Bosons are quantum particles that have integer spin. A
classic example of bosons are photons because they have spin 1. Fermions are quantum
particles that have half integer spin. A classic example of fermions are the electrons
which have spin +/-(1/2) . Because of this bosons and fermions obey different statics in
what it concern mainly their energies. Fermions obey the Pauli exclusion principle which
forbids more than two opposing spin fermions to be on the same energy level. In
contrast, bosons do not need separate energy levels to exists. they can occupy all the
same energy level. In 1925, for the first time, starting from some observations that
Bose made by calculating the statistics of bosons energies and predicted for the first
time that boson particles could condense all in the lower energy state allowed occupying
thus all a single energy level. This state was named the Bose-Einstein condensate.
Experimentally this sate was observed in liquid Helium (liquefaction temperature 4.2 K)
below a so named critical temperature of about 2.17 K. Helium atoms are bosons. In this
state of Helium (below 2.17 K) a Bose-Einstein condensate forms and quantum effects are
observed at the larger scale of the entire gas. For example the liquid becomes
super-fluid (it has no viscosity) and the heat capacity has a big
gap.
There has been made a parallel between the
super-fluidity of liquid Helium and the superconductivity (which happens also below a
certain critical temperature) of certain solids. In both cases a Bose-Einstein
condensate forms, with the difference that the bosons that make the condensate in
superconductivity are made from weak coupled fermions.
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