superfluid

A superfluid is described as a macroscopically coherent quantum system $S_F$ that exhibits `unusual' properties such as negligible losses or no resistance to flows. Therefore, it is defined as a phase that exhibits most unusual effects, previously considered to be anomalous from a Thermodynamic viewpoint, that can be however explained in terms of quantum statistical theory/quantum statistical mechanics and long-range coherence of coupled boson systems. Such theories are, for example, L. D. Landau's phenomenological/ microscopic theory of superfluidity in liquid $^4$He (the 1962 Nobel Prize in Physics), and Sir Anthony Leggett's quantum statistical theory of superfluidity in liquid $^3$He (the 2003 Nobel Prize in Physics shared with Alexei A. Abrikosov).

As an example, in a superconductor the long-ranged coupled Cooper electron pairs form a superfluid that sustains very high electric currents without any significant heating of the metal. Liquid $^3$He is another example of a superfluid where the liquid flow can take place upwards in the absence of externally applied pressure and no apparent resistance to flow-no viscosity.



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