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In the field of
cryogenics In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...
,
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
eis utilized for a variety of reasons. The combination of helium’s extremely low molecular weight and weak interatomic reactions yield interesting properties when helium is cooled below its critical temperature of 5.2 K to form a liquid. Even at absolute zero (0K), helium does not condense to form a solid under ambient pressure. In this state, the zero point vibrational energies of helium are comparable to very weak interatomic binding interactions, thus preventing lattice formation and giving helium its fluid characteristics. Within this liquid state, helium has two phases referred to as helium I and
helium II Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. It ...
. Helium I displays thermodynamic and hydrodynamic properties of classical fluids, along with quantum characteristics. However, below its
lambda point The lambda point is the temperature at which normal fluid helium (helium I) makes the transition to superfluid helium II (approximately 2.17 K at 1 atmosphere). The lowest pressure at which He-I and He-II can coexist is the vapor−He-I−He-II ...
of 2.17 K, helium transitions to He II and becomes a quantum
superfluid Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortices that continue to rotate indefinitely. Superfluidity occurs in two ...
with zero viscosity. Under extreme conditions such as when cooled beyond Tλ, helium has the ability to form a new state of matter, known as a
Bose–Einstein condensate In condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low densities is cooled to temperatures very close to absolute zero (−273.15 °C or −459.6 ...
(BEC), in which the atoms virtually lose all their energy. Without energy to transfer between molecules, the atoms begin to aggregate creating a volume of equivalent density and energy. From observations, liquid helium only exhibits super-fluidity because it contains isolated islands of BECs, which have well-defined magnitude and phase, as well as well-defined phononroton (P-R) modes. A phonon refers to a quantum of energy associated with a compressional wave such as the vibration of a crystal lattice while a roton refers to an elementary excitation in superfluid helium. In the BEC’s, the P-R modes have the same energy, which explains the zero point vibrational energies of helium in preventing lattice formation. When helium is below Tλ, the surface of the liquid becomes smoother, indicating the transition from liquid to superfluid.Charlton, T. R., R. M. Dalgliesh, O. Kirichek, S. Langridge, A. Ganshin, and P. V. E. Mcclintock. "Neutron Reflection from a Liquid Helium Surface." Low Temperature Physics 34.4 (2008): 316-19. Print. Experiments involving neutron bombardment correlate with the existence of BEC’s, thereby confirming the source of liquid helium’s unique properties such as super-fluidity and heat transfer. Though seemingly paradoxical, cryogenic helium systems can move heat from a volume of relatively low temperature to a volume of relatively high temperature. Though this phenomenon appears to violate the
second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unles ...
, experiments have shown this to prevail in systems where the volume of low temperature is constantly heated, and the volume of high temperature is constantly cooled. It is believed this phenomenon is related to the heat associated with the phase change between liquid and gaseous helium.


Applications


Superconductors Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...

Liquid helium is used as a coolant for various superconducting applications. Notable are particle accelerators where magnets are used for steering charged particles. If large magnetic fields are required then superconducting magnets are used. In order for superconductors to be efficient, they must be kept below their respective critical temperature. This requires very efficient heat transfer. Because of the reasons discussed previously, superfluid helium can be used to effectively transfer heat away from superconductors.


Quantum computing

One proposed use for superfluid helium is in quantum computing. Quantum computers utilize the quantum states of matter, such as the
electron spin In atomic physics, the electron magnetic moment, or more specifically the electron magnetic dipole moment, is the magnetic moment of an electron resulting from its intrinsic properties of spin and electric charge. The value of the electron magnet ...
, as individual quantum bits (qubits), a quantum analogue of the bit used in traditional computers to store information and perform processing tasks. The spin states of the electrons present on the surface of superfluid helium in a vacuum show promise as excellent qubits. In order to be considered a usable qubit, a closed system of individual quantum objects must be created that interact with each other, but whose interaction with the outside world is minimal. In addition, the quantum objects must be able to be manipulated by the computer, and the quantum system’s properties must be readable by the computer to signal the termination of a computational function. It is believed that in vacuum, superfluid helium satisfies many of these criteria since a closed system of its electrons can be read and easily manipulated by the computer in a similar fashion as electrostatically manipulated electrons in semiconductor heterostructures. Another beneficial aspect of the liquid helium quantum system is that application of an electrical potential to liquid helium in a vacuum can move qubits with little decoherence. In other words, voltage can manipulate qubits with little effect on the ordering of the phase angles in the wave functions between the components of the liquid helium quantum system.


X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

The advent of high-flux X-rays provides a useful tool for developing high-resolution structures of proteins. However, higher energy crystallography incurs radiation damage to the proteins studied. Cryogenic helium systems can be used with greater efficacy than nitrogen cryogenic systems to prevent radical damage to protein crystals.Cryogenic (<20 K) helium cooling mitigates radiation damage to protein crystals” Acta Crystallographica Section D. 2007 63 (4) 486-492


See also

Dilution refrigerator A 3He/4He dilution refrigerator is a cryogenic device that provides continuous cooling to temperatures as low as 2  mK, with no moving parts in the low-temperature region. The cooling power is provided by the heat of mixing of the Hel ...


References

{{Reflist, colwidth=30em Cryogenics Helium Superfluidity