Scientists can now reliably chill specimens near absolute zero for over 10 hours while taking images resolved to the level of individual atoms with an electron microscope.
The new capability comes from a liquid-helium-cooled sample holder designed by a team of scientists and engineers at the University of Michigan and Harvard University, whose work was federally funded by the Department of Energy and National Science Foundation.
Conventional instruments can usually maintain such an extreme temperature, about -423 degrees Fahrenheit or 36 degrees above absolute zero, for a few minutes, capping out at a few hours. But longer periods of time are needed to take atomic-resolution images of candidate materials for advanced technologies.
These include superconductors that conduct electricity with no heat losses, quantum computers that potentially run millions of times faster than conventional computers for some calculations, as well as neuromorphic computers that improve speed and efficiency by mimicking the human brain. Such candidate materials typically don't have their strange and useful properties unless they are in extreme cold.
"When the atoms get that cold, they don't move much, and that radically changes the behavior of the material," said Robert Hovden , U-M associate professor of materials science and engineering and corresponding author of the study published in the Proceedings of the National Academy of Sciences.
"A lot of really cool things happen. Metals can become insulators or superconductors, and we can design qubits and new computer memories around them," he said. "If we want to understand how these properties emerge, we need to observe the materials at those low temperatures for the entire duration of an experiment."
While ultracold microscopy at -321 F (77 Kelvin) has enabled scientists to snap photos of materials and proteins resolved to the level of individual atoms, colder temperatures are needed to image some quantum properties and achieve higher resolutions. Liquid helium could take the temperatures even closer to absolute zero (0 K, -460 F), since helium condenses around -452 F (4 K).
But practical issues have prevented scientists from using liquid helium with a microscope for more than a few minutes. In most modern transmission electron microscopy platforms, the sample is held under a microscope with a rod attached to a dewar, a thermos-like container. The dewar cools both the rod and the sample when filled with a super-cold liquid, usually nitrogen or helium, which immediately boils inside the dewar, jostling the sample and reducing the image resolution. These issues are worse with liquid helium because it boils more vigorously and evaporates faster.
"It's like pouring water on hot lava," Hovden said. "Not only do you get all these vibrations from the boiling liquid, but the temperature swings all over the place, so the rod contracts and you can't hold the exact temperature you need."
The researchers' instrument can maintain sample temperatures as low as -423 degrees Fahrenheit (20 Kelvin), for over 10 hours, with only 0.004 degrees Fahrenheit (0.002 Kelvin) of wobble. That level of control, which is 10 times better than existing instruments, allows scientists to expose a sample to a finely controlled temperature gradient while watching its properties change under the microscope.
"Being able to see the atomic arrangement as the material changes could be the key to understanding and harnessing the atomic and nanoscale processes that give quantum materials their amazing properties," said Ismail El Baggari , a materials physicist at Harvard University's Rowland Institute and a corresponding author of the study.
The instrument's steady cooling comes from a heat exchanger attached to the sample holder. The helium evaporates as it is pumped through the heat exchanger, cooling the sample before it exits an exhaust vent. Existing closed-loop sample holders
