Quantum fluids in solid materials

Phase transitions between solid, liquid, gas and plasma are common to most matter and have been extremely well studied. The existence of another, the so-called fifth state of matter is much less known, although it was predicted in the 1920s by Satyendranath Bose and Albert Einstein. It requires identical particles that follow the Bose-Einstein statistics, i.e. having integer spin. In the phase transition from a dilute gas to such a Bose-Einstein condensate (BEC) atoms and molecules lose their individual character and collectively join the lowest quantum state. The long-range coherence between the particles establishes a macroscopic blob of quantum fluid which can exhibit unique properties like superfluidity, much different to the other states of matter.