A supercool new scanning probe microscope

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.