First Bose-Einstein Condensate in a Solid

A Bose-Einstein condensate (BEC) has been observed in a solid material for the first time. The BEC in this case is not a collection of atoms but rather a collection of particle-like excitations in the solid, called “magnons.” In the presence of extremely high magnetic fields, atoms with an intrinsic magnetism of their own (as represented by a spin vector) can be oriented all in one direction if the field strength is larger than a certain value. In this configuration a small input of energy can tilt some of the spins out of the general formation. The successive tilting of spins can take the form of a wave moving through the sample. If also the temperature of the sample is extremely low, then the moving wave can be considered as a particle-like (or quasiparticle) entity, much as mechanical vibrations in a solid can be construed as sound waves or as phonons. A magnon is such a moving magnetic-spin disturbance. What the present experiment observes is a condensation of magnons if the magnetic field is lower than the critical strength and the temperature is below a characteristic value. The work was carried out by a group of scientists from these institutions: Max Planck Institute for Chemical Physics of Solids (MPI, CPfS), Dresden; JINR Lab, Dubna; Oxford University; and Adam Mickiewicz University, Poznan. They used a antiferromagnetic material (in which the spins of neighboring atoms tend to be alternately aligned up and down) with a chemical composition of Cs₂CuCl₄. The temperatures were in the mK range and the external magnetic field used was at high as 12 T (120,000 gauss). In an atomic BEC, dilute vapors of atoms (typically a million or so at a time) are chilled until they enter into a single quantum state, as if all the atoms were one atom. In a magnon BEC what is formed is a monolithic static magnetic alignment in the solid. About 10²³ magnons participate in the condensation. A magnon BEC had been predicted several years ago but not realized unambiguously until this work. The evidence for condensation is that the material undergoes a phase transition at a critical temperature dependent on the size of the external field used. What the researchers look for is a significant change in the measured heat capacity (the energy needed to raise the material’s temperature by a certain amount).

Radu et al., Physical Review Letters, 16 September 2005
Contact Heribert Wilhelm, wilhelm@cpfs.mpg.de