Lowtemperature

Low temperature is a term used to describe conditions in which thermal energy is small compared with other energy scales in a system. In physics and engineering, it generally refers to temperatures far below room temperature and often to temperatures below about 120 kelvin, with cryogenic work extending well below 4 kelvin. The concept is relative; the practical lower bound is set by available cooling methods and instrumentation.

At low temperatures, many materials exhibit properties that are not present at higher temperatures. Quantum effects become pronounced, leading to phenomena such as superconductivity, in which electrical resistance can vanish, and superfluidity, in which liquids flow with negligible viscosity. The heat capacity, magnetic behavior, and structural dynamics of materials can change dramatically, and phase transitions may occur with characteristic signatures in thermodynamic measurements. These effects enable the study of fundamental physics as well as practical applications.

Cooling to low temperatures is achieved with cryogenic techniques. Cryogens such as liquid nitrogen (77 K) and liquid helium (4 K) provide primary cooling in many setups. More advanced systems employ mechanical coolers, including pulse-tube and Stirling refrigerators, or dilution refrigerators that reach the millikelvin range. Accurate temperature measurement in cryogenic environments relies on resistance thermometers, thermocouples, thermistors, and other calibrated sensors designed for low temperatures.

Applications span basic research in condensed matter physics and quantum information, as well as practical uses in superconducting magnets for MRI, NMR, and particle accelerators. Cryogenic instrumentation supports infrared and submillimeter astronomy, space instrumentation, and cryopreservation. Safety considerations include the risk of cold burns and oxygen deficiency from liquid gases, as well as pressure hazards from rapid gas expansion.