DISSIPATION AND DECOHERENCE IN QUANTUM MAGNETIC SYSTEMS

Dissipation and decoherence are reviewed for quantum magnets, in which quantum fluctuations render a semi-classical picture incorrect. This is done for 3 examples in detail. First, the tunneling of magnetic domain walls is considered-this is a typical quantum soliton problem, with dissipation being caused by non-linear magnon couplings. Second, we discuss the topological decoherence caused by 1/2-integer environmental spins on the coherent tunneling of giant spins (e.g. ferromagnetic grain magnetization); here the environmental phase is important, and this very effectively destroys coherence. Finally, the influence of singular gauge interactions on the properties of spin liquids is discussed - these create a new type of conformal symmetry-breaking environment, with very strong dissipative properties at low energies, which is thought to describe the normal state high-T(c) superconductors. From these examples one sees that quantum magnets possess rather novel and even bizarre low-temperature properties, with interesting experimental consequences.