New JILA apparatus measures fast nanoscale motions
A new nanoscale apparatus developed at JILA - a
tiny gold beam whose 40 million vibrations per second are measured by
hopping electrons - offers the potential for a 500-fold increase in
the speed of scanning tunneling microscopes (STM), perhaps paving the
way for scientists to watch atoms vibrate in high definition in real
time.
The new device measures the wiggling of the beam,
or, more precisely, the space between it and an electrically
conducting point just a single atom wide, based on the speed of
electrons “tunneling” across the gap. The work is the first use of an
“atomic point contact,” the business end of an STM, to sense a
nanomechanical device oscillating at its “resonant” frequency, where
it naturally vibrates like a tuning fork. JILA is a joint venture of
the National Institute of Standards and Technology (NIST) and the
University of Colorado at Boulder.
This slow-motion simulation of
the JILA nanoscale motion detector shows the wiggling of a floppy
metal beam, just 100 nanometers thick, as it is struck by an
electric current at the dot. Red indicates the greatest change in
position from the rest state.
Credit: K. Lehnert/JILA
Although the JILA technique, described in the
March 2 issue of Physical Review Letters, is not necessarily as
precise as more complex and much colder methods of measuring very fast
motions of ultra-small devices, it incorporates several innovative
attributes. These include the ability to minimize unwanted random
electronic “noise” as well as to measure the random shaking of the
beam caused by back-action or recoil (similar to what happens when a
gun is fired). This level of sensitivity is possible because the
atomic point contact acts as an amplifier for these otherwise
imperceptible factors, and the gold beam is tiny and floppy enough -
just 100 nanometers (nm) thick, and 5.6 micrometers long by 220 nm
wide - to respond to single electrons.
The new method involves bringing the sharp point
within one nanometer of the gold beam. A current is applied through
the point across the gap, until an increase in resistance indicates
that electrons are “tunneling” across the gap (a phenomenon observed
only at atomic dimensions). The size of the gap is then monitored
based on variations in the current. The beam’s undulations were
measured with tens to hundreds of times greater precision than a
typical STM result. That’s because the oscillations are measured using
microwave electronics, which are much faster than the audio frequency
technology typically used with STMs, thus enabling greater precision.
The microwave measurement technique could potentially be applied to
STMs.
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