Superfluidity

Superfluidity was discovered in helium-4 by Pyotr Kapitsa[2] and independently by John F. Allen and Don Misener[3] in 1937. Onnes possibly observed the superfluid phase transition on August 2 1911, the same day that he observed superconductivity in mercury.[4] It has since been described through phenomenology and microscopic theories.

In liquid helium-4, the superfluidity occurs at far higher temperatures than it does in helium-3. Each atom of helium-4 is a boson particle, by virtue of its integer spin. A helium-3 atom is a fermion particle; it can form bosons only by pairing with another particle like itself at much lower temperatures. The discovery of superfluidity in helium-3 was the basis for the award of the 1996 Nobel Prize in Physics.[1] This process is similar to the electron pairing in superconductivity.

Superfluidity in an ultracold fermionic gas was experimentally proven by Wolfgang Ketterle and his team who observed quantum vortices in lithium-6 at a temperature of 50 nK at MIT in April 2005.[5][6] Such vortices had previously been observed in an ultracold bosonic gas using rubidium-87 in 2000,[7] and more recently in two-dimensional gases.[8] As early as 1999, Lene Hau created such a condensate using sodium atoms[9] for the purpose of slowing light, and later stopping it completely.[10] Her team subsequently used this system of compressed light[11] to generate the superfluid analogue of shock waves and tornadoes:[12]

These dramatic excitations result in the formation of solitons that in turn decay into quantized vortices—created far out of equilibrium, in pairs of opposite circulation—revealing directly the process of superfluid breakdown in Bose–Einstein condensates. With a double light-roadblock setup, we can generate controlled collisions between shock waves resulting in completely unexpected, nonlinear excitations. We have observed hybrid structures consisting of vortex rings embedded in dark solitonic shells. The vortex rings act as 'phantom propellers' leading to very rich excitation dynamics.

— Lene Hau, SIAM Conference on Nonlinear Waves and Coherent Structures

The idea that superfluidity exists inside neutron stars was first proposed by Arkady Migdal.[13][14] By analogy with electrons inside superconductors forming Cooper pairs because of electron-lattice interaction, it is expected that nucleons in a neutron star at sufficiently high density and low temperature can also form Cooper pairs because of the long-range attractive nuclear force and lead to superfluidity and superconductivity.[15]

Superfluid vacuum theory (SVT) is an approach in theoretical physics and quantum mechanics where the physical vacuum is viewed as superfluid.

The ultimate goal of the approach is to develop scientific models that unify quantum mechanics (describing three of the four known fundamental interactions) with gravity. This makes SVT a candidate for the theory of quantum gravity and an extension of the Standard Model.

It is hoped that development of such theory would unify into a single consistent model of all fundamental interactions, and to describe all known interactions and elementary particles as different manifestations of the same entity, superfluid vacuum.

On the macro-scale a larger similar phenomenon has been suggested as happening in the murmurations of starlings. The rapidity of change in flight patterns mimics the phase change leading to superfluidity in some liquid states.[16]

Light behaves like a superfluid in various applications such as Poisson's Spot. As the liquid helium shown above, light will travel along the surface of an obstacle before continuing along its trajectory. Since light is not affected by local gravity its "level" becomes its own trajectory and velocity. Another example is how a beam of light travels through the hole of an aperture and along its backside before diffraction.

• Boojum (superfluidity)
• Condensed matter physics
• Macroscopic quantum phenomena
• Quantum hydrodynamics
• Slow light
• Superconductivity
• Supersolid

• Khalatnikov, Isaac M. (2018). An introduction to the theory of superfluidity. CRC Press. ISBN 978-0-42-997144-0.
• Annett, James F. (2005). Superconductivity, superfluids, and condensates. Oxford: Oxford Univ. Press. ISBN 978-0-19-850756-7.
• Guénault, Tony (2003). Basic superfluids. London: Taylor & Francis. ISBN 0-7484-0891-6.
• Svistunov, B. V., Babaev E. S., Prokof'ev N. V. Superfluid States of Matter
• Volovik, Grigory E. (2003). The Universe in a helium droplet. Int. Ser. Monogr. Phys. Vol. 117. pp. 1–507. ISBN 978-0-19-850782-6.

• Quotations related to Superfluidity at Wikiquote
• Media related to Superfluidity at Wikimedia Commons
• Video: Demonstration of superfluid helium (Alfred Leitner, 1963, 38 min.)
• Superfluidity seen in a 2d fermi gas recent 2021 observation relevant for Cuprate superconductors