The value of the absolute zero of temperature, which is the lowest temperature possible to attain, is −273.15°C. Low-temperature physics is the study of techniques for obtaining temperatures close to this point. It also includes the investigation of unusual phenomena at these temperatures.
Producing low temperatures
There are a number of techniques that can be used to obtain low temperatures. A simple way, evaporative cooling, is the reduction in temperature that occurs when a liquid changes from liquid to vapour. This happens because the liquid loses the more energetic molecules, with a consequent overall reduction in temperature. Evaporative cooling is the basic process in the vapour-compression cycle of a domestic refrigerator (see refrigeration)
Much of the pioneering work on low-temperature physics was done by the Dutch physicist Heike Kamerlingh Onnes (1853–1926), who was the first to liquefy helium in 1904 using the Linde process. This method essentially exploits the Joule–Thomson effect, in which gases become cooler as they expand due to the work done by the gas in overcoming the attractive intermolecular forces between the molecules of the gas. Temperatures below 1 K can be obtained in this way. See also liquefaction of gases.
Even lower temperatures can be achieved by adiabatic demagnetization. In this procedure a paramagnetic substance is placed in a magnetic field and the heat produced by magnetization is removed using liquid helium. The substance is then demagnetized when the field is switched off, resulting in a fall in temperature. Adiabatic demagnetization allows temperatures of a small fraction of a degree kelvin (about 0.005 K) to be obtained.
Another technique used in low-temperature physics is dilution refrigeration, which was first suggested by the German physicist Heinz London (1907–70) in 1950. It uses a mixture of the helium–3 and helium–4 isotopes. At a temperature below about 0.87 K a phase transition occurs and the mixture separates into a 3He-rich phase and a 3He-poor phase. A helium-dilution refrigerator is a device in which 3He atoms move from the rich phase to the poor phase—a change that requires energy—and cooling occurs. The process is analogous to evaporative cooling. Dilution refrigerators usually work on a continuous cycle. Thus helium–3 is continuously extracted from the dilute phase, so as to avoid saturation, and resupplied to the rich phase. Continuous dilution refrigerators are used extensively in laboratories to produce temperatures of a few millikelvins.
It is possible to reach temperatures of less than a millionth of a kelvin above absolute zero by using laser cooling. This technique makes use of the Doppler effect to reduce the speeds of atoms. If an atom is moving in the direction of a photon that is coming towards it, the frequency of the photon is higher because of the Doppler effect. If the atom is moving at exactly the right speed to absorb the photon then the impact of the photon slows down the atom. This can be done by tuning the frequency of a laser to correspond to slightly less than an energy gap between quantized energy levels of the atom. Since atoms in a gas move randomly in all directions this means that it is necessary to have the gas that is to be cooled confined to a small region by a magnetic trap and surrounded by six laser beams, which are arranged in three mutually perpendicular pairs pointing in opposite directions.
Laser cooling of trapped atoms has been used to produce temperatures as low as 500 picokelvins.
Quantum effects
Some of the most striking effects in low-temperature physics involve phase transitions to unusual states of matter that do not exist at higher temperatures. These are the result of quantum effects—as temperature decreases, the average speed of a particle in a system also decreases, and quantum effects become more important. This happens because the de Broglie wavelength of a particle, which characterizes its quantum-mechanical wave nature, is given by λ=h/(m v), where h is the Planck constant, m is the mass of the particle, and v the speed. Since low temperatures correspond to low average particle speeds, they are also associated with large de Broglie wavelengths, with the consequence that the quantum-mechanical wave aspects of the behaviour of systems are more apparent at low temperatures.
Bose–Einstein condensation
This is a specifically quantum-mechanical phenomenon occurring at very low temperatures in which a gas of identical bosons forms a system in which all the atoms in the gas have a single quantum state—a Bose–Einstein condensate. It was thought for many years that it would not be possible to produce such systems experimentally because the attractive interatomic forces would cause normal condensation to a liquid at a higher temperature than the very low temperatures required to produce a Bose–Einstein condensate. In 1995, two groups of experimental physicists succeeded in producing condensates by making the gas sufficiently dilute to reduce the interatomic forces.
Superfluidity
A striking manifestation of the importance of quantum statistics in determining the physical properties of matter is given by the contrast between the isotopes helium–4 and helium–3. The very common isotope helium–4 is a boson, whereas the much rarer isotope helium–3 is a fermion. Helium can be liquefied at a temperature of about 4 K. When helium–4 is cooled to a temperature of about 2 K it undergoes a phase transition to a state that has some remarkable physical properties. One such property is superfluidity, i.e. the ability of the liquid to flow without resistance. An essential ingredient in understanding superfluidity is that there is a Bose–Einstein condensate formed from helium–4 atoms.
Since helium–3 atoms are fermions, there is not a phase transition to a superfluid state at about 2 K. However, at very low temperatures the interatomic attractions between helium–3 atoms enable pairs of helium–3 atoms to form. These pairs act as bosons and undergo a Bose–Einstein condensation to superfluid helium–3 at about two thousandths of a kelvin above absolute zero.
Superconductivity
When many metals are cooled beneath a certain critical temperature all their electrical resistance vanishes; the metals are called superconductors. Superconductivity was understood in 1957 when John Bardeen, Leon Cooper, and Robert Schrieffer put forward a theory usually referred to as the BCS theory. The key realization was that there can be a net attraction between two electrons to form what are called Cooper pairs because of interactions between the electrons and photons. These Cooper pairs act as bosons, with their Bose–Einstein condensate being the superconducting state. Since the mid‐1980s many high-temperature superconductors, i.e. superconductors with much higher transition temperatures than the 25 K which is the maximum found for BCS superconductors, have been discovered. The mechanism for high‐temperature superconductivity has not yet been established.