Editing Helium-3 (section)

== Physical properties ==
Due to its low atomic mass of 3.016&nbsp;[[dalton (unit)|u]], helium-3 has some [[Physical property|physical properties]] different from those of helium-4, with a mass of 4.0026&nbsp;u. On account of the weak, induced [[dipole–dipole interaction]] between the helium atoms, their microscopic physical properties are mainly determined by their [[zero-point energy]]. Also, the microscopic properties of helium-3 cause it to have a higher zero-point energy than helium-4. This implies that helium-3 can overcome dipole–dipole interactions with less [[thermal energy]] than helium-4 can.

The [[quantum mechanical]] effects on helium-3 and helium-4 are significantly different because with two [[proton]]s, two [[neutron]]s, and two [[electron]]s, helium-4 has an overall [[Spin (physics)|spin]] of zero, making it a [[boson]], but with one fewer neutron, helium-3 has an overall spin of one half, making it a [[fermion]].

Pure helium-3 gas boils at 3.19 [[kelvin|K]] compared with helium-4 at 4.23 K, and its [[critical point (thermodynamics)|critical point]] is also lower at 3.35 K, compared with helium-4 at 5.2 K. Helium-3 has less than half the density of helium-4 when it is at its boiling point: 59 g/L compared to 125 g/L of helium-4 at a pressure of one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026 [[kilojoule per mole|kJ/mol]] compared with the 0.0829 kJ/mol of helium-4.<ref>[http://www.trgn.com/database/cryogen.html Teragon's Summary of Cryogen Properties] {{Webarchive|url=https://web.archive.org/web/20170809013754/http://www.trgn.com/database/cryogen.html |date=2017-08-09 }} Teragon Research, 2005</ref><ref>{{cite journal|last=Chase|first=C. E.|author2=Zimmerman, G. O.|date=1973|title=Measurements of P-V-T and Critical Indices of He<sup>3</sup> |journal=[[Journal of Low Temperature Physics]]|volume=11|issue=5–6|pages=551|bibcode=1973JLTP...11..551C|doi=10.1007/BF00654447|s2cid=123038029}}</ref>

=== Superfluidity ===
[[File:Phase diagram of helium-3 (1975) 0.002 K region-en.svg|thumb|Phase diagram for Helium-3. "Bcc" indicates a [[body-centered cubic]] crystal lattice.]]
An important property of helium-3, which distinguishes it from the more common helium-4, is that its nucleus is a [[fermion]] since it contains an odd number of spin {{frac|1|2}} particles. Helium-4 nuclei are [[boson]]s, containing an even number of spin {{frac|1|2}} particles. This is a direct result of the [[Angular momentum quantum number#Addition of quantized angular momenta|addition rules]] for quantized angular momentum. At low temperatures (about 2.17 K), helium-4 undergoes a [[phase transition]]: A fraction of it enters a [[superfluid]] [[phase (matter)|phase]] that can be roughly understood as a type of [[Bose–Einstein condensate]]. Such a mechanism is not available for helium-3 atoms, which are fermions. Many speculated that helium-3 could also become a superfluid at much lower temperatures, if the atoms formed into ''pairs'' analogous to [[Cooper pair]]s in the [[BCS theory]] of [[superconductivity]]. Each Cooper pair, having integer spin, can be thought of as a boson. During the 1970s, [[David Lee (physicist)|David Lee]], [[Douglas Osheroff]] and [[Robert Coleman Richardson]] discovered two phase transitions along the melting curve, which were soon realized to be the two superfluid phases of helium-3.<ref>{{cite journal|last=Osheroff|first=D. D. |author2=Richardson, R. C. |author3=Lee, D. M. |date=1972|title=Evidence for a New Phase of Solid He<sup>3</sup> |journal=[[Physical Review Letters]]|volume=28|issue=14|pages=885–888|doi=10.1103/PhysRevLett.28.885|bibcode= 1972PhRvL..28..885O|doi-access=free}}</ref><ref>{{cite journal|last=Osheroff|first=D. D.|author2=Gully, W. J. |author3=Richardson, R. C. |author4= Lee, D. M. |date=1972|title=New Magnetic Phenomena in Liquid He<sup>3</sup> below 3 mK|journal=Physical Review Letters |volume=29|issue=14|pages=920–923|doi=10.1103/PhysRevLett.29.920|bibcode=1972PhRvL..29..920O}}</ref> The transition to a superfluid occurs at 2.491 millikelvins on the melting curve. They were awarded the 1996 [[Nobel Prize in Physics]] for their discovery. [[Alexei Alexeyevich Abrikosov|Alexei Abrikosov]], [[Vitaly Lazarevich Ginzburg|Vitaly Ginzburg]], and [[Anthony James Leggett|Tony Leggett]] won the 2003 Nobel Prize in Physics for their work on refining understanding of the superfluid phase of helium-3.<ref>{{cite journal|last=Leggett|first=A. J.|date=1972 |title=Interpretation of Recent Results on He<sup>3</sup> below 3 mK: A New Liquid Phase?|journal=Physical Review Letters |volume=29|issue=18|pages=1227–1230|doi=10.1103/PhysRevLett.29.1227|bibcode=1972PhRvL..29.1227L}}</ref>

In a zero magnetic field, there are two distinct superfluid phases of <sup>3</sup>He, the A-phase and the B-phase. The B-phase is the low-temperature, low-pressure phase which has an isotropic energy gap. The A-phase is the higher temperature, higher pressure phase that is further stabilized by a magnetic field and has two point nodes in its gap. The presence of two phases is a clear indication that <sup>3</sup>He is an unconventional superfluid (superconductor), since the presence of two phases requires an additional symmetry, other than gauge symmetry, to be broken. In fact, it is a ''p''-wave superfluid, with spin one, '''S'''=1, and angular momentum one, '''L'''=1. The ground state corresponds to total angular momentum zero, '''J'''='''S'''+'''L'''=0 (vector addition). Excited states are possible with non-zero total angular momentum, '''J'''>0, which are excited pair collective modes. These collective modes have been studied with much greater precision than in any other unconventional pairing system, because of the extreme purity of superfluid <sup>3</sup>He. This purity is due to all <sup>4</sup>He phase separating entirely and all other materials solidifying and sinking to the bottom of the liquid, making the A- and B-phases of <sup>3</sup>He the most pure condensed matter state possible.