Helium-4
Bosons such as helium-4 follow Bose-Einstein statistics which, among other things, means that under certain circumstances they condense in the state that possesses the least energy. A phase transition process in which this occurs is termed Bose-Einstein condensation. Super fluid helium-4 is a Bose-Einstein condensate. This state of matter was first predicted by Satyendra Nath Bose and Albert Einstein in 1924-25. The nuclei of helium-4 atoms consist of an even number of neutrons and protons. Each of these particles has a quantum spin of ½. The nucleus as a whole has a whole-integer spin. Bosons are elementary particles that have a whole integer spin and several bosons can occupy the same quantum state at the same time. Examples of bosons are photons, theoretical gravitons, gauge bosons and theoretical Higgs bosons. They are often force carrier particles. Helium-4 nuclei are not true bosons but they can act like bosons if they are cold enough. The wave functions of bosons like to overlap and when the wave functions of helium-4 atoms begin to overlap, they begin to align themselves with one another forming a synchronized wave function. A wave function describes the quantum state of a particle, namely its spin, location and velocity. The individual atoms in this sense lose their individual identity and a Bose-Einstein condensate forms.
As a super fluid, helium-4 is in a macroscopic quantum state. This is very unique. Its macroscopic qualities such as thermal conductivity and viscosity, qualities we can observe, are governed by quantum mechanics rather than classical mechanics, as they are in ordinary fluids. Within all matter, atoms jiggle about randomly. This is called Brownian or thermal motion. As heat is applied to the matter, its atoms move about faster and bounce off each other more often and with more force. That is why gases, with more energetic atoms, are less dense than liquids and solids, with slower atoms, can maintain a lattice-like atomic structure. The quantum nature of these atoms is masked by their energetic kinetic behaviour. You cannot observe their quantum nature directly. However, there is so little thermal motion of helium atoms in the superliquid phase that it can no longer mask the underlying quantum nature of its atoms. When in this super fluid state, all the helium atoms, no matter how large the sample, behave as one single entity.They all move together as one particle, somewhat like people in a marching band.
As a super fluid, helium-4 is in a macroscopic quantum state. This is very unique. Its macroscopic qualities such as thermal conductivity and viscosity, qualities we can observe, are governed by quantum mechanics rather than classical mechanics, as they are in ordinary fluids. Within all matter, atoms jiggle about randomly. This is called Brownian or thermal motion. As heat is applied to the matter, its atoms move about faster and bounce off each other more often and with more force. That is why gases, with more energetic atoms, are less dense than liquids and solids, with slower atoms, can maintain a lattice-like atomic structure. The quantum nature of these atoms is masked by their energetic kinetic behaviour. You cannot observe their quantum nature directly. However, there is so little thermal motion of helium atoms in the superliquid phase that it can no longer mask the underlying quantum nature of its atoms. When in this super fluid state, all the helium atoms, no matter how large the sample, behave as one single entity.They all move together as one particle, somewhat like people in a marching band.