Microscope dangles from 'bungee' cords


Rebecca Pool

Wednesday, April 11, 2018 - 08:45
Image: Professor Ulrike Diebold and colleagues with their suspended STM/AFM [TU Wien]
Researchers at the Vienna University of Technology, Austria, have developed a vibration damping system to minimise the impact of environmental vibrations on their high-resolution scanning tunneling microscope/non-contact atomic-force microscope.
Using 36 vibration-damping elastic cords, Professor Ulrike Diebold and colleagues have suspended their 1 tonne microscope from the ceiling and claim picometre resolution is easily achieved. 
The STM/AFM is housed in a widely used building surrounded by heavy street traffic and railroads.
As Diebold points out: "Other research groups operate similar microscopes in separate basements, or in specially designed buildings."
"When I mention at conferences that we run our instrument in a high-rise building in the middle of Vienna, directly above the subway, colleagues are amazed," she adds.
Single atoms - without vibration damping (left) and with vibration damping (right)
As part of the system, the cords are intertwined to simultaneously dampen vibrations that come from different directions.
The instrument is suspended only 2 mm above the ground, and the distance is monitored with position sensors.
An autolevelling electronic control system adjusts suspension, keeping the instrument horizontal, without any detectable vibrations, even with changing load distribution.
The system automatically readjusts the position of the microscope by pulling on elastic cords with one of three separate electrical motors.
"This is important because weight is shifting around during the experiments," explains Diebold's colleague, Michael Schmid. "We use liquid nitrogen to cool our samples. When nitrogen evaporates, one part becomes lighter, but the overall construction must remain exactly horizontal."
Measurements indicate that the resonance peaks induced by the vibration isolation are much lower than for spring-suspended systems and do not compromise the resolution of the instrument.
The high-performance microscope is dangling from 36 vibration-damping elastic cords.
Prior to system development, Schmid had analysed building vibrations, discovering that wind causes the building to oscillate at a frequency of a few Hertz, and the subway excites vibrations each time it passes underneath.
"We quickly realized that conventional vibration damping would not be enough in our case," says Schmid. "Commercially available solutions filter high-frequency vibrations, but it is hard to get rid of the low frequencies."
He also uncovered the origin of a mysterious 20 Hertz vibration that was so strong it made measurements impossible; but only at certain times of the day.
"It took us a while to realize that this vibration is caused by compressors in the basement that are used to liquefy helium," explains Schmid.
Numerous measurements have been carried out with the special vibration damping and have already resulted in several scientific publications, and the system has now been patented.
"Of course we hope that other institutions will take up our idea and also improve their results as drastically as we did," says Michael Schmid.
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