Quantum fluid

Quantum fluid is a fluid whose properties are determined by quantum effects . Near absolute zero , according to the ideas of classical physics, the movement of atoms should stop and the substance should turn into a crystal, which does not happen with some substances with a small atomic mass, large zero energy (and, accordingly, significant zero vibrations ) and weak interaction between atoms - that they are liquids, due to quantum effects ^[1] , hinder the formation of crystal lattice - at normal pressure helium remains liquid down to an absolute well I, crystalline helium can only be obtained at elevated pressures up to 25 atmospheres. A liquid becomes quantum when the thermal de Broglie wavelength of its particles becomes comparable with the distance between them ( quantum degeneration of the liquid occurs ^[2] . Depending on whether the particles making up the liquid are bosons or fermions , the liquids are called bosonic or fermionic (bosonic liquid or Fermi liquid ).

Quantum fluids were discovered by Peter Kapitsa and John Allen in 1938. In principle, electrons in metals and semiconductors, excitons in dielectrics and nucleons in atomic nuclei form quantum liquids, however, liquid helium-4 and helium-3 , which are bosonic and fermionic liquids, are considered classical examples of such liquids.

Quantum liquids exhibit their unusual properties in states close to the ground quantum state of minimum energy . In this case, the excited state of the liquid can be described as a gas of elementary excitations - quasiparticles , which in turn can be bosons (which arise one at a time) or fermions (which arise in pairs, since the angular momentum of the liquid can only change by an integer h ). Bose quasiparticles arise in both types of liquids, Fermi ones only in Fermi liquids. Unlike liquid atoms, quasiparticles are constantly born and disappear in interactions with each other, while their distribution in the equilibrium state is given by the corresponding statistics with a finite temperature.

The peculiarity of the properties of quantum liquids is associated with the shape of the spectrum of elementary excitations, that is, with the dependence of the energy of a quasiparticle on its momentum. For example, Bose liquids exhibit a superfluidity property associated with a linear dependence of the elementary excitation energy on the pulse at low momenta, and in Fermi liquids the sound attenuation increases with decreasing temperature, so that at absolute zero ordinary sound in Fermi liquids (transferred by Fermi quasiparticles ) cannot propagate, but there exists and can propagate the so-called zero sound carried by Bose excitations of a quantum Fermi liquid.

Another effect arising in Fermi quantum liquids is the pairing of quasiparticles, which occurs at low temperatures if the quasiparticles are attracted to each other. In this case, below a certain temperature, quasiparticles with oppositely directed momenta form pairs that behave like bosons and, accordingly, exhibit superfluidity. Conductivity electrons in a metal are a kind of Fermi liquid, which is affected by the periodic field of the crystal lattice ^[1] . At extremely low temperatures, electrons can condense into a quantum liquid of Cooper pairs with superconductivity .

Superfluid liquids contain the Bose condensate of their constituent particles, moreover, it is described by a macroscopic wave function. The macroscopic coherence size of this component of the condensate allows its use for high-precision measurements, for example, in SQUIDs .

Neutrons in neutron stars , most likely, will also form a quantum liquid, possibly superfluid.