Cryogenic temperatures are extremely cold conditions, often described as below about 120 kelvin, or minus 153 degrees Celsius. Researchers and engineers create them using specialized cooling systems and very cold liquids called cryogens. Liquid nitrogen boils near 77 K at normal pressure, while liquid helium boils near 4.2 K.
Many superconductors need cryogenic cooling before their resistance disappears. A material that transitions at 77 K can be studied with liquid-nitrogen cooling under suitable conditions, whereas one that requires much lower temperatures may need helium or a closed-cycle refrigerator. The cooling method affects the cost and complexity of experiments and devices.
A superconductor described as high temperature is not necessarily warm by everyday standards. For example, 151 K is about minus 122 degrees Celsius: warmer than the conventional cryogenic boundary of roughly 120 K, but still much colder than room temperature. Different scientific communities use cryogenic and high-temperature labels for different comparisons.
When comparing superconducting materials, record the actual temperature in kelvin and the required cooling equipment rather than relying on broad labels. Low-temperature systems also need careful insulation, thermal management and safety procedures for cryogens, including protection from cold burns and oxygen displacement in enclosed spaces.