A supercool new scanning probe microscope
GAITHERSBURG, Md. – A new ultra-low-temperature scanning probe microscope is being billed
by its creators as the most advanced in the world. The microscope, dubbed ULTSPM,
was developed by researchers at the National Institute of Standards and Technology
(NIST), the University of Maryland, Janis Research Co. Inc. and Seoul National University.
The ULTSPM operates at higher magnetic fields and lower temperatures
than any other microscope of its kind, enabling it to resolve energy levels separated
by as little as one-millionth of an electron volt. The probe’s tip must be
mechanically positioned about two atoms’ distance from the surface of the
sample, and this precision must be combined with ultrahigh vacuum, cryogenics and
vibration isolation to obtain measurements of this type.
The ULTSPM lab rests on a separate
110-ton concrete slab (1), supported by pneumatic isolators. Inner (2) and outer
(3) enclosures shield from acoustic noise, with the inner enclosure also acting
as a radio-frequency shield. The microscope is mounted on a 6-ton granite table
(4), also supported by pneumatic isolators. The cryostat (5) is mounted in a hole
in the granite table and in the concrete slab on a third set of pneumatic isolators.
Inside the cryostat, the dilution refrigerator insert (6) hangs immersed in the
liquid helium bath. Samples from the processing lab enter the enclosed room through
two hatches on the right via a central vacuum line (7). Courtesy of NIST.
Vibration control was achieved by building the ULTSPM facility
atop a separate 110-ton concrete block and buffering it by six computer-controlled
air springs. The ULTSPM unit sits on a 6-ton table made of granite and is isolated
from the concrete block by a separate set of air springs, also computer-controlled.
To overcome temperature limitations that existed with previous designs, the researchers
made piezoelectric actuators that expand with atomic-scale precision upon application
of voltage. A low-noise dilution refrigerator supplements the device’s 3-m-deep,
250-liter liquid helium bath to allow the ULTSPM to operate at 10 mK.
A “railroad” system that is vacuum-sealed and that
can be disconnected from the microscope’s chamber allows researchers to prepare
samples and probes without the risk of disturbing measurements in progress.
“The ability to create these kinds of experimental conditions
opens up a whole new frontier in nanoscale physics,” said Robert Celotta,
founding director of the NIST Center for Nanoscale Science and Technology. “This
instrument has been five years in the making, and we can’t help but be excited
about all the discoveries waiting to be made.”
The researchers’ paper, “A 10 mK scanning probe microscopy
facility,” is in press at the Review of Scientific Instruments.
- A framework or latticework having an even arrangement of rods, or any other long narrow objects with interstices between them, used to disperse light or other radiation by interference between wave trains from the interstices. The ability of a grating to separate wavelengths (chromatic resolving power) is expressed as being equal to the number of lines in the grating.
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