FZ-Jülich
It is shown that in solids one has to distinguish between the discrete symmetry on atomistic length scale and the continuous symmetry (translational invariance) of the infinite solid. Associated with the two symmetries are characteristic (quasi)particles with particular excitation spectra. The excitations of the atomistic symmetry are well known as phonons and magnons. On the other hand it was shown by K.G. Wilson in his famous renormalization group (RG) theory that in the vicinity of the magnetic phase transition magnets behave like a continuum. Universality of the critical exponents is a consequence of the continuous symmetry. Continuous dynamic symmetry holds not only in the critical range but down to T ->0 and requests field theories instead of spin wave theories for description. The field particles of the continuous magnet we call GSW bosons after Goldstone, Salam and Weinberg. These bosons can be viewed as magnetic density waves. The GSW bosons have dispersion relations different from the atomistic magnons. In particular, they have lower dispersion energy than magnons and therefore are relevant for the dynamics in the sense of RG theory. Although GSW bosons are difficult to identify experimentally the observed universality provides strong evidence for their existence. Distinction between atomistic interactions and excitations of the continuous solid is essential also in diamagnets and in superconductors. It is argued that Debye bosons are the excitations of the continuous diamagnetic solid while Cooper pairs are the quasi particles of the atomistic superconductor. The excitations of the continuous superconductor we call SC bosons. SC bosons can be viewed as charge density waves while Debye bosons are elastic density waves.