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Bose-Einstein condensation of collective modes (phonons or magnons) represents a fifth state of matter where all constituents are in resonance and fall into a single quantum state. Its room temperature realizations may open new technological and medical breakthroughs. BEC requires quantum degeneracy, which takes place when the average distance between quasiparticles (ξ) becomes shorter than their wavelength (Λ). One of the ways of creating phonon BEC is using flat optical phonon branch, ω(k)≅const, with temperature-independent average value of Λ and increasing the temperature to achieve shorter ξ.
STM imaging phonon BEC (Sci. Rep. 2017) was accomplished using quasi-freestanding WSe2 monolayers on graphene by means of mapping phonon interference field developed around intercalation defects. Two types of interference patterns were observed, as shown in the STM cross-sections below, corresponding to different excitation states of phonon BEC.
Literature
I. Altfeder et al., Scanning Tunneling Microscopy Observation of Phonon Condensate.
I. Nardecchia et al., Out-of-Equilibrium Collective Oscillation as Phonon Condensation in a Model Protein.
Recent events:
International workshop “Bose-Einstein Condensation in Inorganic and Organic Matter”, Marseille, France, April 2019