Electrical Properties of Atom Wires Unravelled
UT and Delft scientists reveal new behavior of
electrons in atomic wires with unparalleled resolution
See the Physics
News Update from the American Physical Society!
February 22, 2006
UT’s Hanno Weitering
and his fellow scientists from Stichting FOM and the Kavli
Institute of Nanoscience Delft (The Netherlands) have clarified the
way electrons confined in an atom wire behave. Such wires, being only
one atom wide, are a source of fundamental knowledge on electrical conduction
and open up a vista on a new generation of chips that are orders of magnitude
smaller than the current generation. It was already known that electrons
in atom wires arrange themselves in a smart way. Dr. Weitering and the
Delft scientists have discovered that it is possible to have more than
one different arrangement of electrons in a single atom wire. The team
also succeeded in visualizing a fundamental quantum mechanical property
of these ultra-small wires at the atomic scale by using a Scanning Tunneling
Microscope. This finding is of vital importance for the research on ultra-small
chips that are governed by the laws of quantum mechanics. The results
will be published in the leading journal Physical Review Letters.
Necklace of atoms
Atom wires generate a lot of attention because of their dimensions: an
atom is typically on the order of 0.5 nanometer (a nanometer is one-billionth
of a meter). Depending on their properties they could serve as interconnects
in future atomic or molecular scale computer chips. Instead of neatly
placing the atoms in a row with a Scanning Tunneling Microscope, there
is a smarter way: make nature do the job for you. This principle is called
self-organization.
The team, consisting of Dr. Weitering and Paul Snijders and Sven Rogge
from Delft, produced atom wires using this approach. Gold atoms were evaporated
in an ultra-high vacuum and deposited on a special kind of silicon surface.
This surface is sawed from a silicon crystal in a deliberately slanted
way in order to generate a periodic “flight of stairs” made
of the planes of atoms arriving under an angle at the surface. It appears
that in this environment the gold atoms organize themselves in perfect
rows parallel to the steps of the stairway – atom wires have been
fabricated, resembling the chain of beads in a necklace, but instead made
of atoms.
Electrons in an atom wire
Electrons always have a negative charge and therefore avoid each other.
But in the atom wires studied here they cannot dodge one another because
the diameter of the wire, being only one atom wide, is too small for evasion.
A phenomenon that can occur in such wires is the formation of a so-called
Charge Density Wave (CDW) – a periodic modulation of the density
of charge. Electrons cluster in a spatial undulating pattern, with a wavelength
exactly fitting an integer amount the interatomic distance in the wires.
Generally, by transforming to this smart arrangement, the system attains
a more favorable state with a lower energy; the so-called “ground
state” has been reached. But the measurements in Delft show that
this is not always the case.
Two transitions
The experiments in Delft were performed with a Scanning Tunneling Microscope
capable of imaging surfaces with atomic resolution. These measurements
showed, surprisingly, that in these self-organized wires two transitions
with different wavelengths take place, one after the other. A doubling
of the wavelength was observed when the system was cooled down from room
temperature. The measurements indicate that this is a conventional Charge
Density Wave. But fully unexpected at lower temperatures was the observation
of another transition in the same atomic wire, this time having a tripled
wavelength. At intermediate temperatures both states can even be observed
to coexist in the wires. It is the first time that two subsequent Charge
Density Wave transitions are observed in one chain.
Fractional electrons
The UT-Delft way of studying atom wires with an STM generates yet another
unique result: the observation of phase slips. Phase slips are locations
where the periodicity of the CDW makes a “mistake;” in between
wavelengths of three atomic distances suddenly one period of four atomic
distances is located. Nobel laureate Schrieffer already predicted in 1981
that these “mistakes” possess a charge equal to ?1/3 or ?2/3
of an electron charge – despite the fact that electrons cannot be
split at all! Also the magnetic moment of the electron (spin) exhibits
this peculiarity; these phase slips either have no spin at all or half
a spin. The exceptionality of these experiments is that this is the very
first time that such fractional quantum numbers are accessible for further
research in a spatially resolved experiment.
More information:
Hanno Weitering (University of Tennessee Physics)
(865) 974-1911
hanno@utk.edu
P.C. Snijders (Kavli Institute of Nanoscience, Delft)
015-2785187
p.c.snijders@tnw.tudelft.nl
Competing periodicities in fractionally filled one-dimensional bands
P.C. Snijders, S. Rogge, H.H. Weitering
Physical Review Letters (February 24, 2006)
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