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low-temperature physics,science concerned with the production and maintenance of temperatures much below normal, down to almost absolute zero, and with various phenomena that occur only at such temperatures. The temperaturetemperature,
measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used.
..... Click the link for more information. scale used in low-temperature physics is the Kelvin temperature scaleKelvin temperature scale,
a temperature scale having an absolute zero below which temperatures do not exist. Absolute zero, or 0°K;, is the temperature at which molecular energy is a minimum, and it corresponds to a temperature of −273.
..... Click the link for more information. , or absolute temperature scale, which is based on the behavior of an idealized gas (see gas lawsgas laws,
physical laws describing the behavior of a gas under various conditions of pressure, volume, and temperature. Experimental results indicate that all real gases behave in approximately the same manner, having their volume reduced by about the same proportion of the
..... Click the link for more information. ; kinetic-molecular theory of gaseskinetic-molecular theory of gases,
physical theory that explains the behavior of gases on the basis of the following assumptions: (1) Any gas is composed of a very large number of very tiny particles called molecules; (2) The molecules are very far apart compared to their sizes,
..... Click the link for more information. ). Low-temperature physics is also known as cryogenics, from the Greek meaning "producing cold." Low temperatures are achieved by removing energy from a substance. This may be done in various ways. The simplest way to cool a substance is to bring it into contact with another substance that is already at a low temperature. Ordinary ice, dry ice (solid carbon dioxide), and liquid airliquid air,
ordinary air that has been liquefied by compression and cooling to extremely low temperatures (see liquefaction). Its commercial preparation involves purification by washing to remove soluble impurities and by passage over calcium oxide (lime) to remove the carbon
..... Click the link for more information. may be used successively to cool a substance down to about 80°K; (about −190°C;). The heat is removed by conductionconduction,
transfer of heat or electricity through a substance, resulting from a difference in temperature between different parts of the substance, in the case of heat, or from a difference in electric potential, in the case of electricity.
..... Click the link for more information. , passing from the substance to be cooled to the colder substance in contact with it. If the colder substance is a liquefied gas (see liquefactionliquefaction,
change of a substance from the solid or the gaseous state to the liquid state. Since the different states of matter correspond to different amounts of energy of the molecules making up the substance, energy in the form of heat must either be supplied to a substance
..... Click the link for more information. ), considerable heat can be removed as the liquid reverts to its gaseous state, since it will absorb its latent heatlatent heat,
heat change associated with a change of state or phase (see states of matter). Latent heat, also called heat of transformation, is the heat given up or absorbed by a unit mass of a substance as it changes from a solid to a liquid, from a liquid to a gas, or the
..... Click the link for more information. of vaporization during the transition. Various liquefied gases can be used in this manner to cool a substance to as low as 4.2°K;, the boiling pointboiling point,
temperature at which a substance changes its state from liquid to gas. A stricter definition of boiling point is the temperature at which the liquid and vapor (gas) phases of a substance can exist in equilibrium.
..... Click the link for more information. of liquid helium. If the vapor over the liquid helium is continually pumped away, even lower temperatures, down to less than 1°K;, can be achieved because more helium must evaporate to maintain the proper vapor pressurevapor pressure,
pressure exerted by a vapor that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapor and some molecules of vapor are condensing to form liquid.
..... Click the link for more information. of the liquid helium. Most processes used to reduce the temperature below this level involve the heat energy that is associated with magnetization (see magnetismmagnetism,
force of attraction or repulsion between various substances, especially those made of iron and certain other metals; ultimately it is due to the motion of electric charges.
..... Click the link for more information. ). Successive magnetization and demagnetization under the proper combination of conditions can lower the temperature to only about a millionth of a degree above absolute zero. Reaching such low temperatures becomes increasingly difficult, as each temperature drop requires finding some kind of energy within the substance and then devising a means of removing this energy. Moreover, according to the third law of thermodynamicsthermodynamics,
branch of science concerned with the nature of heat and its conversion to mechanical, electric, and chemical energy. Historically, it grew out of efforts to construct more efficient heat engines—devices for extracting useful work from expanding hot gases.
..... Click the link for more information. , it is theoretically impossible to reduce a substance to absolute zero by any finite number of processes. Superconductivitysuperconductivity,
abnormally high electrical conductivity of certain substances. The phenomenon was discovered in 1911 by Heike Kamerlingh Onnes, who found that the resistance of mercury dropped suddenly to zero at a temperature of about 4.
..... Click the link for more information. and superfluiditysuperfluidity,
tendency of liquid helium below a temperature of 2.19°K; to flow freely, even upward, with little apparent friction. Helium becomes a liquid when it is cooled to 4.2°K;.
..... Click the link for more information. have traditionally been thought of as phenomena that occur only at temperatures near absolute zero, but by the late 1980s several materials that exhibit superconductivity at temperatures exceeding 100°K; had been found. Superconductivity is the vanishing of all electrical resistance in certain substances when they reach a transition temperature that varies from one substance to another; this effect can be used to produce powerful superconducting magnets. Superfluidity occurs in liquid helium and leads to the tendency of liquid helium to flow over the sides of any container it is placed in without being stopped by friction or gravity.
See A. C. Helden, The Coldest Spot on Earth (1989).
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 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 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 Cryogenics, Electrical units and standards, Nuclear orientation, Phase transitions