In General > s.a. bose-einstein condensation; particle statistics.
* Properties: Superfluids exhibit zero viscosity, and quantized vortices when rotated or subject to a temperature gradient.
* Method: Study using second-waves, regions with different concentrations of ordinary/superfluid components.
@ General references: Feynman RMP(57); Khalatnikov 65; SA(76)dec; Collins PT(92)jun; news pn(96)oct; Guénault 03; Adams & Bry PhyA(04); Annett 04 [intro]; Brandão NJP(05) [order parameter and entanglement]; Balibar CP(07); Pilati et al PRL(08) [critical T, 2D and 3D]; Yu AP(08) [as a Bose exchange effect]; Sewell & Wreszinski JPA(09) [mathematical theory]; Dupuis PRL(09) [unified picture]; Roberts CP(09) [drag forces on moving objects]; Tsubota et al PRP(13) [rev]; Andrianopoli PLB(14) [field-theoretical description]; Schmitt LNP-a1404 [intro, field-theoretical approach and applications]; Wreszinski a1506-conf [rev].
@ History: Andronikashvili 90; Donnelly PT(95)jul; Balibar phy/06, Griffin pw(08)aug [discovery]; Kadanoff JSP(13)-a1303 [Lev Landau and John Bardeen, and the importance of the condensate].

Types / Examples > s.a. knots [quantum knots in a superfluid]; Quasiparticles; sound; turbulence.
* Helium: Known and studied since 1938; In 4He, pairs of atoms condense into a macroscopically coherent quantum state (Bose-Einstein condensation) at 2.18 K, which manifests itself as a frictionless fluid; In 3He, the situation is not so simple, and is usually described by the two-fluid model invented by Laszo Tisza and Lev Landau in the late 1930s; He II (0 to 2.172 K) is a superfluid, highly heat-conductive by friction-free convection and described by the Euler equation for an ideal inviscid fluid; He I (2.172 to 4.2 K) is an ordinary fluid, governed by the Navier–Stokes equation for viscous flow; The superfluid can also be treated, as proposed by Fritz London, Lars Onsager, and Richard Feynman, as a macroscopic quantum state characterized by a complex wavefunction.
* Other examples: 2005, Evidence seen in solid hydrogen [@ news pn(05)mar].
* In curved spacetime: Superfluids in intense gravitational fields are assumed to be present in neutron star and quark star cores.
@ 3He: Bunkov et al PRL(00) [sets of 4 atoms?]; Finne et al Nat(03)aug + pn(03)aug [criterion for the onset of turbulence]; Volovik JLTP(08)cm/07 [history]; Ma & Wang PhyA(08) [new models]; Golovko a1103 [leaking out of an open container]; Volovik & Krusius Phy(12) [coherent quantum states of different chirality].
@ 4He: Pollet et al PRL(08), comment Balibar Phy(08) [solid]; Guo et al PRL(10) + Barenghi Phy(10) [visualization of turbulent behavior of normal-fluid component]; news sn(17)mar [simulations and area law for entropy]; > s.a. condensed matter [supersolid].
@ In general relativity and cosmology: Zurek Nat(85)oct [topological defects in cosmology and superfluidity]; Carter gq/99-ln [vortex dynamics], G&C(00)ap [neutron stars]; Casini & Montemayor gq/99 [covariant]; Volovik PRP(01)gq/00 [analogs]; Garcia de Andrade gq/05 [with torsion]; Villegas a1511 [effect of spacetime curvature]; Huang 16 [the universe as a quantum superfluid].
@ Examples: Donnelly pw(97)feb [rotons]; Kapusta PRL(04)ht [for Dirac neutrinos]; Bulgac et al PRL(06) [spin-1/2 fermions]; Kastrinakis AP(14)-a0901 [new states]; news PhysOrg(10)oct [light]; Enss Phy(14) [transition between bosonic and fermionic superfluidity in 2D quantum fluids of ultracold atoms]; López et al PRL(15) [using self-propelling bacteria].
@ Applications: Sato & Packard PT(12)oct [superfluid He quantum interference device, SHeQUID].

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