low-temperature physics

(cryogenics) A study of the properties of gross matter at low temperatures, especially at temperatures so low that the quantum character of the substance becomes observable in effects such as superconductivity, superfluid liquid helium, magnetic cooling, and nuclear orientation.

A branch of physics dealing with physical properties of matter at temperatures such that thermal fluctuations are greatly reduced and effects of interactions at the quantum-mechanical level can be observed. As the temperature is lowered, order sets in (either in space or in motion), and quantum-mechanical phenomena can be observed on a macroscopic scale.

Some of the most interesting manifestations of low temperatures have been investigated in the temperature range from 4 K (−452°F) down to less than a nanokelvin above absolute zero. (1 K is equal to 1.8°F above absolute zero, or −459.67°F.) Certain metals become superconducting, losing their electrical resistance entirely; hence persistent currents can flow indefinitely in a superconducting ring or coil, displaying quantum-mechanical coherence over large distances. The liquids helium-3 (³He) and helium-4 (⁴He) remain liquid down to absolute zero under their own vapor pressure due to the large zero-point energy of these light atoms. (To overcome the large zero-point energy in liquid ³He and liquid ⁴He, a large pressure, approximately 30 atm or 3 megapascals, must be applied to cause these systems to solidify.) Liquid ⁴He becomes superfluid, exhibiting no resistance to flow under certain conditions; when set in circulation, the fluid current persists indefinitely. Liquid ³He also becomes superfluid at a much lower temperature with interesting magnetic and orbital effects. At sufficiently low temperatures, nuclear magnetic ordering has been observed in solid ³He, in magnetic insulators, and in metallic systems. Silver becomes a nuclear antiferromagnet in the nanokelvin range as a result of quantum-mechanical exchange interactions. Considerable attention has been addressed to the general problem of ordering in disordered systems leading to studies of spin glasses, localization, and lower dimensionality. Quantum statistics are investigated in atomic hydrogen and deuterium, stabilized in states known as spin-polarized hydrogen (H↓) and spin-polarized deuterium (D↓). Because of its light mass and weak interactions, spin-polarized hydrogen is expected to remain gaseous down to absolute zero, whereas spin-polarized deuterium might liquefy at low temperatures. See also Hydrogen; Liquid helium; Superfluidity.

Low-temperature research also deals with problems of thermometry and heat transfer between systems and within systems. Many practical applications have emerged, including the use of superconductivity for large magnets, ultrafast electronics for computers, and low-noise and high-sensitivity instrumentation. This type of instrumentation has opened new areas of research in biophysics, and in fundamental problems such as the search for magnetic monopoles, gravity waves, and quarks. See also Low-temperature thermometry; Superconducting devices; Superconductivity.

The development of low-temperature techniques has revealed a wide range of other phenomena. The behavior of oriented nuclei is studied by observing the distribution of gamma-ray emission of radioactive nuclei oriented in a magnetic field. Other areas of study include surfaces of liquid ³He and liquid ⁴He, ³He–⁴He mixtures, cryogenics, acoustic microscopy, phonon spectroscopy, monolayer helium films, molecular hydrogen, determination of the voltage standard, and phase transitions. See also Acoustic microscope; Cryogenics; Electrical units and standards; Nuclear orientation; Phase transitions.