rather premature, as the researchers can鈥檛 yet
code information in a way that would allow it to be sent from one focus of the
ellipse to the other. But there appear to be plenty of possibilities. 鈥淲e could
use the location of the atom, the energies of the electrons, the location of the
electrons, the spins of the electrons, or a complex mixture of those,鈥� says
Lutz.

Upping the level

Other STM physicists were impressed when they learned of this latest work
from Eigler鈥檚 lab. 鈥淢y first reaction was that this is simply fantastic,鈥� says
Wolf-Dieter Schneider. 鈥淚t鈥檚 a marvellous and novel aspect of how magnetic atoms
behave.鈥� Paul McEuen says that Eigler鈥檚 team has 鈥渦pped the level of the game鈥�
for precise manipulation of atoms. 鈥淭his shows that you can tailor the
interactions between different atoms with an exquisite degree of control,鈥� he
says.

In the past few months, Manoharan and his colleagues have been doing what he
calls 鈥渁ll kinds of weird experiments.鈥� Their tinkering was motivated by the
musings of physicist Charles Rettner, a colleague at Almaden, that perhaps
another atom could form a molecule with the mirage. After all, the mirage is the
projection of an electronic structure. 鈥淲e know that those structures underlie
chemical bonding,鈥� Manoharan explains. 鈥淚f we brought a real atom next to the
mirage, would it make a chemical bond?鈥�

Though this initially struck Manoharan as unlikely, subsequent research has
muted his scepticism. In one experiment the team observed what appears to be a
magnetic interaction between two real atoms within the corral. Manoharan says
that this interplay is 鈥渁 step below chemical interactions鈥�, suggesting that a
real chemical bond might be possible.

The team has not yet attempted to form a full chemical bond between a real
atom and the mirage. 鈥淚t might work, depending on how much of the electronic
structure we鈥檙e projecting,鈥� says Manoharan. 鈥淏ut we are not projecting the
nucleus of the atom or the orbitals of the electrons.鈥�

Ultimately, turning quantum corrals into electronic components will require
mass production on some sort of atomic assembly line. Eigler鈥檚 group doesn鈥檛
plan to try that, at least not soon. But across the continent at the National
Institute of Standards and Technology in Gaithersburg, Maryland, another STM
group is moving in that direction. Joseph Stroscio and his colleagues have
completed a machine based on a cryogenic microscope that will assemble corrals
and other structures containing many thousands of atoms. The system will start
by randomly depositing atoms on a surface. Then, armed with blueprints of the
desired structures, a computer will direct the STM tip to build them in the most
efficient way. 鈥淚t will do it overnight while we鈥檙e sleeping,鈥� Stroscio
says.

Once it鈥檚 working, this set-up will let physicists produce countless atomic
structures with subtly different configurations, allowing them to investigate
how their behaviour varies. Stroscio鈥檚 team also plans to apply strong magnetic
fields to alter the dynamics of electrons in their corrals, which he expects
will be 10 times the size of those in Eigler鈥檚 lab. The hope is that physicists
will be able to use these 鈥渜uantum laboratories鈥� to work out how electrons
behave in different environments.

It鈥檚 not easy to surprise physicists who routinely play with single atoms as
if they were marbles. Yet the new frontier of manipulating the electronic
properties of atoms has energised the STM community. 鈥淚鈥檓 amazed each day that I
can build these structures,鈥� Manoharan says. 鈥淭he prospect of what we will
discover excites me to no end. We can go in any direction we want.鈥�