Unambiguous evidence for a magnon

Bose—Einstein condensate has been seen in a crystal of cesium copper chloride. For a collection of atoms that have an intrinsic magnetism, the spin vectors can all be oriented in one direction if the field strength is larger than a certain value. In such a configuration, a small input of energy can tilt some of the spins out of the general formation. The tilting can propagate and act like a wave moving through the sample. If the temperature of the sample is extremely low, then the moving wave can be considered as a quasiparticle—a magnon. Further, a monolithic, static spin tilting is regarded as a BEC of magnons. A group of scientists from Germany, Russia, the UK, and Poland used the antiferromagnetic material Cs₂CuCl₄, in which the tendency to acquire a static spin tilting competes with both an applied magnetic field and thermal fluctuations. The physicists observed a condensation of magnons when the magnetic field was lower than the critical strength of 8.51 T and the temperature was below a characteristic value in the millikelvin range. Within the crystal, about 10²³ magnons participated in the BEC. (T. Radu et al. , Phys. Rev. Lett. 95, 127202, 2005.)