Electrons as Waves
Although scientists talk about the dual wave and
particle properties of electrons, many nonscientists
still believe that electrons are only tiny particles.
Rooted as we are in the macroscopic world, it can
be difficult for some to picture a particle as also
being a wave. One look at the accompanying picture, however, should help change that. What looks
like ripples surrounding two barely submerged pebbles in a pool of water is really the surface of a
copper crystal.
Although they are true believers in the wave
nature of electrons, the physicists at the IBM Almaden Research Center in San Jose, California,
were genuinely surprised when their scanning tunneling microscope (STM) produced this image of
the copper surface. “We looked at the surface with
all these waves and thought, ‘Is our machine bro-
ken?’” says Michael Crommie, one of the IBM
physicists. But the researchers soon realized that
the waves were produced by electrons confined to
the metal’s surface that bounced off impurities (the
two pits). Because the electrons are waves, they
form interference patterns after reflecting off the
impurities, producing standing waves.
To further explore this behavior, the IBM scientists constructed a “quantum corral” by using their
STM to place 48 iron atoms on a copper surface in
a circle approximately 14 nm in diameter. Then using the STM to study electron behavior on the copper surface inside the corral, they observed the
standing electron waves shown in the photo on the
right. This image provides a unique visual confirmation of what the Schrödinger equation predicts.
Electrons are wavelike. Seeing is believing! ■
(left) The electrons form interference patterns, the ripples shown here, and produce standing waves.
(right) Iron atoms in a circular “corral” cause electron standing waves.